Only legislation and a radical change in the Scottish diet will halt a national epidemic of obesity, and help to stem a huge rise in cancers and other life- threatening diseases, one of America's leading nutritionists has cautioned.

Walter Willett, who is Professor of Epidemiology and Nutrition at Harvard's School of Public Health, has called for more of the pioneering legislation that has seen damaging trans fats banned in New York restaurants and controlled in Denmark.

In a lecture given at the University of Edinburgh, Professor Willett called for a break with bad eating habits that have persisted over centuries in northern Europe and America and are a throwback to a time when food was scarce and people relied heavily on dairy products and red meats.

“It works very well if you need to live to 45, but if you want to live to 75 or 80 in good condition, it's not so good,” Professor Willett said.

His alternative diet minimises the consumption of such Scottish staples as white bread and potatoes, because they are refined carbohydrates with a high glycaemia index, which mean they immediately turn to glucose in the body, contributing to the risk of heart disease and type 2 diabetes.

Scots should instead turn to wholemeal bread and pasta, said Dr Willett and urged a reduction in red meat intake in favour of fish, poultry and nuts. Only two servings of dairy products are permitted, while large quantities of fruit and vegetables are encouraged.

He also rejects the notion that all fats are “bad”, and all carbohydrates are “good”. “People have been told by nutritionists that you can't get fat eating carbohydrates. That's a huge misconception. We shouldn't be gorging ourselves on starch,” he said. Equally, a blanket ban on fats is bad for health, he argued, because some fats, such as olive oil, promote good health.

Professor Willett said that among the “bad” fats widely used in the food industry, trans fats were the worst. These were used increasingly from the 1940s to create margarine and vegetable shortenings. They have been linked to obesity, type 2 diabetes, higher rates of LDL cholesterol and an increase in heart disease.

In 2003 Denmark introduced laws to control the sale of foods containing trans fats and 18 months ago trans fats were banned from restaurants by New York City's board of health. Legislation of this sort could help to penetrate the poorest communities, where dietary failings were felt most, he argued.

“This is a really important move and should be widely adopted. So many dietary changes require a generational approach, like encouraging people to eat a high-fibre diet. But trans fats are part of the manufacturing process and can be easily replaced with healthier fats,” Professor Willett said.

The link between obesity and cancer, which he said had emerged over the past ten years, had led to it becoming set to “overtake smoking as the most common avoidable cause of cancer”, he added. Research has linked excess weight with breast, kidney, colon and oesophageal cancers, leukaemia and lymphomas.

Overweight world

400 million adults worldwide are obese and 1.6 billion overweight

155 million children worldwide are overweight, including

30-45 million obese children

31 per cent of adult males and

33 per cent of adult females in

the United States are obese

Scotland's proportion of obese men rose from 16 per cent to

22 per cent between 1995 and 2003, and from 17 per cent to

24 per cent for women

Croatia has the largest proportion of obese men in Europe, at 31 per cent, and Albania the most obese women, 36 per cent

The Western Pacific has most obese nations: in Nauru 80 per cent of men and 78 per cent of women are obese; in Tonga it is

47 per cent of men and 70 per cent of women

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