Obesity is associated with an increased risk of developing several types of cancer. It may also adversely affect cancer detection, treatment, recurrence, quality of life, and survival, although the magnitude of these effects varies considerably according to cancer type, stage, treatment, body-fat distribution, and accompanying metabolic disorders. The term excess body weight generally includes both overweight and obesity. https://www.cancer.gov/about-cancer/causes-prevention/risk/obesity/obesity-fact-sheet
Cancers Associated with Excess Body Fat
The International Agency for Research on Cancer concluded that there is sufficient evidence that the absence of excess body fat reduces the risk of at least 13 types of cancer:
The association with breast cancer primarily applies to postmenopausal breast cancer; the relationship between obesity and premenopausal breast cancer is more complex and may differ according to tumor subtype and the timing of weight gain. Similarly, the established association with esophageal cancer relates specifically to adenocarcinoma, not to all histological forms of esophageal cancer. https://www.cancer.gov/about-cancer/causes-prevention/risk/obesity/obesity-fact-sheet
Evidence also suggests that excess body fat may increase the risk of fatal or advanced prostate cancer, diffuse large B-cell lymphoma, and male breast cancer. However, the evidence for these malignancies is less conclusive than for the 13 established obesity-associated cancers. Therefore, prostate cancer and non-Hodgkin lymphoma should not be presented as having the same level of evidence as endometrial, colorectal, or postmenopausal breast cancer. (IARC)
The strength of the association varies by cancer type and generally increases with the degree and duration of excess body fat. Compared with people of a healthy weight, individuals with severe obesity have approximately seven times the risk of type 1 endometrial cancer and nearly five times the risk of esophageal adenocarcinoma. For most other obesity-associated cancers, the estimated increase in risk ranges from approximately 10% to a twofold increase. https://www.cancer.gov/about-cancer/causes-prevention/risk/obesity/obesity-fact-sheet
A global analysis estimated that approximately 481,000 new cancer cases in 2012, corresponding to 3.6% of all new cancers, were attributable to a BMI of at least 25 kg/m². The attributable proportion was higher among women than among men—5.4% compared with 1.9%—largely because of the strong associations with endometrial and postmenopausal breast cancer. Endometrial, postmenopausal breast, and colorectal cancers accounted for almost two-thirds of the global cancer burden attributable to excess body weight. (IARC)
More recent Global Burden of Disease estimates indicate that excess body weight was responsible for approximately 463,000 cancer deaths worldwide in 2019, representing about 4.5% of all cancer deaths. The estimated proportion of cancer deaths attributable to excess body weight was approximately 5.2% among women and 4.1% among men. Around 40% of deaths from uterine cancer, 19% of kidney-cancer deaths, and 18% of esophageal-cancer deaths were attributed to excess body weight. (Krebsforschung)
In the United States, an American Cancer Society analysis of data from adults aged 30 years and older estimated that excess body weight accounted for 7.6% of new cancer cases and 7.3% of cancer deaths in 2019. It was therefore the second-largest modifiable contributor to cancer incidence and mortality in that analysis, after cigarette smoking. This ranking is not universally applicable: globally, infections and other risk factors account for different proportions of the cancer burden depending on the population and region. (PubMed)
Population-attributable fractions should not be interpreted as the probability that an individual person’s cancer was caused by obesity. Cancer is usually multifactorial, and these estimates describe the theoretical proportion of cases or deaths that might have been avoided at the population level if excess body weight had not occurred.
Several interacting mechanisms may explain the relationship between obesity and cancer.
Sex hormones: Adipose tissue produces estrogen. After menopause, it becomes a major source of estrogen production, and increased estrogen exposure can promote the development of hormone-sensitive breast and endometrial cancers.
Insulin and growth-factor signalling: Obesity is frequently associated with insulin resistance, hyperinsulinemia, and changes in insulin-like growth factor 1. These pathways may promote cellular proliferation and inhibit programmed cell death.
Chronic inflammation: Visceral adipose tissue can produce pro-inflammatory cytokines and other mediators. Persistent low-grade inflammation may cause DNA damage, promote abnormal cell growth, and create a tissue environment that supports tumor development.
Adipokines and the tumor microenvironment: Altered concentrations of leptin, adiponectin, and other adipose-tissue mediators may influence cell proliferation, angiogenesis, immune responses, and tumor metabolism.
Organ-specific pathways: Obesity increases the likelihood of gastroesophageal reflux, which is a risk factor for esophageal adenocarcinoma. It is also closely associated with metabolic dysfunction-associated steatotic liver disease, which can progress to cirrhosis and hepatocellular carcinoma. Obesity-related gallstone disease and chronic gallbladder inflammation may contribute to gallbladder cancer risk. https://www.cancer.gov/about-cancer/causes-prevention/risk/obesity/obesity-fact-sheet
Obesity at the time of diagnosis has been associated with poorer outcomes in several malignancies, particularly breast, colorectal, prostate, and some hematological cancers. Possible consequences include an increased risk of recurrence, disease progression, second primary cancers, treatment-related complications, and reduced quality of life. However, these associations are not uniform, and BMI alone does not adequately describe body composition, visceral fat, muscle mass, or nutritional status. Unintentional weight loss caused by an undiagnosed cancer can also complicate the interpretation of observational studies. https://www.cancer.gov/about-cancer/causes-prevention/risk/obesity/obesity-fact-sheet
Obesity may create practical challenges in diagnostic imaging, surgery, radiotherapy planning, and systemic treatment. Nevertheless, anticancer medication should not automatically be underdosed because a patient has obesity. The American Society of Clinical Oncology recommends full, weight-based cytotoxic chemotherapy doses and the usual approved doses of immunotherapies and targeted treatments, with subsequent dose adjustments based on toxicity in the same manner as for patients without obesity. (PubMed)
Avoiding excessive weight gain throughout adulthood and maintaining a healthy body weight are important components of cancer prevention. Regular physical activity, limiting sedentary time, and following a dietary pattern rich in whole grains, vegetables, fruit, and legumes can support healthy weight management. Consumption of highly processed, energy-dense foods and sugar-sweetened beverages should be limited. Avoiding tobacco and reducing or avoiding alcohol provide additional cancer-prevention benefits. (World Cancer Research Fund)
Observational studies suggest that intentional and sustained weight loss may reduce the risk of some obesity-associated cancers, particularly postmenopausal breast and endometrial cancer. Bariatric surgery has also been associated with lower cancer incidence and mortality in people with severe obesity. However, randomized evidence demonstrating that weight loss directly prevents cancer remains limited, and weight-management interventions should be individualized and medically supervised when necessary. https://www.cancer.gov/about-cancer/causes-prevention/risk/obesity/obesity-fact-sheet
Weight management should complement, rather than replace, established cancer-prevention measures and recommended screening programmes. People with obesity should receive the same evidence-based screening, diagnostic assessment, and oncological care as other patients, without stigmatization or inappropriate treatment restrictions.