Exercise and malignancy

Exercise puts nearly every part of the human body to work, but can it reduce your risk of cancer?
Exercise and malignancy
By Harvard Medical International
Fri 03 Aug 2007 12:00 AM

Exercise puts nearly every part of the human body to work, and nearly every part of the body responds to regular exercise by getting stronger and healthier. Exercise stokes up the metabolism, the tissues become more sensitive to insulin, so blood sugar levels fall and less insulin is needed to keep sugar levels in balance.

Regular exercise boosts the HDL ("good") cholesterol and lowers the LDL ("bad") cholesterol. By burning calories, exercise reduces body fat - and it's particularly good at reducing abdominal obesity, which is especially hazardous to health.

Exercise has a clear benefit against some major malignancies and a possible benefit against others.

Exercise improves the circulation. The heart muscle gets stronger, so it can pump more blood with each beat. Regular exercise helps the heart resist abnormal rhythms, including the serious arrhythmia that can trigger sudden death. Arteries also benefit; their ability to dilate (widen) is enhanced, and they resist the stiffening that accompanies aging in sedentary people. Muscles and bones also gain from physical activity too. Endurance activities such as walking and biking improve muscular stamina and enhance the muscles' ability to extract oxygen from the blood. Resistance exercises, such as weight lifting, build muscle mass and strength. Resistance and weight-bearing exercises increase bone calcium, reducing the risk of osteoporosis ("thin bones"). Exercise improves the nervous system. Nerve impulses travel faster and reflexes improve. Some aspects of mental function become sharper. Regular exercise can improve the personality, reducing anxiety and fighting depression.

People who exercise regularly look and feel better and can do much more physical work than their sedentary peers. And they are also healthier, enjoying a reduced risk of heart attack, stroke, high blood pressure, cognitive decline in old age, diabetes, obesity, depression, and fractures. Physically active people also have a lower incidence of musculoskeletal disability than folks who are sedentary.

People who exercise live longer than those who don't. Most of the gain comes from a substantial protection against our nation's first and third leading causes of death, heart disease and stroke. But how about America's second leading killer? Can exercise reduce your risk of cancer even though malignancies have little in common with cardiovascular diseases? The answer is a qualified yes, qualified because cancer is not one disease but many. Exercise has a clear benefit against some major malignancies and a possible benefit against others, but it offers no protection against many cancers.

Preventive mechanisms

About 1.4 million Americans will be diagnosed with cancer this year, and some 560,000 will die from the disease. Physically active individuals have a lower overall risk of cancer than their sedentary peers. In part, it may be that active people tend to have healthy lifestyles; eating well, avoiding tobacco and excess alcohol, and getting regular preventive check-ups all help fight cancer. But there are also biological explanations for the benefits of exercise. Most important, exercise reduces body fat, and obesity is an important 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 lowers insulin levels, 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. Additional possibilities include the potential antioxidant and immune-enhancing properties of exercise, but these remain speculative.

Coloncancer

The evidence is best for colon cancer. Taking the lives of about 28,000 American men and an equal number of women, it ranks second to lung cancer among the leading cancer killers. Early detection and treatment can prevent many of these deaths, but regular exercise can reduce your risk of getting colon cancer in the first place. More than 50 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%-40% reduction in risk. As in the case of heart disease, moderate exercise will confer excellent benefits. For example, just 30 or 40 minutes of brisk walking nearly every day will help you walk away from colon cancer.

Why is exercise so effective against colon cancer? In addition to the general mechanisms of protection already discussed, there are specific local effects that appear to help. Most important, exercise speeds waste material through the colon. As a result, potentially cancer-causing chemicals in feces spend less time in contact with the vulnerable cells that line the colon. Exercise may also reduce the influence of irritants, such as bile acids and certain prostaglandins. But although exercise can reduce the risk of colon cancer, it has no effect on rectal cancer, probably because the rectum is normally empty except during bowel movements.

Women's world

Breast cancer is the other big target of physical activity. Although the evidence is not quite as secure as it is for colon cancer, more than 60 studies have investigated the question. Together, they suggest strongly that women who exercise regularly can expect a 20%-30% reduction in the chance of getting breast cancer. Considering that more than 215,000 Americans will be diagnosed with breast cancer this year, that's a big benefit - and it applies about equally to pre- and postmenopausal women. Exercise will never replace mammograms, but it can reduce the number of positive results.

Exercise may also help prevent breast cancer by reducing estrogen levels, decreasing tissue responsiveness to this female hormone, or both. Moderate exercise will confer most of the benefit, but intensive exercise appears to add some additional protection.

Like the breast, the uterus and ovaries respond to estrogen. Several studies suggest that women who exercise have a reduced risk of cancer of the endometrium (the lining of the uterus) but the evidence is incomplete. Unfortunately, exercise does not appear to reduce the risk of ovarian cancer.
Prostate cancer

The prostate is also responsive to hormones, but in this case the driving force is testosterone, the major male sex hormone. Prostate cancer displays much more clinical variability than breast cancer and most other malignancies, and investigations of exercise and prostate cancer have produced widely variable results.

