The concern with fast weight loss is that it usually takes extraordinary efforts in diet and exercise — efforts that could be unhealthy and that you probably can't maintain as permanent lifestyle changes.
A weight loss of one to two pounds a week is the typical recommendation. Although that may seem like a slow pace for weight loss, it's more likely to help you maintain your weight loss for the long term.
Remember that one pound (0.45 kilogram) of fat contains 3,500 calories. So to lose one pound a week, you need to burn 500 more calories than you eat each day (500 calories x 7 days = 3,500 calories).
Also, if you lose a lot of weight very quickly, you may not lose as much fat as you would with a more modest rate of weight loss. Instead, you might lose water weight or even lean tissue, since it's hard to burn that many fat calories in a short period.
In some situations, however, faster weight loss can be safe if it's done the right way. For example, doctors might prescribe very low calorie diets for rapid weight loss if obesity is causing serious health problems. But an extreme diet such as this requires medical supervision. In addition, it can be difficult to keep this weight off.
Some diets include an initiation phase to help you jump-start your weight loss. For example, the Mayo Clinic Diet has a quick-start phase in which you might lose six to 10 pounds in the first two weeks. You can lose weight quickly with an approach like this because it combines many healthy and safe strategies at once — no gimmicks or extreme dieting.
After the initial two-week period, you transition into the recommended weight loss of one or two pounds a week, which gives you time to adopt the necessary lifestyle changes, such as eating a healthy diet and increasing your physical activity, necessary for maintaining weight loss over the long term.