Vitamins and Phytonutrients - InDepth
Carotenoids - InDepth; Flavonoids - InDepth; Phytochemicals - InDepth
An in-depth report on the dietary importance of vitamins and other nutrients.
Highlights
Overview
- A balanced, healthy diet is the best way to obtain vitamins, minerals, and other plant-derived nutrients known as phytochemicals. Fresh fruits and vegetables, along with whole grains, are the primary sources of these nutrients, in addition to dietary fiber.
- Dietary supplements are governed by the Food and Drug Administration (FDA) current Good Manufacturing Practice (cGMP) regulations. These regulations require that manufacturers evaluate their products through testing for identity, purity, strength, and composition. Once on the market, side effects or false claims may prompt restrictions or withdrawal of a supplement.
- In general, supplements are helpful only for certain people (for example, during pregnancy or for people with deficiencies or certain illnesses), and they may actually be harmful to other people. More than one-half of American adults use supplements, most often multivitamins and minerals.
Vitamins and Health
- Many fresh fruits and vegetables contain antioxidants and phytochemicals that may help to inhibit carcinogenesis, including lung, breast, colon, and prostate cancers.
- Although Vitamin D has been the focus of hundreds of studies, the question of whether supplementation with vitamin D is beneficial has not been answered.
- In a 12-year trial of nearly 6,000 male physicians over age 65, a daily multivitamin was not shown to provide any cognitive benefits compared to not taking a daily multivitamin.
- According to the U.S. Preventive Services Task Force (USPSTF), there is not enough evidence to assess the balance of benefits and harms for the use of daily multivitamins, or single or paired nutrient supplements, for the prevention of cardiovascular disease or cancer in healthy adults. The USPSTF recommends avoiding vitamin E or beta-carotene supplementation for the prevention of heart disease or cancer.
- A combination of zinc and antioxidants, including vitamins C, E, and beta-carotene (a vitamin A precursor), slows the progression of age-related macular degeneration (AMD). The combination can be helpful in people with intermediate or advanced AMD in one eye. However, these supplements will not prevent macular degeneration from developing in the first place.
- A review of several trials showed that vitamin C supplementation does not prevent the average person from getting a cold, but it may have a protective effect for some athletes who perform strenuous physical exercise.
New Recommendation
- Healthy postmenopausal women with good diets may not benefit from low-dose vitamin D and calcium supplements. The National Osteoporosis Foundation stresses the importance of calcium and vitamin D for strong bones, and the use of medications along with these nutrients to prevent fractures.
- However, according to the USPSTF, daily low-dose amounts of vitamin D and calcium supplements do not prevent fractures and can increase the risk of developing kidney stones. Talk with your doctor and registered dietitian about whether you need these supplements and how to obtain calcium and vitamin D from your diet.
Introduction
Vitamins are essential nutrients that the human body needs in small amounts for normal function and good health. A well-balanced diet should provide most of the vitamins people need to stay healthy and prevent disease. Vitamin D is the only exception since it is mostly obtained through sun exposure. Vitamin supplements can be helpful in some situations, such as pregnancy and certain illnesses. Strict vegetarians may benefit from taking vitamin B12 supplements.
Unlike carbohydrates, fats, and proteins, vitamins are not sources of energy. Instead, vitamins are involved in the body's metabolism, cell production, tissue repair, and other vital processes.
Vitamins are either fat-soluble or water-soluble.
- Fat-soluble vitamins, which include A, D, E, and K, are stored in the liver and used up very slowly. This means that deficiencies arise only very slowly. However, because the body stores fat-soluble vitamins, they can be dangerous when taken in large amounts.
- Water-soluble vitamins include vitamin C and the B vitamins. The body uses these vitamins very quickly. Excess amounts are excreted in the urine.
Guidelines for Adequate Intake of Vitamins
The Recommended Dietary Allowance (RDA) for vitamins, set by the Food and Nutrition Board of the National Academy of Sciences-National Research Council, has been used for years as a guideline for determining the amounts of vitamins needed to prevent deficiencies and diseases.
You may also see other terms such as Dietary Reference Intake (DRI). The DRI is a general term used to describe the types and amounts of nutrients healthy people need.
The DRI values are based on four categories:
- Recommended dietary allowance (RDA). This is the current rating on most vitamins.
- Estimated average requirement (EAR). This is the amount of the vitamin needed to meet the nutritional requirements for 50% of the population.
- Adequate intake (AI). This amount is used if there are not enough data to calculate the RDA.
- Tolerable upper intake level (UL). This is the maximum dose that is likely to be safe in 98% of the population.
Food and supplement labels now typically list the Daily Value (DV) for consumers. The DV is based on the recommended daily amount of a nutrient for an adult person taking in 2,000 calories a day.
Regulating Quality
Regulation of dietary supplements by the U.S. Food and Drug Administration (FDA) is a complex issue. Under the Dietary Supplement Health and Education Act of 1994, the FDA is required to use a different set of regulations for supplements than for conventional over-the-counter or prescription drugs. This law stipulates that the supplement manufacturer is responsible for making sure the supplement is safe before it is marketed. The FDA is responsible for taking action only if a supplement is found to be unsafe after it reaches the market.
Labels for vitamins and other dietary supplements now include nutrient information and list all of the ingredients in the supplement, including the parts of plants from which the ingredients are taken. Unlike drug labels, vitamin and supplement labels cannot claim to treat any specific disease, but may describe how the nutrient may prevent the disease.
Vitamin and supplement labels may include one of the following:
Health claim.
Description of how the substance may reduce the risk for a health-related condition.Nutrient claim.
Description and amounts of nutrients in the product.Structure or function claim.
Description of how the product may affect organs or systems of the body, without claiming to treat specific diseases.
Dietary supplements are not standardized by the FDA, meaning that the amounts or quality of nutrients may vary depending on the batch or manufacturer. So, more expensive supplements are not necessarily better than less expensive ones. Current legislation limits government regulation of the booming supplement industry. Some companies voluntarily stick to rigorous quality controls, while others do not.
The U.S. Pharmacopeia (USP), an independent organization that sets quality standards for drugs, has also implemented standards for vitamins. Consumers may look for the USP label on products made by companies that follow these standards.
USP verification means the following:
- What is in the bottle matches what is listed on the label.
- There are no harmful levels of contaminants.
- The supplement will be properly absorbed into the body.
- The supplement has been produced according to good manufacturing standards.
The FDA does not require manufacturers to provide any scientific evidence that dietary supplements are safe and effective before a product is sold (unlike drugs, which must be proven both safe and effective through clinical trials). If a supplement causes side effects in people once it is sold, the government may place restrictions on the supplement or withdraw it from the market. The FDA may also withdraw products from the market if their labels are false or misleading.
People Who Should Take Vitamin Supplements
Dietary supplement use is on the rise. More than one-half of American adults use supplements, most often multivitamins and minerals. In particular, more Americans are taking vitamin D and calcium supplements than in the past. Although most experts recommend against routine use of supplements, some believe that shipping, storing, and cooking techniques deplete the nutritional value of food and that it is difficult to meet nutritional needs in a busy American lifestyle. For some, a multivitamin is a simple way to ensure appropriate amounts of substances like folic acid and vitamin D.
