One pound of body weight represents about 3,500 calories. For most people, it would be difficult, if not impossible, to cut out or burn that many calories every day. It would also likely be dangerous.
When you eat fewer calories than you burn over the course of a day, you create a calorie deficit. Your body makes up the difference by burning fat, and that's how you lose weight. To lose 1 lb. a day, you would need a deficit of 3,500 calories every single day.
Your body needs a certain amount of calories a day just to keep you alive, even if you do not exercise. For example, according to the U.S. Department of Agriculture, a 5-foot-6, 140-lb. woman needs about 2,300 calories a day. Even if she ate nothing, she couldn't lose 1 lb. a day. She would have to burn an extra 1,200 a day through exercise --- on an empty stomach.
According to the Weight-Control Information Network of the National Institutes of Health, a safe rate of weight loss is more like 0.5 lb. to 2 lb. a week. Experts interviewed by the "New York Times" say risks of rapid weight loss include a weakened heart and dangerous nutritional deficiencies.