The most common side effect of losing more than 2 pounds a week is weight regain, especially in the absence of exercise. This is the primary reason that low-calorie dieting is not effective in the long-term if you’re trying to control your body weight.
Your body quickly adapts to a lower food intake by reducing its metabolic rate (the rate at which food is burned for energy) in order to preserve calories. In addition, since muscle tissue is made up of proteins, your body will begin stripping this tissue down for additional calories to replace the ones that you are restricting, even if you’re on a high-protein, low-calorie diet.
These bodily adaptations prevent the loss of body fat, which is the ultimate goal of weight loss. This is a primal, protective mechanism carried out by the body in an effort to prevent starvation and is very similar to the built in response of hibernating animals (also called the "starvation response").
Here’s an example. If you lose just 10 pounds in a short period of time (less than a month) with low-calorie dieting your body will adapt causing a reduction in your metabolic rate. Eventually, when you increase your food intake, even though you’re still eating less than you were before dieting, your body will treat the increase in calories as excess and store it as fat. It’s all part of the protective mechanism.
But it doesn’t stop there. When you resume your normal eating habits your body will begin to give you every signal possible to further increase your food intake (hunger pangs, headaches, enhanced sense of smell) so that it can continue to hoard calories in order to counteract future starvation. In this state, all food looks tasty whether it’s fast food, cookies, chips, or candy—Whatever it takes to get more calories to store as fat.
With all of this said if you want to lose body fat, do not attempt to lose more than 2 pounds each week. Losses greater than this will trigger the starvation response leading to decreases in your metabolic rate and losses of muscle tissue. A pound of body fat has approximately 3,500 calories, so a daily dietary restriction of 250-500 calories will total about 1,750-3,500 calories a week, which equates to 1/2-1 pound a week.
When you combine this with regular exercise in the form of cardio exercise (to burn calories) and resistance training (to preserve muscle tissue), you can easily lose 1-2 pounds of fat each week and keep it off, indefinitely. Now, if you’re extremely overweight, you may initially experience rapid weight loss for up to four weeks (usually around 5 pounds per week); however, your weight loss should start to taper off after this time.
Disclaimer: The information provided is for educational and informational purposes only and is not intended to diagnose, treat, cure or prevent any disease. Any reader who is concerned about his or her health should contact a physician for advice.