It’s a much debated question, and there are many sides to consider, but perhaps the best answer comes from the ongoing data provided by the National Weight Control Registry (NWCR). As the largest ongoing investigational study of long-term successful weight loss maintenance, the NWCR is currently tracking the exercise and diet habits of over 10,000 individuals. So far, the average NWCR participant has lost an average of 66 pounds, and has kept it off for over five years.
Established in 1994 by Rena Wing, Ph.D from Brown Medical School, and James O. Hill, Ph.D. from the University of Colorado, the NWCR was developed to identify and investigate the characteristics of individuals who have succeeded at long-term weight control.
So, in answer to, “What's More Important - Exercise or Diet?” of the 10,000 NWCR participants:
• 1 percent succeed with exercise alone
• 10 percent succeed with diet alone
• 89 percent succeed with a combination of exercise and diet
As for the “Why?” part of the question, consider these three weight loss fundamentals.
• Losing weight requires a calorie deficit (using more calories than you consume). You can create this deficit by eating fewer calories, by making your body burn a greater number of calories daily, or through a combination of both methods.
• A healthy, portion-controlled diet means you take in fewer saturated and trans fats, added sugars and refined flours, which are conducive to weight gain. In addition, a healthy diet reduces your risk of developing chronic diseases and improves your energy. Losing just five to ten percent of your body weight can have positive health implications. However, portion control is also essential because you could easily overeat even healthy foods - thereby creating a calorie surplus and gaining weight.
• Exercise helps you burn calories and retain lean muscle mass. With physical activity, you burn more energy and can create a bigger calorie deficit. Adding lean muscle mass also helps burn more calories because it is a more metabolically active tissue than fat. (A person with a greater percentage of muscle mass burns more calories at rest.) In addition, when you lose weight, especially quickly, you lose lean muscle tissue along with fat. Then, should you gain weight back, you tend to gain fat. If you consistently lose and regain weight, you end up with a higher percentage of fat than when you started. Exercise can help counteract this problem.
To summarize: On the one hand, you can easily out-eat your best exercise efforts. On the other hand, without dietary control, weight loss will not happen. On top of that, if you do not exercise, you can only cut calories so much, for only so long, before you start to starve your body of essential nutrients and risk damage.
All of which - thanks to the NWCR - brings us to the realization that, given only one option, diet is more important; however, overwhelmingly, the best possible answer is that diet and exercise together are best.