Effects of Repetitive Low Calorie Dieting on Metabolic Rate
If we lose weight, we always lose a little muscle tissue. And if we follow very-low-calorie diets, this loss of muscle increases. In either case, our metabolic rate tends to fall, making it easier to gain weight in future because we now need fewer calories.
Now when we stop dieting and revert to a higher calorie-intake, it's common to regain weight. This weight gain is exclusively fat.
Thus as you can see, while dieting typically leads to a loss of fat AND muscle, weight regain is
just fat. Therefore, if we engage in yo-yo dieting (weight cycling), gradually our body fat percentage gets bigger, our muscle percentage gets smaller, and our metabolic rate gets lower and lower. This means our calorie-needs get smaller and smaller. So each time we quit dieting and return to eating, we are likely to gain weight.
Source : Diet-I