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In-Depth Reports Home

In This Report

	Highlights
	Introduction
	Carotenoids
	Phytochemicals
	Healthy Foods
	Dietary Health Benefits
	Resources
	References

Related Reports

	Smoking
	Alcoholism

Encyclopedia

	Vitamin A
	Vitamin B12
	Vitamin D

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Vitamins

Highlights

Cancer

Growing evidence suggests that vitamins and micronutrients, especially from foods, may play important roles in the prevention or treatment of certain cancers:

• One study found that the risk of prostate cancer risk dropped as consumption of vegetables high in vitamin C, such as broccoli and bell peppers, rose.
• A diet high in cruciferous vegetables has been found to reduce the risk of kidney cancer; low consumption of cruciferous vegetables increases the risk.

On the other hand, high amounts of folic acid (a B vitamin) may be associated with colorectal cancer, and beta-carotene supplements are associated with increased lung cancer risk in smokers and people exposed to asbestos.

Macular Degeneration

In 2007, the National Eye Institute recommended that people with intermediate or advanced macular degeneration in one eye take a vitamin formula shown to reduce the risk of macular degeneration in the other eye by 25%. The formula contains vitamin C, vitamin E, beta-carotene, and zinc.

Osteoporosis

Vitamin K is widely used in Japan to treat osteoporosis, and studies suggest it also may be effective in treating rheumatoid arthritis.

Heart Disease

Although people with high levels of homocysteine are prone to developing blood clots in their arteries and veins, a 2007 study found that lowering homocysteine with B vitamins and folic acid does not reduce the incidence of deep vein thrombosis (DVT). A 2007 trial in adults with stable coronary artery disease found that lowering homocysteine levels 33% with folic acid and other B vitamins had no effect on arterial inflammation, meaning that lower levels were unlikely to offer protection against heart attack or stroke.

Introduction

Vitamins do not share a common chemistry, but they do share certain characteristics. They are all organic nutrients that are necessary in small amounts for normal metabolism and good health. Your diet or supplements provide most vitamins. The body can manufacture only three vitamins (D, K, and the B vitamin biotin) from nondietary sources. Unlike carbohydrates, fats, and proteins, vitamins are not sources of energy. Instead, vitamins are 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.

Guidelines for Vitamin Adequate Intake

The Recommended Daily 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 guide for determining the amount of vitamins needed for a healthy diet. The RDA refers to an estimate of the average daily requirement. It is not completely adequate, however, for informing people about the amounts of vitamins they may need.

The RDA is gradually being enhanced using a new standard called the Dietary Reference Intake (DRI). The DRI is based on the following ratings, which will eventually appear on labels:

• The recommended daily allowance (RDA). This is the current rating on most vitamins.
• The estimated average requirement (EAR). This is the amount adequate for 50% of all people, which will be put on labels when it can be calculated.
• Adequate intake (AI). This is an amount that will be used if there is insufficient data to calculate the EAR.
• Tolerable upper intake level (UL). This is the maximum dose likely to be safe in nearly all individuals. It will be included on labels if this amount is known.

Food and supplement labels now typically list the Daily Value (DV). This is the percentage of the amount of a nutrient that experts believe a person needs in their daily diet. On food labels it is usually based on one serving size for a person who takes 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.

Labels on vitamins and other dietary supplements now include nutrient information and list all ingredients, including identifying parts of plants from which ingredients may be taken. Unlike the labels for drugs, however, labels for vitamins and supplements may not claim to prevent or treat any specific disease. Labels for vitamins and supplements include one of the following:

• Health claim -- description of how the substance may reduce the risk of a health-related condition
• Nutrient claim -- description of the amount of the nutrient in the product or
• Structure or function claim -- description of how the product may affect organs or systems of the body, without claiming to prevent or treat specific disease

The quality of dietary supplements depends on the manufacturer and is not ensured by FDA. The U.S. government does not require that supplements be standardized, meaning that the amounts or quality of nutrients may vary depending on the batch. So, more expensive supplements are not necessarily better than the less expensive ones. Government regulations are in the process of catching up to the boom in the supplement industry. In the meantime, some companies voluntarily adhere 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 of companies that adhere to 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 absorbed properly into the body.
• It has been produced according to good manufacturing standards.

