Packing on the pounds gets a well-deserved bad rap. Most Americans understand that excess weight contributes to heart disease and diabetes, not to mention the urge to hide behind the kids in family photos. But obesity as a risk factor for cancer?
That seems to be the case. An increasing number of studies are finding that overweight and obese people are more likely to develop cancer of various kinds. At least half a dozen types of cancer are believed to be directly affected by weight.
"As time goes on, we're realizing that obesity is related to more cancers than we originally suspected," said Dr. Donald Hensrud, an associate professor of preventive medicine and nutrition at the Mayo Clinic College of Medicine.
Researchers are unable to prove that obesity actually causes cancer because requiring people to either gain weight or keep their weight down in clinical trials would be impossible. Most of the data come from observational studies, in which people who are thinner are probably doing many things differently than their heavier counterparts. Any number of those factors might be responsible for the difference in cancer rates.
Still, the evidence is "convincing" for a cause-and-effect relationship between obesity and postmenopausal breast, colon, endometrial, esophageal, kidney and pancreas cancer, according to a 2007 report from the World Cancer Research Fund and the American Institute for Cancer Research. The report also cited obesity as a "probable" cause of gallbladder cancer.
Scientists aren't sure how obesity might affect cancer risk, but "there are some plausible biological mechanisms by which this may occur," said Dr. Patricia Ganz, director of cancer prevention and control research at UCLA's Jonsson Comprehensive Cancer Center.
One popular explanation is that extra weight boosts the body's production of hormones such as estrogen, insulin and insulin-like growth factor 1 — all of which have the potential to promote the growth of certain tumors. Another possibility is that fatness contributes to cancer growth by causing cells to divide more rapidly.
Mechanical factors may play a role in certain types of cancer. In the case of esophageal cancer, the culprit seems to be acid reflux. People who are overweight are more likely to experience chronic reflux, which can lead to precancerous changes by eroding the lining of the esophagus.
The suspected higher risk of gallbladder cancer might be explained by the increased tendency of obese people to develop gallstones. These stones cause inflammation that could promote cancer.
Putting a number on it
Rates of obesity have steadily increased over the past few decades, more than doubling from 15% of adults in the early 1970s to 34% of adults in 2005-06, according to data from the National Health and Nutrition Examination Survey. Cancer rates also increased somewhat during this period, from a rate of 4 in 1,000 in 1975 to 4.56 per 1,000 in 2006 —although rates peaked in 1992 and have since been on the decline.
Scientists don't know how much of this increase in cancer is real. Much of it appears to reflect the fact that we now regularly go looking for cancer with mammograms and prostate specific antigen tests, which is one more reason why the relationship between obesity and cancer is so difficult to study.
The World Cancer Research Fund and the American Institute for Cancer Research made an attempt to quantify the relationship in a 2009 policy report. The report concluded that excess body weight has the largest effect on endometrial cancer, causing an estimated 49% of cases. This translates into an extra 20,700 people with endometrial cancer per year.
The policy report also calculated that being overweight or obese causes 35% of esophageal cancers (5,800 people per year), 28% of pancreatic cancers (11,900 people per year), 24% of kidney cancers (13,900 people per year), 21% of gallbladder cancers (2,000 people per year), 17% of breast cancers (33,000 people per year) and 9% of colon cancers (13,200 people per year) .
Dr. Moshe Shike, an attending physician at Memorial Sloan-Kettering Cancer Center whose research focuses on cancer prevention, said he was skeptical about the idea of putting a number on something so slippery, saying that this implies accuracy where none exists. "We don't know the magnitude of the effect," he said.
Dr. Michael Thun, head of epidemiologic research for the American Cancer Society, agreed that the percentages are imprecise, but pointed out that numbers are often the best way to get results. "Numbers carry a strong message, just as estimates of death caused by smoking were very important to tobacco control," he said.
Most of the studies on weight and cancer risk define obesity using body mass index (BMI), a number that accounts for weight in relation to height. Someone who stands 5-foot-7 is considered "overweight" at 159 pounds and "obese" at 191 pounds. A third of Americans are overweight, and another third are obese.