By Noy Birger, BS and Brandel France de Bravo, MPH
Updated January 2013
There are many reasons why being overweight is bad for our health, but most people don’t realize that cancer is one of them. Of course, excess body weight can contribute to serious medical conditions such as heart disease and type II diabetes, but new research shows that the habits that lead to excess body fat might also increase the risk of developing certain types of cancer.
Dr. Andrew Renehan and his colleagues at several European universities estimated that more than 124,000 of newly diagnosed cancer cases in 30 European countries in 2008 were due to overweight or obesity. which was a 77% increase compared to 2002 because of the increase in obesity in recent decades. The risk of obesity-related cancer for women in 2008 was almost three times the risk for men (almost 9% compared to over 3%), even though the BMIs of the men and women studied were quite similar, with the men slightly more overweight than the women, on average. Three cancers – two of which only affect women – accounted for almost two-thirds of all new obesity-related cancers: endometrial cancer, post-menopausal breast cancer, and colorectal (colon) cancer.
How does obesity increase the risk of developing cancer?
What does excess body weight have to do with cancer, and why is being overweight even riskier for women? Researchers think that the danger of excess weight is partly due to the hormones secreted by fat tissues. One of those hormones is estrogen, which is known to increase the risk of breast cancer  and endometrial cancer (also called cancer of the uterus) in women. Fat cells also trigger a type of chronic immune response in the body because it recognizes that the extra fat is not supposed to be there, and so the immune system responds, causing inflammation in the body. Long-term inflammation can prevent the immune system from working properly, making the body less able to fight off disease. In addition, chronic inflammation may cause insulin resistance, which along with high blood sugar levels, has been proposed as another explanation for why obesity increases the risk of cancer. The higher the insulin level of a breast cancer patient, the greater the chance of death: insulin is believed to play a role in promoting tumor growth and in inhibiting cancer treatment.
How new is this news?
Researchers have been documenting the link between obesity and cancer for several years. Even before this new study, the National Cancer Institute estimated that 20 to 30% of the most common cancers in the United States “may be related to being overweight and/or lack of physical activity” and might also “increase the risk of death from many cancers, accounting for up to 14% of cancer deaths in men and 20% of cancer deaths in women.” Every new study fills in another piece of the puzzle, helping to explain which cancers “feed” off fat, and how it is that fat fuels tumor growth.
In fact, Renehan’s study is based on an earlier meta-analysis done by him and many of the same co-authors, in which they looked at over two hundred comparable datasets gathered from different countries around the world. The meta-analysis, published in 2008, found that excess weight in men was most strongly associated with cancer of the esophagus (esophageal adenocarcinoma), the thyroid, colon cancer, and renal cancer (cancer of the kidneys). Being overweight did not appear to increase a man’s risk of prostate cancer (although a study in the U.S. has found that an overweight man with prostate cancer is more likely to die of it than a man with prostate cancer who is not overweight). According to the meta-analysis, excess weight in women is most strongly associated with endometrial cancer, cancer of the gallbladder and the esophagus, and kidney cancer.A few other cancers were also associated with being overweight for both men and women, such as leukemia, multiple myeloma, and non-Hodgkins lymphoma, but the link was weaker. In men, rectal cancer and malignant melanoma also seemed to be related to weight, and women with higher BMI were slightly more likely to be diagnosed with post-menopausal breast cancer, cancers of the pancreas and thyroid, and colon cancer.
Renehans’s conclusions are quite consistent with the findings of other recent studies not included in the meta analysis. Similarly, the American Institute for Cancer Research (AICR) estimated that excess body fat is responsible for 49% of endometrial cancers; 35% of esophageal cancers; 28% of pancreatic cancers; 24% of kidney cancers; 21% of gallbladder cancers; 17% of breast cancers; and 9% of colon cancers. U.S. obesity rates have recently reached a plateau of 34% for adults, and AICR estimates that will result in over 100,000 new cases of cancer each year.
There is one study that is not consistent with Renehan’s findings; Renehan did not find any significant link between obesity and ovarian cancer, but a recent U.S. study did.
Does losing weight reduce your risk of cancer?
Obesity is caused by too many calories in proportion to physical activity but researchers are still not certain whether certain types of food, weight (rather than specific foods), or lack of physical activity is the major culprit, or if two or three of these factors interact to increase cancer risk. Several studies show that high dietary fat intake increases the risk of post-menopausal breast cancer, prostate cancer, and pancreatic cancer. Other studies have indicated that high fat diets increase the risk of death from cancer and that low fat diets reduce the chances of cancer recurrence. Dietary fat intake is, of course, also likely to cause obesity, so it is very difficult to determine whether weight or diet is a stronger predictor of increased cancer risk. In other words, is it the fat you eat or the fat on your body that puts you at the greatest risk of cancer? And not all body fat is equal: abdominal fat, for instance, is associated with colon cancer.
Can losing weight prevent you from getting cancer? The evidence is clear for some cancers but not for others. Losing weight may reduce the risk of prostate cancer and definitely reduces the risk of reflux, an important precursor to esophageal adenocarcinoma. Several studies have found that intentional weight loss can reduce the risk of breast cancer, particularly when the weight loss is after menopause.,, In addition, a study by Andrew Renehan showed a decreased cancer incidence in a group of formerly very obese patients who had undergone bariatric surgery to reduce the size of their stomach and lose weight. All of these studies provide preliminary evidence that the cancer risk from being overweight or obese may be reversible and may not only be related to the types of food eaten or exercise habits, but also to the excess fat tissue. In some of these studies, however, the people participating changed several habits at once, such as diet and exercise, so it is not always possible to determine if the reduction in cancer risk was related primarily to the physical weight loss, or to the change in diet or exercise, or both.
Excess body weight doesn’t only increase the risk of developing cancer, it can also make treatment more difficult and cause tumors to grow more quickly. If a person is very overweight, veins are not easy to find, making it hard for health workers to give intravenous medication. More importantly, doctors have difficulty determining the correct dose of medicine, and as a result, obese patients often do not receive sufficient chemotherapy. Given these complications and the likelihood of other chronic health problems such as type II diabetes and heart disease, it is not surprising that cancer death rates are usually higher for people who are very overweight.
Overall, the research indicates that when weight goes up, so does the risk of many cancers. Similarly, when weight goes down, or is at least maintained throughout adulthood,the risk of certain cancers also goes down. These are studies of large numbers of people- looking at patterns and associations- so the findings do not mean that every obese person will get cancer or that slender people are protected from it. The exact relationship between cancer and excess body weight has not been firmly established. We still do not understand how an individual person’s diet, level of physical activity, and genes can all be factored together to calculate his or her cancer risk. What we do know is that there are many health benefits to achieving and maintaining a healthy body weight, including lowering the risk of cancer and improving cancer treatment outcomes. After giving up tobacco, watching your weight and staying active are your best forms of health insurance. For guidelines and tips on living a healthy lifestyle, read Eating Habits to Improve Health and Lower Body Mass Index.
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