Before selling any supplement introduced after 1994, manufacturers must submit information as to why the product is considered safe for people. The FDA may refuse to allow it on the market if it finds the evidence insufficient. 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 for sale, 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 misleading or false.

People Who Should Take Vitamin Supplements

About 30% of Americans take at least one vitamin or mineral supplement daily. 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. Most people who have a healthy diet do not need vitamins, but there are some exceptions.

Pregnant and Breast-Feeding Women. Women who are pregnant or who are breast-feeding generally need additional vitamins. Vitamins B6, B12, and folic acid are particularly important. Women who are vegetarians must be sure to avoid deficiencies, which can harm their offspring. Folic acid reduces the risk for neural tube defects and possibly facial abnormalities, such as cleft palate. Studies also show that low folate levels during pregnancy are associated with low birth weight, a risk factor for the development of cardiovascular disease in adulthood. Multivitamins that contain folic acid also appear to be somewhat protective. A woman's best approach is to take extra folic acid plus multivitamin supplements (which have additional benefits), starting them before becomming pregnant.

The human body stores several years' worth of vitamin B12, so nutritional deficiency of this vitamin is extremely rare. Although, people who follow a strict vegetarian diet and do not consume eggs or dairy products may require vitamin B12 supplements.

Pregnant women with healthy diets may have low folate levels and need to take supplements. Requirements are as follows:

• The recommended daily allowance (RDA) for folic acid prior to conception and during pregnancy is 400 mcg.
• During breast-feeding 260 - 280 mcg is recommended.

The following vitamins may have some value for pregnant women:

• Choline, another vitamin B, is also essential for pregnant (450 mg) and nursing women (550 mg). Choline plays a key role in brain development. Not getting enough during pregnancy can lead to memory and cognitive defects in the baby. Choline supplements can also lessen the cognitive defects of prenatal alcohol exposure.
• Vitamin B6 may help improve morning sickness.
• Vitamin C may reduce the risk of urinary tract infections during pregnancy.
• Vitamin D may help prevent preeclampsia.
• 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 (international units) 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 and Children. 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 - 300 IU are recommended. Formulas are required to contain sufficient vitamins and minerals. One study suggests that vitamin supplements for infants under 1 year of age may help protect them from developing type 1 diabetes later on. Beyond infancy, most American children receive all the vitamins they need from their diet unless they are living in severely deprived circumstances.

Smokers. Smoking interferes with absorption of several vitamins, importantly vitamins C and D. In one study nearly 25% of female smokers and 31% of male smokers were deficient in vitamin C. Smoking can interfere with the metabolims of vitamin D, resulting in poor muscle function. Taking high doses of antioxidant vitamins, however, may actually be harmful in smokers, especially beta carotene. Instead of taking supplements, most smokers should be sure their diets are rich in fresh fruits and vegetables and whole grains. Of course, smoking cessation is the most important intervention of all.

Click the icon to see an image of sources for vitamin C.

Alcoholics. 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. Low levels of B6 are associated with increased risk of colorectal cancer in men who drink large amounts of alcohol.

Overweight Adults. Overweight and obesity causes many problems that often result in metabolic syndrome or type 2 diabetes. Evidence suggests that isoflavones can help regulate cholesterol levels and reduce body weight and fat mass. Because some medications used to control blood sugar levels reduce folic acid and vitamin B12, some people may need vitamin supplements.

People Who Have Had Gastric Bypass Surgery. Vitamin deficiency is a recognized complication of gastric bypass surgery. Women, African-Americans of both sexes, and adults who have had laparoscopic Roux-en-Y bypass surgery are at highest risk. The deficiency is treated with water-soluble vitamin supplements.

Strict Vegetarians. Strict vegetarians need supplements of vitamin B12, unless they get enough of it from fortified cereals and other grain products.

Dieters and Vegetarians. 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 may need riboflavin, vitamin B12, and vitamin D supplements. Vegans, who do not eat dairy or eggs as well as meat, may be at further risk for vitamin A deficiencies if they do not also have plenty of dark colored fruits and vegetables. Those who eat eggs and dairy products need only watch their iron levels.

