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Dietary Health Benefits
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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 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 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.
Some evidence also exists that high doses of vitamin C may speed up atherosclerosis, or hardening of the arteries. 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 vitamin, then, may block that action. Research published in 2005 suggests that those who take large amounts of vitamin E (1500 IU/day) may slightly increase their risk for heart failure and death, but this evidence is not considered conclusive. Further study is necessary.
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