Meat-eaters who switch to a vegetarian diet gain slightly less weight than those who do not make major changes to their diets, a study suggests.
Scientists for Cancer Research UK compared the weight gains of 22,000 meat eaters, fish eaters, vegetarians and vegans over a five-year period.
On average people gained two kilos but those who had switched to a diet with less animal foods gained a little less.
Obesity is a major cause of cancer and other diseases.
The research team, led by Professor Tim Key, of Oxford University, studied the eating habits of 22,000 people recruited to the European Prospective Investigation into Cancer and Nutrition (EPIC) between 1994 and 1999.
"This confirms that the best way to prevent obesity is to combine a healthy diet with exercise"
They were weighed and measured and asked to give details of their diet and lifestyle.
The team then followed their subjects up an average of five years later (between 2000 and 2003) and asked them the same questions.
They found that everyone gained an average of 2kg, but those who had switched to a vegetarian diet from a meat-eating diet gained around 0.5kg less.
Those switching to a vegan diet showed slightly smaller weight gains.
Professor Tim Key said it had been known for some time that vegetarians and vegans tended to be slimmer than meat-eaters but they had never been followed over a number of years.
He also said: "Contrary to the current popular views that a diet low in carbohydrates and high in proteins keeps weight down, we found that the lowest weight gain came in people with high intake of carbohydrate and low intake of protein."
Obesity risks
He also stressed that, in line with current understanding, the highest weight gain came in people who did very little exercise.
"This confirms that the best way to prevent obesity is to combine a healthy diet with exercise," he added.
Cancer Research UK conducted the study because of the known links between obesity and cancer.
The charity's medical director, Professor John Toy, said: "EPIC continues to highlight the importance of diet and exercise in achieving a healthy weight.
'Balanced diet'
"We know that obesity contributes to the increasing risk of cancer as it does to heart disease and diabetes."
Dr Colin Waine, president of the National Obesity Forum, said: "Whatever diet you are on, if your calorie intake exceeds your calorie output, you will gain weight."
He suggested that those on a vegetarian diet put on less weight because they were more likely to achieve a balanced diet and eat less saturated fat.
But he added: "This research shouldn't lead to advocating vegetarianism as an answer to obesity.
"We know if you have a balanced diet and exercise to a reasonable degree - that is the best way to lose weight."
Ursula Arens, spokeswoman for the British Dietetic Association, said the study did not prove that switching to a vegetarian diet led to weight loss in someone who was overweight.
"A diet of chips and chocolate is 'vegetarian' but not healthy or likely to help slimming," she said.
But she added that vegetarians were often interested in nutrition and made healthy choices such as fruit, vegetables, pulses and whole grains.
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