Many people follow a special diet in an effort to lose weight, treat a medical condition, feel healthier or simply to follow a trend. In fact, a study from earlier this year shows that 20 percent of Americans reported that they were dieting to lose weight.

No doubt many more have changed what they eat for other health-related reasons. This is smart, since nutrition plays an important role in body weight and other health indicators such as blood glucose and cholesterol. But people who only change their diet are missing important benefits of another health behavior – regular physical activity.

The health benefits of dietary changes are well-established through both research and practical experience. For example, research shows that reducing calories leads to weight loss. In fact, it is possible to predict how much weight a person will lose if you know how many fewer calories they are eating.

Similarly, studies have shown that moderating carbohydrate intake leads to better blood glucose control, something that is of particular interest to diabetics.

The personal experience of people following these types of diets also shows the same effects. In some ways, individual “research” is even more compelling.

Consider a person who is dieting to lose weight. For several weeks they follow the diet faithfully and lose weight. Then they go on a vacation for a week and, despite their best efforts, eat more than they should and end up gaining a few pounds. They go back on the diet when they return, and the weight loss resumes.

Clearly, changing what a person eats can have a big impact on their body weight, blood glucose and most other health factors. Research also shows that exercise is important for the same conditions, both independently and when combined with diet.

As an example, both diet and exercise alone lead to weight loss, but diet combined with exercise leads to even greater results.

Another benefit is that exercise can act as a “buffer” to offset an imperfect diet. This is one reason why exercise is seen as essential for long-term weight management.

In some cases, exercise is necessary for the nutrition intervention to be effective. Calcium is important for optimal bone density and increasing calcium intake through food and supplements is used as a treatment for osteoporosis. It turns out that eating more calcium doesn’t necessarily lead to stronger bones.

Exercise, especially weight-bearing activities and lifting weights, stimulates bone growth. Combining exercise with increased calcium in the diet maximizes bone formation and improves bone density better than either treatment alone.

For some conditions, exercise can be a more effective treatment than diet. A diet low in sodium is commonly recommended to treat high blood pressure. But the effect of reducing sodium intake is, on average, modest and varies greatly among people.

Exercise has a stronger and more consistent effect on lowering blood pressure. Cutting back on salt is a good recommendation for everyone, but adding exercise is really necessary to significantly lower blood pressure.

Another thing to keep in mind is that both diet and exercise are essential for treating most chronic health conditions, even when medications are used. Not only do diet and exercise add to the effectiveness of drugs but these lifestyle modifications are the key to eventually stopping the medication, which should be the goal.

Additionally, the expected effectiveness of drugs for weight loss, blood pressure, cholesterol, and diabetes may be based on studies in which the subjects also modified their diet and activity. Not following these changes could undermine the effectiveness of the drug.

Brian Parr, Ph.D., is an associate professor in the Department of Exercise and Sports Science at USC Aiken where he teaches courses in exercise physiology, nutrition and health behavior.