Deficiencies in vegetarian children may be particularly harmful. (One study, for example, reported that adolescents who had been on macrobiotic diets before age 6 and were deficient in vitamin B12 scored lower on psychological tests.) Pregnant and breast-feeding women who are vegetarians must be sure to have sufficient vitamins. Of special note, maternal deficiencies in vitamin B12 may cause delayed growth and neurologic problems in their newborns.

Click the icon to see an image of sources for vitamin D.

Older Adults. 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 important B vitamins. (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.

People Who Need to Avoid Sunlight. People who need to avoid sunlight or are housebound, and whose diet is low in foods that contain vitamin D should take supplements. People with darker skin are at higher risk for deficiencies than those with whiter skin. (Note: vitamin D is toxic in high doses, and no one should exceed the recommended daily intake of vitamin D except under the direction of a physician.)

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 are proving to be very important for health. Beta carotene is the most widely studied carotenoid, but others are proving to be of great interest. As with some, but not all, carotenoids, beta carotene is known as a provitamin A because it converts to the vitamin in the body.

They are categorized as either xanthophylls or carotenes according to their chemical composition.

Carotenes

Carotenes are hydrocarbons and most 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 fruits and vegetables. Beta carotene is the most widely studied carotenoid. Evidence now strongly suggests that when taken as a separate supplement it can have harmful effects.
• 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 may have important cancer fighting properties and other health benefits.
• The beneficial actions of most carotenes such as those tomatoes, corn, and carrots, appear to be enhanced by cooking them, especially in oil (preferably olive, canola, or another monounsaturated oil). (Note: Cooking can also destroy certain nutrients, such as vitamin C, in these vegetables.)

Xanthophylls

Xanthophylls contain oxygen and most are found in green vegetables, such as broccoli, cabbage, and kale. They are also in yellow fruits and vegetables. Xanthophylls include lutein and zeaxanthin, which are both stored in the retina of the eye. Neither converts to vitamin A. Both are powerful antioxidants and may be very important for healthy eyes. Unlike carotenes, cooking may reduce the antioxidant activity of some xanthophylls in foods, although probably not to any significant degree.

Phytochemicals

The word phytochemicals means plant chemicals. Hundreds of phytochemicals are being studied. 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 specific phytochemicals are not necessarily applicable to the vegetables or fruits 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 any single substance.

The benefits of individual phytochemical supplements are largely unproven. Furthermore, they are not regulated and high concentrations of some may behave like drugs and can be toxic and possibly even contribute to cancer cell growth.

Polyphenols and Flavonoids

Polyphenols are important phytochemicals, and flavonoids (or catechins) are members of the polyphenol family that may have significant health benefits. Laboratory studies have shown that specific flavonoids suppress tumor growth, interfere with sexual hormones, prevent blood clots, and have anti-inflammatory properties. In general, 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 common berries contain flavonoids and are particularly rich in potent antioxidants.

Among the important flavonoids are resveratrol, quercetin, and catechin. Evidence suggests that resveratrol (found in red wine, grapes, olive oil) may be extremely potent. In laboratory studies, it increases cell survival and has been shown to increase the life span of worms and fruit flies. Catechins are the primary flavonoids in tea and may be responsible for its possible beneficial effects. Flavonoids in dark chocolate may also be health protective.

Isoflavones (Phytoestrogens)

Isoflavones, commonly known as phytoestrogens, have actions that are similar to the female hormone estrogen. A high consumption of soy, which is primarily composed of isoflavones, may reduce symptoms resulting from estrogen depletion during menopause. In a recent study, supplementation with isoflavones decreased hot flashes by 57% and night sweats by 43%, but other research is less favorable.

Lignan is another phytoestrogen and is found in the fiber layers of whole-grains, berries, some seeds, some vegetables, and a few fruits.

