Overweight and cancer risk

Carrying excess body fat is one of the most consistently identified modifiable risk factors for cancer, second only to tobacco in many population estimates. Higher body mass index (BMI) is linked with increased risk of at least 13 cancers, including postmenopausal breast, endometrial, colorectal, kidney, liver, pancreatic, oesophageal (adenocarcinoma), gallbladder, ovarian, thyroid, gastric cardia, multiple myeloma, and meningioma. The connection appears strongest for hormone-related and digestive-tract cancers.

Adipose tissue is not inert storage; it is a hormonally and immunologically active organ. Excess fat raises circulating oestrogen (produced by aromatase in fat cells), which can drive growth in oestrogen-sensitive tissues such as the breast and endometrium. Obesity also elevates insulin and insulin-like growth factor 1 (IGF-1), which signal through PI3K/AKT/mTOR pathways and encourage cell proliferation while suppressing apoptosis. Chronic low-grade inflammation, marked by elevated interleukin-6, tumour necrosis factor alpha, and leptin (with reduced adiponectin), creates a pro-tumour microenvironment. Growth hormone receptor (GHR) signalling, gut microbiome dysbiosis, and altered bile acid metabolism appear to add further tumour-supportive pressure. Emerging spatial transcriptomics work suggests obesity reshapes the immune and stromal compartments of early breast lesions, influencing the transition from in situ to invasive disease.

A large real-world analysis of 79,271 patients receiving systemic anticancer therapy in England (2013 to 2023) found that obesity prevalence at first treatment substantially understates lifetime exposure, suggesting epidemiological studies may underestimate obesity's contribution. The Swedish ODDS pooled cohort, following more than 620,000 adults with repeated weight measurements between ages 17 and 60, linked earlier obesity onset and sustained weight gain to higher cause-specific mortality. Diabetes, which often coexists with obesity, has been consistently associated with higher incidence and worse survival in pancreatic, liver, colorectal, breast, and endometrial cancers. In endometrial cancer, where roughly half of cases are attributable to excess weight, insulin resistance and PI3K pathway activation appear to mediate much of the risk. Spatial molecular profiling of early breast cancers shows distinct obesity-driven transcriptional signatures separating ductal carcinoma in situ from invasive ductal carcinoma. Exploratory work in the MA.32 adjuvant breast cancer trial suggested that BMI and obesity-associated blood markers may identify subgroups (notably HER2-positive) where metformin offers benefit, hinting at a metabolic axis that is therapeutically tractable.

The risk is not uniform. Postmenopausal women carry the heaviest hormone-driven burden because adipose-derived oestrogen dominates after the ovaries stop producing it. Endometrial cancer risk rises sharply with BMI, with relative risks above 5 reported for severe obesity. Men show stronger associations for oesophageal adenocarcinoma, kidney, and liver cancer. Earlier onset of obesity, particularly before age 30, and longer duration of excess weight appear to compound risk more than weight at any single time point. People with type 2 diabetes, metabolic syndrome, or polycystic ovary syndrome carry layered risk because insulin resistance amplifies the same pathways. Genetic susceptibility (for example, Lynch syndrome for endometrial and colorectal cancer) interacts with adiposity to push absolute risk higher.

The evidence base is large and convergent: prospective cohorts, Mendelian randomisation studies, mechanistic biology, and emerging interventional data all point in the same direction. Causality is widely accepted by the International Agency for Research on Cancer for the 13 cancers above. Important uncertainties remain around how much intentional weight loss reduces cancer risk, whether the new GLP-1 receptor agonists (such as semaglutide and tirzepatide) will translate into measurable cancer reductions, and how to identify people most likely to benefit from weight-targeted prevention. Bariatric surgery cohorts suggest substantial risk reductions, particularly for hormone-related cancers, but residual confounding cannot be fully excluded. Confounders include physical activity, diet quality, alcohol, and socioeconomic factors that track with weight.

The weight of evidence indicates that excess body fat meaningfully raises the risk of more than a dozen cancers, with the strongest signals in hormone-driven and digestive cancers. Modest, sustained weight loss appears to lower risk markers and may reduce incidence, although the magnitude is still being quantified. Prevention discussions are increasingly framed around metabolic health rather than weight alone, since insulin resistance and inflammation seem to do much of the biological work. For people already living with overweight, the takeaway is not panic but perspective: risk is shaped by duration, distribution of fat, and overall metabolic state, and even partial improvements in any of these appear to matter.