How fast you should lose

The most effective method for weight loss is reducing the number of calories you consume while increasing the number of calories you burn through physical activity. To lose 1 pound, you need an expenditure of approximately 3,500 calories. You can achieve this either by cutting back on your food intake, by increasing physical activity, or ideally, by doing both.
For example, if you consume 500 extra calories per day for one week without changing your activity level, you will gain 1 pound in weight seven days multiplied by 500 calories equals 3,500 calories, or the number of calories resulting in a 1-pound weight gain. Likewise, if you eat 500 fewer calories each day for a week or burn 500 calories per day through exercise for one week, you will lose 1 pound. Starvation diets may result in rapid weight loss, but this weight loss is almost impossible to maintain for most people. When food intake is severely restricted (below approximately 1,200 calories per day), the body begins to adapt to this state of poor nutrition by reducing its metabolic rate, potentially making it even more difficult to lose weight. It is also possible to experience hunger pangs, bouts of hypoglycemia, headaches, and mood changes from overly stringent dieting. These symptoms can result in binge eating and weight gain. Since a highly restrictive diet is almost impossible to maintain for a long time, people who attempt to starve themselves thin often start to gain weight again when they stop dieting.