Healthy fat link to bowel disease
A high intake of polyunsaturated fat in the diet, while good for the heart, may lead to inflammatory bowel disease, say researchers. Experts believe a high intake of linoleic acid, found in foods like "healthy" margarines, may be implicated in a third of ulcerative colitis cases.
The researchers base their findings, due to be published in Gut, on food diaries from more than 200,000 people.
If the link proves to be causal, some people might want to modify their diet.
The researchers also found that a diet rich in another type of fat, omega 3 fatty acid found in oily fish such as salmon and herring, reduced the likelihood of developing ulcerative colitis by 77%.
Linoleic acid is a naturally occurring essential fatty acid, present in a variety of foods, including the oils of seeds and nuts, such as sunflower, safflower, soya, corn seeds or walnut oils.
The multinational team working on the EPIC (European Prospective Investigation into Cancer and Nutrition) study say there is a plausible biological mechanism that could explain why linoleic acid is linked with this bowel condition.
Once in the body, linoleic acid is converted to arachidonic acid, which is a component of the cell membranes in the bowel.
Arachidonic acid can then be converted into various inflammatory chemicals, high levels of which have been found in the bowel tissue of patients with ulcerative colitis.
In all, 126 of the people in the study developed ulcerative colitis after an average period of four years.
After adjusting for other factors likely to influence the results, including smoking, age, total energy intake, and use of aspirin, those whose diets included the most linoleic acid were more than twice as likely to develop the condition as those whose diets contained the least.
Lead researcher Dr Andrew Hart of the University of East Anglia, Norwich, said: "There are no dietary modifications of benefit in patients with ulcerative colitis, although, based on this study's findings, a diet low in linoleic acid may merit investigation."
In the UK, people consume on average about 10g per day of linoleic acid, found in around nine level teaspoons of polyunsaturated margarine or three teaspoons of sunflower oil.
In the study, the people who consumed the most linoleic acid had a daily intake three times this or more.
Dr Anton Emmanual, medical director of the digestive disorders charity Core, stressed that the study did not prove that linoleic acid caused bowel disorders, and warned that dietary diaries could be unreliable.
However, he said: "Nevertheless there is good biological plausibility of why linoleic acid can cause inflamation, and certainly Western diets are often excessive in this kind of fat.
"The omega 3 fish oils counteract the harmful effects of lineloic acid it would be helpful to see whether diets high in fish oils reduce colitis.
"Lineloic acid may have small part to play in some patients, but factors such as smoking, bacteria and stress are likely to be at least as important."
Professor Jon Rhodes, of the British Society of Gastroenterology, said the study was interesting, but also stressed it did not prove cause and effect - further tightly controlled studies would be needed to do that.
Dr Elisabeth Weichselbaum, of the British Nutrition Foundation, said the study was interesting.
But added: "The results need to be interpreted with caution.
"People who have very high intakes of omega-6 fats are likely to have a generally different diet from those with low intakes. Therefore, it may as well be possible that there are other factors that could have an effect."