Nutritionists may have to re-think their advice on vitamins, which is based on recommended daily levels for a normal person, because there may be no such thing as a "normal" population when it comes to dietary needs, according to researchers at Trinity College, Dublin.
A study of one vitamin, folic acid, has shown that one in eight people in Britain have a genetic mutation which reduces their capacity to absorb it from food. Folic acid, contained in green vegetables and as an additive in many breakfast cereals, is essential for women in pregnancy to prevent spina bifida and related birth defects, and is also believed to protect against heart disease.
Tests on 500 women showed that those with mutant copies of the gene that controls the absorption of folate by the body had significantly lower levels of the vitamin in their tissues. The gene codes for an enzyme that enables the folate to act within the cells and the mutant version greatly reduces the efficiency of the enzyme. The less efficient the enzyme, the greater the quantity of folic acid required in the diet.
Professor John Scott, head of the department of biochemistry at Trinity College, who led the research team, said the findings, reported in the Lancet, had changed his attitude to vitamin supplements. "Until recently I took the view that anyone who ate a normal balanced diet did not need vitamin supplements. I no longer think that. We know people have differences in the way they metabolise drugs but this is the first time anyone has shown that people have differing requirements for nutrients."
The findings will strengthen the argument for adding folate to the national diet as introduced in the US. More than eight out of ten women who have spina bifida babies consume normal levels of folate but if those are increased to "high normal" levels they can cut their risk of a subsequent spina bifida baby tenfold, from eight per 1000 births to 0.8.
Professor Scott said it was likely that people's capacity to absorb other vitamins such as A, C and D could be affected by other genetic mutations.