Statistics have shown that those who are overweight or obese have a greater risk of contracting cancer than those who are a healthy weight. But until recently, researchers did not know biologically why this is the case.

Researchers from the American Institute for Cancer Research (AICR) have found that the bodies’ of overweight and obese people release high levels of insulin and other hormones known as “growth factors.” This already has been linked to cardiovascular disease and other chronic diseases.

“Research has demonstrated that insulin and other growth factors encourage cells to divide more rapidly,” said Helen Norman, PhD, lead author of AICR’s review of the scientific literature on obesity and cancer risk. Because the tissues of many overweight, inactive people are constantly exposed to high levels of insulin, their cells reproduce quickly and frequently. The more often cells reproduce, the greater the likelihood that a random mutation could occur, which could lead to cancer.

The pancreas produces low levels of insulin to help metabolize glucose and carbohydrates. However, in many people, as weight increases and physical activity drops, the tissues of the body become less sensitive to insulin and these individuals become insulin resistant. They then experience greater difficulty converting glucose into energy. The body’s attempt to compensate for this is to produce more insulin (hyperinsulinemia), which spurs the cells to divide more often.

Not all overweight and obese people are insulin resistant, but the condition is more common among these groups of people. In the literature review, Norman found that regular exercise helps prevent the buildup of fat tissue, which keeps insulin levels in check. In addition, growing evidence suggests that exercise releases binding proteins that “soak up” any excess growth factors in the blood.