He is currently working at the Britain's Cancer Research UK charity and the University of Oxford. Scientists said that meat eaters who changed to a vegetarian or vegan diet gained the least weight.
It was also found that lowest weight gain came in people with high intake of carbohydrates and low intake of protein.
The research compared weight gain among meat eaters, fish eaters, vegetarians and vegans who eat no animal products.
The results were published in the International Journal of Obesity. It showed that on an average people gained 2 kilos over five years.
The weight gain was less among vegetarians when compared to that of the meat-eaters. The lowest weight gain was in people who changed their diet to eat fewer animal products.
Key and his colleagues said that exercise was another important factor in controlling weight. Key said that the investigation was mainly from the British arm of EPIC (European Prospective Investigation into Cancer and Nutrition).
Their aim is to study the effect of diet in relationship to cancer. They studied the diets of 500,000 people in 10 different countries.
The EPIC study has revealed that diabetic patients have three times the normal risk of developing colorectal cancer.
Diet with tobacco and alcohol are a deadly combination and result in about a third of cancer cases in developed countries.