Obesity rates have been rising
Weight-loss surgery could help women - but not men - reduce their risk of cancer, research suggests.
Obesity is known to increase the risk of many types of cancer, but it was unclear whether surgery to address the problem also cut the risk of cancer.
Now a Swedish study, published in Lancet Oncology, has shown that weight-loss surgery is associated with a 42% reduction in cancer levels in women.
Experts believe the surgery's impact on hormone levels could be key.
It is estimated that obesity is linked to 20% of all cancer deaths in women, and 14% in men in the western world.
A team from Sahlgrenska University Hospital, Gothenburg, set out to discover if weight-loss (bariatric) surgery could help to cut the risk.
BODY MASS INDEX
Calculated by dividing weight in kilograms by height in metres squared
Normal: 18.5 - 24.9
Overweight: 25 - 29.9
Obese: Above 30
Over an average period of 10.9 years, the researchers followed 2,010 obese patients who had undergone weight-loss surgery, comparing them with 2,037 obese who received other forms of treatment, or no treatment.
Over the study period, patients who had surgery lost an average of 19.9kg in weight, compared to an average of 1.3kg in the group who did not have surgery.
Among women, the number of first-time cancers was significantly lower (79) in the surgery group than in the non-surgery group (130).
But surgery appeared to have no effect on men's cancer risk, with 38 cases recorded in the surgery group, and 39 in the non-surgery group.
The beneficial effect of weight-loss surgery on women seemed to apply to a wide range of cancers.
However, exactly why the surgery had a beneficial effect remains a mystery - analysis could find no direct link with losing weight, or reducing food intake.
This suggests the surgery has a more subtle impact on cancer risk.
Dr Andrew Renehan, a cancer expert at the UK's University of Manchester, said the most likely explanation was that weight-loss surgery had an impact on hormone levels in the body.
Several common cancers are known to be linked to the female sex hormone oestrogen in particular.
Dr Renehan said it was possible that weight-loss surgery might also cut cancer risk for men, but that the effect might take many years to become apparent.
Dr Ian Campbell, medical director of the charity Weight Concern, agreed that hormone levels were probably the key, with weight-loss surgery reducing the amount of hormone-producing fat cells in the body.
"In obese men, the types of cancer most common are not so hormone sensitive and therefore not so directly influenced by weight loss," he said.
"However, it may also be due to the development of cancers at later stage in life in men.
"In men, obesity is often goes hand in hand with a nutrient-poor diet, and lack of exercise and so even when weight loss has been achieved through surgery, unless these lifestyle issues are addressed, significant increased risk of some cancers will remain."
Dr Julie Sharp, of the charity Cancer Research UK, said less drastic measures, such as healthy eating and taking exercise, were the best way to control weight.
"In the UK around 13,000 people a year could avoid cancer by maintaining a healthy bodyweight," she added.