WORK IT OUT: We've all heard it's good for us, but what exactly does exercise do for the body?
iStockPhoto/Eric Gevaert

The Department of Health and Human Services (HHS) late this year released its new Physical Activity Guidelines for Americans, calling for adults between the ages of 18 and 64 to exercise moderately (such as brisk walking or water aerobics) for at least two hours and 30 minutes or vigorously (running, swimming, or  cycling 10 mph or faster) for at least an hour and 15 minutes weekly.

The longer, harder and more often you exercise, the greater the health benefits, including reducing the risk of diseases such as cancer and diabetes, according to the recommendations, which were based on a decade of scientific research.

Studies have shown that people who engage in the amount of exercise recommended by the feds live an average of three to seven years longer than couch potatoes, according to William Haskell, a medical professor at Stanford University who chaired the HHS advisory committee. But how exactly does exercise accomplish this? And what about claims by  naysayers that exercise not only isn't healthy but may actually be bad for you? Is there any truth to them?

Good for the heart and blood vessels
In the past decade or so, various studies involving  thousands of participants have shown that workouts lower the risk of heart disease. "Exercise has a favorable effect on virtually all risk factors of cardiovascular disease," says Jonathan Meyers, a health research scientist at the Palo Alto Veterans Affairs Health System in California. The reason, he says: when a person exercises, the heart muscle contracts forcefully and frequently, increasing blood flow through the arteries. This leads to subtle changes in the autonomic nervous system, which controls the contraction and relaxation of these vessels. This fine-tuning leads to a lower resting heart rate (fewer beats to pump blood through the body), lower blood pressure and a more variable heart rate, all factors that lower the risk of developing cardiovascular disease, he says. 

Meyers says that exercise also limits inflammation associated with heart trouble, such as arteriosclerosis or hardening of the arteries around the heart, which may lead to heart attacks. Many recent studies have focused on C-reactive protein, a marker of inflammation. Meyers says that research showed that sedentary folks who embarked on three- to six-month exercise programs, on average, experienced a 30 percent dip in their C-reactive protein levels – about the same drop as someone given a statin (a cholesterol and inflammation-lowering drug). In other words, in many people, exercise might be as effective as an Rx in tamping down inflammation, one of the key risk factors for cardiovascular disease.

Exercise also boosts cardiovascular health by decreasing the amount of plasma triglycerides—fatty molecules in the blood that are associated with plaque build-up in the arteries— notes Haskell. What's more, he adds, physical activity helps reduce the particle size of low-density lipoprotein (LDL) or so-called bad cholesterol in the blood, and increase amounts of high-density lipoprotein (HDL), aka good cholesterol, which translates to less artery clogging.