It's a common belief that individual biology makes weight loss easier for some and not for others, and a research team has proved it for the first time in a laboratory setting.
In light of the current debate as to whether or not exercise leads to weight loss, it's important to note that the current study focused on calorie restriction.
Working with 12 men and women defined as obese in the metabolic unit of the Phoenix Epidemiology and Clinical Research Branch (PECRB) at the National Institute of Health in Arizona, the team's first step was to gauge each participant's metabolism.
For this, they calculated energy expenditure based on air samples using a gadget called a whole-room indirect calorimeter, an apparatus that measures oxygen consumed and carbon dioxide produced.
Participants were assessed using the calorimeter after a day of fasting to establish each individual's baseline metabolism and the researchers took into account age, sex, race and baseline weight.
Next, they followed a diet for six weeks involving a 50 percent calorie reduction and because they were staying at the PECRB, no cheating was possible.
Those whose metabolism had decreased during the day of fasting lost the least amount of weight during the six weeks of dieting, indicating that some had a faster metabolism than others.
"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," says Susanne Votruba, Ph.D., study author and PECRB clinical investigator.
At this point the research team still doesn't understand whether individuals are born with the same metabolism or whether it changes pace over time.
"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," says Dr. Votruba.
The study has significant implications for a personalized approach to weight loss, although more research is necessary to learn whether biological differences are innate and how best to overcome a stubborn metabolism.
At present, lead author Dr. Martin Reinhardt, a PECRB postdoctoral fellow is optimistic.
"The results corroborate the idea that some people who are obese may have to work harder to lose weight due to metabolic differences," he says. "But biology is not destiny. Balanced diet and regular physical activity over a long period can be very effective for weight loss."
The study was published in the journal Diabetes.
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