Your body burns fat when you eat fewer calories than you burn.
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Body-fat reduction only happens one way: by eating fewer calories than you expend. Therefore, reducing calories in your diet and increasing physical activity is the most effective way to lose weight. Consuming too few calories can backfire, however, slowing your metabolism to make fat loss harder. That's why finding the right caloric balance is important for weight-loss success.
The Calorie Connection
A calorie is simply a unit of energy. Foods containing fat, protein or carbohydrates contain calories, which you use as fuel for physical activities. Your body stores unused calories in fat cells for energy when you need it, which is why you gain weight when you overeat. Conversely, your body turns to those fat stores for energy when you don't consume enough calories to fuel your activities. That's when your body breaks down, or "burns," fat.
By the Pound
A pound of fat contains about 3,500 calories. Therefore, most people will lose about 1 pound of body fat for every 3,500-calorie deficit they create. When you eat 500 fewer calories than you need for energy each day, you'll lose about 1 pound per week. To customize this formula, multiply your weight in pounds by 15 to estimate your daily energy needs. This assumes that you are moderately active, which you should be if you're trying to lose weight. For example, a moderately active 145-pound person burns about 2,175 calories per day and will burn 1 pound of fat per week eating 1,675 calories per day.
While it may seem logical that cutting more calories will lead to greater fat loss, this isn't always the case. When you create too large a calorie deficit, your body tends to burn more muscle than fat. Calorie deprivation also slows your metabolism -- the rate at which you burn calories -- because your body tries to conserve energy when it feels starved. To avoid such nutritional deprivation, Harvard Health Publications recommends that women eat at least 1,200 calories a day and men eat at least 1,500 calories per day.
For your health, don't let calorie reduction turn into nutrient reduction. Eat plenty of fruits and vegetables, as well as lean proteins such as beans, egg whites and broiled salmon. Choose fiber-rich whole grains, such as whole-wheat bread, popcorn and oatmeal. These options are packed with vitamins and minerals and provide enough food volume to help you feel satisfied even as you eat fewer calories.