Does being overweight increase my risk of cancer, and if so, which types?
Excess weight increases your risk of at least 13 types of cancer, including uterine, colorectal, breast and liver cancer. Losing weight reduces that risk, although it remains unclear whether all methods of weight loss are equally effective for immune defence against cancer.
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.
Based on multiple systematic reviews and meta-analyses (PMID 30445141, 37047163, 39279764, 31722374), a mechanistic review (PMID 27903155), an immunological mouse study (PMID 38565540) and an observational cohort study (PMID 39287872). The evidence is strongest for uterine, colorectal, breast, liver and pancreatic cancer. The data on bariatric surgery come from observational research, not from randomised trials.
Does a large belly increase my risk of cancer more than being simply overweight?
A large belly raises the risk of several types of cancer more than being simply overweight, especially in men. Having your waist circumference measured is therefore more meaningful than watching the scales alone.
Does sitting a lot increase my risk of cancer, even if I exercise?
Sitting a lot increases your risk of certain cancers, even if you exercise. In addition to moving regularly, try to reduce your sitting time itself, because the two only partially compensate for each other.
Does eating a lot of sugar increase my risk of cancer?
Drinking large amounts of sugary drinks is associated with a noticeably higher risk of certain cancers, especially early-onset bowel cancer and liver cancer. The link is associative, but consistent enough to make cutting back on soft drinks and fruit juice a concrete priority.
Does working at night increase my risk of cancer?
Night work probably increases the risk of breast cancer, and possibly of other types of cancer. If you have been working night shifts for years, it is worthwhile to discuss this with your doctor and to stay alert to population screening programmes.
Does having type 2 diabetes increase your risk of certain cancers?
Research shows that type 2 diabetes is associated with a higher risk of certain cancers, and that link runs largely through the excess weight that often accompanies it. For most cancers, commonly used diabetes medicines show no raised risk.
Does poor sleep increase my risk of cancer?
Poor sleep, and sleep apnea and night-shift work in particular, is associated with an increased cancer risk. Whether you personally face a higher risk depends on which sleep problems you have; at the very least, have any apnea symptoms evaluated.