Cancer, the nation-wide’s next leading
killer, is not only one illness. Exercise has clear benefit against some major
malignancies and possible advantage against others, but it offers no defense
against many cancers. About 1,373,000 Americans will be diagnosed with cancer
every year, and some 570,000 will die from the disease. Physically active
individuals have a lower overall risk of cancer than their sedentary peers. In
part, this may be explained by the fact that active people tend to have healthy
lifestyles; eating well, avoiding tobacco and excess alcohol, and getting
regular preventive checkups all help fight cancer. But there are also
biological explanations for the benefits of exercise.
Most important, exercise reduces body fat,
and obesity is a major contributor to many malignancies. Exercise also appears
to reduce the body's production of various growth factors, proteins that may
promote the multiplication of malignant cells. Physical activity also lowers
levels of insulin, another potential growth promoter. Other hormonal effects
help explain how exercise reduces the risk of breast cancer and how it may
protect against certain reproductive tract malignancies. Other possibilities
include potential antioxidant and immune-enhancing properties of exercise, but
these remain speculative.
The evidence is best for colon cancer. More
than fifty studies from around the world show that physically active people are
less likely to develop colon cancer than inactive individuals. The protection
is substantial, amounting to a 30 to 40 percent reduction in risk. And as in
the case of heart disease, moderate exercise will confer excellent benefits.
Breast cancer is the other big target of
physical activity. More than sixty studies suggest that women who exercise
regularly can expect a 20 to 30 percent reduction in the chance of getting
breast cancer. Considering that about 213,000 Americans will be diagnosed with
breast cancer this year, that's a big benefit—and it applies about equally to pre-
and postmenopausal women. And a 2005 Harvard study of 2,987 women with breast
cancer linked moderate exercise, such as walking for twenty-five to forty-five
minutes a day, to a substantial improvement in survival.
Exercise may help prevent breast cancer by
reducing estrogen levels and/or decreasing tissue responsiveness to the female
hormone. Moderate exercise will confer most of the benefit, but intensive
exercise appears to add some additional protection. Like the breast, the female
reproductive organs are estrogen-responsive, and several studies suggest that
women who exercise may have a reduced risk of cancer of the uterus, but not the
ovaries.
The prostate is also a hormone-responsive
organ, but in this case, the driving force is testosterone, the major male sex
hormone. Prostate cancer displays much more clinical variability than most
other malignancies, and investigations of exercise and prostate cancer have
produced widely varied results. Some suggest protection from modest exercise
(such as walking), others indicate benefit only from very intense exercise,
while still others cannot detect any link. More study is needed.
Incomplete evidence is also the watchword
for lung cancer and pancreatic cancer, with several studies hinting that
exercise can help. Lung cancer is common, pancreatic cancer uncommon—but since
both are deadly, even a little protection would be welcome indeed. And although
exercise may not prevent or treat other malignancies, cancer specialists have
learned that appropriate levels of physical activity can help rehabilitate
cancer patients, improving energy, self-confidence, and overall well-being.