Scare after scare appears in the popular press -- with news blaming traces of all sorts of chemicals and "toxins" for causing cancer. But it is becoming ever more apparent that many types of cancer are in fact linked more to lifestyle-related choices such as smoking and, now, obesity.
It is widely recognized that obesity (defined as a Body Mass Index or BMI of 30 or more) is linked to a number of health risks -- e.g., heart disease and type 2 diabetes. It has been less often acknowledged that the occurrence of several types of cancer is more frequent in the obese. A recent meta-analysis in the British journal The Lancet (A. Renehan and colleagues, Lancet 371: 569-578) substantiates such observations and extends the findings of earlier studies.
The meta-analysis combined the data from 141 prospective epidemiologic studies (studies that followed people from before they had any kind of cancer) and examined the occurrence of cancer in 19 different anatomical locations. Unlike many previous reports, the results were examined separately for men and women -- and for some sites, the incidence of cancer differed by gender.
In particular, increases in men's BMI were more strongly associated with cancer of the colon, rectum, and thyroid than they were in women. Conversely, obese women had a greater risk of gallbladder cancer than did obese men for a similar increase in BMI. Other site-specific cancers that were strongly associated with increases in BMI were one type of cancer of the esophagus and kidney in men and endometrial, esophageal, and kidney in women. There were increases in the incidence of some blood cancers (multiple myeloma and leukemia) in both men and women, but these increases weren't as great as those seen for some other cancers.
The authors suggest that at least some if not all of the increased occurrence of cancers may be mediated by hormonal changes brought about by the presence of excess adipose (fat) tissue. Some candidates include changes in insulin, insulin-like growth factor, estrogens, testosterone, and adipokines (hormones secreted by fat cells -- especially those found in the fat around the abdominal organs).
While we don't know the actual mechanism(s) by which obesity increases the risk of the various cancers -- and it isn't necessarily the same for each -- it behooves us to pay less attention to the myriad frights bandied about and more to maintaining a healthy weight and lifestyle.
Ruth Kava, Ph.D., R.D., is Director of Nutrition at the American Council on Science and Health (ACSH.org, HealthFactsAndFears.com).