Does being overweight increase my risk of cancer, and if so, which types?

Excess weight and obesity increase your risk of cancer in at least 13 parts of the body. Worldwide, nearly 12% of all cancers in men and just over 13% in women can be attributed to excess body weight. This is not a suspicion but a conclusion supported by a large and consistent body of research.

The strongest associations have been demonstrated for uterine, colorectal, breast (post-menopausal), liver and pancreatic cancer. The size of the effect is well illustrated by bariatric surgery: women who lost a significant amount of weight afterwards had a 62% lower risk of uterine cancer, 44% lower risk of post-menopausal breast cancer, 65% lower risk of liver cancer, 48% lower risk of pancreatic cancer and 37% lower risk of colorectal cancer compared with people with obesity who did not have the surgery. This sharp drop in risk following weight loss is one of the reasons researchers regard excess weight itself as the cause.

Excess weight is also associated with ovarian, gallbladder, oesophageal, stomach, kidney and thyroid cancer, as well as two rarer conditions: meningioma (cancer of the membranes surrounding the brain) and multiple myeloma (a type of blood cancer). The evidence for these is somewhat less strong. For thyroid, kidney and blood cancers, weight loss through surgery also showed no clear protective effect, which makes the relationship less well understood.

How does excess weight promote cancer? Two mechanisms are well supported. Fatty tissue produces hormones, particularly oestrogens, and causes a low-grade chronic inflammation in the body. That inflammation has been linked to a worse prognosis in breast and tongue cancer. Notably, this fatty-tissue inflammation also occurs in people with a normal weight, so body weight alone does not explain everything. In addition, obesity impairs the function of the immune cells that detect and eliminate cancer cells, meaning the immune system keeps tumours less effectively in check. This second mechanism has so far been demonstrated mainly in mouse studies.

One mouse study deserves separate mention but calls for caution¹: losing weight through dietary restriction restored immune function in mice, whereas losing weight through the weight-loss drug semaglutide did not, even though the animals lost the same amount of weight. This suggests that the method of weight loss may matter for immune function. How this plays out in humans still needs to be investigated in human studies.