Fast Facts – Obesity and Cancer

The chronic and life-threatening diseases of obesity and cancer have a complex relationship. Individuals with obesity are more likely to develop specific types of cancer. Every year about 5% of new cancer cases in men and 10% in women are attributable to excess body weight.³

• The International Agency for Research on Cancer (IARC) has determined that there is sufficient evidence to link excess adiposity to 13 cancers including colorectal cancer, liver cancer, gallbladder cancer, pancreatic cancer, postmenopausal breast cancer, ovarian cancer, thyroid cancer, and several others.⁴
• The way excess body fat leads to cancer is complex and specific to the type of cancer. Chronic inflammation and its effect on the immune system are likely involved to some degree in all types of cancer that are affected by obesity.⁵ Other factors that may link obesity and cancer include increased blood glucose or estrogen levels.⁶
• Obesity treatments may also affect an individual’s risk for cancer. Although bariatric surgery decreases the odds of hormone-related cancers including breast, endometrium, and prostate cancer, gastric bypass was found to increase the odds of colorectal cancer.⁷
  • The mechanisms by which gastric bypass increases the odds of colorectal cancer are not fully understood but may involve a change in the patient’s microbiome or increased inflammation in the gastrointestinal tract.⁷

Obesity is not just related to the incidence of cancer, but also to cancer outcomes. Men with severe obesity (BMI over 40) are 52% more likely to die of their cancer than men with healthy weight (BMI between 18.5 and 30 ) and women with severe obesity are 62% more likely to die of their cancer than women with healthy weight.⁸ 

• Increased mortality in patients with obesity has been observed for esophageal, colorectal, liver, gallbladder, pancreatic, and kidney cancer, as well as non-Hodgkin's lymphoma and multiple myeloma. This effect was also observed in men with stomach or prostate cancer and in women with breast, uterine, cervical, or ovarian cancer.⁸ The effect of weight on cancer-related mortality is not always straightforward. In what is known as the “obesity paradox” some researchers have observed that overweight or obesity can have a protective effect against cancer-related mortality.⁹
  • One potential explanation for this effect is a reliance on BMI to diagnose an unhealthy weight status, as opposed to measurements of body composition or nutrition.⁹ Although those with a BMI over 25 are considered to have excess weight, they may have more muscle mass or healthier diets than those with a BMI under 25, giving them an advantage during cancer care.
  • Other explanations include collider bias (a type of selection bias) or other types of confounding, such as the fact that weight loss may accompany more severe or late-stage cancers, associating lower weights with worse conditions.¹⁰

Cancer patients may gain or lose significant amounts of weight during or after their cancer treatment. The American Society for Clinical Oncology recommends that oncologists take an active role in addressing weight with their patients.¹⁶ 

• Physical activity is an important weight management tool for cancer survivors but may be difficult due to the late effects of cancer treatment. Pain, anxiety, fatigue, lack of stamina, and treatment side effects are among the barriers to exercise most commonly found in cancer survivors.^17,18
• Eating a nutritious diet may be difficult for cancer patients who receive chemotherapy. Many patients undergoing chemotherapy experience malnutrition and weight loss during treatment as a result of nausea, vomiting, or loss of appetite.19