Jeremy Barnes, an associate professor of health management at Southeast Missouri State University, explains.

Regular exercise leads to numerous and varied physiological changes that are beneficial from a health standpoint. They include improved cardio-respiratory function and skeletal muscle function; higher levels of high-density lipoprotein cholesterol (the so-called "good" cholesterol); improved blood pressure, body composition, and bone density; decreased insulin need and improved glucose tolerance; enhanced performance of work, recreational and sport activities; and many positive psychological benefits. These changes, in turn, help lower death rates from illnesses such as cardiovascular disease (including heart attack and stroke); type-2 diabetes; and certain cancers, including colon, breast and lung; and lower disease rates for high blood pressure, obesity, osteoarthritis and osteoporosis. Because of the many benefits of physical activity and exercise, the federal government now encourages all adults to increase their physical activity levels so they accumulate 30 minutes, or more, of moderate instensity physical activity most days of the week.

Improved cardio-respiratory function means that the body is able to perform exercise much more efficiently. This results mainly from the body more effectively getting oxygen into the blood stream and transporting it to the working muscles, where it is needed for the metabolic processing of energy. In other words, the regular exerciser's body is much more proficient at loading, transporting and utilizing oxygen. He thus finds exercise such as climbing stairs far less strenuous than a person who does not exercise and is out of shape. Improvement in cardio-respiratory function does not result from changes in the lung's ability to expand, however. In general regular exercise does not substantially change measures of pulmonary function such as total lung capacity, the volume of air in the lungs after taking the largest breath possible (TLC), and forced vital capacity, the amount of air able to be blown out after taking the largest breath possible (FVC). Studies comparing TLC and FVC show little difference between regular exercisers and nonexercisers, in fact. So even though people often report feeling out of breath or winded during exercise, it is unlikely that pulmonary function limits their ability to exercise, unless they have a disease that specifically impairs lung function such as asthma, bronchitis or emphysema.

One of the largest differences between an exerciser and a nonexerciser concerns the heart's ability to pump blood and consequently deliver oxygen to working muscles. Cardiac output is a major limiting factor for prolonged exercise. In addition, an exerciser typically has a larger blood volume, is better able to extract oxygen from the air in the lungs and is better able to extract oxygen from the blood at the working muscles than a sedentary individual is. Gas exchange involves not only oxygen delivery but also the removal of carbon dioxide, which is a byproduct of energy metabolism, and this process is also more efficient in an exerciser.

When all is said and done, regular exercise produces numerous favorable changes that collectively result in the body being able to work in a far more efficient manner. All of us are born with the ability to increase our physical fitness levels through regular exercise so it is unfortunate that many peoples' sedentary lifestyles and lack of exercise result in unfavorable outcomes in terms of disease. Perhaps one of the greatest challenges we face in developed societies is how to facilitate and encourage healthier lifestyles that include regular physical activity. Left unchecked, this problem will contribute to ever-increasing health care costs and higher disease rates. Regular exercise is not the magic bullet in terms of disease prevention, but, when combined with a healthy diet, it may be the best intervention currently available to any one who is willing to get up off the couch.