Yo-yo dieting refers to repeatedly losing weight and then gaining it back. This is often associated with a very low calorie diet. There is usually an initial success, but the weight loss can't be maintained due to the diet's severe nature.
Yo-yo diets lead to the loss of body fat, but the body also loses lean muscle. With less lean muscle, your metabolic rate is lowered. When you resume your normal eating habits, you add back more body fat and the cycle continues. With added body fat, your overall health deteriorates. This can lead to chronic diseases, such as type 2 diabetes and coronary artery disease.
Also, this type of dieting can take a negative emotional and motivational toll on the dieter. You may feel defeated and unwilling to try to maintain a healthy weight. Remember, the goal of a healthy weight is a good one. But instead of trying fad diets, talk to your doctor or a dietician about how to lose weight the right way. You'll have a better shot at keeping it off.