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How metabolism slows down when you try to lose weight

New research finds that the body may adapt to losing weight by burning hundreds of fewer calories per day.

Many people trying to shed pounds have seen their diets stall after a certain amount of weight loss. A new study shows how the body's metabolism slows as a way to balance the lower amount of calories that are consumed.

An analysis of data from 65 dieting white and Black women, ages 21 to 41, revealed that their bodies could adapt to burn, on average, 50 fewer calories a day. Some of the women, who were initially overweight or obese, adapted to the weight loss to use hundreds of fewer calories per day, according to the report published Thursday in Obesity.    

This “metabolic adaptation” is a response to weight loss by decreasing the resting metabolic rate — that is, the number of calories a person needs to keep critical systems functioning, such as the heart and the lungs.

“Metabolic adaptation during weight loss can make it harder for people to achieve their goals,” said the study’s first author, Catia Martins, an associate professor of nutrition science at the University of Alabama at Birmingham. “In this study we found people with more metabolic adaptation took longer to achieve their weight-loss goals.”

In this case, the women were all trying to get to a body mass index, or BMI, of 25, just a little past what is considered a normal or healthy BMI range of 18.5 to 24.9.

Martins and her colleagues found that dieting took one day longer for every 10-calorie drop in resting metabolic rate.

“We had some women whose resting metabolic rate dropped by close to 700 calories, which means it would take them 70 more days, or about two months longer, to achieve their weight loss goals compared to someone with no metabolic adaptation at all,” Martins said.

To take a closer look at how women’s resting metabolic rates might change during dieting, Martins and her colleagues re-analyzed data from two earlier University of Alabama at Birmingham studies, dubbed ROMEO and JULIET. The researchers focused on patients who were losing weight by diet alone, with a maximum of one day per week of exercise.

During the study, all volunteers were provided with an 800-calorie-a-day diet until they reached their weight loss goals. At that point, a number of measurements were taken, including resting metabolic rate.

Martins and her colleagues determined that 64 percent of the women had completely stuck with their diets. Overall, the women lost an average of 12.5 kg (27.6 pounds) over an average of 22 weeks. When the researchers accounted for factors such as dietary adherence, they found that the greater the change in resting metabolic rate, the longer it took women to reach their weight loss goals.

It’s not about willpower.

Dr. Rekha Kumar, Weill Cornell Medicine

“A person experiencing a lot of metabolic adaptation will experience weight loss plateaus and will struggle to lose those last pounds,” Martins said.

Exercise can help restart weight loss

The study did not look at whether the changes in resting metabolic rate could be avoided. Martins said she suspects that adding exercise, as well as weightlifting, might help. Another strategy, she said, would be to take a short break from the diet.

“Once the person really stabilizes for a while — two weeks would probably be enough — then the effect will go away and they can then restart the diet,” she said.

There are other metabolic challenges to losing weight, said Dr. Rekha Kumar, an associate professor of clinical medicine in the division of endocrinology, diabetes and metabolism at Weill Cornell Medicine in New York.

“The researchers have isolated one aspect of metabolic adaptation,” said Kumar, who was not involved with the new research. “But the important thing is that resting metabolic rate is not the only thing presenting a challenge to people trying to lose weight. There are so many hormones, such as ghrelin and leptin, that go in the wrong direction with weight loss.”

The study does “support what people see in their own experience and what clinicians see in their patients — it’s not about willpower,” Kumar said. “As you lose weight, it becomes harder and harder to achieve your weight loss goal.” 

The brain interprets a reduction in calories as a danger to the body, a possible sign that a famine has begun, Kumar said. “And that’s true no matter how it’s achieved, whether it’s through dieting or weight-loss surgery.”