Exercise may be the single most effective thing you can do to stay healthy, but many people never hear their doctors mention it. Regular, moderate exercise has been shown to protect against conditions ranging from diabetes to depression, from cancer to cardiovascular disease. Yet about half of Americans don't do it. Studies suggest that people are more likely to exercise -- to at least try it -- if following doctors' orders. But health-care providers maintain no established tradition of discussing these benefits with their patients, have no framework for assessing patients' fitness or recommending regimens and specialists. Exercise Is Medicine is working to change that. The four-year-old initiative, organized by the Indianapolis-based American College of Sports Medicine (ACSM) along with the American Medical Association, aims to make exercise assessments and advice as standard a part of doctor's office visits as weigh-ins and blood-pressure checks.