The association between obesity, as defined by a body mass index of 30 or higher, and risk of a variety of types of cancer has received a considerable amount of attention in recent years. Obesity has been associated with an increased risk of esophageal cancer, pancreatic cancer, colorectal cancer, breast cancer (among postmenopausal women), endometrial cancer, kidney cancer, thyroid cancer, liver cancer and gallbladder cancer. Obesity may also lead to increased cancer-related mortality. About 75-80% of all cancers in the United States are preventable, if risk factors are avoided (also see (Cancer prevention). Obesity appears to be the third most important risk factor for cancer in the United States, just behind tobacco and diet. Obesity is the source of about 15% of all preventable cancers. In 2018, Chinese researchers performed a systematic review and comprehensive quantitative meta‐analysis of cohort studies reporting body mass index (BMI) and the risk of 23 cancer types, providing epidemiological evidence supporting the association between BMI and cancer risk. The strongest positive association between BMI and cancer risk was found among patients in North America. The mechanisms by which obesity may increase the risk of cancer are not well understood, but it is believed that the combined effects of the adipose tissue environment and the endocrine alterations that accompany it among obese people both interact to promote tumor initiation and progression. Adipose tissue also creates an inflammatory environment that enhances the ability of tumor cells to metastatize.

The U.S National Cancer Institute indicates that one of the main ways in which obesity can cause cancer is by promoting chronic low-level inflammation, which can, over time, cause DNA damage that leads to cancer. Articles supporting this view were reviewed by Cerda et al. There are also a number of hormonal, metabolic and other changes caused by obesity that may affect carcinogenesis. A review by Tahergorabi et al. summarizes articles indicating that obesity induces changes in angiogenesis, inflammation, interaction of proinflammatory cytokines, endocrine hormones, adipokines including leptin and adiponectin, insulin, growth factors, estrogen, progesterone and cell metabolism. DNA damage appears to be the primary cause of cancer (see Carcinogenesis). It is not clear which factor(s) altered by obesity are major source(s) of the DNA damages causing increased cancer risk in obese individuals. However, both reduced DNA repair and increased DNA damage are observed in obese individuals.

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