In 2016 the International Agency for Research on Cancer (IARC), part of WHO, judged the evidence linking excess body fat to thirteen tumour sites or types sufficient. These include the colon and rectum, oesophageal adenocarcinoma, gastric cardia, liver, gallbladder, pancreas, kidney, ovary, endometrium, postmenopausal breast and thyroid, plus meningioma and multiple myeloma. Eight were additions to the earlier assessment. Not all these tumours are malignant: some meningiomas are benign. Obesity is an important risk factor that can be addressed, but does not mean everyone affected will develop cancer.
How does risk change with each 5-unit increase in BMI?
PTLO summarises meta-analyses associating each 5 kg/m² increase in BMI with around 30-60% higher risk of endometrial cancer, oesophageal adenocarcinoma and kidney cancer, and around 13-18% higher risk of colorectal, pancreatic and postmenopausal breast cancer. These are broad ranges of relative risk, not 13-18 extra cases per 100 people or a personal prediction. Estimates vary by study, sex and tumour site. At 178 cm tall, 5 BMI units correspond to about 16 kg. The table highlights issues to discuss at an appointment.
| Cancer | What is known about the link with obesity | What we ask at the appointment |
|---|
Cancer of the body of the uterus | Associated with higher BMI; hormonal changes are among the factors | Last appointment with a gynaecologist, bleeding after the menopause |
Adenocarcinoma of the oesophagus | The association concerns adenocarcinoma, not every oesophageal tumour | Heartburn going back years, difficulty swallowing, any previous gastroscopy |
Kidney cancer | The strength of association differs between women and men | Blood in the urine, the last abdominal ultrasound |
Bowel cancer | Findings vary by sex and location within the bowel | Colonoscopy after the age of 50, blood in the stool, bowel cancer in the family |
Pancreatic cancer | Excess body fat is one of the risk factors | Pain in the upper abdomen radiating to the back, yellowing of the skin |
Breast cancer after the menopause | The association mainly concerns postmenopausal women | The last mammogram, any lump that can be felt |
What links obesity and tumours?
Insulin resistance and changes in insulin and growth-factor signalling may favour cell proliferation. Aromatase in fat tissue converts androgens to oestrogens, helping explain associations with endometrial and postmenopausal breast cancer. Chronic inflammation and the gut microbiota may also play a role. Reflux, which is more common with obesity, can damage the oesophagus. These mechanisms are complex and do not mean a tumour will develop in every person with obesity.
Can losing weight reduce cancer risk?
The observational SPLENDID study compared 5,053 people who underwent bariatric surgery with 25,265 matched people who did not. Estimated cumulative incidence of obesity-associated cancer at 10 years was 2.9% versus 4.9%; cancer-related mortality was 0.8% versus 1.4%. The weight difference was about 25 kg. Median follow-up was 6.1 years. Without random allocation, this shows an association rather than proving that surgery itself prevented cancer.
A 2025 analysis of 86,632 adults with overweight or obesity found rates of 13.6 versus 16.4 cases per 1,000 person-years for 14 tumour types among GLP-1 receptor agonist users and nonusers. Person-years represent the combined time participants were followed. The set included lung cancer and was not identical to IARC's list. An uncertain signal of potentially higher kidney cancer risk needs further assessment. This was also observational; we do not offer obesity medication as a way to avoid cancer.
Can obesity medicines cause tumours?
Thyroid concerns partly originate from rodent studies in which semaglutide caused thyroid C-cell tumours. European product information describes rodents' particular sensitivity to this mechanism; relevance to humans is considered low but cannot be completely excluded. A 2025 study in six countries compared 98,147 GLP-1 receptor agonist users with nearly 2.5 million DPP-4 inhibitor users, all with type 2 diabetes. The main comparison found no increased thyroid cancer risk, but median follow-up ranged from 1.8 to 3 years, limiting long-term conclusions. TSH assesses thyroid function, not whether cancer is absent. Ultrasound is arranged when indicated, for example for a palpable nodule.
What do these findings mean for our care?
We discuss screening at the initial consultation and at check-ups. Testing depends on age, symptoms and personal and family history. Maintaining a healthy weight is part of prevention, but weight loss does not replace screening or guarantee protection from cancer. For cancer survivors, the plan is agreed with the treating team.