But exercise may not have the same effect on every person's cardiovascular system, notes Arthur Leon, chief cardiologist at the University of Minnesota's Heart Disease Prevention Clinic in Minneapolis. "On average, there is a response but there is great variability, and that variability runs in families," he says. Take, for example, HDL cholesterol. Most broad studies show physical exercise leads to up to a 5 percent increase in HDL levels, but a closer examination shows that the percentages vary from zero to 25 percent, depending on the study subject, he says, noting that only about half of the population seem to experience HDL increases as a result of exercise.
Several studies (including the ongoing federal National Health and Nutrition Examination Survey) following thousands subjects for several years, show that regular exercise lowers the risk for certain cancers, particularly breast and colon cancer, says Demetrius Albanes, a researcher at the National Cancer Institute in Bethesda, Md. Scientists have yet to pinpoint the mechanisms involved but have come up with several plausible explanations.
"Physical activity beneficially affects body weight," says Albanes, noting that leaner people have lower circulating levels of insulin, a hormone produced by the pancreas that helps cells absorb glucose, their primary energy source. Obese and overweight people, are more likely to develop insulin resistance, a condition in which the cells no longer respond to the hormone and absorb glucose. When this happens, the pancreas produces greater amounts to compensate, flooding the bloodstream with insulin; high levels of insulin in the blood have been linked to [some types of] cancer. "Insulin is essentially a growth hormone," Albanes says. "Insulin could create new tumors by increasing rates of cell division, or it could just make small tumors grow."
Albanes says that exercise may also ward off cancer and other diseases because it appears to beef up the body's immune system. Exercise may also help reduce levels of the female hormones estrogen and progesterone in the blood, potentially also lowering the risk of developing breast and uterine cancers linked to high levels of those hormones.
Despite the apparent link between physical exercise and lower odds of cancer, Albanes acknowledges that there could be other factors at work. "[Because] most of these studies are not controlled trials, it could be some other lifestyle factor [that helps explain the lower cancer risk], " he says, noting that people who exercise may also eat healthier diets.
Builds strong bones
Robert Recker, an endocrinologist and current president of the National Osteoporosis Foundation in Washington, D.C., says research indicates that moderate exercise increases and maintains bone mass and reduces the risk of osteoporosis. "The most compelling evidence," he says, "is that if you don't do anything, your fracture risk is much greater."
Like muscles, bones become stronger when forced to bear more weight than normal. "The skeleton is a smart structural organ and knows how much load [force] is being put on it," Recker says. "Pick up a pail of water, and you're loading your arm, your shoulder, your spine, your legs and your hips." That means muscles are contracting, exerting forces on the bones supporting those body parts. This force stimulates the bone to maintain or even build new tissue. But scientists have yet to figure out why. "That's a focus," he says, "of incredibly aggressive research."