Q: If one undereats to lose weight wont the weight come back once the person goes back to his habitual eating patterns? I think one should go on a diet only if its a diet that can be kept for a life time,otherwise he or she will gain back the lost weight.

A: Not really. Suppose a woman maintains her 140 pound weight by eating 2000 calories per day. She goes on a diet, preferably combined with a workout program and perhaps cardio and loses 10 pounds in half a year or so. Because she lost weight gradually and maintained her muscle mass and metabolism, when she goes back to her old 2000-calorie diet, then she will simply maintain her new 130 pound weight. If she managed to gain muscle while losing those 10 pounds (something certainly possible for a beginner) then she can even afford to eat slightly more than her old 2000 calories and still not gain weight. So a well-planned weight loss can certainly be maintained for life (or at least for many years -- old age will almost inevitably slow down metabolism and bring on extra weight) even once the person stops the diet and returns to his/her old eating habits.

The reason why most people who try to diet gain their weight back is because they go about dieting the wrong way. They typically go on overly restrictive diets (a sure way to kill your metabolism and lose muscle), often overdo the cardio (further destroying muscles), and rarely engage in weight-lifting (which could've helped them preserve their precious muscle mass and boost metabolism while losing weight). As the result, they lose a lot of muscle along with fat and their metabolism is in the dumps by the time they quit dieting. So when they return to their old eating habits, the same calories that used to maintain the old weight now cause them to gain weight (mostly in the form of fat, unless they weight-train).