Exercise alone has a modest contribution to weight loss. But when you alter one component, cutting the number of calories you eat in a day to lose weight, doing more exercise than usual, this sets off a cascade of changes in the body that affect how many calories you use up and, in turn, your bodyweight.

Exercise accounts for a small portion of daily calorie burn.

Even when you work out, those extra calories burned only account for a tiny part of your total energy expenditure, only around 10 to 30 percent, depending on the person. It's not nearly equal to food intake.

• How much we move is connected to how much we eat. And exercise, of course, has a way of making us hungry — so hungry that we might consume more calories than we burned off.
• Some people simply slow down after a workout, using less energy on their non-gym activities, because they're tired.

People who have had success losing weight have a few things in common: 

• They weigh themselves at least once a week. 
• They restrict their calorie intake, stay away from high-fat foods, and watch their portion sizes. 
• They also exercise regularly. But note: They use physical activity in addition to calorie counting and other behavioral changes.