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In 2016, the International Agency for Research on Cancer (IARC) reviewed the evidence and concluded that carrying excess body fat causes 13 different cancers. This was not just a statistical link — it was a judgement that the relationship is causal, based on population studies, laboratory mechanisms, and experiments all pointing the same way [1]. A landmark meta-analysis of prospective studies had already shown that cancer rates climb steadily as body-mass index (BMI) rises [4]. Obesity has since become, alongside tobacco, one of the most important cancer risk factors we can actually do something about.

The 13 Cancer Types

IARC found 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 cancers: womb (endometrium), oesophagus (adenocarcinoma), upper stomach, liver, kidney, gallbladder, pancreas, colorectum (bowel), breast (after menopause), ovary, thyroid, a brain-lining tumour called meningioma, and the blood cancer multiple myeloma [1]. More recent analyses have added evidence for further sites, including prostate cancer and certain types of non-Hodgkin lymphoma.

Mechanisms: Why Body Weight Drives Cancer Risk

Body fat is not just stored energy. The fat packed around the abdomen, in particular, behaves like an active hormone-producing organ, and researchers have identified at least four overlapping ways it feeds cancer biology [5].

Insulin and IGF-1: Belly fat makes the body less responsive to insulin, so insulin levels stay high. High insulin pushes the liver to make more of a growth hormone called IGF-1, and both insulin and IGF-1 switch on growth signals (the PI3K/Akt/mTOR and RAS/MAPK pathways) that tell cells to multiply and resist dying [2,5]. A 2024 genetic (Mendelian randomisation) study showed that higher BMI causally raises IGF-1, which in turn accounts for a meaningful share of the link between body weight and bowel cancer [2].

Oestrogen: After menopause, body fat becomes the main place the body makes oestrogen, by converting other hormones using an enzyme called aromatase. This extra oestrogen is the leading reason obesity raises the risk of breast and womb cancers [6]. It is also why aromatase inhibitors — drugs that block this conversion — are used to treat hormone-sensitive breast cancer [6].

Adipokines and inflammation: Enlarged fat cells release a pro-inflammatory mix of signalling molecules: more leptin (which encourages cells to grow), less adiponectin (which normally protects against tumours and improves insulin handling), and more inflammatory messengers such as IL-6 and TNF-α. A 2015 review traced how this imbalance feeds into cancer-promoting pathways in the bowel [3,5].

Gut bacteria and bile acids: Obesity also shifts the balance of gut bacteria and bile acids toward more inflammatory forms, which can help tumours get started in the digestive tract [5].

Where the Fat Sits Matters

Fat around the organs (visceral, or abdominal, fat) drives cancer risk more strongly than fat just under the skin — which is why waist size can predict some cancer risks better than BMI alone. Waist circumference and waist-to-hip ratio are increasingly used alongside BMI to estimate risk.

Weight Loss and Cancer Risk Reduction

The strongest evidence that losing weight lowers cancer risk comes from weight-loss (bariatric) surgery studies. In the large SPLENDID study, people who had bariatric surgery had about a third lower rate of obesity-related cancers over 10 years than similar people who did not (roughly 2.9% versus 4.9%), along with lower cancer death rates [7]. Even modest lifestyle weight loss of 5–10% of body weight produces measurable improvements in insulin sensitivity, IGF-1 levels, and inflammation markers within months.

References

1. Lauby-Secretan B et al. (2016) Body fatness and cancer — viewpoint of the IARC Working Group. N Engl J Med 375:794–798.
2. Bouras E et al. (2024) Genetically predicted circulating IGF-1 and colorectal cancer. Int J Epidemiol 53:dyae067.
3. Tandon P et al. (2015) Obesity and the metabolic syndrome as risk factors for colorectal cancer. World J Gastroenterol 21:1371–1380.
4. Renehan AG et al. (2008) Body-mass index and incidence of cancer: a systematic review and meta-analysis of prospective observational studies. Lancet 371:569–578. doi:10.1016/S0140-6736(08)60269-X.
5. Avgerinos KI et al. (2019) Obesity and cancer risk: emerging biological mechanisms and perspectives. Metabolism 92:121–135. doi:10.1016/j.metabol.2018.11.001.
6. Brown KA (2021) Metabolic pathways in obesity-related breast cancer. Nat Rev Endocrinol 17:350–363. doi:10.1038/s41574-021-00487-0.
7. Aminian A et al. (2022) Association of bariatric surgery with cancer risk and mortality in adults with obesity (SPLENDID). JAMA 327:2423–2433. doi:10.1001/jama.2022.9009.

Featured image created using Google Gemini AI.