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Obesity and Cancer

© David Olle

Excess body weight has long been considered a risk factor in developing cancers. Since the 1970's, epidemiological (population) studies have been conducted in order to evaluate the association. These studies, also known as observational studies, have been criticized as being performed "after the fact." Patients with cancer were identified (the "cases"), as well as a group of "control" persons that were similar in all respects except for the presence of cancer. Medical records and patients' recollections regarding their weight status in the past were collected. Associations between weight and cancer were then sought. In these studies, it is difficult to determine cause and effect.

Body-mass index, which is weight adjusted for height, is commonly used for studies of this type, since it allows for estimates of adiposity that can be compared across studies and populations. In 2003, Calle and associates reported on a landmark study that indicated that overweight and obesity are associated with mortality from a wide range of cancer types. This was a prospective study using 900,000 participants initially free of cancer that were assigned to various groups based on differing body-mass indices. Confounding variables, such as smoking, diet, and race that could influence the results were carefully considered. The incidence of various cancers in the groups was followed over a period of 16 years.

What are the risk factors of obesity with cancers?

The heaviest persons on the study had death rates from all cancers combined that were 52% higher (for men) and 62 % higher (for women) than the rates in men and women of normal weight. In both men and women, increasing body-mass index was significantly associated with higher rates of death due to cancer of the esophagus, colon and rectum, liver, gallbladder, pancreas, and kidney, as well as non-Hodgkin's lymphoma and multiple myeloma. Higher body-mass index values were associated with increasing risk of death from stomach and prostate cancers in men, and for death from cancers of the breast, uterus, cervix, and ovary in women. The authors estimated that overweight and obesity could account for 14% of all deaths from cancer in men and 20% of those in women.

Why does obesity increase cancer risk?

Adipose (fatty) tissue is an active organ that receives hormone and metabolic signals from other organs, and acts upon these signals by modifying the breakdown of fat into fatty acids, as well as by the secretion of hormones. These activities can give rise to the phenomenon of insulin resistance, where there tissues begin to lose their response to insulin. The pancreas then compensates by increasing its secretion of insulin. The increased circulation of insulin can stimulate the synthesis of insulin-like growth factor 1 (IGF1). Both insulin and IGF1 can bind to receptors in tissues to promote cellular proliferation and inhibit apoptosis (programmed cell death), the hallmarks of cancer.

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