A specific condition that occurs in the bodies of many inactive, overweight individuals may actually encourage the cancer process, according to researchers at the American Institute for Cancer Research (AICR).
Research has shown that overweight, inactive individuals have a higher risk for many cancers, but a clear biological explanation for exactly why this is so has been more elusive.
Now, however, one likely reason is beginning to emerge from the research. It has been shown that being overweight and inactive can cause the body to secrete higher and higher amounts of insulin and other growth factors. Cells exposed to high levels of these substances over an extended period of time may begin to reproduce so quickly that cancer becomes more likely.
But AICR researchers stress that this potentially dangerous condition is reversible. Regular physical activity can help reset the body's hormonal systems and regulate the secretion of insulin.
Losing weight and keeping it off also helps restore insulin levels to normal. As research continues, scientists will likely identify additional biological reasons that excess weight and inactivity increase risk. It is already clear, however, that diet, physical activity and weight management play key roles in cancer prevention.
The AICR researchers say that being overweight and inactive can cause the body to release high levels of insulin and other hormones known as "growth factors." High insulin levels-along with several other conditions collectively known as "metabolic syndrome" or "syndrome X"-have already been linked to cardiovascular disease and other chronic diseases.
Many cancer scientists now believe this same state of affairs is one reason cancer is more common among the overweight and obese.
"Research has demonstrated that insulin and other growth factors encourage cells to divide more rapidly," said Helen Norman, Ph.D., lead author of a new AICR review of the scientific literature on obesity and cancer risk.
"Under normal circumstances, release of insulin is tightly regulated and doesn't pose much of a risk. But in many overweight, inactive people, the tissues are constantly exposed to high levels of insulin, which causes their cells to reproduce quickly and often."
When cells reproduce rapidly, Norman said, the chances increase that something could go wrong in the process, such as the kind of random mutation that can lead to cancer.
The emerging focus on the role of insulin is based on research showing that two related conditions called insulin resistance and hyperinsulinemia may be linked to several kinds of cancer, including breast cancer and colon cancer.
In non-obese, non-diabetic individuals, the pancreas produces low levels of insulin to help metabolize certain components of the diet - glucose and carbohydrates. But studies have shown that in many individuals, the tissues of the body become less sensitive to insulin as weight increases and physical activity drops. This phenomenon is known as insulin resistance.
As their bodies become less sensitive to insulin's effects, overweight and obese individuals experience greater difficulty converting glucose into energy. The body attempts to compensate for this by producing more insulin, a condition called hyperinsulinemia, which in turn spurs cells to divide and raises cancer risk.
It appears that whether or not a person becomes resistant to insulin is influenced by genetic factors as well as weight status and level of physical activity. Thus, not all overweight people are insulin resistant. This may be the reason it has taken scientists so long to document insulin's role in disease risk.
Norman also added that a recent review of the scientific literature on obesity and cancer risk suggests that physical activity may prove to have an even greater role in cancer prevention than was previously thought.
"Regular exercise helps to prevent the buildup of fat tissue in the first place, which in turn keeps the levels of insulin and other hormones in check," she says. "There is also growing evidence that exercise releases binding proteins that 'soak up' any excess growth factors in the blood."
Physical activity also speeds the passage of dietary and environmental carcinogens from the body. Studies suggest that this reduces the risk of colon cancer in particular because these potential carcinogens spend little time in direct contact with colorectal tissues.
Interestingly, researchers say that the colorectal cancer risk associated with excess insulin seem to be more pronounced in men. This is likely because men tend to store fat around the abdomen, and abdominal fat has been shown to secrete insulin at high levels.
Dr. Rowan Chlebowski, of the Harbor-UCLA Research and Education Institute, believes there will soon prove to be several biological reasons that people who are overweight and inactive tend to have higher cancer rates. "The role of insulin resistance in cancer risk needs to be studied further, but it certainly represents one of the most promising avenues of research to come along in a while."
SOURCE:
American Institute for Cancer Research (http://www.aicr.org)