Isothiocyanates

Isothiocyanates and related substances, indoles, are also known as mustard oils and are responsible for the sharp taste in cruciferous (also called brassica) vegetables. Such vegetables include broccoli, cabbage, Brussels sprouts, cauliflower, collards, kale, kohlrabi, mustard greens, rutabaga, turnips, and bok choy. Isothiocyanates also 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

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.

Organosulfur Compounds

Organosulfurs are part of the allium family of phytochemicals. Compounds, such as allicin, may have benefits on the immune system, assist the liver in rendering carcinogens harmless, and reduce production of cholesterol in the liver. These compounds are found in garlic, leeks, onions, chives, scallions, and shallots.

Capsaicin

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

Sterols

Sterols, which include sitosterol, stigmasterol, campesterol, and squalene, are found in vegetable oils. Sitosterol is the most studied and appears to have cholesterol-lowering effects.

Beta-sitosterols may help improve urine flow and urinary symptoms in men with enlarged prostate glands (benign prostatic hyperplasia, or BPH). A recent review study of five randomized trials (519 men) found that urinary flow and other urinary symptoms improved in men who took the herbal remedy from 4 - 26 weeks. The studys authors cautioned that while beta-sitosterols show effectiveness in the short term, their long-term effectiveness, ability to prevent complications from BPH, and safety are not known. More research is necessary. Beta-sitosterols come from South African star grass, Hypoxis rooperi, or species of Pinus and Picea.

Healthy Foods

Evidence increasingly suggests that a varied diet, not individual food chemicals, is essential for basic health and a longer life. Such diets are rich in fresh fruits and vegetables and whole grains, and low in saturated fats.

Dietary Health Benefits

The benefits of any dietary factors are 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. An exception is lutein, which is known to reduce the risk of macular degeneration. However, a diet 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

Description of Oxygen-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 particles known as oxygen-free radicals (also sometimes called oxidants). These chemically active particles are by-products of many of the body's normal chemical processes. Their numbers are increased by environmental assaults, such as smoking, chemicals, toxins, and stress. In higher levels, oxidants can be very harmful in the following way:

• 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 itself.
• Oxygen-free radicals can also enhance the dangerous properties of low-density lipoprotein (LDL) cholesterol, a major player in the development of atherosclerosis.

Description of Antioxidants and Warnings on High-Dose Supplements. Antioxidant vitamins (A, C, and E), carotenoids, and many phytochemicals can neutralize free radicals. Unfortunately, although it is clear that vitamins are required to prevent deficiency diseases, high doses of vitamin C, vitamin E, and beta carotene supplements may also have pro-oxidant effects, which can be harmful in patients with cancer. In these people, high doses of antioxidant vitamins may actually protect cancer cells just as they do healthy cells.

The strongest evidence on negative effects to date comes from studies reporting an increase in lung cancer and overall mortality rates among smokers who took beta carotene supplements. In determining reasons for this disturbing effect, one animal study suggested that beta carotene increased enzymes in the lungs that actually promote cancerous changes. One study also reported a higher risk for cancer in male smokers who took multivitamins plus A, C, or E.

Some evidence also indicates that high doses of vitamin C may speed up atherosclerosis, or hardening of the arteries. In one study, women with heart disease who took antioxidant vitamins had a higher risk for heart attack or death than those who didn't take one.

Another study also reported a higher incidence and greater severity of respiratory infections in older adults who took 200 mg of vitamin E daily. Some researchers speculate that certain immune factors generate oxidants to fight bacteria. This antioxidant vitamin, then, may block that action. Research published in 2005 suggests that those who take large amounts of vitamin E (1,500 IU/day) may slightly increase their risk for heart failure and death, but this evidence is not considered conclusive. Further study is necessary.

Protection against Heart Disease

Vitamins and Heart Protection.

