Cancer Researchers Warn About Obesity-Cancer LinkAmerican Institute for Cancer Research New Review of Scientific Literature Establishes Clear Links to the Most Common Cancers Researchers at the American Institute for Cancer Research (AICR) warned today that carrying extra weight means carrying extra cancer risk. They added that the nation's current epidemic of overweight and obesity is likely to drive up cancer rates in coming years. The researchers issued the warning after completing a thorough review of existing scientific literature on the obesity-cancer link. This review found a rapidly growing number of studies showing that obesity is clearly a factor in some of the most common cancers in the U.S.  "This is a warning that needs to be heard," said Melanie Polk, RD, 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 a recent AICR survey, only 35 percent of Americans believed obesity to be linked to cancer. Yet over 60 percent of Americans are overweight or obese, according to the Centers for Disease Control and Prevention. It was the need to clarify and communicate the association between obesity and cancer, in fact, which motivated AICR to do a comprehensive analysis of the available science. Researchers collected and analyzed individual studies, review articles, international reports and white papers covering all aspects of obesity and cancer risk. The researchers distinguished between overweight and obesity, which are both commonly defined by a formula that combines height and weight measurements. An individual's Body Mass Index (BMI) is the ratio of weight (in kilograms) to height (in meters squared). A BMI between 18.5 and 25 is considered healthy. Between 25 and 30 is considered overweight, and above 30 is considered obese. (It should be noted that BMI is only a general measurement, and may not be an accurate gauge of obesity for athletes, pregnant women, adolescents and other individuals due to differences in body proportions and composition.) Based on a new review of the scientific literature, AICR researchers have concluded that obesity increases the risk for many of the most common cancers worldwide, and perhaps cancer in general. This is of particular concern not only in industrialized countries like the US where obesity is already epidemic among both adults and children, but also in the developing world where obesity rates are rising. If obesity continues to increase, the long-term impact on cancer rates is likely to be profound and costly. The evidence to date has consistently linked obesity to POST-MENOPAUSAL BREAST CANCER, COLON CANCER, ENDOMETRIAL CANCER, PROSTATE CANCER and KIDNEY CANCER. There are several likely explanations for the observed increases in risk, including behavioral conditions (poor diet, lack of exercise) and biological conditions that coincide with excess weight (insulin resistance, hormonal changes, trapping of carcinogens in fat deposits, and slow movement of carcinogens through the colon.) Based on their analysis of the available research, the AICR researchers issued the following recommendations: - Limit weight gain during adulthood to no more than 11 pounds. Although the strongest evidence implicates obesity as a cancer risk, the AICR researchers noted that there is some consensus in the literature that overweight individuals also have higher cancer risk, and that cancer risk generally increases as BMI increases. The AICR researchers also stressed that because the area of obesity and cancer has only recently become the focus of widespread research, certain kinds of cancers have been studied more thoroughly than others. According to the researchers, evidence that obesity increases risk is most consistent for breast cancer in post-menopausal women. Results in regard to colon cancer have been more variable, but there is a general consensus that obesity presents an increased risk, particularly in men. Obese individuals are also clearly at increased risk for cancers of the endometrium (lining of the uterus), prostate and kidney. These are among the most common types of cancer in the US and the world. For each of the cancers mentioned above, researchers have developed theories to explain the precise reason or reasons why obesity increases risk. Because cancer is a complex disease that can arise in different tissues for different reasons, these theories tend to involve several different biological and behavioral factors. Yet as the scope of this research widens, researchers say the consistent patterns that have emerged so far suggest that overweight and obesity are likely to factor in other cancers, and in cancer in general. (One large study cited in the AICR review, for example, concluded that 10% of all cancer deaths among non-smokers in the US are caused by overweight.) Important Finding: Exercise Provides Strong Protection Against Cancer One other important factor that has emerged from AICR's new analysis of the scientific literature is the central role that regular physical activity plays in reducing cancer risk. In an overwhelming majority of studies, exercise was consistently associated with lower risks for many different types of cancer. In their review of the available evidence from laboratory studies, clinical studies and large-scale epidemiological studies, the AICR researchers reached several conclusions about obesity's link to cancer at different sites. Obesity Causes Bodily Changes that Seem to Fuel Many Cancers Obesity and Breast Cancer: Breast cancer is the most common cancer among women in the US. Researchers point out that most of the studies that have looked at obesity and breast cancer have focused on post-menopausal 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. But exposure to estrogen has also 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. But 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, researchers say. 