Traditionally people have been advised to reduce animal fats, but the biggest ever study has shown they do not increase the risk of stroke, heart disease or diabetes. However, trans fats, found in processed foods such as margarine, raise the risk of death by 34 per cent in less than a decade.

"For years everyone has been advised to cut out fats," said study lead author Dr Russell de Souza, who is an assistant professor in the Department of Clinical Epidemiology and Biostatistics, at McMaster University in Canada.

"Trans fats have no health benefits and pose a significant risk for heart disease, but the case for saturated fat is less clear.

"That said, we aren't advocating an increase of the allowance for saturated fats in dietary guidelines, as we don't see evidence that higher limits would be specifically beneficial to health."

Saturated fats come mainly from animal products such as butter, cows' milk, meat, and egg yolks, and some plant products such as chocolate and palm oils. Trans-unsaturated fats - or trans fats - are mainly produced industrially from plant oils for use in margarine, snack foods and packaged goods.

Guidelines recommend that saturated fats are limited to less than 10 per cent, and trans fats to less than 1 per cent of energy, to reduce risk of heart disease and stroke. However, the new research, which looked at 50 studies involving more than 1 million people, found there was no evidence that saturated fat was bad for health.

The "vilification" of saturated fats dates back to the 1950s when research suggested a link between high dietary saturated fat intake and deaths from heart disease. But the study author looked at data from only six countries.

The new research found no clear association between higher intake of saturated fats and conditions such as coronary heart disease, cardiovascular disease, or type 2 diabetes.

In contrast, consumption of trans fats was associated with a 34 per cent increase in death over an 8½-year period, a 28 per cent increased risk of death from coronary heart disease, and a 21 per cent increase in the risk of cardiovascular disease. Despite the research, health experts cautioned against changing to a diet which was high in saturated fat.

Professor Tom Sanders, emeritus professor of nutrition and dietetics, at King's College London, said: "It would be foolish to interpret these findings to suggest that it is OK to eat lots of fatty meat, lashings of cream and oodles of butter.

"Death rates from cardiovascular disease have fallen in the UK by about 55 per cent since 1997 despite the rise in obesity for reasons that remain uncertain but this may in part be due to changes in the food supply particularly fewer trans and more omega-3 fatty acids."

The research was published in the British Medical Journal. The Daily Telegraph, UK