Three of the most common changes people wish to make to improve their health are losing weight, becoming more active and quitting smoking. Considering that two-thirds of American adults are overweight and only about 50 percent meet minimum recommendations for physical activity, many people may want to change more than one of these factors. About 20 percent of U.S. adults smoke, so there are some who need to change all three. Since quitting smoking, losing weight and regular exercise are among the most difficult behaviors to change, especially at the same time, some people focus on one change to begin with. Obviously, changing all three of these behaviors is ideal, but if you are only willing to change one, which should you take on first to have the biggest impact on your overall health?

You might think that quitting smoking would be the most important change to make initially. Smoking is the primary cause of lung cancer and other respiratory diseases such as emphysema and chronic bronchitis. Smoking also increases the risk of most other cancers and is a major contributor to heart attacks and strokes. Quitting smoking greatly reduces these risks with beneficial changes that begin within days of quitting. Despite this, if you only want to change one behavior, smoking isn’t the place to start.

Being overweight is a leading cause of diabetes, high blood pressure, heart disease and some cancers. In fact, the negative health effects of obesity are now thought to contribute to more deaths than smoking. If you are overweight, losing just 10 percent of your body weight (20 pounds for a 200-pound person) can significantly reduce the severity of these conditions. Maintaining a healthy body weight can prevent many of these health problems. However, losing weight is not the first change you should make.

It turns out that becoming physically active is the most important change you can make to improve your overall health. Decades of research shows that regular physical activity reduces the risk of most chronic diseases including diabetes, heart disease and some cancers and can extend the lifespan by up to five years. In fact, the health risks of inactivity are equal to or greater than that of obesity or smoking. Regular activity also improves muscular strength, aerobic fitness, bone density, cognitive function and memory. There is no other single intervention – drugs included – that has that many health benefits.

Research also shows that the negative health effects of being overweight and obesity are, in part, caused by inactivity and poor fitness. Your risk of death is lower if you are overweight but physically fit than if you are at a “healthy” weight but unfit. Regular exercise can reduce the risk of diabetes in people who are overweight, whether they lose weight or not. Furthermore, studies of “successful losers” shows that daily exercise is a requirement for long-term weight loss, so becoming active now can help you lose weight later.

Obviously, you should change all three of these behaviors for optimal health. But if you are looking for one change to make that will have the biggest impact, start by becoming more active. A good initial goal is to reduce the time you spend being sedentary (sitting) and to get a minimum of 30 minutes of moderate-intensity activity such as brisk walking each day. You can get greater benefits by participating in more intense exercise, including strength training, three or more days per week. And once you have established a routine of regular activity, you will be ready to make other health changes.

Brian B. Parr, Ph.D. ACSM Certified Clinical Exercise Specialist and an associate professor in Department of Exercise and Sports Science at USC Aiken