Extra weight adds cancer risk - Obesity

Researchers at the American Institute for Cancer Research (AICR) warn that carrying extra weight means carrying extra cancer risk. Moreover, the nation's current epidemic of overweight and obesity is likely to drive up cancer rates in coming years.

"This is a warning that needs to be heard," emphasizes Melanie Polk, AICR Director of Nutrition Education. "Most Americans know that obesity is related to cardiovascular disease and diabetes, but far fewer are aware that shedding pounds could reduce their risk of getting cancer."

In their review of the available evidence from laboratory, clinical, and large-scale epidemiological studies, the AICR researchers reached several conclusions about obesity's link to cancer at different sites:

Breast cancer is the most-common cancer among women in the U.S. Most of the studies that have looked at obesity and breast cancer have focused on postmenopausal breast cancer. Scientists believe that fat tissue exhibits one particular biological property that may play a significant role in cancer development.

According to the AICR researchers, it has long been established that, during a woman's reproductive years, the ovaries produce estrogen, a hormone that regulates the menstrual cycle and other sexual functions. Exposure to estrogen has been shown to stimulate the body's cells to reproduce more rapidly, which is associated with increased risk for cancer. Another source of estrogen is fat tissue, but the amount of estrogen produced by fat is much lower than that produced by the ovaries.

Although the ovaries stop secreting estrogen at menopause, studies have shown that fat tissue continues to pump the hormone (and other so-called "growth factors") into the bloodstream unabated. Thus, overweight and obese women continue to expose their bodies to estrogen's effects for as long they carry significant amounts of fat tissue. It is believed that this extended lifelong exposure to estrogen is linked to the increased breast cancer risk seen after menopause.

There is also support in the scientific literature for a link between a condition called insulin resistance and breast cancer (and, perhaps, cancer in general). Under normal circumstances, the pancreas produces low levels of insulin to help metabolize certain components of the diet--glucose and carbohydrates Studies have shown that, in overweight and obese individuals, the tissues of the body become less sensitive to insulin. As a result, these individuals have difficulty converting glucose into energy. The body attempts to compensate for this by producing more insulin, Studies have shown that insulin and insulin-like "growth factors" stimulate cells to divide more rapidly, in much the same way that estrogen does. This rapid proliferation, it is believed, increases cancer risk Some studies suggest that, in obese postmenopausal women, excess estrogen and excess insulin may actually work together to exert a particularly strong influence on breast cells, increasing cancer risk significantly.

Several studies seem to indicate that the time of life at which weight is gained plays an important role in breast cancer risk. In particular, avoiding significant weight gain throughout adulthood seems to offer considerable protection. Another preventive strategy that seems particularly effective against breast cancer at all stages of life is regular physical activity, which helps to prevent the buildup of fat tissue in the first place. In addition, intense exercise before and during adolescence can delay the start of menstruation and thus decrease the total number of ovulations that occur over a lifetime, which has been associated with reduced risk for cancer later in life, Some studies suggest that a lifetime of regular exercise decreases an individual's total level of exposure to estrogen and other hormones.

Colorectal cancer is the third-most-common cancer in the U.S. Many of the first studies to explore the link between obesity and colorectal cancer focused on total caloric intake alone, and the ones that compare different countries regularly find that those populations consuming diets high in calories and fat have higher risk of colorectal cancer. However, it now appears that the total number of calories consumed is less important than maintaining energy balance--that is, making sure that the number of calories taken in is more or less equal to the number of calories burned.

When more calories are consistently taken in than are burned, overweight and obesity eventually result. In study after study, consuming excess calories has been associated with increased colon cancer risk. Researchers suspect that, under these conditions, the dietary and environmental carcinogens that are normally secreted before they can do any damage may become trapped in the body's fat stores.

Excess fat also seems to result in a condition of insulin resistance, which causes the body to produce higher and higher amounts of insulin and other growth factors. This, it is believed, stimulates colon cells to reproduce rapidly and increases cancer risk. Researchers say that the colorectal cancer risk associated with excess insulin seem to be more pronounced in men, likely because they tend to store fat around the abdomen, and abdominal fat has been shown to secrete insulin at high levels.