Obesity and Cancer: The Evidence Behind 13 Cancer Types

In 2016, the International Agency for Research on Cancer (IARC) reviewed the evidence and classified excess body weight as a cause of 13 distinct cancer types. This was not a statistical association — it was a causal determination based on epidemiological, mechanistic, and experimental evidence converging across multiple biological systems. Obesity has since become, alongside tobacco, one of the most significant modifiable cancer risk factors identified.

The 13 Cancer Types

IARC identified sufficient evidence for a causal role of excess body weight — defined as a BMI above 25 kg/m² for overweight and above 30 kg/m² for obesity — in the following cancer types: endometrium, oesophagus (adenocarcinoma), stomach (cardia), liver, kidney (renal cell), gallbladder, pancreas, colorectum, breast (postmenopausal), ovary, thyroid, meningioma, and multiple myeloma [1]. More recent analyses have added evidence for additional sites including prostate cancer and certain subtypes of non-Hodgkin lymphoma.

Mechanisms: Why Body Weight Drives Cancer Risk

Adipose tissue is not metabolically inert. Excess visceral fat functions as an active endocrine and paracrine organ, contributing to cancer biology through at least four interacting systems.

Insulin and IGF-1 axis: Visceral adiposity drives insulin resistance and compensatory hyperinsulinaemia. Elevated insulin stimulates hepatic IGF-1 production, and IGF-1 acting on IGF-1R activates the PI3K/Akt/mTOR and RAS/MAPK pathways — both of which promote cell proliferation and suppress apoptosis. A 2024 Mendelian randomisation study demonstrated that genetically predicted higher BMI causally increased IGF-1 levels, which in turn mediated a significant proportion of the BMI-colorectal cancer association [2].

Oestrogen: In postmenopausal women, adipose tissue is the primary site of oestrogen biosynthesis via aromatase-mediated conversion of androgens to oestrogens. Obesity-associated oestrogen excess is the dominant mechanism linking excess weight to postmenopausal breast and endometrial cancers. Aromatase inhibitors, which block this conversion, are used therapeutically for this reason.

Adipokines and inflammation: Hypertrophic adipocytes secrete a pro-inflammatory adipokine profile: elevated leptin (which promotes cell proliferation and inhibits apoptosis), reduced adiponectin (which has anti-tumour and insulin-sensitising properties), and elevated IL-6 and TNF-α. A 2015 review characterised these mechanisms for colorectal cancer, documenting how the leptin-to-adiponectin ratio predicts cancer risk and how adipokine dysregulation feeds into the JAK/STAT and NF-κB oncogenic cascades [3].

Microbiome and bile acids: Obesity-associated microbiome dysbiosis shifts bile acid metabolism toward pro-inflammatory secondary bile acids, which activate NF-κB in colonocytes and promote tumour initiation in the gastrointestinal tract.

Cancer Risk Is Not Uniformly Distributed by Fat Distribution

Visceral (abdominal) adiposity drives cancer risk more potently than subcutaneous adiposity — waist circumference is a stronger predictor of many obesity-related cancer risks than BMI alone. Waist-to-hip ratio and waist circumference are increasingly recommended as cancer risk stratification tools alongside BMI.

Weight Loss and Cancer Risk Reduction

Bariatric surgery studies provide the strongest evidence that intentional weight loss reduces cancer incidence. Large cohort studies following bariatric surgery patients show approximately 30–40% reductions in obesity-associated cancer risk at 10+ year follow-up, with the largest reductions in endometrial and breast cancer. Lifestyle-induced weight loss of 5–10% of body weight produces measurable improvements in insulin sensitivity, IGF-1 levels, and inflammatory biomarkers within months.

References

1. Lauby-Secretan B, Scoccianti C, Loomis D, Grosse Y, Bianchini F and Straif K (2016) Body fatness and cancer — viewpoint of the IARC Working Group. N Engl J Med 375:794–798. doi:10.1056/NEJMsr1606602.
2. Bouras E et al. (2024) Genetically predicted circulating IGF-1 and colorectal cancer: a Mendelian randomisation study. Int J Epidemiol 53:dyae067. doi:10.1093/ije/dyae067.
3. Tandon P et al. (2015) Obesity and the metabolic syndrome as risk factors for colorectal cancer. World J Gastroenterol 21:1371–1380. doi:10.3748/wjg.v21.i5.1371.