• Antioxidant Vitamins A, C, and E. Deficiencies in vitamins A, C, E, and beta carotene have been linked to heart disease. All of these nutrients have antioxidant effects and other properties that should benefit the heart. A study in patients with heart failure has shown that vitamin C can work with dobutamine, a powerful intravenous medication, to strengthen the hearts ability to contract following a heart attack. In fact, a 2005 study has found that taking high doses of vitamin E is associated with an increased risk of heart failure. In 2007, the Womens Antioxidant Cardiovascular Study failed to find that vitamins C, E, and beta carotene could reduce the risk of heart attack, stroke, need for revascularization, or cardiovascular death in women. According to the U.S. Preventive Service Task Force, evidence is insufficient to confirm or refute the benefits of supplements of any of these vitamins in protecting against heart disease.
• Folate and B12 Vitamins. Deficiencies in the B vitamins folate (known also as folic acid) and B12 have been associated with elevated blood levels of homocysteine, an amino acid that has been associated with a higher risk for heart disease, stroke, and heart failure. One study, reported lower failure rates after heart surgery in patients who took folic acid and vitamins B12 and B6. And a major 2002 study suggested that lowering homocysteine levels with folic acid would reduce the risk for heart disease by 16% and stroke by 24%. However, a 2007 trial in adults with stable coronary artery disease found that lowering homocysteine levels 33% with B vitamins and folic acid had no effect on arterial inflammation, meaning that lower levels were unlikely to offer protection against heart attack or stroke. More evidence is needed to determine whether homocysteine plays a causal role in cardiovascular disease and whether the B vitamins are protective. Folate improves blood flow through the arteries, which may be important for the heart, regardless of its effect on homocysteine. Although people with high levels of homocysteine are prone to damaging blood clots in their arteries and veins, a 2007 study found that lowering homocysteine with folic acid and other B vitamins does not reduce the incidence of blood clots in the peripheral veins (deep venous thrombosis).
• Niacin. Niacin (vitamin B3) is used for lowering unhealthy cholesterol levels. Although vitamin B3 is available over the counter, it can have significant side effects. A physician should prescribe niacin in order to ensure its safety and effectiveness. [See In-Depth Report #23, Cholesterol.]

Carotenoids and Heart Protection. Studies have reported that a diet high in fruits and vegetables containing beta carotene, lycopene, and other carotenoids may reduce the risk of heart attack. A small Finish study found that a diet high in tomatoes reduced total cholesterol and LDL ("bad") cholesterol. Diets low in lycopene (particularly from tomatoes) were associated with a significantly higher risk of heart disease and stroke.

Atherosclerosis is a disease of the arteries in which fatty material is deposited in the vessel wall, resulting in narrowing and eventual impairment of blood flow. Severely restricted blood flow in the arteries to the heart muscle leads to symptoms such as chest pain. Atherosclerosis shows no symptoms until a complication occurs.

Phytochemicals and Heart Protection. Several phytochemicals are associated with heart protection.

• Flavonoids. Certain flavonoids, found in both black and green tea, dark chocolate, onions, red wine or red grape juice, and apples, appear to be strongly heart protective. In one study, people who consumed the most flavonoids in foods had a 20% lower risk for heart disease than those with low consumption. Flavonoids may protect against damage done by cholesterol and help prevent blood clots. A number of studies have now reported heart protection from the flavonoid catechin, which is found in both black and green tea. The flavonoid resveratrol, which is found in grape skin, appears to be responsible for the well-known heart protective effects in red wine and purple grape juice.
• Organosulfurs. Organosulfurs found in onions and garlic have been under investigation for possible beneficial effects on cholesterol levels. One study reported an association between taking garlic capsules and significantly lower cholesterol-build up in the arteries of older women but not in older men. In the study, daily garlic supplements dramatically reduced the build-up of newly formed plaque in the arteries, while having much less effect on older, harder plaque deposits. Garlic supplements for cardiovascular disease may be most beneficial when used during earlier years among men and later years among women.
• Isoflavones. Soy protein is the most studied source of isoflavones (known as phytoestrogens, or plant estrogens). Not all studies are consistent, but the majority has shown an improvement in at least one of the cholesterol components in people who consumed at least 25 grams of soy protein. A 2007 meta-analysis of all soy protein studies performed from 1990 - 2006 found that soy protein significantly decreased total cholesterol and LDL cholesterol, but had no effect on HDL or triglycerides. The effect was particularly evident in people with hypercholesterolemia. A 2007 study found that 12 weeks of soy supplement lowered total cholesterol and LDL levels in both Caucasian and African-American postmenopausal women. Soy may also reduce other heart risk factors, at least in certain populations. For example, in one 2002 study, soy was beneficial for controlling blood sugar and lowering LDL in postmenopausal women with type 2 diabetes. In a 2007 study of overweight men and postmenopausal women, soy protein reduced blood pressure and arterial stiffness. In another study, soy protein was associated with lower systolic blood pressure in men. The best sources are soy products (tofu, soy milk) or whole soy protein. Powdered soy protein that contains at least 60 mg of isoflavones may provide similar benefits.
• Sterols. The plant sterols, including sitosterol, are also proving to be potent cholesterol fighters by blocking the absorption of cholesterol in the intestine. Sitostanol, a derivative of sitosterol, is being used in new margarine products to lower cholesterol levels. Sterols and stanols are now found in breads, cereals, yogurt, and fruit juices.