However, many studies have found no difference in mortality rates between people who take vitamin supplements and those who do not. While nutrients from food have proven protective effects against many diseases, several studies show the same nutrients taken in supplement form are not beneficial. Most people who eat a healthy diet rich in a variety of fruits and vegetables do not need vitamins, but there are some exceptions.
Pregnant and Breastfeeding Women
Women who are pregnant or breastfeeding have an increased requirement for some vitamins and minerals. Folate (vitamin B9), vitamin B6, and vitamin B12 are particularly important during pregnancy. Since the best sources of vitamin B12 are found in meat and animal products, pregnant women who are vegetarian or vegan should take extra care that they are not deficient in this vitamin, which can harm their baby. Folate occurs naturally in dark leafy greens, beans, peas, and nuts. The synthetic form of folate found in supplements is called folic acid. Adequate intake of folate or folic acid reduces the risk for neural tube defects, which affect the baby's brain and spinal cord. Studies also link low folate levels during pregnancy with low birth weight, which may increase the risk of developing heart disease in adulthood. The CDC recommends that all women of childbearing age take a folic acid supplement.
Pregnant women who eat a healthy diet may still have low folate levels and need to take folic acid supplements. Requirements for folic acid are as follows:
- Before getting pregnant: 400 mcg
- During pregnancy: 600 mcg
- Breastfeeding: 500 mcg
The human body stores several years' worth of vitamin B12, so deficiency in this vitamin is extremely rare. However, people who follow a strict vegetarian diet and do not eat eggs or dairy products may need to take vitamin B12 supplements to ensure adequate intake.
Some women have low vitamin A reserves in their liver. However, getting too much vitamin A from food or supplements significantly increases the risk for birth defects. Experts recommend that pregnant women get no more than 3,000 mcg of vitamin A each day. Pregnant women should be careful in taking extra amounts of any of the fat-soluble vitamins, including vitamins A, D, E, and K.
Infants and Children
Infants who are breastfed by healthy mothers should receive enough of most vitamins. However, in some cases, infants may not get enough of vitamins K and D.
- Human milk contains low levels of vitamin K, and newborn babies do not yet have the gut bacteria that produces vitamin K within the body. Most babies are given an injection of this vitamin at birth. In the absence of this injection, babies are at risk for vitamin K deficiency bleeding (VKDB).
- Infants who are breastfed or who do not get enough sunlight exposure are at risk for low levels of vitamin D. Therefore, daily supplements of 400 international units (IU) or 10 micrograms are recommended until the child switches to vitamin D-fortified formula or milk.
- The AAP recommends the use of iron supplements for breastfed infants beginning at the age of 4 months.
Formulas are required to contain enough vitamins and minerals. After infancy, most American children receive all the vitamins they need from their diet, unless they are severely deprived. However, research suggests that many healthy children ages 1 to 11, especially African American and Hispanic children, are not getting enough vitamin D. It is recommended that children and adolescents take a vitamin D supplement of 600 IU (15 mcg) daily if they are not getting this amount through fortified foods and milk. This recommended dosage may increase in the coming years.
Smokers
Smoking interferes with the absorption of several vitamins, especially vitamins C and D. Smoking can also interfere with the metabolism and utilization of vitamin D, resulting in poor muscle function.
Taking antioxidant supplements s, especially beta-carotene (a precursor to vitamin A), is harmful to smokers. Instead of taking supplements, smokers should eat a diet rich in fresh fruits, vegetables, and whole grains. However, smoking cessation is the most important intervention.
Alcoholic Use Disorder
People with alcohol use disorder are at increased risk for several vitamin deficiencies. The most dangerous deficiencies are:
- Vitamin B1 (thiamin)
- Vitamin B2 (riboflavin)
- Vitamin B6 (pyridoxine)
- Folic acid
- Vitamin C
People Who Have Had Gastric Bypass Surgery
Vitamin deficiencies, particularly in vitamin B12, folate, and vitamin D are a common complication of gastric bypass surgery. Adults who have had Roux-en-Y bypass surgery are at highest risk as this procedure affects the absorption of nutrients by the intestinal tract. Deficiencies are routinely treated with supplements to avoid secondary complications, such as neuropathy, anemia, and osteoporosis.
Strict Vegetarians
Strict vegetarians need to take vitamin B12 supplements, unless they get enough of this vitamin from fortified cereals and other grain products. They may also need to take vitamin D and riboflavin supplements, or a multivitamin, and watch their iron levels. Vegans who do not eat dairy, eggs, fish, or meat may be at risk for vitamin A deficiencies if they do not eat plenty of dark-colored fruits and vegetables.
Vitamin deficiencies may be particularly harmful in vegetarian children. Pregnant and breastfeeding women who are vegetarians must get enough vitamins. If mothers do not get enough vitamin B12, growth and nervous system problems can occur in their newborns.
Dieters
People who are on weight-reduction diets of fewer than 1,000 calories a day should probably take a multivitamin. They should also check in regularly with a physician to make sure they are getting enough nutrients. This extremely low calorie intake is not recommended for most people.
Older Adults
Almost one-third of older people do not get enough of certain vitamins and important minerals. Often their dietary habits slip and they fail to regularly eat balanced meals. In addition, older adults are more likely to take medications that prevent the absorption of certain vitamins.
Common vitamin deficiencies in older people include the following:
- Older people, particularly if they are not exposed to sunlight, may be deficient in vitamin D. Older adults should get at least 800 IU (20 mcg) of vitamin D a day. People who are obese, or who have osteoporosis, limited sun exposure, or poor nutrient absorption may need to increase their intake to as much as 2,000 IU per day.
- Seniors also may have low levels of important B vitamins. Older adults with symptoms of dementia should be tested for a B12 deficiency.
Seniors need to use caution when taking vitamin supplements. Because metabolism slows with age, it takes the liver longer to remove vitamins from the body. Therefore, the effect of some vitamin supplements may be intensified in older adults. For example, a dose of vitamin A that might be harmless in a younger adult could be toxic in an older person.
People Who Avoid Sunlight
People who avoid sunlight or who are housebound, and those whose diet is low in foods that contain vitamin D, should take supplements. People with darker skin are at higher risk for vitamin D deficiencies than those with lighter skin. It should be noted that vitamin D is toxic in excessive doses, and no one should exceed the RDA of vitamin D except under a physician's direction.
