Can folic acid cause cancer?
Supplements of vitamin B are thought to increase the risk of bowel cancer, warns Jennifer Swift
Shirley Sepstrup’s busy job in a hospital lab in East Sussex left her with little time to cook, so she often relied on convenience foods, hoping that her daily multivitamin pill would make up for any dietary deficiencies. But when a colonoscopy showed that, at 52, she had developed bowel cancer – like her father, uncle and grandfather – she had to radically change her eating habits.
Out went the ready meals, and Shirley began to eat wholefoods, drink 1.5 litres of water a day, and avoid known risks for bowel cancer – red meat, preserved meats such as ham and bacon, sugar and processed foods. But recently she learnt of an unexpected risk: a vitamin.
Folic acid is the synthetic form of a naturally occurring B vitamin, folate. Women who have good levels of folate in their diet, or take folic acid supplements, are far less likely to have babies affected by the birth defect spina bifida. America and Canada started adding folic acid to flour in 1998 and the Food Standards Agency (FSA) in the UK has called for a similar fortification here. But recent research has linked high folic acid consumption with an increased risk of bowel cancer; the modest-sounding annual increase of one per cent could, in fact, amount to an extra 3,000 cases per year in the UK. Other evidence points to an increased risk of breast or prostate cancers.
Natural folate protects against cancer because it allows the body to copy DNA accurately.
“But many middle-aged and elderly people have tiny pre?malignant lesions,” says Prof Young-Im Kim of the University of Toronto. “Excess folate, especially in the form of folic acid, can fuel lesion growth, accelerating progression into life-threatening cancers, because high levels of the vitamin make it easier for tumour cells to copy themselves.
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He says that soon after fortification of flour began in North America, the rate of bowel cancer – then in decline – abruptly increased. The FSA’s advisers were sufficiently concerned to recommend holding off a similar initiative until the results of two further studies emerge later this year.
Overdosing on B vitamins was thought to be impossible because they are water-soluble and any excess is excreted in the urine. But evidence is mounting that folic acid circumvents the body’s natural mechanisms for limiting folate absorption in the gut. Folic acid goes directly to the liver, which is easily saturated, and the excess spills out into the body.
“People with a high intake end up with unmetabolised folic acid floating in their bloodstream,” says Dr Siân Astley of the Institute for Food Research in Norwich. “We don’t really know what its consequences might be.”
The recommended daily intake for folate is 200 micrograms (mcg), and most multivitamins contain this amount of folic acid. But it is also added to breakfast cereals, snack bars and some margarines. Official government advice puts the safe upper limit for folic acid at 1,000mcg per day, but the leading vitamin B expert, Prof David Smith of Oxford University, thinks there is now sufficient evidence to cut that down to 500mcg in general and 400mcg for cancer survivors.
“If you eat a lot of fortified cereals, you may want to rethink your daily multivitamin. Or you could stick with the vitamin pill and switch to wholegrains without added synthetic vitamins, such as porridge or muesli,” says Dr Astley. “Fortification is an overly broad approach that increases everyone’s folic acid intake, instead of targeting those who need it.”
