Greasy does it

For decades, we've been told by dietitians, doctors and public health authorities to avoid saturated fats such as butter or lard in favour of unsaturated vegetable oils like safflower and soya bean.

Studies conducted in the 1960s and '70s suggested that dietary saturated fatty acids raise the concentrations of total and low-density lipoprotein, or "bad" cholesterol, while polyunsaturated fatty acids do the opposite - lower cholesterol and decrease one's risk of heart disease.

But research published earlier this month in the British Medical Journal has stirred controversy. Scientists from the US National Institutes of Health analysed recovered data from the Sydney Diet Heart Study conducted from 1966 to 1973 and found that substituting dietary omega-6 linoleic acid - the most abundant polyunsaturated fatty acid in a Western diet - for saturated fats increased the number of deaths from coronary heart disease and cardiovascular disease.

The study, which involved 458 men, showed no evidence of polyunsaturated fatty acids' long-preached heart-healthy benefits. Participants were provided with liquid safflower oil (which is 75 per cent linoleic acid) and a safflower oil-based margarine to be used in place of animal fats for cooking, baking and spreading.

Many experts have disputed the findings. Bill Shrapnel, deputy chairman of the Sydney University Nutrition Research Foundation, says the study used Miracle margarine as a source of polyunsaturated fat, a product which in the 1960s contained about 15 per cent trans-fatty acids, which have the worst effect on heart disease risk of any fat.

"The adverse effect of the intervention in this study was almost certainly due to the increase in trans-fatty acids in the diet," says Shrapnel, who was a consultant for Unilever, makers of Miracle margarine. "Recent, well-conducted studies indicate that replacing saturated fat with polyunsaturated fat lowers the risk of heart disease, and this is widely accepted. Trans-fatty acids were largely removed from Australian margarines in the mid-1990s when their adverse effects on heart health became apparent."