Vitamins do not share a common chemistry, but they share certain characteristics. They are all organic nutrients that are necessary in small amounts for normal metabolism and good health. Most vitamins must be provided by the diet or by supplements; only three vitamins (D, K, and the B vitamin biotin) can be manufactured in the body from nondietary sources. Vitamins are not sources of energy as are carbohydrates, fats, and proteins. Instead, vitamins serve as chemical partners for the enzymes involved in the body's metabolism, cell production, tissue repair, and other vital processes. Vitamins are either fat soluble or water soluble. The fat-soluble vitamins, which include A, D, E, and K, are absorbed by the body using processes that closely parallel the absorption of fat. They are stored in the liver and used up by the body very slowly. The water-soluble vitamins include C and the B complex vitamins. The body uses these vitamins very quickly; excess amounts are eliminated in urine.

The Recommended Daily Allowance (RDA) for vitamins, set by the Food and Nutrition Board of the National Academy of Sciences-National Research Council, is gradually being enhanced using a new standard called the Dietary Reference Intake (DRI). The DRI is a general term for four different rating sets that will apply to vitamins, minerals, and proteins taken by men or women in specific age groups. (1) The Estimated Average Requirement (EAR) is the daily intake that meets the requirements of 50% of the population group. (2) If the EAR is unknown a rating called Adequate Intake (AI) is used, which is an estimate of average intake that seems to be healthy and not dangerous. (3) The Recommended Daily Allowance (RDA) is the amount of a supplement sufficient to meet nearly all needs of men or women in certain age groups. (4) The Tolerable Upper Intake Level (UL) is the maximum dose likely to be safe in nearly all individuals. The full list of nutrients is expected to be completed by 2000. Another standard being developed called Daily Reference Values (DRVs) will cover nutrients and other food components, such as phytoestrogens, fiber, and other nutritional factors, which do not yet have any standards.

Because vitamins and other dietary supplements are regulated as food and not drugs by the government, however, no standards exist for effectiveness. Brands differ in quality and in the ability of vitamins to be absorbed by the body; it is nearly impossible for the average consumer to compare them wisely. Shopping for the lowest-priced vitamins is the wisest move; there appears to be no significant differences in quality between the vitamins in the lowest and those in the highest price ranges. The FDA is now requiring that vitamins and other dietary supplements include labels similar to those now found on all commercial foods. The label will include nutrient information and list all ingredients, including identifying parts of plants from which ingredients may be taken, but the quality of the supplement will not be evaluated. The essential problem remains there is no control over what goes into a dietary supplement and not enough known about long term effects and risks. Studies on ginseng, for example, found that some supplements did not contain any active ingredients and that others varied widely in the amount they contained. The U.S. Pharmacopoeia, an independent organization that sets standards for drugs, has implemented standards for vitamins; consumers should look for the USP label on products of companies that adhere to these standards.

Vitamin A is part of a group of compounds called retinoids and is essential for growth, bone development, night vision, reproduction, and healthy skin. Vitamin A is present in animal products, such as liver, dairy products, eggs, and fish liver oil. It is also converted from carotenoids, found in dark red, green, and yellow vegetables (see Cartenoids below). Vitamin A is best absorbed in the presence of some dietary fat. Average DRI is 875 mcg. Supplements are rarely recommended for people with normal diets because of the risk of toxicity.

Deficiencies of Vitamin A. Deficiencies of vitamin A can cause skin disorders and eye damage; in less developed countries severe deficiencies cause blindness in 250,000 children each year. Diets low in vitamin A may also increase the risk of developing cancer. Deficiencies in the U.S. are rare but can result from inadequate diets in preschool children or in women who are pregnant or breast feeding, from intestinal disorders such as cystic fibrosis, steatorrhea, and biliary obstruction, from hyperthyroidism, and from liver diseases such as cirrhosis.

Toxicity of Vitamin A. Vitamin A can be very toxic when taken in high-dose supplements for long periods of time and can affect almost every part of the body, including eyes, bones, blood, skin, central nervous system, liver, and genital and urinary tracts. Symptoms include dizziness, nausea, vomiting, headache, skin damage, mental disturbances, and, in women, infrequent periods. Severe toxicity can cause blindness and may even be life threatening. Liver damage can occur in children who take RDA-approved adults levels over prolonged periods of time or in adults who take as little as five times the RDA-approved amount for seven to ten years. In children, chronic overdose can cause fluid on the brain and other symptoms similar to those in adults. Pregnant women who take amounts not much higher than RDA levels increase the risk for birth defects in their children. High consumption of vitamin A may also increase the risk of gastric cancer and the risk of osteoporosis and fractures in women.

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. Diets rich in B vitamins are particularly important for pregnant and breast-feeding women and for other people who require more energy, such as athletes and heavy-labor workers.

General Deficiencies of Vitamin B. Deficiencies of B vitamins 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. Alcohol interferes with these vitamins, and some of the physical and mental problems that alcoholics experience may be attributed to a deficiency of B vitamins. Elderly people are also at risk for deficiencies, because of inadequate diets and potential interference with B-vitamin absorption by medications.

General Toxicity of Vitamin B. Because the B vitamins are water-soluble and eliminated in the urine, toxic reactions from oral administration of most of them are extremely rare. (Exceptions are niacin and B6.) (For specific adverse reactions from deficiencies or high dosage levels, see individual discussions of B vitamins, below). It should be noted that substances known as B15 (pangamic acid) and B17 (laetrile) are neither vitamins nor nutrients; both chemicals are highly dangerous and have no proven nutritional or other value.

