Vitamins are nutrients essential for the body. Vitamins are divided into two groups: water-soluble (B-complex and C) and fat-soluble (A, D, E and K).
Unlike water-soluble vitamins, steadily metabolised in the body, fat-soluble vitamins are stored in the liver and fatty tissues. Their turn-over-rates are significantly lower than that of water-soluble vitamins. As fat-soluble vitamins are stored for long periods, they generally pose a greater risk for hypervitaminous toxicity than water-soluble vitamins when. A well-balanced diet will probably not cause toxicity in healthy individuals. Since, generally, traces of vitamins are sufficient for the body accessory pharmaceutical vitamin supplementation appears dubious.

While diseases caused by a lack of fat-soluble vitamins are rare in the United States, symptoms of mild deficiency can be observed rather frequently. Additionally, some fat metabolism dysfunctions may, in turn, decrease the absorption of vitamins A, D, E and K.

Data from a large trial indicate that a daily intake of 600 IU of natural-source vitamin E provided no diminished risk for major cardiovascular diseases or cancer. It did not affect total mortality and decreased cardiovascular mortality in healthy women, nor can supplementary vitamin E uptake be recommended in order to prevent these diseases ( Lee at al., 2005, JAMA).

In a scientific report, published 2007 in The Lancet Medical Journal, 20 years of studies from more than 170.000 persons with high risk of developing gastrointestinal cancers were pooled. Antioxidant supplements were investigated, amongst others, vitamin A, C and E, as well as selenium. "The antioxidant pills - except selenium - are useless for prevention of gastrointestinal cancers," argued Dr. Goran Bjelakovic, leader of the study for the Cochrane Heptao-Billary Group at the Copenhagen University Hospital (Denmark). The Cochrane organisation is a well-regarded international network of experts publishing systematic reviews of evidence on a variety of health interventions.
Bjelakovic mentioned that the potential inhibitory effect of selenium against cancer of the liver needed a closer look, but that further studies investigating the cancer inhibitory effects of vitamin A, C and E would be a waste of time.

The assumed correlation between health and antioxidants attracted attention since they were suspected to compensate oxidative stress on the cardiovascular system and other tissues, giving rise to enhanced cancer risk. Thus, a significant coherence between a diet rich in antioxidant-loaded fruits and vegetables and decreased risk of cancer and heart disease could be found.
However, these effects could not be verified in more recent studies comparing antioxidants with placebos. Some researchers argue that antioxidants might work only as food-ingredients, or that people nourishing vitamin-rich are healthier simply due to better care of themselves.

The latest investigation on cancers of the colon, pancreas, stomach and gullet, comes on the heels of a similar review past year by the U.S. Preventive Services Task Force. That group found fault with lacking evidence required for recommendation or dissuasion of vitamin supplements as cancer and heart disease prevention.

"The conclusions are consistent with other reviews that, in general, there is not convincing evidence that the use of antioxidant supplements, at least for short periods of time, provides any health benefit with respect to cancer," claimed Eric Jacobs, senior epidemiologist at the American Cancer Society.

"Usually, these trials will test the effect of taking vitamins for a relatively short period of time, and they will test the effect of perhaps one or two individual antioxidants," noted Jacobs, who was not involved in the research. "It might be an error to conclude that they are all useless. We don not know what the effects are of various combinations over long periods of time."

Annette Dickinson, president of the Council for Responsible Nutrition, an American trade group for makers of dietary supplements, is not surprised by the lacking significance of studies testing persons already at high disease risk.

"Testing a single antioxidant in a high risk population does not necessarily equate to testing a rich intake of a variety of antioxidants as a preventive measure in healthy people," she said. "It's crazy to say the question is closed. The question has not really been fully addressed."

Another expert, Jeffrey Blumberg, chief of the antioxidants research lab at Tufts University in Boston, worries that doubts raised by the lack of positive findings may dissuade consumers from more readily accepting established benefits of dietary supplements, such as vitamins C and E and lutein for age-related eye degeneration; calcium and vitamin D for osteoporosis and colorectal cancer; omega-3 fatty acids for cardiovascular disease and folic acid for neural tube birth defects.
"I think it is premature to write off antioxidant supplements for cancer prevention", Blumberg said.

(CBS News, Febr. 28 07)