Dietary Health Benefits
The benefits of any dietary factors are very difficult to prove, and, to date, there is no hard evidence that any supplement protects against any major disease. 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.
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 bodys 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, to date, there is no strong evidence that antioxidant supplements offer any real protection. Everyone should try to obtain these nutrients from dark colored fresh fruits and vegetables, which contain balanced teams of antioxidant vitamins and nutrients.
In fact, there is some increasing evidence that high doses of vitamin C, vitamin E, and beta carotene supplements may have pro-oxidant effects, which can be harmful in certain cases.
The strongest evidence on negative effects to date are 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. A 2000 study also reported a higher risk for cancer in male smokers who took multivitamins plus A, C, or E.
And, even more worrisome, in people with existing cancer, high doses of antioxidant vitamins, such as vitamin C or beta carotene, may actually protect cancer cells (just as they do healthy cells). For example, a 2003 study reported a higher risk in melanoma in people with vitamin-C rich diets.
Some evidence also exists that high doses of vitamin C may speed up atherosclerosis. In a 2003 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.
A 2002 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 vitamins then may block that action.
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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. However, a number of studies have found no reductions in heart disease in people who have taken antioxidant vitamins. In 2003, the US Preventive Service Task Force concluded that, to date, evidence is insufficient to confirm or refute the benefits of supplements of any of these vitamins in protecting against heart disease. It is important to note, in addition, that each of these supplements may even be harmful to the heart in high doses.
- 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. Some studies in 2002, suggest that any risk posed by homocysteine or benefits from folic acid for heart disease are at most modest. One study, however, 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%. 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.
- 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. [For more information, seeWell-Connected Report #23, Cholesterol.]
Carotenoids and Heart Protection. Studies have reported that a high intake of fruits and vegetables containing beta carotene, lycopene, and other carotenoids may reduce the risk of heart attack. For example, lycopene-poor diets (particularly lycopene in tomatoes) were associated with a significantly higher risk of heart disease and stroke. In another study involving animals, lutein protected against early hardening of the arteries.
Phytochemicals and Heart Protection. A number of phytochemicals have been 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 a 2003 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. (Studies on tea-drinking however have been mixed. For example, the British consume a lot of tea but have high rates of heart disease.) 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. A glass or two of red wine a day may be healthful. For people who cannot drink alcohol, juice from red grapes may be beneficial.
- Organosulfurs. Organosulfurs found in onions and garlic have been under investigation for possible beneficial effects on cholesterol levels. Two well-conducted studies found no heart-benefits from taking capsules equivalent to between one and one and a half garlic cloves a day. The preparation of these products, however, may be responsible for the lack of effect. On a more positive note, a 2000 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, then, 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. 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 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. Tablets of individual isoflavones found in soy do not appear to offer any advantages and may be harmful.
- Sterols. The plant sterols, including sitosterol, are also proving to be potent cholesterol fighters. Sitostanol, a derivative of sitosterol, is being used in new margarine products to lower cholesterol levels.
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 very important 2001 study reported no protection stroke with vitamins A, E or beta carotene.
The vitamin B, folate (usually in the form of folic acid), may protect against stroke. 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 major 2002 study suggested that lowering homocysteine levels with folic acid would reduce the risk for heart disease by 16% and stroke by 24%.
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 (e.g., cabbage, Brussels sprouts, broccoli).
- Tomatoes (which contain lycopene).
- Carrots (which contain alpha carotene).
There is some evidence that antioxidants may enhance the anticancer effects of chemotherapy. In a 2000 study, patients who maintained their antioxidant levels were better able to withstand the high stress caused by chemotherapy compared to those with low antioxidant levels. Antioxidant nutrients that may have properties 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.
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. To date, no individual supplements have offered any protection against cancer, and high doses may even be hazardous.
- 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 no 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. A 2000 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.)
- 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.
- 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.)
Note: In 2003, the US Preventive Services Task Force concluded that, to date, evidence is insufficient to confirm or refute the benefits of supplements of any of these vitamins in protecting against cancer.