Deficiencies and Toxicity of Vitamin B1 (Thiamin). Thiamin is essential for converting blood sugar into energy and is involved in metabolic activities in nerves, heart, and muscles and in the production of red blood cells. Thiamin is found in almost all foods, but the best source is pork and good sources are dried fortified cereals, oatmeal, corn, nuts, cauliflower, and sunflower seeds. Recommended RDA is 1.2 mg per day for men and 1.1 mg for women. Supplements for people with normal diets and health are unnecessary. Severe vitamin B1 deficiency, known as beriberi, is rare in the U.S., but can occur in alcoholics, in severely malnourished people, or in those receiving long-term dialysis or intravenous feeding. Symptoms may include visual disturbances, paralysis, staggering, loss of sensation in the legs and feet, psychosis, and congestive heart failure. No toxic effects have been reported for thiamin.

Deficiencies and Toxicity of Vitamin B2 (Riboflavin). Like thiamin, riboflavin is important in the production of energy. Some dietary sources are liver, dried fortified cereals, and low fat yogurt. Recommended DRI is 1.7 mg. Supplements for people with normal diets and health are unnecessary. Deficiencies affect the skin and mucous membranes and can cause cracks on the lips or corners of the mouth, eczema of the face and genitals, a burning sensation on the tongue, or eye irritation. Previously, no toxic effects had been reported even from large doses of riboflavin However, one new study indicated that high consumption of vitamin B2 may increase the risk of stomach cancer. (In the same study, vitamins B1, B3, and B6 were protective).

Deficiencies and Toxicity of Vitamin B3 (Niacin). Niacin, also known as nicotinic acid, helps break down blood sugar for energy and also acts as a vasodilator, widening blood vessels and increasing blood flow. Dietary sources are mackerel, swordfish, chicken, veal, dried fortified cereals, pork, salmon, and beef liver. Current DRI recommendations are 20 mg. This vitamin may be prescribed for improving cholesterol levels, but supplements are unnecessary in people with normal health and diets. A deficiency of niacin causes pellagra; symptoms can include eczema, intestinal and stomach distress, depression, headache, thinning of the hair, and excess saliva production. Anemia may occur, although this is probably due to an accompanying deficiency in folic acid (see Folic Acid, below). Even mildly high doses of niacin can cause hot flushing of the face and shoulders, headache, itchiness, and stomach problems. Some report heart disturbances and temporarily lowered blood pressure. Large doses may produce ulcers, gout, diabetes, and liver damage, which are usually reversed when high doses are discontinued.

Deficiencies and Toxicity of Vitamin B5 (Pantothenic Acid.) Pantothenic acid, like many B vitamins, plays a wide role in the body, including metabolism of fats, carbohydrates, and proteins, as well as production of steroid hormones and other important chemicals. Dietary sources are whole grains, beans, milk, eggs, and liver. Recommended adequate intake (AI) is 4 to 7 mg. Supplements are unnecessary in people with normal health and diets. Deficiency is unlikely except in company with other B vitamin deficiencies. Symptoms include abdominal distress, burning sensation in the heels, and sleep problems. Although no toxicity has been reported in humans, high dosages have caused liver damage in rats.

Deficiencies and Toxicity of Vitamin B6 (Pyridoxine). Vitamin B6, or pyridoxine, has an effect on over 60 proteins in the body, importantly, those that play a role in the nervous system, in red and white blood cell production, and in heart disease. Food sources of B6 are meats, oily fish, poultry, whole grains, dried fortified cereals, soybeans, avocados, baked potatoes with skins, watermelon, plantains, bananas, peanuts, and brewer's yeast. Current DRI recommendations are 2 mg in all adults, although for heart protection some experts recommend 3 to 6 mg. Deficiencies are very uncommon but can cause skin problems and nervous system disorders, including impaired memory and concentration. They may also increase the risk for kidney stones. In rare cases, infants are born unable to metabolize pyridoxine; in such cases seizures or convulsions can occur, and vitamin B6 must be administered. Deficiency is associated with increased levels of the chemical homocysteine, which is turn has been associated with heart disease, birth defects, and possibly Alzheimer's disease and dementia. (It is not known if reducing homocysteine levels will have any protective effect.) A diet rich in fruits and vegetables may reduce homocysteine levels. Very high doses (2,000 mg per day) can cause nerve damage with symptoms of instability and numbness in the feet and hands, which may be permanent in some cases. In some cases, the damage may be permanent. Pyridoxine also reduces the effects of L-dopa, the drug used for Parkinson's disease.

Deficiencies and Toxicity of Vitamin B12 (Cobalamin). Vitamin B12 is essential for the production of blood cells, manufacturing genetic material, and for healthy functioning of the nervous system. The only natural dietary sources of B12 are animal products, meats, dairy products, eggs, and fish (clams and oily fish are very high in B12); like other B vitamins, however, B12 is added to commercial dried cereals. The RDA is 2.4 mcg a day. Deficiencies are rare in young people, although the elderly may have trouble absorbing natural vitamin B12 and require synthetic forms from supplements and fortified foods. Symptoms of mild B12 deficiency include memory loss, instability, disorientation, and decreased reflexes, and possibly and hearing loss. Deficiencies also elevate homocysteine (see vitamin B6, above). When vitamin B12 deficiencies are caused by a genetic defect, in which a protein known as gastric intrinsic factor is lacking, a serious disorder known as pernicious anemia can develop, which must be treated with injections of vitamin B12 or else neurologic damage may occur.

