To lose a pound a person needs to eat 3,000 calories less than the body needs. The average woman requires approximately 2,000 calories a day, while the average man needs about 2,600. If only 500 calories are consumed in a day, a woman would lose a pound in two days, a man would lose the same amount in a day and a half.
A diet that restricts calories to only 500 a day isn't considered safe. The body goes into starvation mode, which results in a loss of some body functions to preserve the brain and heart. In starvation mode, the body's metabolism slows to conserve energy and lean muscle is burned along with fat. With the loss of lean muscle, metabolism slows further.
Weight lost during a diet restricted to 500 calories a day is quickly regained when the dieter returns to regular eating habits. To lose weight permanently, a person should burn 500 calories more a day than they take in. A woman who needs 2,000 calories a day would eat 1,500 calories. This results in weight loss of about 1 pound a week. Calories should come from fruits, vegetables, lean meat, whole grains and fat-free dairy products.