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Healthy eating just got a little more complicated.

New research from the University of Toronto shows certain vegetable oils that claim to be healthy may actually increase the risk of heart disease.

And the results mean Health Canada should reconsider cholesterol-lowering claims on food labelling, says Dr. Richard Bazinet, lead author of the new study which is available online now at the .

“This is important information for people buying certain foods because of the heart benefits when really, that’s not accurate,” says Bazinet, of U of T’s department of nutritional sciences. “While most of these foods are a good choice, there are a few notable exceptions.”

Bazinet and his team report that replacing saturated animal fats with polyunsaturated vegetable oils had become common practice for consumers, based on the understanding that such oils reduce serum cholesterol levels and help prevent heart disease. Since 2012, Health Canada's Food Directorate has allowed the food industry to use a label on the oils – and foods containing the oils – claiming “a reduced risk of heart disease by lowering blood cholesterol levels."

But researchers say it’s more complicated than the label suggests – and the problem lies in the ratio of two kinds of polyunsaturates fatty acids found in the oils.

"Careful evaluation of recent evidence, however, suggests that allowing a health claim for vegetable oils rich in omega-6 linoleic acid but relatively poor in omega-3 α-linolenic acid may not be warranted," write Bazinet and Michael Chu, Lawson Health Research Institute and Division of Cardiac Surgery at Western University in London, Ontario.

Corn and safflower oil, which are rich in omega-6 linoleic acid but contain almost no omega-3 α-linolenic acid, are not associated with beneficial effects on heart health, Bazinet says.

The authors cite a study published earlier this year in February 2013 in which "...the intervention group replaced saturated fat with sources of safflower oil or safflower oil margarine (rich in omega-6 linoleic acid but low in omega-3 α-linoleic acid). They found that the intervention group had serum cholesterol levels that were significantly decreased (by about 8%–13%) relative to baseline and the control group, which is consistent with the health claim."

However, rates of death from all causes of cardiovascular disease and coronary artery disease significantly increased in the treatment group, says Bazinet.

"When the new results were added to a meta-analysis, the net result was a borderline 33 per cent increase in heart disease risk for oils rich in omega-6 and poor in omega-3, with absolutely no evidence of a benefit as is implied by the health claim," Bazinet says.

In Canada, omega-6 linoleic acid is found in corn and safflower oils as well as foods such as mayonnaise, creamy dressings, margarine, chips and nuts. Canola and soybean oils, which contain both linoleic and α-linolenic acids, are the most common forms of oil in the Canadian diet.

“We suggest that the health claim be modified such that foods rich in omega-6 linoleic acid but poor in omega-3 α-linolenic acid be excluded," conclude the authors.

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Michael Kennedy is a writer with University Relations at the University of Toronto.