When you go into a calorie deficit, the body compensates in a few different ways. The stomach releases more of the hunger hormone ghrelin, while levels of the fullness hormone leptin drop.
At the same time, your metabolism slows, so you burn fewer calories — and the difference can be significant. That’s because you now have less fat and muscle mass using up those calories.
Unfortunately, this slowdown can last for years.
One landmark study on contestants from The Biggest Loser found that their resting metabolic rate (how many calories we burn daily just to stay alive) still remained significantly lower six years after rapidly losing extreme amounts of weight on the show.
However, keep in mind that set-point theory is still simply a theory. “It’s widely accepted. But the evidence for it is very poor,” says David A. Levitsky, PhD, a professor of nutrition and psychology at Cornell University in Ithaca, New York, who researches the regulation of body weight.
Velasquez agrees, explaining that, although there have been animal and preclinical studies, scientists haven’t conducted strong research on humans to definitively prove this is how weight regulation works. Yet while other hypotheses have been proposed, none have gained much traction.
“There are competing theories, but I don’t think that it’s something we see in clinical medicine,” Dr. Sowa says.
Still, Dr. Levitsky argues that we can’t ignore the psychological factors that play a role in regaining weight, particularly the difficulty of changing ingrained habits.
“We learn very early how much to eat, and that will determine what you’re going to weigh,” he says. “If you return to the same habits that you had before you dieted, you’re going to return to your [old] body weight.”