Weight loss, it's often said, is all about calories in, calories out, and burning more than you take in—except when it's not. Eat too many calories, and you'll gain weight. But eat too few calories, the theory goes, and your body will enter what's commonly referred to as "starvation mode," where your metabolism stops burning calories and instead converts everything you eat to fat in preparation for a famine.
The theory of starvation mode is something most people on a diet have heard about; if you don't eat enough calories, your metabolism will effectively shut down and cause you to gain weight just like if you ate too much.
But the theory isn't just pseudoscience touted by fitness magazines and weight-loss blogs; researchers and doctors have studied it, too. And it's a little more complicated than just calories in, calories out. The more scientifically accurate term is hypometabolism, says Holly Lofton, MD, director of the Medical Weight Management Program at NYU Langone Health. Hypometabolism is a slow metabolic rate that can occur once you start to cut calories below what your body and metabolism need to function properly.
This metabolic state is meant to help us with survival, Lofton says—it's an evolutionary response that traces back to prehistoric times when the supply of food was more unpredictable. Our metabolism evolved to slow down in order to conserve energy, prepare for long periods of time without food, and naturally suppress feelings of hunger. Today, however, that same survival mechanism can backfire when we're trying to get fit.
"When one is going well under what their metabolic needs are to maintain their weight, the body can slow down its usage of food as a primary resource and go to fat," Lofton says. Although your body turns to fat for energy, drastically cutting back on calories doesn't mean you'll actually shed pounds. "You'd have to really be under pretty extreme situations when one would stop losing weight," she says, "but that can definitely occur." For someone 5-foot-2 or taller, Lofton says that dipping below 900 calories per day over an extended period of time will usually cause their body to enter a state of hypometabolism.
Even if you've been losing weight consistently with a healthy calorie deficit, but all of a sudden hit a plateau, it could be from a related metabolic state called adaptive thermogenesis—a theory that states as you lose weight, your body adjusts to expend less energy. This change in metabolic processes is believed to be a factor in why 80 to 90 percent of people who lose weight gain it back, according to a review published in the International Journal of Obesity.
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These findings are the same as those reached in a more famous study published in the journal Obesity in 2016 that followed contestants from The Biggest Loser six years after they appeared on the show. The contestants ate a low calorie deficit and expended hundreds—if not thousands—of calories through hours of exercise over the course of months. The researchers had expected that the contestants' respective metabolisms would slow down, but the actual results were well beyond their expectations. For instance, they found Danny Cahill, the winner of season 8 who lost 239 pounds, burned 800 fewer calories per day than other men of his size, according to the New York Times. Cahill has regained a little over 100 pounds since the show ended.
Put your body under enough stress by drastically cutting calories, and you'll lose weight—just not necessarily the weight you want to. For every pound you lose, you'll also lose a little bit of water, a little bit of fat, and a little bit of muscle, Lofton explains. When you're eating a balanced diet—these are the most up to date guidelines—these ratios work out more in your favor—you tend to burn 70 to 80 percent fat and only 20 percent muscle as you exercise. But when you cut your calories too much, your body starts eating away at muscle, especially if you're on a very low-protein diet, she says.
"Muscle mass is one of the determinants of your energy expenditure, and thus your metabolic rate," Lofton says. "So if you're losing muscle, your metabolism slows much more quickly than if you're losing fat.' One way doctors can test for hypometabolism is using what's known as a bioelectric impedance analysis. If you are indeed in a hypometabolic state, simply increasing your food intake by just a couple hundred calories a day could be all you need to help reset your metabolism and start losing weight again.
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