Deficiencies and Toxicity of Biotin. Biotin, also a B vitamin, is involved in the production of amino acid proteins and fatty acids. It is obtained not only in diet but is also produced by bacteria in the intestines. Dietary sources are eggs, milk, liver, mushrooms, bananas, tomatoes, whole grains, nuts, and brewer's yeast. There is no DRI for biotin; some experts suggest 30-100 mcg. Deficiencies are almost unheard of. There is no evidence of toxicity with this vitamin.

Deficiencies and Toxicity of Folate. Folate is a B vitamin compound that is important for many metabolic processes in the body and is used in the manufacture of neurotransmitters (chemical messengers in the brain), in preventing heart disease, and for synthesizing DNA. Folate is best found in avocado, bananas, orange juice, cold cereal, asparagus, fruits, green, leafy vegetables, dried beans and peas, and yeast. The synthetic form, folic acid, is now added to commercial grain products. Vitamins are usually made from folic acid, which is about twice as potent as folate. Many experts now recommend that adults have 400 mcg of folic acid daily, which is considerably higher than the DRI recommendations of 400 mcg of folate, which does not take into consideration the possible benefits of folate on the heart.

Low levels of folate during pregnancy are common without supplements; deficiencies at that time increase the risk of neural tube defects in newborns. Women who are planning to be pregnant should take 400 mcg of folic acid before conception as well as when they are pregnant or breast feeding. Folate deficiency can cause megaloblastic anemia and affect concentration and memory and may cause hearing loss. As with vitamins B6 and B12, deficiencies also elevate homocysteine, and recent studies are suggesting a strong link between low folate levels and the onset of heart disease. Poor diet coupled with alcoholism is the most common cause of folate deficiency. Any condition that disturbs the small intestine can contribute to folate deficiency. High-dose aspirin, smoking, treatment for seizures, and oral contraceptives may increase the risk for folate deficiency. Some studies have reported a link between high doses and central nervous system disorders, zinc deficiency, and seizures in epileptics. This risk appears to be low, but these results indicate that megadoses of folic acid should be avoided. High amounts in the elderly may also mask symptoms of vitamin B12 deficiencies.

Deficiencies and Toxicity of Choline. Choline has recently been recognized as an important B vitamin. It is particularly essential for fetal brain development and for learning and memory. Peanuts, eggs, cauliflower, and meats, especially liver, are good sources of choline. Recommended daily doses are 425 mg for nonpregnant women, 450 mg for pregnant women, and 550 mg for nursing women. Excessive doses can cause intestinal problems, and there is also some concern that high doses can be carcinogenic.

Vitamin C acts as an antioxidant. It is essential for the production of collagen, the substance that forms the body's connective tissues (bones, cartilage, tendons, and ligaments), and it may help to boost the immune system. Best sources are citrus fruits and juices, papayas, hot chili peppers, bell peppers, broccoli, potatoes, kale, red cabbage, cauliflower, cantaloupe, sweet potatoes, and Brussels sprouts. Good sources are tomatoes and potatoes. The National Institutes of Health now recommend 100 to 200 mg a day. (Smokers need the higher amount.)

Deficiencies of Vitamin C. Scurvy is the major vitamin C deficiency disease and affects most body tissues, particularly bones, teeth, and blood vessels. Early symptoms include tiredness, weakness, irritability, weight loss, and vague muscle aches. Later symptoms are bleeding gums, wounds that won't heal, rough skin, and wasting away of the muscles. Low dietary intake of vitamin C may also increase lead levels in the blood. Deficiency has been uncommon in the U.S., usually occurring in the elderly, alcoholics, cancer patients, and some food faddists. Surprisingly, a recent study suggested that many healthy middle-class Americans were deficient in vitamin C. High doses of aspirin taken over a long period of time can interfere with vitamin C and may cause a deficiency.

Toxicity of Vitamin C. Adverse effects from vitamin C supplements are uncommon, but high doses (more than 1000 mg per day) may cause headaches and intestinal and urinary problems, including kidney stones. Studies have suggested that in doses of only 500 mg or more vitamin C may actually have pro-oxidant effects (as opposed to being a beneficial antioxidant in normal levels) and can damage cells. Because ascorbic acid increases iron absorption, people with certain blood disorders, such as hemochromatosis, thalassemia, or sideroblastic anemia, should particularly avoid high doses. Large doses can also interfere with anticoagulant medications, blood tests used in diabetes, and stool tests for diagnosing colon cancer. A condition called rebound scurvy is caused by abrupt withdrawal from long-term ingestion of large doses of the vitamin. In rare cases, infants of pregnant women who have taken large doses of vitamin C have been born with rebound scurvy.

Vitamin D is actually a term for several hormones that are stored mainly in the liver and also in fat and muscle tissue. Vitamin D is essential for maintaining healthy bone structure because of its role in the absorption and metabolism of calcium. It is easily absorbed during digestion and is excreted from the body very slowly. It is manufactured in the body from a chemical reaction to the ultraviolet radiation in sunlight and is found in a few food sources, including vitamin D fortified milk, fatty fish, egg yolk, and liver. The AI (adequate intake) level is 200 IU (5 micrograms) per day for children and most adults, 400 IU (10 micrograms) for people over 50, and 600 IU (15 micrograms) for those over 70 who do not have sufficient exposure to sunlight. Some experts believe that many people now may require supplements to achieve the recommended levels.

