‘It’s not about willpower.’ How metabolism slows down when you try to lose weight

Many people who are trying to lose pounds find that their diets stop after a certain amount of weight loss. A new study shows how the body’s metabolism slows as a way to balance fewer calories.

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

This “metabolic adaptation” is a response to weight loss by reducing the resting metabolic rate – the number of calories a person needs to keep vital systems, such as the heart and lungs, working.

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

In this case, all of the women were trying to reach their BMI, or BMI, of 25, shortly after what is considered a normal or healthy range for a BMI of 18.5 to 24.9.

Martins and her colleagues found that for every 10 calories a day took longer, dieting decreased resting metabolic rate.

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

To take a closer look at how women’s resting metabolic rates change while dieting, Martins and her colleagues reanalyzed data from two previous University of Alabama studies at Birmingham, called ROMEO and JULIET. The researchers focused on patients who lost weight through diet alone, up to a maximum of one day per week of exercise.

During the study, all volunteers were fed a diet of 800 calories per day until they reached their weight loss goals. At that point, a number of measurements were taken, including your resting metabolic rate.

Martins and her colleagues determined that 64 percent of women had fully stick to their diets. Overall, the women lost an average of 12.5 kg (27.6 lb) 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 for women to reach their weight loss goals.

A person with a lot of metabolic adaptation will experience consistent weight loss and will experience recent weight loss,” Martins said.

Exercise can help restore weight loss

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

“Once a person is really stable for a while – maybe two weeks is enough – the effect will wear off and they can then restart the diet,” she said.

There are other metabolic challenges to weight loss, said Dr. Rika Kumar, associate clinical professor of 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 in the new research. But the important thing is that your resting metabolic rate isn’t the only thing that’s challenging for people trying to lose weight. There are a lot of hormones, like ghrelin and leptin, that go in the wrong direction with weight loss.”

Kumar said the study “supports what people see in their own experience and what clinicians see in their patients — it’s not about willpower.” “When you lose weight, achieving your weight loss goal becomes more and more difficult.”

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

This story first appeared NBCNews.com.

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