Vitamin A and Provitamin A Carotenoids (Beta-Carotene) | |
Benefits | Essential for:
Beta-carotene is an antioxidant that helps protect the body against the damaging effects on cells that can lead to disease. |
Recommended dietary allowance (RDA) or Adequate Intake (AI)(mcg = micrograms) | Infants (AI):
Children:
Nonpregnant adolescent girls and women:
Pregnant women:
Warning: The use of preformed vitamin A, including the acne medication tretinoin (a vitamin A derivative), during pregnancy can cause birth defects. Breastfeeding women:
Adolescent boys and men:
Note: Some experts recommend also getting 3 to 6 mg of beta-carotene, which converts to vitamin A. Vitamin A is now measured with a unit called the Retinol Activity Equivalent (RAE). One RAE is equal to:
Retinol is the most active form of vitamin A. |
Foods containing the vitamin |
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Effects of deficiencies |
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People at risk for deficiencies |
The liver in healthy adults can store a year's worth of vitamin A. A temporary lack of nutrients is unlikely to cause a serious vitamin A deficiency. |
Toxicities | Vitamin A is very toxic when taken in high doses (more than 25,000 IU a day) for long periods of time. Even mild excesses increase the risk for fractures. The tolerable upper intake for healthy adults is 3000 mcg. Large amounts of beta-carotene will not make people sick, but they can turn the skin yellow or orange. Excess vitamin A can affect almost every part of the body, including the eyes, bones, blood, skin, central nervous system, liver, and genital and urinary tracts. Symptoms of vitamin A overdose include:
Severe toxicity can cause blindness and may even be life-threatening. High doses of vitamin A may also increase the risk for lung cancer, osteoporosis, and hip fractures. In children, long-term vitamin A overdose can cause fluid on the brain and liver damage, as well as the same complications seen in adults. Pregnant women who take amounts of vitamin A that are not much higher than the RDA increase the risk for birth defects in their children. |
B Vitamins, part 1 | ||
B Vitamins: General Information | Vitamin B1 (thiamin) | |
Benefits | The B vitamins have a wide and varied range of functions in the human body. Most B vitamins are involved in the process of converting blood sugar into energy. | Vitamin B1 (thiamin) is essential for converting carbohydrates from food into energy. It is also involved in heart, muscle, and nerve function. |
Recommended dietary allowance (RDA) or Adequate Intake (AI)(mcg = micrograms, mg = milligrams) | Infants (AI):
Children:
Adolescents and adults:
Pregnant and breastfeeding women (all ages): 1.4 mg/day. | |
Foods containing the vitamin | Good sources are fortified cereals and breads, dried milk, nuts, legumes (dried beans), cauliflower, lean meats, and sunflower seeds. People who eat a normal diet and are in good health do not need to take supplements. | |
Effects of deficiencies | Deficiencies are uncommon in the U.S., but when they occur, they usually involve several B vitamins, since many of them come from the same food groups. | Vitamin B1 deficiency may cause:
Severe vitamin B1 deficiency is known as beriberi. |
People at risk for deficiencies |
| See general vitamin B description. |
Toxicities | Because the B vitamins are water-soluble and removed in the urine, toxic reactions from taking most of them by mouth are extremely rare. (Exceptions are niacin and vitamin B6.) Note: Substances known as B15 (pangamic acid) and B17 (laetrile) are not vitamins or nutrients. Both chemicals are highly dangerous and have no proven nutritional or health value. | No toxic effects have been reported from thiamin. |
B Vitamins, part 2 | |||
Vitamin B2 (riboflavin) | Vitamin B3 (niacin) also known as nicotinic acid | Vitamin B5 (pantothenic acid) | |
Benefits | Important for growth, red blood cell production, and the conversion of carbohydrates into energy. | Helps break down food for energy. Helps the digestive system, skin, and nerves function. Widens blood vessels and increases blood flow. Early studies suggest a possible benefit for preventing skin cancer. May be prescribed in very high doses for improving cholesterol levels, but medical supervision is recommended. | Important for helping the body break down fats, carbohydrates, and proteins. Essential for growth, and the production of steroid hormones and cholesterol. |
Recommended dietary allowance (RDA) or Adequate Intake (AI)(mcg = micrograms, mg = milligrams) | Infants (AI):
Children:
Adolescents and adults:
Pregnant women (all ages): 1.4 mg/day Breastfeeding women (all ages): 1.6 mg/day | Infants (AI):
Children:
Adolescents and adults:
Pregnant women (all ages): 18 mg/day Breastfeeding women (all ages): 17 mg/day Some people take 1 to 3 grams of niacin per day to treat low HDL (good) cholesterol and high LDL (bad) cholesterol and triglycerides. | Infants (AI):
Children (AI):
Adolescents and adults (AI):
Pregnant women (all ages) (AI): 6 mg/day Breastfeeding women (all ages) (AI): 7 mg/day |
Foods containing the vitamin | Liver, fortified breads and cereals, milk and other dairy products, eggs, nuts, fish, and some dark green leafy vegetables. People who eat a normal diet and are in good health do not need to take supplements. | Fish, chicken, veal, beans, nuts, fortified breads and cereals, dairy products, eggs, pork, salmon, and beef liver. People who eat a normal diet and are in good health do not need to take supplements. | Whole-grain cereals, beans, milk, avocado, broccoli and other vegetables in the cabbage family, milk and liver. People who eat a normal diet and are in good health do not need to take supplements. |
Effects of deficiencies |
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Niacin deficiency is called pellagra. | Deficiency is unlikely to occur, except together with other B vitamin deficiencies. Symptoms of a deficiency include:
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People at risk for deficiencies | See general vitamin B description. | Alcoholics and people who are malnourished. | Alcoholics and people who are malnourished. |
Toxicities | No toxic effects have been reported, even from large doses of riboflavin. | Even mildly high doses of niacin can cause hot flushes of the face and shoulders, headaches, itchiness, and stomach problems. Large doses may trigger erectile dysfunction, ulcers, gout, impaired glucose tolerance, and liver damage. These symptoms are usually reversed when high doses are stopped. | Although no toxic effects have been reported in humans, high doses have caused liver damage in rats. |
B Vitamins, part 3 | ||
Vitamin B6 (pyridoxine) | Vitamin B12 (cobalamin) | |
Benefits | Vitamin B6 affects over 60 proteins in the body, including proteins that play a role in the nervous system, red and white blood cell production, the immune system, and heart function. | Vitamin B12 is essential for the production of red blood cells, manufacture of genetic material, and function of the nervous system. |
Recommended dietary allowance (RDA) or Adequate Intake (AI)(mcg = micrograms, mg = milligrams) | Infants (AI):
Children:
Adolescents and adults:
Pregnant women (all ages): 1.9 mg/day Breastfeeding women (all ages): 2.0 mg/day | Infants (AI):
Children:
Adolescents and adults:
Pregnant women (all ages): 2.6 mcg/day Breastfeeding women (all ages): 2.8 mcg/day |
Foods containing the vitamin | Meats, oily fish, poultry, whole grains, fortified cereals, soybeans, avocados, baked potatoes with skins, watermelon, plantains, bananas, peanuts, and brewer's yeast. | The only natural dietary sources are animal products, including meats, milk and other dairy products, eggs, and fish (clams and oily fish are very high in vitamin B12). Like other B vitamins, B12 is added to fortified cereals and grain products. |
Effects of deficiencies |
In unborn children, a lack of vitamin B6, vitamin B12, and folic acid may cause defects such as cleft lip and palate and spina bifida. Women should take B vitamin supplements before and during their pregnancy. Note: People who have been regularly taking more than 50 mg of vitamin B6 and stop suddenly are at risk for a rebound deficiency. That is why people should taper off vitamin B6 slowly. |
Children who lack this vitamin may not grow properly. Deficiencies in pregnant and breastfeeding women may cause nervous system problems in their babies. Vitamin B12 deficiency anemia is a decrease in red blood cells that occurs when the intestines cannot properly absorb vitamin B12. Pernicious anemia is a type of vitamin B12 deficiency anemia that results from lack of intrinsic factor, which is a stomach-produced substance required for vitamin B12 absorption. Pernicious anemia is a serious disorder that causes rapid heart rate, shortness of breath, dizziness, weakness, and fatigue. It must be treated with injections of vitamin B12 or very high oral doses to prevent nervous system damage. |