Deficiencies of Vitamin D. Deficiencies result in rickets in children and osteomalacia in adults; both diseases are characterized by a softening of the bones caused by low amounts of calcium and phosphorous. Vitamin D deficiency also increases the risk for hip fractures in postmenopausal women. It is also associated with a higher risk for prostate cancer and breast cancer risk, especially in older women. There has been some concern that underexposure to sunlight in older people, particularly if they live in the North, may put them at risk for low levels of vitamin D. A 1998 study suggests, in fact, that vitamin D deficiency may actually be a significant problem in the general U.S. population even among younger adults. Experts in the study suggested that adults may need higher daily amounts than the RDA recommendations. Exposure to sunlight for about 15 to 20 minutes at mid-morning or mid-afternoon three times a week is also recommended for most people who live in temperate climates.

Toxicity of Vitamin D. Vitamin D is very toxic in high doses. Infants can be severely damaged with daily amounts higher than 1000 IU, which can cause mental and growth retardation, kidney failure, and death. In children and adults, daily amounts over 50,000 IU can cause weakness, anorexia, vomiting, diarrhea, and mental changes. Prolonged use of megadoses can cause calcification of soft tissue and life-threatening kidney failure. Low-calcium diets and withdrawal from the vitamin can usually reverse the side effects except for kidney failure.

Vitamin E is a fat-soluble antioxidant vitamin that helps prevent cell membrane damage, and by inhibiting the oxidation of LDL cholesterol, may reduce the risk of atherosclerosis and heart attacks. Vitamin E is found in vegetable oils (particularly wheat germ oil), sweet potatoes, avocados, nuts, sunflower seeds, and soybeans. Deficiencies have not been established. DRI is 30 IU, although some groups recommend between 200 to 400 IU. Contrary to current belief, some research now indicates that vitamin E supplements provide few health benefits and may even become pro-oxidants in large doses and have damaging effects. Some experts believe that the beneficial form of vitamin E is gamma-tocopherol, which is found in foods, while supplements of vitamin E, which are usually alpha-tocopherol, are not protective and, in fact, replace the beneficial gamma-tocopherol. Some people taking large amounts have experienced fatigue, nausea, and diarrhea. Vitamin E may also cause bleeding problems, particularly in people taking anti-clotting medications.

The most important function of vitamin K is its role in blood clotting and prevention of bleeding. The vitamin also contributes to maintaining healthy bones and healing fractures. Best dietary sources are canola oil, cruciferous vegetables, and soybean oil. Good sources are beef liver, bran, and olive oil. RDA is 60 to 65 micrograms (women) and 70 to 80 micrograms (men).

Deficiencies of Vitamin K. Because vitamin K is produced by bacteria in the intestines, deficiency is very uncommon but may occur in patients who have problems absorbing fats, such as those with cirrhosis, people who are on long-term antibiotic therapy, or who are taking other medications, including cholestyramine, Dilantin, and phenobarbital. Symptoms of vitamin K deficiency are easy bruising. A low dietary intake of vitamin K seems to increase the risk of hip fractures in women.

Toxicity of Vitamin K. Allergic-type responses, including rash and itching, to high doses have been reported. Those who are taking Coumadin, an anticoagulant, should not take vitamin K without consulting a physician.

Carotenoids are a group of about 600 nutrients; they are neither vitamins nor phytochemicals. Because they are converted in the body to vitamin A, however, they are discussed in the vitamin section. The most widely studied carotenoid is beta carotene. Others, however, including lycopene, lutein, and zeaxantin, may offer health protection equal to or better than beta carotene. Experts now believe that carotenoids work best as a team along with phytochemicals and vitamins. They are found in foods such as carrots, pumpkins, sweet potatoes, tomatoes, and other deep green, yellow, orange, and red fruits and vegetables.

Phytochemicals (the word simply means plant chemicals) have a major positive impact on human health. Many contribute to the bright and vivid colors found in fruits and vegetables. Hundreds of phytochemicals are currently being studied. The results of such studies are not necessarily applicable to the vegetables or fruit that harbor small concentrations of these chemicals. Nevertheless, it is obvious that vegetables and fruits are healthful, which is probably due to some balance of phytochemicals, carotenoids, vitamins, fibers, and minerals rather than a single substance. It should be stressed that very little has been proven concerning the benefits of phytochemical supplements sold in health food stores. Furthermore, high concentrations of some of these chemicals may behave like drugs and can be toxic, and possibly even contribute to cancer cell growth.

Polyphenols include flavonoids, or catechins, and appear to be powerful antioxidants. Certain flavonoids, including quercetin, are more active than others are. Laboratory studies have shown that they suppress tumor growth, interfere with sexual hormones, may help prevent blood clots, and have anti-inflammatory properties. Flavonoids are found in celery, cranberries, onions, kale, broccoli, apples, cherries, berries, tea, red wine, parsley, soybeans, tomatoes, eggplant, and thyme. Most common berries, which contain flavonoids, are particularly rich in potent antioxidants.

Isothiocyanates and related substances, indoles, are also known as mustard oils and are responsible for the sharp taste in cruciferous vegetables (broccoli, cabbage, Brussels sprouts, cauliflower, collards, kale, kohlrabi, mustard greens, rutabaga, turnips, bok choy). They stimulate enzymes that convert estrogen to a more benign form and may block steroid hormones that promote breast and prostate cancers. (Cruciferous vegetables are also high in fiber, vitamin C, and selenium.)