Protection against Stroke

A healthy diet rich in fruits and vegetables and low in salt and saturated fats may significantly lower the risk for a first stroke, perhaps by helping to protect against high blood pressure -- a major risk factor for stroke.

Vitamins and Stroke Protection. The effects of antioxidant vitamins and carotenoids on stroke, dementia, or both are being studied. Studies are conflicting, however. A 2007 study of 8,171 women with cardiovascular disease reported that vitamins C, E, and beta carotene offered no protection against heart attack and stroke.

The B vitamin folate (usually in the form of folic acid) may protect against stroke. However, exactly which people benefit from this therapy has yet to be determined. Studies have suggested that people who have higher blood levels of folate have a lower than average risk for stroke. Its primary benefit in this case appears to be to reduce levels of homocysteine, an amino acid that has been strongly linked to an increased risk of coronary artery disease, stroke, and Alzheimer's disease. A 2007 meta-analysis of 8 trials found that folate supplements decreased homocysteine 20% and lowered stroke risk 18%. Interestingly, lowering homocysteine with folic acid and B vitamins had no effect on heart attack, strokes, amputations, need for dialysis, or death in patients with chronic or end-stage kidney disease.

Carotenoids and Stroke Protection. Some, but not all, studies have reported a lower risk of stroke from carotenoids, including beta carotene and lycopene.

Protection against Cancer

Many fresh fruits and vegetables contain chemicals that may fight many cancers, including lung, breast, colon, and prostate cancers. Examples of important cancer fighting foods include the following:

• Cruciferous vegetables (such as cabbage, Brussels sprouts, and broccoli)
• Tomatoes (which contain lycopene)
• Carrots (which contain alpha carotene)

Some evidence suggests that antioxidants may enhance the anticancer effects of chemotherapy. In multiple studies, patients who maintained their antioxidant levels were better able to withstand the high stress caused by chemotherapy or radiation therapy compared to those with low antioxidant levels. Antioxidant nutrients that may help reduce the side effects of chemotherapy include vitamins E and C, beta carotene, genistein and daidzein (isoflavones found in soy), and quercetin (found in red wine an purple grape juice).

Any protective effects of vitamins or specific phytochemical against cancer, however, appear to depend on the cooperative effort among them. Individual supplements of any vitamin or food chemical have not as yet shown any benefits.

Additionally, certain supplements may actually encourage tumor growth, particularly when taken in large amounts. Two 2007 studies found a connection between folate supplements and colorectal cancer. In one study, which was designed to evaluate the benefits of folic acid in patients who had previous colorectal adenomas (precancerous polyps), the researchers instead found that folic acid was associated with a higher risk of having 3 or more adenomas and noncolorectal cancers. In another study, it was noted that the downward trend in colorectal cancer diagnoses abruptly started to rise in 1996 when mandatory folate enrichment of grains within the U.S. and Canada began. Rates continue to exceed pre-1996 levels. Additionally, a large 2007 National Cancer Institute/AARP study found an increased risk of advanced and fatal prostate cancer in men who took more than 7 multivitamins a week, but no association between multivitamin use and localized prostate cancer.

