Scientists question RDA of B vitamin

Low levels of vitamin B6 in smokers may raise their risk of cancer, according to researchers in the US, who found that even moderate smokers had dangerously low levels of the vitamin.

Vitamin B6 is believed to be protective against the DNA damage that can lead to cancers and is vital in DNA synthesis and repair.

In a small study by researchers at Washington State University, four of six young, healthy moderate smokers - men and women who smoked less than a pack a day - had unacceptably low blood levels of vitamin B6 at baseline. Only one of the otherwise comparable non-smokers had unacceptable vitamin B-6 status.

After three months of consuming increasingly high levels of dietary and/or supplemental vitamin B6, the smokers eventually reached acceptable levels. But they never caught up with their non-smoking counterparts, reported study author Dr Terry Shultz at the Experimental Biology 2003 meeting in San Diego, this week.

The results further convinced the researchers that current RDA for this critical vitamin is too low for even moderate smokers and could be too low for the US population as a whole.

In several large population studies, people with a higher intake of vitamin B6 were found to have a lower risk of colon, prostate, lung, gastric and pancreatic cancers, noted Dr Schultz. Importantly, he added that the study shows how quickly raising levels of the nutrient in the diet can improve both smokers' and non-smokers' vitamin B6 status and, equally rapidly, decrease the number of DNA strand breaks in both groups.

The body uses vitamin B6 to convert the vitamin folate to a form that can produce thymine, a component of DNA. If the body does not have enough vitamin B6, it cannot make enough thymine and it tries to make do by substituting uracil. Uracil is not a normal component of DNA, and the normal DNA repair mechanisms of the cell become stressed. This inefficiency in the normal repair mechanisms leads to breaks in DNA strands and instability of chromosomes - possible first steps in the development of cancerous cells.

After measuring baseline vitamin B6 status and DNA strand breaks, six smokers and six non-smokers were given carefully controlled diets containing only marginal amounts of the vitamin for 28 days. The diet was designed to be low in vitamin B6, so foods high in the vitamin, such as cereals, beef, chicken, fish, legumes, soy products and bananas, were limited. The diet was composed of other commonly consumed foods and was adequate in all other nutrients.

At the end of this depletion period, the researchers found that all subjects had lower vitamin B6 levels and higher numbers of DNA strand breaks. The smokers' already low vitamin B6 levels had fallen even lower, but the two groups appeared similar in the number of DNA strand breaks, possibly because the researchers only measured the total lymphocyte profile rather than looking at subsets, which may have been affected differently, they reported.

During the second month, the subjects ate only a carefully controlled diet that included 1.4mg of vitamin B6 (dietary plus some supplementation), approximately the RDA. During the third month, vitamin B6 intake was raised to 2.2mg per day, and during the fourth and final month, subjects were allowed to eat whatever they wanted, but they were supplemented with 10.3mg of vitamin B6 per day, more than seven times the recommended RDA.

As the amount of vitamin B6 in the diet went up, body levels of vitamin B6 went up and DNA strand breaks went down, beginning as early as the first month of supplementation with the vitamin.

Vitamin B6 has been identified as a nutrient with a high prevalence of inadequate dietary intake in the general population, said Dr Schultz, but the study suggests smokers are at even higher risk for low vitamin B6 status, and therefore for the problems it presents.