MINNEAPOLIS, June 19, 2008—As Americans have gotten heavier, they have been diagnosed with more cases of obesity-related cancer.

According to the June issue of Mayo Clinic Health Letter, an estimated 14 percent of cancer deaths in older men and 20 percent in older women can be attributed to excess body fat.

It's not fully understood why excess body fat increases the risk of cancer. Four theories have been put forward:

• Insulin. Obesity and inactivity generally lead to higher levels of insulin circulating in the blood. Excess insulin is believed to fuel the growth of cancer cells. In addition, it increases circulating levels of other hormones that likely play a role in cancer development and growth, such as estrogen.
• Estrogen. Adding to insulin's influence on estrogen levels, fat tissue also produces this hormone. Estrogen levels are 50 percent to 100 percent higher in postmenopausal women who are overweight versus those who are lean. It's believed that this change increases the risk of estrogen-sensitive cancers such as certain types of breast cancers.
• Immune system. Excess fat is thought to release proteins that may suppress the immune system and increase inflammation. Both may play a role in cancer development and progression.
• Other risks of obesity. Obesity-related problems such as acid reflux, high blood pressure, gallstones and fatty liver may damage tissues of the esophagus, kidney, gallbladder and liver, respectively. This may set the stage for cancer development.

Although there's evidence that gaining weight increases the risk of cancer, there has been almost no research that demonstrates whether losing weight will reduce that risk. Two major components of weight loss — a healthy diet and exercise — appear to be excellent ways to reduce cancer risk. 

Mayo Clinic news release