High consumption of cruciferous vegetables (at least once per week) was associated with lower risk of kidney cancer, and low consumption (less than once per month) of cruciferous vegetables was associated with higher risk of kidney cancer in a multinational 2007 European study. Cruciferous vegetables also appear to offer protection against head and neck cancer resulting from chemical toxins found in cigarettes and alcohol, for example.

Vitamins and Cancer Protection. Because many cancers are thought to be initiated by the effects of oxygen-free radicals on DNA, the antioxidants A, C, and E and beta carotene have been intensively studied. A major study found that men who took selenium for 6 or 7 years reduced their risk of prostate cancer by 52%. Nevertheless, most individual supplements have not been proven to protect against cancer, and high doses may be dangerous.

A 2007 review of the diets of men exposed to asbestos found a decreased risk of prostate cancer associated with increasing intakes of vitamin C-rich vegetables, but not fruits and vegetables high in vitamin A. The chemopreventive role of silymarin (Silybum marianum), found in milk thistle extract, has been extensively studied and has shown anticancer efficacy against various cancers, especially prostate and skin, by inhibiting UVB radiation.

A review of 13 cancer registries found 416,134 cases of skin cancer and 3,776,501 cases of non-skin cancer as a first cancer. Rates from cancer registries in sunny countries (such as Australia and Spain) and less sunny countries (such as Canada and Iceland) were compared. The researchers concluded that vitamin D production in the skin decreases the risk of several solid cancers, especially stomach, colorectal, liver and gallbladder, pancreas, lung, female breast, prostate, bladder, and kidney cancers. The apparently protective effect of sun exposure against second primary cancer is more pronounced after non-melanoma skin cancers than melanoma.

Consumption of aflatoxins, a common fungus-related toxin infecting cereal grains, oil seeds, spices, tree nuts, and the milk of animals fed contaminated feed, is known to cause hepatocellular carcinoma, a deadly form of liver cancer. Rodent studies have shown that phenolic antioxidants, dithiolethiones, isothiocyanates, and triterpenoids may act as chemopreventive agents, dispersing aflatoxins and protecting against hepatocellular carcinoma. Human trials are planned. A similar study found that several isothiocyanates, diallyl sulfide, and polyphenolic compounds can prevent esophageal dysplasia from progressing to squamous cell carcinoma.

A review of all articles on vitamins and cancer published through February 2007 found that multivitamin/mineral supplement use may prevent cancer in individuals with poor or suboptimal nutritional status. One trial on poorly nourished Chinese showed supplementation with combined Beta-carotene, vitamin E and selenium reduced gastric cancer incidence and mortality, and overall cancer mortality. In a French trial, combined vitamin C, vitamin E, beta-carotene, selenium, and zinc reduced cancer risk in men but not in women. With few exceptions, neither beta-carotene nor vitamin E had benefits for preventing cancer. Beta-carotene supplementation increased lung cancer risk in smokers and persons exposed to asbestos.

A 2007 study of nearly 82,000 men and women in Sweden found that high intake of methionine was associated with reduced risk of pancreatic cancer. The same relationship was not seen with vitamin B6 or folate.

• Vitamin A, C, and E. Although some studies have reported an association between low blood levels of these antioxidant vitamins and a higher risk for cancer, supplements of vitamins A, C, and E appear to have few advantages in most cases. And there are some studies finding higher cancer risks with high intakes of antioxidants. For example, a 2003 study reported a higher risk in melanoma in people with vitamin-C rich diets. Another study also reported a higher risk for cancer in male smokers who took multivitamins plus A, C, or E. (Vitamin E may be protective against bladder cancer and ovarian cancer.)
• Vitamin D. Some studies have suggested that certain vitamin D compounds may inhibit certain cancer cells, specifically prostate cancer, from proliferating. More research is needed. In 2007, the National Cancer Institute confirmed that ultraviolet (UV) radiation exposure may reduce the risk of developing non-Hodgkin lymphoma (NHL), but only in patients with certain variations in the D vitamin receptor gene. A second 2007 study found that variations in this gene increase the risk of diffuse large B-cell lymphoma. A 2007 prospective analysis of 31,500 women in the Womens Health Study evaluated calcium and vitamin D intake. The researchers found a moderately lower risk of premenopausal, but not postmenopausal, breast cancer with higher intakes of total calcium and vitamin D. A 2007 review of breast cancer cases reported in Ontario, Canada, found reduced breast cancer risks were associated with increasing sun exposure in women ages 10 - 19, less evidence for associations in women ages 20 - 29, and no evidence for ages 45 - 54. Researchers concluded that sun exposure earlier in life, particularly during breast development, may be key in the connection between vitamin D exposure and breast cancer risk.
• Folic acid and B12. These B vitamins 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 regularly drink alcohol. (In the study, folic acid had no other effect on breast cancer.)