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. In fact, some studies suggest that in obese post-menopausal women, excess estrogen and excess insulin my actually work together to exert a particularly strong influence on breast cells, increasing breast cancer risk significantly. Finally, 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 against breast cancer. Another preventive strategy that seems particularly effective against breast cancer at all stages of life is regular physical activity. Regular exercise helps to prevent the buildup of fat tissue in the first place. In addition, intense exercise before and during adolescence can also 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. Obesity and Colon Cancer: Colorectal cancer is the third most common cancer in the US. The AICR researchers point out that many of the first studies to explore the link between obesity and colorectal cancer focused on total caloric intake alone. And indeed, studies that compare the diets of different countries regularly find that those populations consuming diets high in calories and fat have higher risk of colorectal cancer. But now it 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. And 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 excreted 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. 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. For both men and women, however, regular physical activity has been strongly associated with reduced colorectal cancer risk. There is now a clear consensus in the scientific literature that exercising regularly offers significant protection against colon cancer. It has been theorized that this is so because exercise prevents the buildup of fat tissue and regulates the levels of insulin and other hormones in the blood. Other studies suggest that physical activity speeds the passage of dietary and environmental carcinogens from the body and reduces the amount of time these substances spend in contact with colorectal tissues. Obesity and Cancer of the Endometrium (Lining of the Uterus): Endometrial cancer is generally a post-menopausal cancer, and the fourth most common cancer among women. Like breast cancer, endometrial cancer has been associated in several studies with a set of hormonal conditions unique to overweight and obese bodies, described above. Insulin resistance may also have a role. Some studies suggest that it is obesity late in life that particularly increases risk, and that avoiding weight gain during adulthood could decrease risk of this cancer by as much as 10 percent. Obesity and Prostate Cancer: Prostate cancer is the most common cancer among men. In several studies, cancer of the prostate seems to be associated with a similar set of hormonal conditions mentioned above. Once again, abdominal fat has been shown to be associated with higher levels of insulin, testosterone and other growth factors that may stimulate prostate cells to divide more quickly, increasing risk for cancer. A majority of studies have found regular physical activity to be strongly associated with lower risk for prostate cancer. The duration and intensity of exercise needed to afford significant protection is still the subject of considerable debate. Obesity and Renal (Kidney) Cancer: Researchers note that kidney cancer has been consistently associated with overweight and obesity. This relationship seems to be slightly stronger in women than in men. This, scientists say, suggests that estrogen exposure may increase risk further by stimulating kidney cells to grow and divide more quickly. Obesity and Other Cancers: In several studies, obesity has been associated with cancer of the esophagus. There may be an indirect physical explanation for this: obesity is associated with gastroesophageal reflux, a condition that exposes the lining of the esophagus to damage that is in turn associated with increased risk for esophageal cancer. Researchers Issue Conclusions and Recommendations Based on New Review "AICR researchers have concluded that obesity increases the risk for many of the most common cancers worldwide, and perhaps cancer in general," said Melanie Polk, AICR Director of Nutrition Education. "This is of particular concern not only in industrialized countries like the US where obesity is already epidemic among both adults and children, but also in the developing world where obesity rates are rising. If obesity continues to increase, the long-term impact on cancer rates is likely to be profound and costly."
For more information, or to contact American Institute for Cancer Research, see their website at: www.aicr.org |
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