Monoterpenes have two important phytochemicals, perillyl alcohol and limonene. They block proteins that stimulate cell growth and reproduction and are being tested for actions against cancer. Limonene is found in the peels of citrus fruits.

Phytoestrogens, also called isoflavones, have actions that are similar to the female hormone estrogen. Important phytoestrogens are genistein, daidzein, enterolactone, and equol. They act as antioxidants and tumor suppressors. These compounds may improve cholesterol, prevent bone loss, and suppress enzymes that stimulate breast cancer. Phytoestrogens are found in soy products (not soy sauce), whole grains, berries, fruit, vegetables, and flax seed.

Organosulfurs, which include allicin, boost the immune system, assist the liver in rendering carcinogens harmless, and may reduce production of cholesterol in the liver. These compounds are found in garlic, leeks, onions, chives, scallions, and shallots.

Saponins are forms of carbohydrates that neutralize enzymes in the intestines that may cause cancer. They also may boost the immune system and promote wound healing. Saponins are found in ginseng, beans, including soy beans, and whole grains.

Capsaicin 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 it may inhibit cancer-generating substances. It is found in hot red peppers.

Sterols, which include sitosterol and squalene, are found in vegetable oils and may have cholesterol-lowering effects.

The benefits of any dietary factors are very difficult to prove. Studies on population groups may not consider other lifestyle or genetic factors. They often rely on people self-reporting their own dietary habits and often such surveys only reflect short-term eating habits. Other studies are done in the laboratory on animals or blood samples, which may not reflect the effects of nutrients on humans. Nevertheless, it is never wrong to eat plenty of fresh fruits and vegetables and whole grains, the primary sources of vitamins, carotenoids, and vitamins, as well as of fiber and important minerals.

Antioxidant Properties. Currently, the most important benefit claimed for vitamins A, E, C, and many of the carotenoids and phytochemicals is their role as antioxidants, which are scavengers of particles known as oxygen-free radicals. These unstable particles are by-products of many of the body's normal chemical processes and are increased by smoking, environmental toxins, and stress. They can damage cell membranes and interact with genetic material, possibly contributing to the development of a number of disorders including cancer, heart disease, cataracts, and even the aging process. Oxygen-free radicals can also enhance the dangerous properties of low-density lipoprotein (LDL) cholesterol, a major player in the development of atherosclerosis. Antioxidant vitamins and many phytochemicals can neutralize free radicals and may reduce or even prevent some of their damage. Although it is clear that vitamins are required to prevent deficiency diseases, the possible benefits of higher-dose supplements are still unproven in most cases. In fact, there is some evidence that in high doses vitamin C, vitamin E, and beta carotene have pro-oxidant effects that can be harmful. Everyone should eat plenty of dark colored fresh fruits and vegetables, which contain combinations of antioxidant vitamins and nutrients.

Vitamins and Carotenoids. Deficiencies in Vitamins C, E, A, and beta carotene have been linked to heart disease. All of these nutrients have antioxidant effects and other properties that may benefit the heart. For example, vitamin E may prevent blood clots and the formation of fatty plaques and cell proliferation on the walls of the arteries. Vitamin C may help maintain blood vessel flexibility and so have benefits on blood pressure. Generally such findings have occurred in the laboratory. Many studies have been undertaken to determine if such actions are powerful enough protect the heart in people. The evidence for protection from vitamin or other supplements, however, is quite weak, and in fact each of these supplements in high doses may be harmful. Long-term studies of people who took vitamin E supplements (which are usually composed of the form alpha tocopherol) are mixed. While population studies, such as a major study of nurses, reported benefit for the heart from high vitamin E levels (usually only found in supplements), others are finding little or no benefits for men or women and some have even reported harmful effects. On the other hand, studies are fairly consistent in indicating that eating foods rich in natural vitamin E (known as gamma tocopherol) may be protective. (Such foods, which include nuts, monounsaturated oils, and wheat germ, may also have other beneficial properties that account for these results.) The evidence for heart protection from vitamin C and beta carotene supplements is even weaker. In two major studies that demonstrated some benefit from vitamin E, vitamin C was not protective. One study found some benefits against stroke but not heart disease. Studies have reported that a high intake of beta carotene and other carotenoids from fruits and vegetables may reduce the risk of heart attack. It should be stressed, however, that studies are continuing to indicate that high doses of supplements of any of these antioxidants have pro-oxidant effects that can harm the arteries and incur other damage.

Several important studies have demonstrated a link between deficiencies in the B vitamins folic acid, B6, and B12 and elevated blood levels of an amino acid homocysteine, a risk factor for atherosclerosis. Studies further indicate that taking these vitamins can decrease homocysteine levels. It is not clear yet, however, that reducing homocysteine levels will actually protect against heart disease. Another B vitamin, niacin (vitamin B3), is used for lowering unhealthy cholesterol levels; although vitamin B3 is available over the counter, a physician should prescribe niacin in order to ensure its safety and effectiveness.