In 2006, a study for the National Institutes of Health reviewed randomized trials evaluating the effectiveness and safety of multivitamin and mineral supplements in preventing cancer and chronic disease. The studies had mixed results, and some supplements reduced cancer rates in certain populations. However, the reviewers concluded that current evidence is not sufficient to determine whether multivitamin and mineral supplements may prevent cancer and chronic disease.

Carotenoids and Cancer Protection. A number of studies have reported that fruits and vegetables rich in carotenoids are associated with protection against many cancers. Lycopene, found in tomatoes, may have particular value in protection against prostate, colon, lung, and bladder cancer. A 2005 study found that in one out of four men with genetic variations that cause them to be more sensitive to oxidative stress, supplementation with selenium, vitamin E, and lycopene significantly reduces the risk of prostate cancer. Individual supplements, however, do not offer any advantage. In fact, evidence now strongly suggests that beta carotene supplements increase the risk for lung cancer in smokers and people exposed to asbestos

Phytochemicals and Cancer Protection. The following phytochemicals appear to have cancer-protecting properties.

• Isothiocyanates. Isothiocyanates and sulforaphane, found in cruciferous vegetables, may block the effects of carcinogens and suppress tumor growth. In one study, for example, women with the highest consumption of cruciferous vegetables had a 24% lower risk of breast cancer than women with the lowest consumption.
• Isoflavones. Isoflavones, found in soy beans and flax seed, behave like estrogen in some ways and not in others. Researchers are very interested, then, in their effects on hormone-related cancers, including breast and prostate cancers. Much research has focused on soy. In general, a number of Asian studies have reported an association between a higher intake of soy and a lower incidence of reproductive and breast cancers. The effects of phytoestrogens, however, in all women are far from settled. Some evidence suggests the genistein in soy may have properties that are protective against lung cancer. Nonfermented soy products (tofu, soy milk) also may protect against stomach cancer, while fermented soy products (miso, soy paste) appears to increase the risk.
• Organosulfurs. The organosulfur compounds found in the onion and garlic family may have very potent properties in suppressing or blocking carcinogenic substances. A 2007 study found that synthetic organosulfur compounds act as selective inhibitors of growth in breast cancer cells. Studies indicate that people who regularly consume fresh or cooked garlic have about half the risk of developing stomach cancer and two thirds the risk of colorectal cancer as people who eat little or no garlic. One possible explanation for garlic's anti-cancer effect in the stomach is its antibacterial action against H. pylori, which can promote stomach cancer. Taking garlic supplements, however, did not offer these benefits.

It should be noted that 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, as they can easily be complicated by factors such as diet, exercise, the presence of healthy or unhealthy lifestyle behaviors, environmental factors, and more.

Resources

• http://fnic.nal.usda.gov -- The Food and Nutrition Information Center
• http://dietary-supplements.info.nih.gov -- Office of Dietary Supplements, National Institutes of Health
• www.ars.usda.gov/ba/bhnrc/ndl -- Nutrient Data Laboratory
• www.fda.gov -- Food and Drug Administration
• www.eatright.org -- The American Dietetic Association
• www.acsh.org -- American Council on Science and Health
• www.aicr.org -- American Institute for Cancer Research
• www.nutritiondata.com -- Information on vitamins and nutrients in foods
• www.consumerlab.com -- Independent testing of nutritional supplements' contents and quality
• www.usp.org -- US Pharmacopeia
• www.herbs.org -- Herb Research Foundation

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