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Exercise is one way to lose weight. Reducing calorie intake is another. FreshSplash/Getty Images
  • Many people have experienced the frustration of not being able to lose additional weight once some initial pounds have been shed.
  • Experts say this might be due to something called metabolic adaptation.
  • This is the prehistoric process that signals our bodies to slow metabolism to avoid excessive weight loss due to food scarcity.
  • Experts say the best way to avoid this adaptation is to lose 1 to 2 pounds a week over an extended period of time.

If you’re not reaching your weight loss goals, the reason may be something called metabolic adaptation.

Metabolic adaptation is one of your body’s survival mechanisms that occurs when you’re losing a significant amount of weight.

A new study conducted by the Department of Nutrition Sciences at the University of Alabama at Birmingham concludes that metabolic adaptation was associated with more time to reach weight loss goals.

This might explain why it can become more difficult to lose weight after you’ve lost some initial pounds.

In their study, researchers looked at women who were premenopausal and above the healthy weight range listed for body mass index (BMI).

Participants lost an average of 16 percent of their weight over an average of 5 months. Researchers said metabolic adaptation after the 16 percent weight reduction increased the time it took to lose more weight.

The study participants included 65 premenopausal women with overweight; 36 white women and 29 Black women. Both groups followed an 800-calorie diet until they hit a specific BMI.

One group was considered sedentary (only exercising once per week) while the other group engaged in routine exercise. All participants did not smoke and reported regular menstrual cycles.

All participants also:

  • had normal glucose levels
  • had a family history of overweight and obesity in at least one first-degree relative
  • were not taking medications that could affect body composition or metabolism

On average, adherence to the diet was about 64 percent. These results were consistent after adjusting for other causes that could be interfering with weight loss.

Metabolic adaptation happens when our body adjusts to reduce our resting metabolic rate (RMR).

The RMR is how many calories our body needs to function properly and maintain weight, explained Caroline West Passerrello, MS, RDN, LDN, a spokesperson for the Academy of Nutrition and Dietetics, as well as community coordinator and instructor at the University of Pittsburgh and owner at Caroline West LLC.

This rate is different for everyone and changes throughout our lives since it’s based on factors such as age, height, weight, activity level, and body composition, Passerrello told Healthline.

“We know from research that RMR does change with weight loss,” she said. “However, there are many variables that determine if the change in RMR is clinically significant and if the change to RMR will persist once a person’s weight has stabilized.”

Andy De Santis, a registered dietitian and weight loss expert based in Toronto, Canada, said metabolic adaptation really amounts to the idea of your metabolism “slowing down” in response to calorie restriction and weight loss.

A smaller body tends to spend less energy at rest. Food requires energy to digest, so when both food intake and weight drop, so does the amount of energy your body spends on a daily basis, he explained to Healthline.

So it’s hard to say these study findings change our knowledge that much, he noted.

“The idea that some people are affected by metabolic adaptation more than others and that subsequently could extend the amount of time it takes for different people to lose weight is interesting and could be one of many variables that explain differing success rates/durations of weight loss endeavors,” De Santis said.

And it could be a valuable insight to someone who feels their efforts trying to lose weight are substantial, but their results aren’t matching, given that metabolic adaptation plays a role in weight loss success even when dietary adherence is high, he added.

“Human physiology has many safeguards in place to keep our body processes functioning,” Passerrello said. “Many of these safeguards also make it difficult for an individual to maintain lost weight.”

If you are interested in losing weight, Passerrello said the general recommendation is to adjust your energy balance.

This means reducing the number of calories consumed or increasing the number of calories expended to result in a weight loss of about 1 to 2 pounds per week.

Many experts agree that losing 1 or 2 pounds per week is a healthy and safe rate. The Centers for Disease Control and Prevention states that people who lose weight gradually and steadily are more successful at keeping weight off.

Losing more than 2 pounds per week is more likely to result in greater reductions to our RMR, said Passerrello. In other words, losing more than that per week can be a signal to your body that a food supply has become scarce.

“I hope this is a reminder to folks that there are factors we can control, but weight loss isn’t solely about willpower and personal choices,” said Passerrello.

Participants were put on a diet that consisted of just 800 calories a day over the course of the study.

This 800-calorie diet isn’t designed for every body.

One diet company, The Fast 800, says eating 800 calories a day is an intensive way to start your weight loss journey and “reset your metabolism,” helping people to lose weight “very fast,” lower blood pressure, and potentially reverse type 2 diabetes.

De Santis said that 800 calories a day is 33–50 percent less than the average caloric needs we might expect in premenopausal women.

“The adherence to the diet, as per the paper, was only around 64 percent, which means only a bit more than half the participants could pull it off,” he said.

De Santis added that this result speaks to the practicality of the diet.

Then again, the goal of the study was to measure metabolic adaptation to weight loss, so the context is a bit different from a standard diet you might see being marketed.

“It’s safe to say I don’t advocate for 800-calorie diets,” he said.

He urges people to talk with their doctor before starting a weight loss program.