Three weeks does not represent a lot of time for you to lose weight, and it's definitely not enough time for you to develop new, healthy eating habits that can last a lifetime. But if you absolutely need to shed pounds in 21 days -- maybe for a once-in-a-lifetime special event -- you do have some strategies you can use to help you get slimmer in time. The amount of weight you ultimately lose will depend on which strategy you choose.
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Medically Supervised Diets
If you have a medical reason that explains your urgent weight-loss needs, you may be eligible to enroll in a medically supervised diet program, according to the University of Texas-Southwestern Medical Center. These programs, generally run by universities or major medical centers, generally use liquid diets that consist of about 800 calories each day. They also include counseling aimed at helping you change your lifestyle to address the reasons you're overweight in the first place. If your body normally burns about 2,000 to 2,500 calories each day, you can lose up to 10 lbs. in 21 days on this type of diet.
Low-carbohydrate diets work well initially to help you lose weight fast -- in fact, you tend to drop pounds more quickly on a low-carb eating program than you do on a low-fat type of diet, according to the University of Maryland Medical Center. In the first phase of the popular Atkins diet program, for example, participants can eat no more than 20 g of carbs each day -- and it's fairly common to see weight loss of 5 lbs. a week in people following the diet carefully. Therefore, if you follow this type of low-carb diet for 21 days, you potentially can lose up to 15 lbs. Be careful, though, since researchers haven't finished evaluating the long-term effects of very low-carb diets.
Diet and Exercise
Simple diet changes -- substituting carrot sticks for potato chips -- can have a surprisingly quick effect on your weight, as can instituting a new exercise program, according to Columbia University. Although you're unlikely to see Atkins-style weight-loss results of 15 lbs. in 21 days if you just start eating healthier and exercising, you will see some results -- and you'll be able to sustain those results more easily. If you carve 400 calories a day out of your normal diet and burn another 400 calories each day through your new fitness routine, you'll lose about 5 lbs. in three weeks.
You're actually better off to start a diet and fitness program slowly and carefully than to jump in with both feet, according to Columbia University. You risk fewer injuries that way and also reduce your chances of burning out on the program before you see real results. It's possible to lose up to 15 lbs. in 21 days, but you're more likely to keep the weight off if you only lose about 5 lbs. Regardless of which weight-loss method you choose, discuss your plans and your current state of health with your physician before beginning any program.