When considering weight loss, it is always best to plan ahead. It takes a significant amount of time to lose weight, especially if you do it in a way that does not harm your body. Safe weight loss never exceeds 2 lbs. per week -- any diet that causes weight loss greater than that amount should be supervised by a doctor.
To figure out how many pounds you can lose in a three-day period, you need to know how many calories you burn each day. A moderately active man can multiply his weight by 15, and a relatively inactive man can multiply his weight by 13 to get this number. If you are female, multiply your weight by 12 if you are moderately active and by 10 if you are relatively inactive. Once you know how many calories you need each day to maintain your weight, you will know how many calories you can safely cut. To diet healthily, don't go below 1,200 calories. For each deficit of 3,500 calories you create, you will lose 1 lb.
If you are trying to lose a large amount of weight in three days, you might be tempted to fast or go on a very low calorie diet. If you do so, you may lose 2 or 3 lbs. For example, a man who requires 3,000 calories a day to maintain his weight who goes on a fast will lose almost 3 lbs. Unfortunately, much of this weight loss will come from muscle. Whenever your body senses that it is entering a period of starvation, it will cannibalize its own muscle. This can set you up to regain weight later, as muscle burns more calories than fat. You will also have a flabbier appearance.
It is possible you may lose 2 or 3 lbs. of water weight in addition to fat during your three-day diet. If you strictly limit calories, you will almost always limit the amount of sodium in your diet as well. A low-sodium diet can cause your body to release excess water. If you fast or eat such a low-calorie diet that your body burns muscle for energy, your muscles will also release water. Keep in mind that any water weight you lose will return as soon as you begin eating a normal diet.
Diet and Exercise
The way to lose weight in a safe and healthy manner is to eat a healthy diet and get a reasonable amount of exercise. A person who normally requires 2,100 calories a day to maintain her weight and who burns 300 calories a day through additional exercise on a 1,200-calorie diet can expect to lose 1 lb. of fat at the end of a three-day period. This weight loss is much more likely to be permanent than weight loss achieved through a more extreme diet.