Make the Lifestyle Change and Feel Better About
Obesity's Role in Cancer
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
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
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.
But the real culprit is fat, not weight, so a football player-type with
lots of muscle and little flab would not be at increased risk even if
his BMI fell into the "overweight" category.
The effects of weight loss
If excess weight increases the risk of cancer, can losing weight reduce
the risk? Preliminary research suggests that it might. At least two
large, published studies have found that people who undergo gastric
bypass surgery are significantly less likely to develop cancer or die
from it than severely obese people who don't undergo the weight-loss
People who have been diagnosed with certain cancers, including breast,
prostate and colorectal cancer, also seem to have a worse prognosis if
they're overweight or obese, according to Dr. Jeffrey Meyerhardt, an
assistant professor of medicine at Harvard Medical School. Preliminary
studiessuggest that weight loss might reduce this risk.
For example, a clinical trial called the Women's Intervention Nutrition
Study that was published in the Journal of the National Cancer Institute
in 2006 randomly assigned breast-cancer survivors to either a low-fat
diet or their regular diet. Not only did the women on the low-fat diet
wind up an average of six pounds lighter than their control-group
counterparts, they had a reduced risk of cancer recurrence at five
Studies like this one fail to answer the question of which played a
bigger role, the types of foods eaten or the amount of weight lost. But
as randomized trials fail to find cancer-fighting benefits for specific
dietary components, such as fiber (long suspected of decreasing the risk
of colon cancer), the evidence increasingly points toward weight as the
"A healthy diet is good for avoiding obesity, but it's not clear that
diet itself impacts the prognosis of most cancers," said Meyerhardt. A
possible exception, he said, is colon cancer; his own research has
linked recurrence and reduced survival to a diet high in red meat,
refined grains and sugary desserts.
Shike said that the No. 1 lifestyle measure people can take to reduce
their risk of cancer is to avoid smoking and secondhand smoke, followed
by maintaining a healthy weight.
Physical activity and diet also play a role in cancer prevention, he
said, primarily through their effect on weight. In addition, the
American Cancer Society recommends no more than one alcoholic drink per
day for women or two per day for men.
Now, the exceptions
Much remains unknown about the overall role obesity plays in cancer
risk. For some cancers, of course, it appears to play little or no role.
And for a few types of cancer, the incidence is actually lower in
Contrary to its relationship with postmenopausal breast cancer, extra
weight seems to offer women some protection against premenopausal breast
cancer. A possible explanation is that obesity interferes with ovarian
function in younger women, causing reductions in estrogen. Only about
20% to 25% of breast cancer cases occur before menopause, however, so
weight control is still related to an overall reduction in breast cancer
Another exception to the rule is lung cancer, for which the risk
steadily increases as weight drops. The reason? "Smokers tend to be
leaner than nonsmokers," said Dr. Elizabeth Platz, an associate
professor at the Johns Hopkins Bloomberg School of Public Health.
Cigarette-induced slimness comes at a steep price, though: a
dramatically increased risk of lung cancer, among other diseases.
The incidence of prostate cancer also seems to be lower for overweight
men. In this case, the apparent decrease may occur because the extra
weight makes screening and diagnostic tests less sensitive, according to
Platz. If so, delays in diagnosis might explain why research has linked
obesity to an increased risk of aggressive prostate cancer.
Childhood cancers and those of the brain, nervous system and
musculoskeletal system appear to be unaffected or little affected by
weight, according to the World Cancer Research Fund and the American
Institute for Cancer Research policy report.
But don't forget
No matter what researchers ultimately reveal about the role of weight in
cancer, weight control remains an essential part of staying healthy.
"If body fatness were totally unrelated to cancer, the message would
still be the same, because of the importance of weight control for heart
disease, stroke, diabetes, joint pain and other conditions," said Dr.
Tim Byers, a professor of epidemiology at the Colorado School of Public
He also emphasized that staying trim is no guarantee of a cancer-free
life. "It's a risk factor, that's all, just like bad brakes and drunk
driving are risk factors for traffic accidents."