Diet and exercise have been components of weight loss programs for decades. Debates exist, however, over which is more important for weight loss and health outcomes. Most experts believe both are important for optimal health and weight management, however, studies of dieting for weight loss generally yield different results than studies of the effects of exercise for weight loss.
To achieve effective weight loss of about 1 Ib. to 2 Ib. per week, a calorie deficit of 500 to 1,000 calories per day is required. This deficit can be achieved through a reduction of calorie intake, increased physical activity or both. However, reducing your daily calorie intake to achieve a significant calorie deficit is generally more practical for many people than exercising at the duration and intensity required to burn the same number of calories.
The consensus on effects of dieting vs. exercising on weight loss is fairly consistent: dieting is most important for weight loss. According to the National Institutes of Health, increasing your physical activity is not significantly more effective for weight loss than dieting alone. A 2010 study published in the "JAMA" found that after 12 months, weight loss was similar between two groups of obese subjects. The first group dieted and exercised for the full 12 months; the other group dieted for 12 months but exercised only in the last 6 months. The American Dietetic Association notes that moderate physical activity by itself has minimal effects on weight loss in most cases, simply because it's difficult for most Americans to produce an energy deficit of 500 to 1,000 calories per day using exercise without a calorie reduction.
Although diet is generally most important for weight loss, exercise plays an important role in maintaining a healthy body weight after a weight loss. The National Institutes of Health reports that regular physical activity is most important to preventing weight regain, and the Academy of Nutrition and Dietetics goes further, stating that physical activity is crucial to prevent weight regain. However, maintaining a healthy diet plays an important role in weight maintenance as well.
Exercise and diet are both important to maintaining optimal health outcomes. The Academy of Nutrition and Dietetics notes that exercising can help improve outcomes related to heart disease, cancer and diabetes, and that regardless of body weight and BMI, physical activity is associated with a reduced risk of death. Improving your diet can also help reduce certain health conditions. For example, losing weight if you’re overweight, increasing your fiber intake, increasing dietary stanols and sterols, and limiting saturated and trans fats can help reduce blood cholesterol levels and heart disease risk, according to the U.S. Department of Health and Human Services.