It’s an excuse so well-worn that most people have stopped disputing it when it comes from the mouths of would-be dieters: “I can’t lose weight because I have a slow metabolism!” Though easy to dismiss as a lack of willpower or hustle, it’s harder to ignore when two people of the same approximate build eat the same diet, but only one sees physiological changes. Now, researchers at the National Institute of Health have conducted a study that finds, for the first time ever, that differences in physiology can affect how someone responds to caloric intake – the so-called “slow metabolism.”
“When people who are obese decrease the amount of food they eat, metabolic responses vary greatly, with a ‘thrifty’ metabolism possibly contributing to less weight lost,” said Susanne Votruba, Ph.D., study author and PECRB clinical investigator. “While behavioral factors such as adherence to diet affect weight loss to an extent, our study suggests we should consider a larger picture that includes individual physiology – and that weight loss is one situation where being thrifty doesn’t pay.”
The study was conducted at the Phoenix Epidemiology and Clinical Research Branch (PECRB) and included 12 men and women diagnosed with obesity. To see how participants responded to changes in diet, the researchers used a stunning piece of technology: A whole-room indirect calorimeter, which actually allows researchers to calculate an individuals caloric expenditure from air samples. Researchers first established a baseline by measuring participants’ energy expenditure after a day of fasting. Next, they embarked on a six-week inpatient test phase, where their calorie consumption was reduced by 50%.
After accounting for age, sex, race and baseline weight, the effect was clear: Those who lost the least weight during the calorie-reduced period were those whose metabolism decreased the most during fasting. It’s commonly held that the human metabolism will slow down in response to reduced calorie intake, but the effect is more pronounced in some than it is in others, apparently. Those whose metabolisms slowed down the most were labeled as having “thrifty” metabolisms, so-named for the way they made the most of their caloric intake.
“The results corroborate the idea that some people who are obese may have to work harder to lose weight due to metabolic differences,” said Martin Reinhardt, M.D., lead author and PECRB postdoctoral fellow. “But biology is not destiny. Balanced diet and regular physical activity over a long period can be very effective for weight loss.”
Indeed, if weight loss is, roughly speaking, a matter of energy in versus energy out, it’s technically true that anyone can and in fact must lose weight if they diet long enough. However, such an observation ignores the fact that willpower is a finite resource, and that a lack of results can have a significant psychological impact on a person’s ability to continue with a diet. Perhaps the finding that physiology may play a not-insignificant role will encourage dieters who aren’t seeing the lbs roll off as fast as they’d like.
Two-thirds of Americans are considered overweight, and more than one-third are obese. Complications from obesity can include heart disease, type 2 diabetes and certain types of cancer, which health professionals note are some of the most common causes of preventable death.