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. 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.
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.
- Flavonoids. Flavonoids and polyphenols, including those found in apples, dark chocolate, onions, tea, and red wine, are coming under strong 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. Resveratrol is a particularly potent polyphenol found in grapes and red wine. It has been found have tumor-suppressing properties. In studies on mice it has reduced tumor promotion and progression. Quercetin, another polyphenol, may also be protective.
- Organosulfurs. The organosulfur compounds found in the onion and garlic family may have very potent properties in suppressing or blocking carcinogenic substances. 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.
Evidence of Protection Against Other Diseases with Vitamins, Carotenoids, and Phytochemicals
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Disease or Condition
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Vitamins
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Carotenoids, Phytochemicals, and Healthy Foods
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Alzheimer's Disease
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Vitamin E. Some reports, including a large 2002 population study, have suggested that vitamin E intake, from food or supplements, may protect against mental decline. (One study suggested that the vitamin protected only those who carried the apoE4 gene. No strong evidence to date has found any protection from antioxidant supplements.)
B Vitamins. Some studies suggest that deficiencies of the B vitamins B6, B12, and folate may be a risk factor for Alzheimer' diseases, possibly because deficiencies elevate homocysteine levels, which some research now associated with a higher risk for Alzheimer's disease.
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According to several studies, eating plenty of darkly colored fruits and vegetables may slow brain aging. Of interest was a 1999 study on animals, in which extracts taken from blueberries and strawberries actually reversed age-related decline in brain function. Blueberries were the most effective.
The estrogen-like properties in isoflavones are of interest in the study of Alzheimer's disease. Animal studies suggest that soy might be protective against AD, particularly in postmenopausal women. Of some concern, however, were one population and a few animal studies suggesting that soy intake may pose a risk for greater mental decline among older men. More research is needed to confirm the effects of soy on the aging brain and to determine if there are gender differences.
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Infectious Disease
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Studies are mixed whether vitamin supplements protect against upper respiratory infections. Large doses of vitamin C, for example, may help reduce the duration of a cold, but they do not appear to protect against one in the first place, even after exposure to a cold virus. Two studies in 2002 on multivitamins reported opposite results, with one finding fewer infections and one finding no difference. It is possible that vitamin C or multivitamin supplements may be helpful in specific people, such those who are vitamin deficient or have medical problems that impair their immune systems.
Studies on vitamin E specifically have been largely negative. A 2002 study, in fact, reported a higher incidence and greater severity of respiratory infections in older adults who took 200 mg of vitamin E daily.
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Lycopene, found in tomatoes, appears to have properties that protect infection-fighting white blood cells.
Saponins extracted from ginseng and allicin (found in garlic) have properties that boost the immune system. Both ginseng and garlic have long been traditionally used for their health benefits.
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Asthma
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Vitamin C from diet has been associated with lower risk for asthma. Supplements do not appear to provide benefit. In one study, some people with exercise-induced asthma benefited from taking vitamin C one hour before strenuous physical activity.
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A study in 2001 suggested that flavonoids found in apples and red wine may help lower the risk for asthma. Some evidence indicates that a low dietary intake of antioxidant nutrients could increase the risk for lung damage. Such nutrients should be obtained from fresh, deep green and yellow-orange fruits and vegetables.
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Eye Disorder
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Cataracts and Macular Degeneration. Oxygen-free radicals play a role in cataract formation and age related macular degeneration, the most common cause of irreversible blindness in the elderly.
Low levels of vitamin C in the lens of the eye have been particularly strong predictors of cataracts. Studies on protection against cataracts using antioxidant supplements have been mixed, including two identically conducted studies that reported opposite results. Vitamin C currently has the strongest evidence for protection, but even with this antioxidant studies are not consistent.
A combination of zinc and antioxidants, including vitamin C and E, may slow the progression of macular degeneration) (Vitamin E alone does not appear to be protective.)