Phytochemicals. Flavonoids, particularly those found in both black and green tea, onions, red wine, and apples, may protect against damage done by cholesterol and help prevent blood clots. The first trial of flavonoid supplements reported that they had a significant effect on preventing blood clots. Drinking as little as a cup of tea a day appeared to help protect against heart disease. Some studies have suggested that the organosulfurs found in onions and garlic may have beneficial effects on cholesterol levels, although two well-conducted recent ones found no heart-benefits from taking capsules equivalent to between one and one and half garlic cloves a day. The preparation of these products, however, may be responsible for the lack of effect. Another recent study reported that heating garlic blocks its health-protective effects. By allowing crushed fresh garlic to stand 10 minutes before heating, however, beneficial chemicals are released and not lost when the garlic is cooked. It is known that human estrogens have a beneficial effect on cholesterol levels, and soy protein, high in the phytoestrogens (plant estrogens) isoflavones, has been associated in some studies with lower levels of total cholesterol, LDL, and triglycerides. Some researchers suggest that the well-known protective effect of red wine against cardiovascular disease may be due, in part, to the phytoestrogen resveratrol, which is found in grape skin.

Some studies including a 1999 one on stroke patients suggested that vitamin E may protect against ischemic stroke (caused by blocked arteries) and an animal study reported that high levels of vitamin E in the diet reduced the amount of brain tissue injured by a stroke. (This vitamin may increase the risk for hemorrhagic stroke -- a far less common form of stroke.) Studies are also indicating that people who have higher blood levels of folic acid have a lower than average risk for stroke. Two 1995 studies found that high intake vitamin C intake was associated with a lower risk for death from stroke (but not from coronary artery disease).

Any protective effects of vitamins or specific phytochemicals against cancer appear to depend on a cooperative effort between them and other nutrients found diets rich in fresh fruits and vegetables. For example, although premenopausal women who ate foods rich in vitamin A and carotenes appeared to have a reduced risk of breast cancer, supplements of vitamins A, C, and E had no effect.

Vitamins and Carotenoids. Folic acid and B12 convert the amino acid homocysteine to methionine, a substance that helps prevent cells from becoming malignant. Folic acid may provide some protection against cervical and colon cancer. One small study showed a reduction of lung cancer cells in smokers taking folic acid and vitamin B12, but the study was very small, of short duration, and other factors might have biased the results. Still another study reported that folic acid may reduce the risk for breast cancer among women who regulary drink alcohol. (In the study, folic acid had no other effect on breast cancer.)

Because many cancers are thought to be initiated by the effects of oxygen-free radicals on DNA, the antioxidant vitamins E, C, and A and beta carotene have been intensively studied. One study reported that people with the lowest blood levels of vitamin E are at higher risk for all types of cancer than those with the highest levels. One study found that tocotrienol, a vitamin E compound found in unrefined palm oil, inhibits cancer cell proliferation and causes cancer cell death. (The standard vitamin E supplement, tocopherol, had much weaker effects.) Palm oil sold in the U.S. is refined and does not contain tocotrienol. Most studies, however, have found no evidence that vitamin E or C supplements protect people from a first or a recurring cancer. In fact, studies are now reporting that excessive use of a single supplement, such as beta carotene, may interfere with other nutrients or convert into pro-oxidants and become harmful. Synthetic forms of vitamin A are being studied for their protective effects and even for treating some cancers. It should be stressed, however, that vitamin A supplements can be very toxic, even in amounts not much higher than the RDA levels.

Of particular concern are the many studies that have found an increase in lung cancer and overall mortality rate among smokers who took beta carotene supplements. In determining reasons for this disturbing effect, a recent animal study reported that beta carotene increased enzymes in the lungs that actually promote cancerous changes. Another study suggested that beta carotene taken as a supplement may interfere with other, protective forms of vitamin A. Fruits and vegetables rich in carotenoids, show protection against cancer. The carotenoid lycopene, found in tomatoes -- particularly when they are cooked -- may have particular value in protection against prostate, colon, and bladder cancer.

Phytochemicals. Isothiocyanates and sulforaphane found in cruciferous vegetables may block the effects of carcinogens and suppress tumor growth. Young sprouts of broccoli and cauliflower, compared to the mature counterparts, contain much higher levels of isothiocyanates.

Plant estrogens, including daidzein, equol, and enterolactone, which are found in soy beans, whole wheat, berries, and flax seed, may help reduce the risk for certain hormone-stimulated cancers, such as breast and prostate, by replacing human estrogen with the benign plant estrogens they contain. They may also inhibit the proliferation of existing cancer cells. Because of the lower risk for reproductive and breast cancers in Asian women, most studies on plant estrogens are being conducted using soy. The effects on cancer are not entirely clear, however. One reported that when taken with high doses of estrogen replacement therapy, soy protects against breast and uterine cell proliferation. However, when combined with low doses of estrogen replacement therapy, soy causes breast cell proliferation. Other studies have indicated that some phytoestrogens, such as black cohosh, also cause cell proliferation in the uterus under certain circumstances. Some experts advise that until more is known about when or even if phytoestrogens protect against breast cancer, women who are on hormone replacement therapy and at risk for breast cancer should also avoid plant products with known high levels of phytoestrogens. Some evidence suggests the genistein in soy may have properties that are protective against lung cancer.

Flavonoids, including those found in apples, onions, tea, and red wine, are coming under scrutiny for possible cancer fighting properties. In one 24-year study, people who ate flavonoid-rich foods had a 20% lower risk for cancer in general. (For lung cancer the risk was reduced by 46%.) Laboratory and animal studies have indicated that compounds in green tea have the capacity to selectively kill cancer cells. Some experts recommend that drinking four cups of green tea daily may be protective against certain cancers; black tea does not appear to have these particular benefits. Resveratrol, a substance found in grapes, has been found to fight tumor promotion and progression in mice. The organosulfur compounds found in the onion and garlic family may have very potent properties in suppressing or blocking carcinogenic substances.

