One of the things I try to get across to students in my intro psych class during our unit on motivation and hunger is why people have so much trouble losing weight and then keeping it off for good. Millions of people struggle with this problem but wind up choosing a strategy that doesn't work. A typical diet involves restricting calories, which at first blush seems like the right thing to do. Pounds disappear one after another. Some may even reach their target weight but then put almost all of the weight back on despite following the same diet.
Why does this happen?
The answer lies in how our bodies are designed. Humans evolved in an environment that was very different from the one we now enjoy. The easy access Americans have to fatty, salty, and sweet calorie rich food is completely novel. Our ancestors probably had to travel long distances to acquire the nutrients they needed to survive, let alone thrive. What was available may not have been particularly nutritious or filling. Tubers, nuts, and fruit were staple items - meat a rare delicacy. Famines may have been a regular problem. Those who could slow down their metabolism in response to famine could conserve the fat and muscle they had stored and wait out the "lean times." Our bodies aren't prepared to deal with regular indulgences that we have access to now.
When a dieter consistently restricts calories over a period of time, the person loses weight but also winds up signaling the brain (the hypothalamus) that s/he is in the midst of famine conditions. The brain responds adaptively by slowing metabolism down. If the person eats the same amount, s/he will start gain weight again as the body "saves up." Restricting calories even more will exacerbate the problem and create new ones (malnutrition).
In order to keep the unwanted weight off, the body's "set point" has to be changed. The set point is the genetically determined weight the person tends to have when s/he isn't dieting, overeating or engaging in an exercise program. That set weight point depends on the person's metabolism and this is what has to change in order for the person to permanently keep lost weight off. Metabolism depends on how responsive the hypothalamus is to the various signals that it receives from the body and the concentration of 'hunger hormones' such as leptin, neuropeptide Y, ghrelin, and orexin circulating in the bloodstream.
You can think of the hypothalamus as a thermostat for hunger regulation. Reduced food intake = slow down metabolism. Increased food intake = rev up metabolism. Because the human body evolved in an environment that saw recurrent famines, it is far easier for us to slow our metabolism down than to speed it up. When a dieter loses weight but then starts to gain it all back, that's because the body is doing exactly what it's supposed to do.
So, the real question is how to speed up one's metabolism and permanently reset the body's set point.
One way to try to do this is to get more exercise. Unfortunately, doing this while also cutting calories, releases a whole new wave of metabolic changes that are designed to put the weight right back on! For example, research from the NIH suggests that when a person loses weight and exercises more, thereby creating an energy deficit, ghrelin (an appetite stimulant) kicks into high gear.
That raises the question of whether it's even possible to permanently change a person's set point.