If you want to keep the weight down, switch to a meat-free diet, scientists suggested.

Researchers who studied the eating habits of 22,000 people over five years, including meat eaters and vegetarians, found they all put on a few kilos but meat eaters who changed to a vegetarian or vegan diet gained the least.

"Contrary to current popular views that a diet low in carbohydrates and high in protein keeps weight down, we found that the lowest weight gain came in people with high intake of carbohydrates and low intake of protein," said Professor Tim Key.

The research compared weight gain among meat eaters, fish eaters, vegetarians and vegans -- who eat no animal products -- and is published in the International Journal of Obesity.

It showed that on average people gained 2 kilos (4.4 lb) over five years. None of the volunteers was overweight.

"The weight gain was less in the vegans than in the meat-eaters and somewhere in between in the other groups," said Key, of Britain's Cancer Research UK charity and the University of Oxford, who conducted the study.

"The lowest weight gain was in people who changed their diet to eat fewer animal products," he told Reuters.

Key and his colleagues said exercise was another important factor in controlling weight.

"The data also showed that people who became more physically active during the five-year period gained less weight than people who did very little exercise," Key said.

The findings are from the British arm of EPIC (European Prospective Investigation into Cancer and Nutrition), which is comparing the diets of 500,000 people in 10 countries to discover how diet is linked to cancer.

The EPIC study has already revealed that diabetics have three times the normal risk of developing colorectal cancer, which kills more than 490,000 people worldwide each year.

It also showed that diet is second only to tobacco, as a leading cause of cancer, and, along with alcohol, is responsible for nearly a third of cancer cases in developed countries.

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