The same lifestyle that will reduce your chances of getting cancer will also lower your risk of heart disease, hypertension, stroke, diabetes, and obesity.

One of the most encouraging views comes from the Alpha-Tocopherol Beta Carotene (ATBC) Cancer Prevention Trial. Working together, researchers from Finland and the U.S. National Cancer Institute conducted a study of 29,133 male Finnish smokers, which has produced many important insights. For example, the study found that beta carotene supplements appear to increase the risk of lung cancer in male smokers, which came as a great surprise. It also found that supplements of vitamin E substantially reduce the risk of prostate cancer in these smokers. In nonsmokers, though, other trials have reported inconsistent benefits from vitamin E and no harm from beta carotene. In a follow-up analysis, ATBC researchers found that exercise also seems to lower the risk of prostate cancer. Each man in the study provided detailed data about his physical activity, including both recreational exercise and physical work on the job. The scientists also collected information about age, smoking, pre-existing benign prostatic hyperplasia, and place of residence. During up to nine years of observation, 319 of the men were diagnosed with prostate cancer. When the results were tallied, exercise appeared protective, even after vitamin use and other factors were taken into account. But not all forms of exercise were effective; heavy lifting was not helpful, but walking was.

It's a hopeful finding, but it does not settle the issue. Other studies of exercise and prostate cancer have yielded mixed results. For example, a 1994 study of 53,000 Norwegian men also reported that occupational walking, but not heavy lifting, was associated with a reduced risk of prostate cancer.

A 1996 study from the Cooper Clinic, which evaluated 12,975 men between the ages of 20 and 80, found that men whose weekly exercise exceeded the equivalent of 10 miles of walking enjoyed a reduced risk of prostate cancer. In that research, the most physically active men had 27% less prostate cancer than the men who were least fit. But a 1992 study of 17,719 Harvard alumni set the bar higher, finding a reduced risk of prostate cancer only in men who exercise intensely enough to burn 4,000 calories per week. And a 1998 study of 47,542 health professionals was even less optimistic, finding only a suggestion that high levels of exercise may reduce the risk of metastatic prostate cancer.

Many men would try to live up to the Harvard Alumni Study's standard of walking or jogging 40 miles a week if they could cut their risk of prostate cancer in half - but other studies are less encouraging, finding no clear benefit. That's why the U.S. surgeon general concluded that more research would be needed to clarify the relationship between exercise and the risk of prostate cancer. The ATBC Trial is a step in that direction, but still more steps are in order.

The big picture

Cancer is a complex problem. Despite marvelous advances in basic research, diagnosis, and treatment, much more progress is needed. The same is true for cancer prevention. As for so many diseases, exercise can play a role in prevention. The evidence is best for colon cancer, with breast cancer a close second. The evidence is mixed for female reproductive malignancies and prostate cancer. The same is true for a much less common male reproductive malignancy, testicular cancer.

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 directly help 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.

You can't run away from cancer, nor can you hide. But you can reduce your risk. Exercise will help with certain malignancies, nutrition with many others. Avoid excess sunlight and radiation. Abstain from all forms of tobacco. Stay lean. Be alert for symptoms and get the check-ups that can help detect early disease. It sounds like a lot, but it's really very simple. And the same lifestyle that will reduce your chances of getting cancer will also lower your risk of heart disease, hypertension, stroke, diabetes, and obesity. For all their differences, these serious and distressingly prevalent problems have one thing in common: Prevention works.

Exercise and benign prostatic hyperplasia (BPH)Many men with BPH would like to do less walking or, at least, less walking to the bathroom at night. Can daytime exercise reduce your odds of going on the night patrol? Perhaps, according to a report from researchers at Harvard. The subjects were 30,634 men enrolled in the Health Professionals Follow-up Study in 1986. All the volunteers provided detailed information about their diet, height, weight, alcohol and tobacco use, and medical history as well as their exercise habits. During eight years of observation, 1,890 of the men underwent surgery for BPH and another 1,853 developed moderate to severe BPH symptoms that were treated nonsurgically. When the results were analyzed, physical activity appeared to protect against BPH. When compared to the least active men, the most active were about 25% less likely to develop surgically treated or symptomatically bothersome BPH. Even a little exercise was helpful, but more was better; men who walked two to three hours a week achieved optimal benefit. Conversely, the men who spent the most time parked in front of a TV set had the highest risk of BPH. All in all, the research suggests that men don't have to take BPH lying down.

How does exercise help? In this study, at least, neither diet nor obesity provided the explanation. Hormones might play a role, but it takes much more exercise to lower testosterone levels than it does to reduce BPH. The Harvard scientists suggested a more plausible explanation: Exercise reduces the activity of the sympathetic nervous system, possibly relaxing these muscles in the prostate and bladder neck. Whatever the mechanism, it's yet another way that exercise keeps men healthy.

This article is provided courtesy of Harvard Medical International. © 2006 President and Fellows of Harvard College.

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