A 1993 study found that older people who took a multivitamin had fewer sick days than those who did not. One analysis of studies found that large doses of vitamin C reduced the duration of the cold by 21%, and another found that it reduced the severity and length of respiratory infections in a group of ultramarathon athletes, who are normally at risk for these infections. Fewer incidents of infectious diseases have been observed in people taking large supplements of vitamin E. In a study of elderly people whose immune system had deteriorated, B6 restored them to normal levels, and in those taking large doses the immune response was close to that seen in young people. It was not clear from the study if vitamin B6 actually protected against disease, and it should be noted that large doses of this vitamin can cause nerve damage so should be taken only under advisement from a physician. Major studies in underdeveloped countries where immunization programs are limited and risks for infectious diseases are high, children with infectious diseases who were given vitamin A have lower mortality rates. Vitamin A supplements, however, are toxic in children in amounts close to normal adult doses and should never be given to children with normal diets. Saponins extracted from ginseng and allicin found in garlic also have properties that boost the immune system. Both ginseng and garlic have long been traditionally used for their health benefits. Lycopene, found in tomatoes, appears to have properties that protect infection-fighting white blood cells.

Some people with exercise-induced asthma may benefit from taking vitamin C one hour before strenuous physical activity, although antioxidant vitamins do not appear to reduce the risk of adult-onset asthma. Vitamin C and E may even counteract the negative effects of ozone on lung function in athletes. Studies have indicated that diets rich in antioxidant vitamins and carotenoids improve lung functions in everyone and may provide some protection against lung damage from chronic obstructive pulmonary disease (COPD) among smokers if their diets are healthy throughout their smoking years. (Smokers should never take beta carotene supplements.) One new study reported that vitamin C may lower the risk of emphysema, chronic bronchitis, and other forms of chronic lung disease.

Oxygen-free radicals play a role in cataract formation. Several studies report that the consumption of antioxidant-rich foods is associated with a decreased risk of cataracts. Vitamins E and C and the B vitamins niacin and riboflavin (vitamin B2) might protect against cataracts. Carotenoids, especially lutein and zeaxantin found in dark, green, leafy vegetables, may also help prevent macular degeneration, which is the most common cause of irreversible blindness in the elderly.

There is some evidence that topical products (e.g., lotions and creams) containing vitamins A, E, C, and beta carotene help protect the skin when applied before sun exposure -- although they have no benefits if they are applied afterward. Researchers are also interested in the possible protective effects of combined forms of topical vitamins A and D. One small study found that taking a combination of vitamins C and E may help reduce sunburn reactions, and another reported that such supplements were associated with a lower risk for basal cell carcinoma. (Vitamin A supplements should never be taken, however, to cure skin problems without a doctor's recommendation.) Drinking green or black tea, which contain powerful flavonoids, helped block the carcinogenic effects of UVB radiation in one study.

Postmenopausal women who have very limited exposure to sunlight and low amounts of vitamin D in their diet are at increased risk for osteoporosis and hip fracture. Although everyone, particularly postmenopausal women, should have sufficient vitamin D, doses beyond the recommended daily allowance do not appear to stimulate additional bone mass. One study indicated that 1 mg of additional Vitamin K (ten times the U.S. RDA levels) reduced urinary calcium loss by 33% in postmenopausal women. Supplements are not recommended at this time; clinical trials to confirm this finding and to determine the toxicity levels of vitamin K are still required. In one study bone density increased in postmenopausal women who ate foods rich in soy protein, which contain isoflavones (phytoestrogens). Some experts recommend 25 to 45 milligrams of isoflavones a day. Soy powders, soluble in juice or milk, that list amounts of isoflavones per serving are now available in health food stores. Tofu prepared with calcium may be particularly beneficial. Ipriflavone, a synthetic derivative of isoflavones is currently being investigated for preventing bone loss. Studies are reporting a positive effect on bone density with very mild side effects (mostly gastrointestinal symptoms).

Vitamins B6, B12, and folic acid are important for the production of neurotransmitters, the chemical messengers in the brain that regulate mood and concentration. Deficiencies in these vitamins have been observed in people with depression and dementia and even in those who perform badly on tests. In one study, when subjects were deprived of B6, their memories deteriorated; when B6 was restored, their memories improved. (Large doses of B6, however, can damage nerves.) Some reports have suggested that certain dietary antioxidants, such as vitamin C, E, and selenium may be protective against mental decline. According to a new study, eating plenty of fruits and vegetables may slow brain aging; they are recommended in any case for good health. Foods rich in vitamin E and A are of particular interest for their effects on memory and learning.

According to recent studies, gallstones are half as common in women with high levels of vitamin C in their blood. The antioxidant qualities of vitamin E may help counteract the nerve damage associated with type 2 diabetes. Vitamin B6 in doses of up to 100 mg may help reduce premenstrual symptoms, including depression.

About 40% of Americans take at least one vitamin or mineral supplement daily and another 20% take them occasionally. In a large study that examined the death rates of 11,000 people, however, there was no difference in mortality rate between those who took vitamin supplements and those who didn't.