Glaucoma. Although no evidence exists that antioxidants will prevent glaucoma, some studies reported an association between vitamin E and improved visual fields in patients with glaucoma.
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Several studies report that the consumption of antioxidant-rich foods is associated with a decreased risk for cataracts. Carotenoids, especially lutein lycopene, and zeaxanthin are especially eye-protective and may help prevent cataracts and macular degeneration.
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Skin Disorders and Wrinkles
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One small study found that taking a combination of vitamins oral C and E supplements may help reduce sunburn reactions, although the protection is much less than from sunscreens. Taking the vitamins singly did not have any effect. In fact, a 2002 study reported that oral vitamin C had no effect on sunburn reaction. Of concern, in the same study some natural antioxidants in the body were reduced in people who took the vitamin.
Also of concern are studies reporting no benefits and possibly harm from topical vitamin C in the form of ascorbyl palmitate, which is soluble in fat.
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A 2001 study reported that older adults had fewer wrinkles if they ate whole grains, fresh fruits and vegetables, and the use of healthy oils (such as olive oil). Diet played a role in improving skin regardless of whether the people in the study smoked or lived in sunny countries.
The following foods and phytochemicals may be especially skin protective:
Both green tea and ginger appear to have properties that may provide some protection against skin cancer. Green tea skin care products are now available.
The substance silymarin, found in the milk thistle family (which includes artichokes), may inhibit UVB-promoted cancers in animals.
In one interesting study, eating garlic protected animals very effectively against UVB damage by interfering with urocanic acid in the skin. Whether these results may apply to humans (and what quantities of garlic might be beneficial) is still unknown.
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Osteoporosis
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Vitamin D. Vitamin D is the essential companion to calcium in maintaining strong bones. Supplements may be needed for people who have poor exposure to sunlight. It should be noted that diet supplies most people's need and high amounts of vitamin D can be toxic.
Vitamin K. Studies suggest that vitamin K has properties that protect bone and prevent fracture. Vitamin K2 (menatetrenone), a form of vitamin K, is proving to prevent fractures in people with osteoporosis. Vitamin K affects blood clotting, and supplements are not recommended without specific physician instruction.
Vitamin B12. One study reported that in people with osteoporosis and pernicious anemia, taking vitamin B12 (which is used to treat the anemia) also increased bone density.
Vitamin C and E. There has been some indication of a positive association between vitamin C and E intake and bone density, although evidence proving actual benefits is weak.
Note on Vitamin A. High amounts of dietary vitamin A reduces bone density and may even increase the risk for fracture in both postmenopausal women and men. (A form of vitamin A, retinoic acid, has been found to stimulate bone break down.) Beta carotene does not appear to increase risk.
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Studies suggest that diets rich in fresh fruits and vegetables (which include those high in potassium and magnesium) reduce elimination of calcium from the body and help preserve bones.
Studies are suggesting that isoflavones-rich soy products may actually improve bone density in postmenopausal women. However, some evidence suggests that separate supplements of isoflavones (e.g. genistein and daidzein) derived from soy do not provide the benefits of the whole protein like compounds. In fact, animal studies suggest that taking them separately may cause harm, including a possible increase in estrogen-related cancers. (Studies suggesting this have used animals or laboratory evidence. To date, there is no evidence of harm for humans who eat soy products.) More research is needed.
Flavonoids and other compounds in tea may protect the bones.
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Menstrual Disorders
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Vitamin B6. Limited clinical evidence suggests that vitamin B6 may be beneficial in reducing premenstrual symptoms, including depression. Typically, women take 100 mg per day, although one study suggested that a lower dose (50 mg) may have the same effect.
Vitamin B1. One study reported relief from menstrual pain using vitamin B1 (thiamin).
Vitamin E. Several randomized controlled trials have shown that vitamin E significantly improves both physical and emotional premenstrual symptoms. One study reported that high doses of vitamin E helped reduce menstrual cramps. The doses were much higher than those recommended and could possibly increase the risk for bleeding.
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