Keeping weight off after a diet is no easy feat, and most people who lose weight eventually gain at least some, if not all of it back. This isn't a matter of lack of willpower or effort, but of biology: To maintain weight loss, you are essentially fighting a system that's wired to re-gain lost pounds.
The biology of weight loss, and weight maintenance, is a complex process, but here's a simplified look at the changes that happen following weight loss that can make it all too easy to re-gain weight:
When you lose weight, your energy stores, or fat deposits, decrease. This causes hormones in your body — including one called leptin, which is made by fat cells themselves and normally stops you from feeling hungry — to signal to your brain that your fat stores have fallen below a critical level, according to a 2010 review paper.
Your brain then sets into motion a number of responses across your body aimed at putting the weight back on.
One of these responses is to signal to muscle tissues that they should become more efficient, meaning they burn fewer calories. As a result, you need fewer calories to get through your day than would be expected based on your weight alone. For example, a person who weighed 200 lbs. (91 kilograms), and then lost 20 lbs. (9.1 kg), would require about 300 to 400 fewer calories per day than a person who weighed 180 lbs. (82 kg) without dieting, in order to maintain that weight, said Dr. Michael Rosenbaum, a professor of pediatrics and medicine at Columbia University Medical Center who has studied weight-loss maintenance.
Another change is that areas of the brain involved in the rewarding feeling you get from eating food become more active, and areas of the brain involved in restraining eating become less active, Rosenbaum said. This translates to increased appetite and overconsumption of food compared to the calories you need.
With these responses, "you've created the best possible scenario to re-gain the weight you've lost," Rosenbaum said. [The Best Way to Keep Weight Off]
In other words, your body essentially defends its fat stores, making it much harder to keep weight off, especially in an environment where food is plentiful.
But the biological propensity to re-gain weight should not make you feel defeated. "The overarching message about our biology's response to weight loss should not be misconstrued into a conciliatory surrender to the inevitability of weight regain," researchers from the University of Colorado Anschutz Medical Campus wrote in a 2011 review paper on the body's response to dieting.
"The biological drive to re-gain lost weight can be countered with environmental, behavioral and pharmaceutical interventions," they said.
For example, studies suggest that certain types of foods can reduce hunger without providing a huge number of calories.
High amounts of exercise, on the order of an hour per day, are also helpful. This amount of exercise not only burns calories, but also helps prevent the body's metabolism from slowing down, so you burn more calories when you're at rest, said J. Graham Thomas, an assistant professor of psychiatry and human behavior at the Weight Control and Diabetes Research Center of Miriam Hospital in Rhode Island.
Several weight-loss medications are approved by the Food and Drug Administration. These drugs work in different ways; some work in the brain to suppress appetite, while others alter the functioning of the digestive tract.
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Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.