People at risk for deficiencies |
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People over age 50, and those who have Crohn disease, celiac disease, or who have undergone certain gastrointestinal surgeries may have trouble absorbing vitamin B12 and need to take supplements. |
Toxicities | Very high doses can cause nerve damage, which may be permanent in some cases. The babies of pregnant women who take large doses, such as for morning sickness, may have adverse health effects. | There is no evidence of toxic effects from this vitamin. |
B Vitamins, part 4 | |||
Biotin (a B vitamin) | Choline (a vitamin-like essential nutrient) | Folate, or Folic Acid, its synthetic form (vitamin B9) | |
Benefits | Biotin (B7) is involved in the production of amino acids, proteins, hormones, and fatty acids. | Choline is essential for a baby's brain development and for learning and memory. | Folate (B9) is important for many processes in the body. It is used to produce chemical messengers in the brain, break down and produce proteins, and make DNA. DNA is the genetic information that controls cell functions. Dietary sources of folate may reduce the risk of some forms of cancer, including cancers of the colon, breast, esophagus, cervix, and stomach. Supplements, however, have not demonstrated the same protective effect. It is very important for pregnant women to get enough folic acid to prevent birth defects in their babies. |
Recommended dietary allowance (RDA) or Adequate Intake (AI)(mcg = micrograms, mg = milligrams) | Infants (AI):
Children (AI):
Adolescents and adults (AI):
Breastfeeding women (all ages): 35 mcg/day | Infants (AI):
Children (AI):
Adolescents and adults (AI):
Pregnant women (all ages): 450 mg/day Breastfeeding women (all ages): 550 mg/day | Supplements may be in the form of folate(natural) orfolic acid(synthetic). Folic acid is nearly twice as potent as folate.Infants (AI):
Children:
Adolescents and adults age 14 and older:
Women who are planning to become pregnant should take at least 400 mcg/day of folic acid before conception, 600 mcg/day during pregnancy, and 500 mcg/day while nursing. |
Foods containing the vitamin | Cereal, egg yolks, chocolate, milk, legumes, liver, mushrooms, bananas, tomatoes, whole grains, nuts, and brewer's yeast. Also produced by bacteria in the intestines. | Peanuts, eggs, cauliflower, and meats, especially liver. | Avocado, bananas, beets, oranges and orange juice, cereal, asparagus, broccoli, green leafy vegetables, dried beans and peas, and yeast. Breads and cereals are now supplemented with folic acid. |
Effects of deficiencies | Deficiencies are rare. Symptoms include:
| Low levels during pregnancy increase the risk for birth defects in newborns. | As with vitamins B6 and B12, deficiencies of folate raise levels of homocysteine, an amino acid in the body that may increase the risk for heart disease, and possibly Alzheimer disease. Folic acid supplements lower homocysteine levels, but they have little or no impact on the risk of heart disease. This suggests that homocysteine may be a marker of heart disease, rather than a cause. However, some evidence suggests that folic acid supplementation in patients with low folate levels substantially reduces the risk of a first stroke. Low folate levels during pregnancy increase the risk of birth defects in newborns. Folic acid supplementation plays a key role in preventing neural tube birth defects. Folate deficiencies can also cause depression, anemia, and problems with concentration, memory, and hearing. |
People at risk for deficiencies |
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Toxicities | There are no known toxic effects of biotin. | Excessive doses can cause intestinal problems. There is also some concern that high doses can cause cancer. | There is a possible link between high-dose folic acid supplements and an increased risk for colorectal, prostate, and breast cancers. More research is needed. Researchers have also found a link between high doses of folic acid and central nervous system disorders, zinc deficiency, and seizures in epileptics. This risk appears to be low, but results indicate that people should avoid taking megadoses of folic acid. Large amounts of folic acid may mask symptoms of vitamin B12 deficiency. |
Vitamin C (Ascorbic Acid) | |
Benefits | Vitamin C acts as an antioxidant (it reduces harm from damaging chemical processes in the body) and helps the body absorb iron. Vitamin C is essential for the production of collagen, the basic protein in bones, cartilage, tendons, and ligaments. It also helps with wound healing. Another possible, but still unproven, benefit of vitamin C is protection against narrowing of the airways during exercise in people with asthma. Studies have shown that supplementation does not prevent colds in the average person, but it may have a protective effect for some athletes who perform strenuous physical exercise. |
Recommended dietary allowance (RDA) or Adequate Intake (AI)(mg = milligrams) | Infants (AI):
Children:
Adolescents:
Adults:
Pregnant and breastfeeding women:
Smokers need 35 mg/day more vitamin C than nonsmokers. |
Foods containing the vitamin | Citrus fruits and juices, strawberries, papayas, hot chili peppers, bell peppers, broccoli, potatoes, dark leafy greens, kale, red cabbage, cauliflower, cantaloupe, sweet potatoes, tomatoes, and Brussels sprouts. The best way to get vitamin C from these foods is by eating them raw. Cooking foods that are rich in vitamin C or storing them for long periods of time can reduce the vitamin C content. |
Effects of deficiencies | Symptoms of vitamin C deficiency may include:
Scurvy is the main disease of vitamin C deficiency. It affects most body tissues, especially the bones, teeth, and blood vessels. Symptoms include tiredness, weakness, irritability, weight loss, and muscle wasting. Vitamin C deficiencies may also contribute to gum disease and gallstones. In children, a lack of vitamin C in the diet has been associated with poor lung function. Low vitamin C intake may also increase lead levels in the blood. |
People at risk for deficiencies | True vitamin C deficiency is rare in the United States. It only occasionally occurs in older people, alcoholics, cancer patients, and some people who are on severely limited diets that are low in fresh fruits and vegetables. However, studies now suggest that as many as 16% of middle-aged Americans are low in vitamin C, with the highest risk in smokers and middle-aged men. Taking high doses of aspirin over a long period of time can interfere with vitamin C absorption. |
Toxicities | The upper limit of vitamin C is 2,000 mg/day in adults (the limit is lower in children). High doses may cause headaches and diarrhea. Long-term high doses may increase the risk for kidney stones. Vitamin C increases iron absorption, so people with blood disorders such as hemochromatosis, thalassemia, or sideroblastic anemia should avoid taking high doses. Large doses interfere with blood thinning medications, blood tests used in diabetes, and stool tests. Rebound scurvy can occur after a person stops taking large doses of vitamin C. This rebound effect may be more significant in infants or pregnant women. |
Vitamin D | |
Benefits | Vitamin D is essential for calcium absorption and bone growth. It also plays a role in cell growth, neuromuscular function, and immunity. Vitamin D status, health risk, and the benefits of supplementation continue to be studied. |
Recommended dietary allowance (RDA) or Adequate Intake (AI)(mcg = micrograms, IU = international units) | Infants (AI):
Safe upper limit for infants is 1,000 to 1,500 IU/day. Children:
Safe upper limit for children is 2,500 to 3,000 IU/day. Older children and adults:
Pregnant and breastfeeding women:
Safe upper limit for older children, adults, and pregnant and breastfeeding women is 4,000 IU/day. Breastfed infants, as well as people who are obese, have osteoporosis, limited sun exposure, poor nutrient absorption, or dark skin should take vitamin D supplements. |
How the body gets the vitamin | There are two forms of vitamin D. Vitamin D3 (cholecalciferol) is made in the skin from a chemical reaction triggered by the ultraviolet radiation from sunlight. People can get enough vitamin D by exposing their skin to 10 to 15 minutes of sunshine 3 times a week (without sunscreen). Vitamin D2 (ergocalciferol) is found in a few food sources, including vitamin D-fortified milk, cheese, butter, fatty fish, oysters, egg yolks, and liver. |
Effects of deficiencies |
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People at risk for deficiencies |
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Toxicities | Vitamin D is very toxic in high doses. Too much vitamin D can cause the intestines to absorb excess calcium, leading to high blood calcium levels and possible damage to the heart, blood vessels, and kidneys. Vitamin D toxicity can also lead to heart arrythmias.
