How does metabolism affect weight loss? A new study shows why some diets stall

Many people trying to lose weight have seen their diets stall after a certain amount of weightloss. A new study shows how the body’s metabolism slows down as a way to balance the fewer calories being consumed.

An analysis of data from 65 black and white dieting 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, like the heart and lungs, working.

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

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

Martins and colleagues found that the dieter took one more day for every 10-calorie drop in resting metabolic rate.

“We had some women whose resting metabolic rate dropped by about 700 calories, which means it would take them 70 days longer, or about two months longer, to achieve their weight loss goals compared to someone without any metabolic adaptation,” Martins said.

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To take a closer look at how women’s resting metabolic rates might change during the diet, Martins and colleagues reanalyzed data from two earlier studies from the University of Alabama at Birmingham, called ROMEO and JULIETA. The researchers focused on patients who lost weight on diet alone, with a maximum of one day a week of exercise.

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

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

“A person who experiences a lot of metabolic adaptation will experience plateaus in weight loss and have a hard time shedding those last few pounds,” Martins said.

Exercise can help restart weight loss

The study did not look at whether changes in resting metabolic rate could be prevented. Martins said he suspects adding exercise, in addition to weight lifting, might help. Another strategy, he told her, would be to take a short break from the diet.

“Once the person really settles down for a while, two weeks would probably be enough, then the effect will wear off and then they can restart the diet,” he said.

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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.

“Researchers have isolated one aspect of metabolic adaptation,” said Kumar, who was not involved in the new research. “But the bottom line is that resting metabolic rate isn’t the only thing that presents a challenge for people trying to lose weight. There are so many hormones, such as ghrelin and leptin, going in the wrong direction with weight loss.”

The study “supports what people see in their own experience and what doctors see in their patients, it’s not about willpower,” Kumar said. “As you lose weight, it becomes more and more difficult 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 diet or weight-loss surgery.”

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