Women who are pregnant or who are breast-feeding generally need additional vitamins. The B vitamins are particularly important. Folic acid reduces the risk for neural tube defects and possibly facial abnormalities, such as cleft palate. Multivitamins also appear to protect against certain heart birth defects. In order to be protective, however, both folic acid and multivitamin supplements must be taken before a women actually becomes pregnant. Folate levels from even healthy diets may not be protective enough for pregnant women and supplements are needed. The RDA for folic acid prior to conception and during pregnancy is 400 mcg. During breast feeding between 260 and 280 mcg is recommended. Choline, another vitamin B, is also essential for pregnant (450 mg) and nursing women (550 mg). Vitamin B6 may help improve morning sickness. One study also suggested that if pregnant women took vitamin K supplements, their infants might not need the required injection of this vitamin after birth, but supplements of vitamin K during pregnancy are not currently recommended.

Some women have low vitamin A reserves in their liver; it is important to note, however, that too much vitamin A significantly increases the risk for birth defects. Daily amounts of 10,000 IU of vitamin A in supplements and food (an amount not far above the RDA level) can pose a danger. Experts recommend that pregnant women take in no more than 8,000 IU per day and avoid eating liver.

Infants who are breast fed by healthy mothers receive enough vitamins except, in some cases, vitamins K and D. Human milk has low levels of K, and the newborn's immature intestinal tract may not produce enough of the baby's own supply. Most babies are given an injection of this vitamin at birth. Infants being breast fed by malnourished women or those who lack sufficient exposure to sunlight may be deficient in vitamin D. In these cases, supplements of 200 to 300 IU are recommended. Formulas are required to contain sufficient vitamins and minerals. One study suggests that vitamin supplements for infants under one year of age may help protect them from developing type I diabetes later on. Past infancy, most American children receive all the vitamins they need from their diet unless they are living in severely deprived circumstances.

Smoking interferes with absorption of a number of vitamins, importantly vitamin C. Folic acid supplements may be important for all smokers. Taking high doses of antioxidant vitamins, however, may actually be harmful, especially beta carotene in smokers. Instead of taking supplements, most smokers should be sure their diets are rich in fresh fruits and vegetables and whole grains.

Alcoholics often suffer from multiple vitamin deficiencies. The most dangerous deficiencies are from vitamins B1 (thiamin), folic acid, B6 (pyridoxine), B2 (riboflavin), and vitamin C.

People on weight-reduction diets with less than 1,000 calories a day should probably take a multivitamin and should also check regularly with a physician. Vegetarians who do not eat dairy or meat products may need riboflavin, vitamin B12, and vitamin D supplements. Those who eat eggs and dairy products need only watch their iron levels.

Deficiencies of vitamins and important minerals have been observed in almost a third of elderly people. Often their dietary habits slip and they fail to eat balanced meals regularly. Multiple drug regimens may prevent absorption of some vitamins. Elderly people, particularly if they are not exposed to sunlight, may be deficient in vitamin D. They also may have low levels of vitamins B6, folic acid, and B12. (Older adults showing signs of dementia should be checked for B12 deficiencies as well as other disorders causing mental disturbances.) One study reported that the immune systems of elderly people may benefit from higher levels of vitamin E than the daily recommended dosage. It should be noted, however, that metabolism slows down as a person ages, and in elderly people it takes the liver longer to eliminate drugs and vitamins from the body. The effect of some vitamin supplements, therefore, may be intensified; dosage levels of vitamin A, for instance, which might be harmless in a younger adult, could be toxic in an elderly patient. Nevertheless, experts are increasingly recommending extra vitamin and mineral supplements for older people.

B Vitamins may cut heart disease risk. Harvard Health News, April 1998

B vitamins and the heart : What men can learn from women. Harvard Men's Health Watch, June 1998

Excessive dietary intake of vitamin A is associated with reduced bone mineral density and increased risk for hip fracture. Ann Intern Med, 11/15/98, vol. 129.

Getting enough folate and b6. HealthNews, 3/10/98

Hypovtimainosis D in medical patients. The New England Journal of Medicine, 3/19/98

Lowering blood hommocysteine with folic acid based supplements: Metanalysis of randomized trials. British Medical Journal, 3/21/98

Multivitamin use, folate, and colon cancer in women in the Nurses' Health Study. Ann Intern Med, 10/1/98, Vol. 129

New vitamin B recommendations. HealthNews, 5/11/98

Reduction of plasma homocysteine levels by breakfast cereal fortified with folic acid in patients with coronary heart disease. The New England Journal of Medicine, 4/9/98

Safe amounts of vitamins, HealthNews, 8/26/98

The effect of vitamin B12 deficiency on older veterans and its relationship to health. J Am Geriatr Soc, October 1998, Vol. 46.

Vitamins, minerals, diet, and prostate cancer. Harvard Men's Health Watch, May 1998

Harvey Simon, M.D., Editor-in-Chief
Massachusetts Institute of Technology; Physician, Massachusetts General Hospital

Masha J. Etkin, M.D., Gynecology
Harvard Medical School; Physician, Massachusetts General Hospital

John E. Godine, M.D., Ph.D., Metabolism
Harvard Medical School; Associate Physician, Massachusetts General Hospital

Daniel Heller, M.D., Pediatrics
Harvard Medical School; Associate Pediatrician, Massachusetts General Hospital; Active Staff, Children's Hospital

Irene Kuter, M.D., D. Phil., Oncology
Harvard Medical School; Assistant Physician, Massachusetts General Hospital

Paul C. Shellito, M.D., Surgery
Harvard Medical School; Associate Visiting Surgeon, Massachusetts General Hospital

Theodore A. Stern, M.D., Psychiatry
Harvard Medical School; Psychiatrist and Chief, Psychiatric Consultation Service, Massachusetts General Hospital