Eating a low-calcium diet and stopping the vitamin can usually reverse these side effects, except for kidney failure. |
Other Vitamins | ||
Vitamin E (Tocopherol or Tocotrienol) | Vitamin K | |
Benefits | This vitamin helps protect cells from damage that can lead to disease and premature aging. Vitamin E also helps with the production of red blood cells and prevents clots from forming inside blood vessels. Researchers once thought that vitamin E might protect against heart disease, but this theory has been disproved. It is also clear that vitamin E does not prevent prostate cancer, as previously thought, and one study found that it actually increases the risk. Supplements of vitamin E have also been linked to an increase in respiratory infections and the overall death rate. | The most important function of vitamin K is its role in blood clotting and bleeding prevention. It also helps maintain healthy bones and heal fractures. Vitamin K is widely used in Japan to treat osteoporosis, and studies suggest it may be effective in treating rheumatoid arthritis. |
Recommended dietary allowance (RDA) or Adequate Intake (AI)(mcg = micrograms, mg = milligrams) | Infants (AI):
Children:
Upper limits:
Adolescents and adults:
Breastfeeding women (all ages): 19 mg/day Upper limits:
Vitamin E is composed of eight compounds (four tocopherols and four tocotrienols). It is most often available as dl-alpha-tocopherol (a synthetic form) supplements. People should take vitamin E supplements with some oil or fat to help their body absorb this vitamin most efficiently. | Infants (AI):
Children (AI):
Adolescents and adults:
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Foods containing the vitamin | Vegetable oils (particularly wheat germ oil), sweet potatoes, turnip greens, mangos, avocados, spinach, broccoli, nuts (almonds, peanuts, hazelnuts), sunflower seeds, fortified breakfast cereals, and soybeans. Tocotrienol (a possibly beneficial form) is found in natural tropical oils. Palm oil sold in the U.S. is refined and does not contain tocotrienol. | The best dietary sources are canola oil, green leafy vegetables such as collard, mustard, and turnip greens, kale, spinach, Brussels sprouts, broccoli, soybeans, and soybean oil. Other good sources are beef liver, fish, bran, canola oil, and olive oil. Vitamin K is also produced by bacteria in the intestines. |
Effects of deficiencies | Deficiencies are rare. Effects include:
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People at risk for deficiencies |
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Some evidence suggests that more young people may be deficient than was previously believed. |
Toxicities | Although vitamin E is one of the best-studied vitamins, research has yielded conflicting results. Definitive conclusions about the benefits and risks of vitamin E have not yet been determined. Although vitamin E from foods is not dangerous, large doses in supplement form may cause bleeding problems, particularly in people taking anti-clotting medications. Some research now indicates that vitamin E, like other antioxidants, may have damaging effects. Studies of people who took large amounts of vitamin E supplements indicated a higher risk of heart failure and death. | Allergic responses, including rash and itching, to high doses have been reported. People who are taking Coumadin, an anticoagulant, should not take vitamin K without consulting a physician. |
A Special Note on Vitamin D
While vitamin D has been the focus of hundreds of studies, the question of whether supplementation with vitamin D is beneficial has not been fully answered.
A large analysis of hundreds of systematic reviews and meta-analyses on the benefits of vitamin D supplementation did not yield conclusive evidence for its benefits, including no clear evidence for the prevention of osteoporosis or falls. There appeared to be associations of higher vitamin D status and protection from several neonatal, child, and maternal outcomes (as well as parathyroid hormone in people with chronic kidney disease), but further study is needed.
Supplementation with vitamin D3 (but not D2) was shown to reduce the risk of death from heart disease, cancer, and other causes, among older adults. However, more research is necessary to define the optimal dosing.
Key areas related to vitamin D, such as autoimmune diseases, cancer, infections, and cognition, could benefit from more analysis to help draw conclusions.
Carotenoids
Carotenoids are a group of more than 700 fat-soluble nutrients that produce the colors in foods such as carrots, pumpkins, sweet potatoes, tomatoes, and other deep green, yellow, orange, and red fruits and vegetables. Many carotenoids are proving to be very important for health. Beta-carotene is the most widely studied carotenoid, but others are also of great interest. Some carotenoids, including beta-carotene, are known as provitamin A because they convert to vitamin A in the body.
Carotenoids are categorized as either xanthophylls or carotenes according to their chemical composition. Xanthophylls will be covered under the phytochemicals section of this report.
Carotenes
Most carotenes are found in yellow, orange, and red vegetables. They include beta- and alpha-carotene and lycopene.
Beta-carotene and other provitamin A carotenoids
. Beta-carotene, alpha-carotene, and beta-cryptoxanthin are carotenes that are converted into vitamin A or retinol (the active form of vitamin A) in the body. They are found in many yellow and orange fruits and vegetables. Beta-carotene is the most widely studied carotenoid. Evidence now strongly suggests that when taken as a separate supplement, high doses can have harmful effects in smokers.Lycopene
. Lycopene is responsible for the red color in fruits and vegetables, including tomatoes, red grapes, watermelon, and pink grapefruit. It is also found in papayas and apricots. It does not convert to vitamin A, but it may have important health benefits.
The beneficial actions of most carotenes, such as those in tomatoes, corn, and carrots, appear to be enhanced by cooking these foods in oil (preferably in olive, canola, or another monounsaturated oil). However, cooking can also destroy certain other nutrients, such as vitamin C, in these vegetables.
Phytochemicals
The word phytochemical means plant chemical. Phytochemicals are plant-based nutrients, rather than vitamins. Researchers are studying hundreds of phytochemicals. Many are believed to have a major positive impact on human health. Some contribute to the bright and vivid colors found in fruits and vegetables.
