Wednesday, December 1, 2010 at 6:55AM
From The American Journal of Epidemiology
by Tim Byers
Colorado School of Public Health, University of Colorado Cancer Center
It started 30 years ago with vitamin A: the idea that some cancers might be caused by vitamin deficiencies. Animal experimental models led us to the notion that cancer risk might be “materially” reduced by supplementation with beta-carotene, a retinol precursor. Although that idea was seductive, we were all disappointed when 2 large randomized controlled trials that began in 1985 in Finland and the United States reported an 18% increased risk of lung cancer caused by high-dose beta-carotene supplementation and a 28% increased lung cancer risk caused by a combination of beta-carotene and retinol. The vitamin A era was over.
Next came the B vitamins. Again, based on animal experimental evidence and supported by epidemiologic evidence of connections between diets low in B vitamins and increased cancer risk, a large randomized controlled trial was begun in 1985 in central China, where micronutrient deficiency was common and where rates of cancers of the stomach and esophagus were extraordinarily high. Nonetheless, several years of supplementation with a combination of riboflavin (vitamin B2) and niacin (vitamin B3) had no effect on incidence of upper gastrointestinal cancers. Interest in folic acid (vitamin B9) persisted, though, in part because of its striking effect on neural tube birth defects, coupled with speculation about possible benefits of food fortification for diseases such as colorectal cancer that were inversely associated with diets rich in folate-containing foods and supplements. However, a 7-year randomized controlled trial found that high-dose folic acid supplements actually increased risk of colorectal adenomas. The vitamin B era was over.