A study of almost 7,000 patients found a strong association between bariatric surgery and protection from cancer. Bariatric surgery involves modifications to the stomach or intestines to reduce food intake or calorie absorption.

Procedures include inserting "gastric bands" which restrict access to the stomach, removal of part of the stomach, and gastric bypasses that interfere with digestion. Successful bariatric surgery can allow obese patients to lose up to 70% of their excess weight and not put it back on.

The new research, conducted by scientists in Canada, involved 1,035 patients who had bariatric operations between 1986 and 2002. They were compared with 5,746 patients who had not undergone the procedures.

All were described as "morbidly obese", meaning they were so fat their health was compromised. The first group had 85% fewer cases of diagnosed breast cancer than the second, the researchers found. Colon and pancreatic cancers were 70% less frequent among patients who had bariatric surgery. Rates of several other types of cancer were also distinctly lower in the surgery group.

The findings were presented at the annual meeting of the American Society for Metabolic & Bariatric Surgery in Montreal.

Study leader Professor Nicolas Christou, from McGill University Health Centre in Montreal, said: "The relationship between obesity and many forms of cancer is well established. This is one of the first studies to suggest that bariatric surgery might prevent the risk of cancer for a significant percentage of morbidly obese people.

"Bariatric surgery is an extremely efficient tool in the treatment of morbid obesity and its consequences."

Excess body fat is thought to increase the production of hormones such as oestrogen and insulin that fuel the growth of certain cancers. Obese women also have lower levels of SHBG, or "sex hormone binding globulin", which mops up excess oestrogen in the body.

Experts estimate that in Europe 5% of cancers in women and 3% in men are caused by being overweight or obese.