The results of studies on certain phytochemicals may not necessarily apply to the vegetables or fruits that contain only small amounts, or to supplements containing large amounts of these substances. The health benefits of vegetables and fruits are probably due to some balance of phytochemicals, carotenoids, vitamins, fibers, and minerals rather than to any single substance.
The benefits of individual phytochemical supplements are unproven. Furthermore, these supplements are not regulated. High concentrations of some phytochemicals may act like drugs and be toxic, possibly even contributing to cancer cell growth.
Xanthophylls
Xanthophylls include lutein and zeaxanthin, which are both stored in the retina of the eye. Both lutein and zeaxanthin are powerful antioxidants that may be very important for healthy eyes.
Most xanthophylls are found in green vegetables, such as broccoli, cabbage, and kale. They are also in yellow fruits and vegetables. Cooking may reduce the antioxidant activity of some xanthophylls in foods, although probably not to any significant degree.
Polyphenols and Flavonoids
Polyphenols are important phytochemicals. Flavonoids (or catechins) are members of the polyphenol family that may have significant health benefits. Laboratory (but not human) studies have shown that specific flavonoids suppress tumor growth, interfere with sex hormones, prevent blood clots, and have anti-inflammatory properties. Flavonoids are found in celery, cranberries, onions, kale, dark chocolate, broccoli, apples, cherries, berries, tea, red wine or purple grape juice, parsley, soybeans, tomatoes, eggplant, and thyme. Most berries contain flavonoids and are rich in potent antioxidants.
Resveratrol, quercetin, and catechin are three important flavonoids. Evidence suggests that large doses of resveratrol (found in red wine, grapes, and olive oil) may have important health benefits. In laboratory studies, resveratrol increased cell survival and lengthened the lifespan of worms and fruit flies. Quercetin has been shown to have anti-inflammatory properties and may also enhance the immune system. It is found in onions, tomatoes, cruciferous vegetables, capers, and a variety of other fruits and vegetables. Catechins are the primary flavonoids in tea, and may be responsible for its healthful effects. Flavonoids in dark chocolate may also have health benefits.
Isoflavones (Phytoestrogens)
Isoflavones, commonly known as phytoestrogens, have actions that are similar to the female hormone estrogen. However, no evidence to date indicates that soy-rich foods or phytoestrogen supplements help with hot flashes or other menopause symptoms. Soy has come under some controversy due to its possible role in contributing to breast cancer, thyroid problems, or dementia. However, the most recent studies indicate that isoflavones in soy have either a beneficial or neutral effect for most people.
Lignan is another phytoestrogen. It is found in whole grains, berries, some seeds, some vegetables, and a few fruits.
Isothiocyanates
Isothiocyanates are responsible for the sharp taste in cruciferous vegetables, such as broccoli, cabbage, Brussels sprouts, cauliflower, collards, kale, kohlrabi, mustard greens, rutabaga, turnips, and bok choy. Some isothiocyanates help repress estrogen-stimulated growth of breast cancer cells. Others have been shown to have anti-inflammatory, anti-bacterial, and anti-viral properties. (Cruciferous vegetables are also high in fiber, vitamin C, and selenium.)
Monoterpenes
Monoterpenes contain two important phytochemicals, perillyl alcohol and limonene. They block proteins that stimulate cell growth and reproduction, and are being tested for their ability to fight cancer. Limonene is found in the peels of citrus fruits.
Organosulfur Compounds
Organosulfides are found in garlic, leeks, onions, chives, scallions, and shallots. They are part of the allium family of phytochemicals. Organosulfur compounds such as allicin may benefit the immune system, help detoxify carcinogens, and reduce the production of cholesterol in the liver.
Capsaicin
Capsaicin is found in hot red peppers. It seems to reduce levels of substance P, a compound that contributes to inflammation and the delivery of pain impulses from the central nervous system. Research suggests that it may inhibit cancer-causing substances.
Sterols
Sterols, which include sitosterol, stigmasterol, campesterol, and squalene, are found in vegetable oils. Sitosterol is the best-studied sterol, and it appears to have cholesterol-lowering effects.
Beta-sitosterols may help improve urine flow and urinary symptoms in men with an enlarged prostate gland (benign prostatic hyperplasia, or BPH). Beta-sitosterols come from South African star grass (Hypoxis rooperi) or the Pinus and Picea tree species.
Healthy Foods
Evidence increasingly suggests that it is not individual food chemicals, but a varied diet that is essential for good health and longevity. These types of diets are rich in fresh fruits and vegetables and whole grains, and low in saturated fats.
Some Examples of Healthy Foods | |||
Foods | Phytochemicals and Carotenoids | Vitamins and other valuable food components | Claimed Benefits |
Apples | Flavonoids | May help maintain healthy cholesterol levels. Apples are high in fiber. | |
Beans | Flavonoids | Folate, iron, potassium, and zinc | Beans are high in protein and fiber, and they have beneficial effects on the digestive tract and heart. |
Berries, dark-colored | Ellagic Acid, polyphenols, flavonoids | Vitamin C, minerals | The anthocyanins in berries such as blueberries, cranberries, and elderberries have many health properties, including antioxidant effects. Berries are also high in fiber. Blueberries may help protect the aging brain. |
Broccoli (also kale, Brussels sprouts, and cauliflower) | Flavonoids, isothiocyanates, lutein, beta- and alpha-carotene. Note: Young sprouts of broccoli and cauliflower contain much higher levels of isothiocyanates than their mature forms. | Vitamin C, folate, fiber, and selenium | The nutrients in these vegetables may have anti-cancer properties. All these vegetables have a high fiber content. |
Carrots, sweet potatoes, and other bright yellow or orange vegetables | Lutein, beta-carotene, and other provitamin A carotenoids | Vitamin A (converted from carotenoids), vitamin C | Protects eyes, lungs. (Lightly cooking carrots may increase the potency of their nutrients.) High in fiber. |
Chocolate, dark. Note: Milk chocolate does not have health benefits. | Flavonoids | Heart protective (may help prevent harmful blood clots). Claimed to have protective properties against lung cancer (not other cancers), but this is not proven. | |
Eggs | Lutein | Many B vitamins, vitamin A, vitamin D | Although egg yolks are high in cholesterol, a modest consumption of eggs does not appear harmful. |
Fish, oily (mackerel, salmon, sardines) | Vitamins B3, B12, D and E. Essential fatty acids, selenium | Protects the heart and brain. | |
Garlic | Allium (organosulfurs) | May promote cardiovascular and immune system function. Heating garlic can reduce its benefits. Allowing crushed fresh garlic to stand for 10 minutes before heating, however, may preserve beneficial chemicals while cooking. | |
Ginger | Zingiberaceae | Claimed to have cancer-fighting properties. | |
Grains (whole) | Lignans (phytoestrogens) | Vitamin B, selenium (important antioxidant mineral), fiber, folate | Whole grains appear to reduce the risk for diabetes, heart disease and various digestive disorders. Generally higher in fiber than their refined counterparts. |
Grapes, including purple grape juice and red wine | Flavonoids, (resveratrol, quercetin, and catechin) | Antioxidant effects. | |
Nuts | Vitamin E, vitamin B1, essential fatty acids, folate | Heart-healthy benefits include lowering LDL (bad) cholesterol and reducing the risk of developing blood clots that can lead to a heart attack. | |
Onions | Flavonoids, allium (organosulfurs) | Antioxidant and potential heart protective effects. | |
Oranges | Monoterpenes | Vitamin C, folate, potassium. | Many health benefits, including increasing HDL (good) cholesterol levels. High in fiber. |
Potatoes (sweet) | Beta-carotene, provitamin A carotenoids | Vitamin C, vitamin E, vitamin A | Many health benefits, including antioxidant and anti-inflammatory properties. |
Soy (the best sources are tofu, soy milk, or whole soy protein) | Isoflavones (phytoestrogens), flavonoids, phytosterol, phytate, saponins | May have effects similar to estrogen, including maintaining bone calcium. Studies on the effects of soy and soy compounds on cancer risk had different results. The American Cancer Society recommends that women with breast cancer avoid taking dietary supplements that are high in isoflavones. Isoflavones are a type of phytoestrogen (estrogen-like plant chemical). However, research found no harmful effects of eating soy in breast cancer survivors. | |
Spinach and other dark green leafy vegetables | Zeaxanthin, beta-carotene | Vitamin C, folate, vitamin A (converted from carotenoids) | May protect the lungs and brain. Good source of fiber. |
Tea (Both black and green tea are beneficial, but green tea appears to have the greatest effectiveness.) | Flavonoids (primarily catechins) | Antioxidant properties, particularly in green tea, which may be especially beneficial for smokers. Both black and green tea may protect against heart disease and stroke, although studies are mixed. Tea drinking also may help control weight and prevent osteoporosis. | |
Tomatoes | Lycopene, flavonoids | Vitamin C, biotin, minerals | Lycopene, which is found in tomatoes, has been a target of research. Lycopene may help fight infections. Studies have reported that a diet high in fruits and vegetables containing beta-carotene, lycopene, and other carotenoids may reduce the risk for a heart attack. Diets low in lycopene (particularly from tomatoes) have been associated with a significantly higher risk for heart disease and stroke. Supplements, however, do not reduce these risks. Good source of fiber. |
Note on Organic versus Non-organic Products.There is some evidence that organic produce has higher levels of antioxidants and that some agricultural chemicals may destroy flavonoids. Nevertheless, organic produce is expensive. Fruits and vegetables, no matter how they are grown, are still filled with healthy nutrients. A diet rich in non-organic produce is better than a diet devoid of any produce at all. | |||
Dietary Health Benefits
In general, the best way to get healthy levels of important nutrients is by eating healthy foods rather than taking supplements.
The benefit of any individual food or nutrient is very difficult to prove, and to date, there is little evidence that most dietary supplements protect against major diseases in otherwise healthy people with normal eating habits. A diet that is naturally high in vitamins and minerals can be the best defense against many diseases. Fresh fruits and vegetables and whole grains are the primary sources of vitamins, carotenoids, and vitamins, as well as of fiber and important minerals.
Antioxidants: Pros and Cons
Free Radicals (Oxidants)
Currently, the most important benefit claimed for vitamins A, C, E, and many of the carotenoids and phytochemicals is their role as antioxidants, which are scavengers of free radicals (also sometimes called oxidants), chemically active particles that are byproducts of many of the body's normal chemical processes.
Warnings on High-Dose Antioxidants and Other Supplements
Antioxidant vitamins (A, C, and E), carotenoids, and many phytochemicals can neutralize harmful free radicals. Although it is clear that the modest amounts of these vitamins in foods prevent deficiency diseases, high-dose vitamin C, vitamin E, and beta-carotene supplements may also have pro-oxidant effects, which can be harmful in people with cancer. In these people, high doses of antioxidant vitamins may actually protect cancer cells just as they do healthy cells. Because there is no evidence that antioxidants decrease the risk of developing cancer or heart disease, people should talk with their physician before taking these vitamins or supplements.
Protection against Heart Disease
A low dietary intake of vitamins A, C, E, and beta-carotene has been linked to heart disease. Deficiencies in the B vitamins folate (also known as folic acid) and B12 have been associated with high blood levels of the amino acid homocysteine. Higher homocysteine levels may increase the risk for heart disease, stroke, and heart failure. However, supplements of these vitamins, alone or in combination, havenot
been shown to protect against heart disease.Calcium, which is often taken along with vitamin D to protect bones, may actually increase the risk for heart disease, although more research is needed. Some recent studies suggest that calcium supplements, when taken alone or with vitamin D, increase the likelihood of heart problems, especially heart attack. However, a comprehensive analysis of 31 separate studies on calcium intake and cardiovascular disease found no cause for concern. Older adults and those at risk for osteoporosis should discuss calcium supplements with their doctor.
Atherosclerosis is a disease of the arteries in which fatty materials, calcium, and other components are deposited in the vessel wall, resulting in narrowing and eventual blockage of blood flow. Severely restricted blood flow in the arteries to the heart muscle leads to symptoms such as chest pain. It is important to note that the calcium you consume does not directly affect the amount in your arteries.
Phytochemicals and Heart Protection
Foods containing phytochemicals have been shown to protect the heart. However, the reduced risk for heart disease is more likely due to a generally healthy diet that is low in harmful fats and other unhealthy foods than to any one fruit or vegetable.
Phytochemicals that may benefit the heart:
- Flavonoids. Certain flavonoids, which are found in both black and green teas, dark chocolate, onions, red wine or red grape juice, and apples may protect the heart. Flavonoids may help prevent cholesterol damage and blood clots, but further studies are needed.
- Organosulfurs. Organosulfurs, found in onions and garlic, have been under investigation for their possible cholesterol-lowering effects.
- Isoflavones. Soy protein, found in foods like tofu and soy milk, is the best-studied source of isoflavones (known as phytoestrogens, or plant estrogens), which may help lower LDL (bad) cholesterol and triglycerides.
- Sterols. Plant sterols, including sitosterol, are also proving to be potent cholesterol fighters. They work by blocking the absorption of cholesterol in the intestine.
Protection against Stroke
A healthy diet that is rich in fruits and vegetables and low in salt and saturated fat may significantly lower the risk for a first stroke, perhaps by protecting against high blood pressure, which is a major stroke risk factor.
Vitamins and Stroke Protection
The effects of antioxidant vitamins and carotenoids (vitamins C or E, or beta-carotene) on stroke have been studied extensively. Most studies have found that these vitamins do not protect against stroke. Although B vitamin supplements help lower homocysteine levels, they have no effect on heart disease outcomes. More research is under way to evaluate the effect of vitamins on reducing stroke risk.
Protection against Cancer
Studies have found that diets rich in fresh fruits and vegetables may decrease the risk for many cancers, including cancers of the lung, breast, colon, and prostate. Examples of possible cancer-fighting foods include cruciferous vegetables (such as cabbage, kale, Brussels sprouts, and broccoli), tomatoes (which contain lycopene), and carrots (which contain alpha-carotene).
Because many cancers are thought to be caused by the effects of oxygen-free radicals on DNA, the antioxidants A, C, and E and beta-carotene have been intensively studied for cancer prevention.
For the most part, vitamins and phytochemicals in supplement form have not shown any benefit against cancer. Any protective effects of vitamins or phytochemicals against cancer may depend on the combined effect of these nutrients in foods. A National Institutes of Health study reviewed randomized trials that evaluated the effectiveness and safety of multivitamin and mineral supplements for preventing cancer and chronic disease. The reviewers concluded that current evidence is not sufficient to determine whether use of multivitamin and mineral supplements by healthy people prevents cancer or chronic disease.
Certain supplements may actually encourage tumor growth by protecting cancer cells. Research finds that beta-carotene supplements increase lung cancer risk in smokers and people who have been exposed to asbestos.
Vitamins and breast cancer prevention:
- Eating foods that are rich in calcium and vitamin D (such as low-fat or nonfat yogurt and milk) may modestly reduce the risk for breast cancer in premenopausal women. It is not clear whether fruits and vegetables can prevent breast cancer from developing or returning.
- The American Cancer Society recommends that women with breast cancer avoid taking dietary supplements that contain high amounts of isoflavones. Isoflavones are a type of phytoestrogen (estrogen-like plant chemical). There have been concerns that high intakes of soy may increase the risk for estrogen-responsive cancers such as breast cancer, but research has been inconclusive.
Dietary supplements and other cancers:
- Taking folic acid supplements did not increase or decrease the risk for site-specific cancer, such as large intestine, lung, breast, or any other site, in the first few years of supplementation in a recent study.
- Taking folic acid supplements does not lower the risk of developing colorectal cancer. In fact, supplements appear to increase the risk for repeated colorectal polyps. Eating a diet rich in fruits and vegetables may provide some protection against colorectal cancers, however.
- There is no evidence to support the idea that antioxidant supplements (including vitamins C, E, A, folate, and beta-carotene) decrease the risk for lung cancer. Evidence shows that vitamin C, vitamin E, and beta-carotene supplements increase lung cancer risk in smokers.
- There is no evidence that once someone is diagnosed with cancer, taking large doses of vitamin C will help the treatment. In fact, there is concern that large doses of antioxidants from supplements could interfere with radiation therapy and chemotherapy and, therefore, should be avoided during cancer treatment.
- Evidence has clearly shown that vitamin E does not prevent prostate cancer, and it may actually increase the risk for prostate cancer (and heart failure). While eating soy foods may decrease the risk for prostate cancer, soy or isoflavone supplements have not yet been proven to reduce that risk.
- Selenium supplements do not reduce the risk for bladder or prostate cancer.
- Selenium may help protect against colon and lung cancers, but studies have not confirmed that supplements help. Selenium supplements appear to increase the risk for diabetes.
Studies on the health benefits of vitamins and minerals have some important limitations. Some are held to rigorous standards, while others are not. In most cases, the results of existing research are complex, because they can be complicated by factors such as diet, exercise, healthy or unhealthy lifestyle behaviors, as well as environmental and genetic factors.
Evidence of Protection against Other Diseases from Vitamins, Carotenoids, and Phytochemicals | |||
Disease or Condition | Vitamins | Carotenoids, Phytochemicals, and Healthy Foods | |
Alzheimer Disease | Vitamin E.There is some evidence that foods high in vitamin E might reduce the risks for dementia and Alzheimer disease.B Vitamins.Some studies suggest that deficiencies of vitamins B6, B12, and folate (folic acid) may be a risk factor for Alzheimer disease. However, there is no evidence from randomized, controlled trials that these supplements prevent Alzheimer disease.Vitamin D.There is some evidence that older adults with low vitamin D levels are more likely to experience cognitive decline, including problems with thinking, learning, and memory. Whether supplementing with vitamin D can help reverse these cognitive changes is unclear. | ||
Infectious Disease | Studies are mixed as to whether vitamin supplements protect against upper respiratory infections. The evidence suggests there is little or no benefit. It is possible that vitamin C or multivitamin supplements may be helpful in specific people, such as those who are vitamin deficient or who have medical problems that impair their immune system. | ||
Eye Disorders | Cataracts and Macular Degeneration.Oxygen-free radicals play a role in cataract formation and age-related macular degeneration (AMD), the most common cause of irreversible blindness in older people.A low level of vitamin C in the lens of the eye has been a particularly strong predictor of cataracts. People with cataracts are often deficient in vitamin A, the carotenes, lutein, and zeaxanthin. Studies have not demonstrated that antioxidant vitamin supplements (such as vitamins C and E) prevent cataracts, however. A combination of zinc and antioxidants, including vitamins C, E, and beta-carotene, slows the progression of AMD. This combination can be helpful in people with intermediate or advanced AMD in one eye. However, these supplements will not prevent macular degeneration from developing in the first place. | Several studies have associated antioxidant-rich foods with a decreased risk for cataracts. Carotenoids, especially lutein, lycopene, and zeaxanthin, are especially eye-protective. However, there is not enough evidence to suggest that taking supplements containing carotenoids lowers the risk for cataracts. | |
Osteoporosis | Vitamin D.Adequate amounts of daily calcium and vitamin D can reduce the risk of osteoporosis, but the impact on fracture risk is under debate. To keep bones strong, the National Osteoporosis Foundation (NOF) recommends:
Ideally, adequate calcium and vitamin D will come from food and sun exposure. Doctors are currently reconsidering the use of calcium and vitamin D supplements based on studies suggesting that supplements do not make much difference for bone mineral density protection. The U.S. Preventive Services Task Force (USPSTF) advised that healthy postmenopausal women don't need to take these supplements. According to the USPSTF, taking daily low-dose amounts of vitamin D supplements (400 IU or less), with or without calcium supplements (1,000 mg or less), does not prevent fractures. For higher doses, the USPSTF says there is not enough evidence to make a recommendation. In addition to a possible lack of benefit, these supplements are associated with certain risks, like kidney stones, even at lower doses. Vitamin D and calcium supplements may be appropriate for certain people, including those who do not get enough vitamin D through sunlight exposure and those who do not consume enough calcium in their diet. They are also helpful, along with other medications, for people who have been diagnosed with osteoporosis. | ||
Menstrual Disorders | Vitamin B6.Limited evidence suggests that vitamin B6 may help reduce premenstrual symptoms, including depression. Typically, women take 100 mg of vitamin B6 per day, although one study suggested that a lower dose (50 mg) may have the same effect. | ||
Resources
- Food and Nutrition Information Center -- www.nal.usda.gov/fnic
- Office of Dietary Supplements, National Institutes of Health -- Nutrient Recommendations: Dietary Reference Intakes (DRI) ods.od.nih.gov/Health_Information/Dietary_Reference_Intakes.aspx
- Vitamin and Mineral Supplement Fact Sheets -- ods.od.nih.gov/factsheets/list-VitaminsMinerals
- Food and Drug Administration -- www.fda.gov
- Academy of Nutrition and Dietetics -- www.eatright.org
- American Council on Science and Health -- www.acsh.org
- Independent testing of nutritional supplements' contents and quality -- www.consumerlab.com
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Review Date: 7/14/2020
Reviewed By: Meagan Bridges, RD, University of Virginia Health System, Charlottesville, VA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
All rights reserved.