For the chronically pound-conscious, the end of the holidays means tiptoeing onto scales while murderously eyeing the thin buffet-samplers. So why can some people gorge themselves and still maintain their weight, while others merely nibble, but gain? The answer, revealed in tomorrow's issue of Science, seems to involve how people move in everyday life: who snoozes and slouches, who fidgets and paces.
The researchers--endocrinologists James Levine, Michael Jensen, and their colleagues at the Mayo Clinic in Rochester, Minnesota--wanted to find out the reason for the variations in weight in response to overeating. For their study, the researchers recruited 16 nonobese, relatively sedentary people, and for 8 weeks made sure that these 12 men and four women consumed an extra 1000 calories per day--an experience the volunteers described as 2 months worth of Thanksgiving meals. During that time, strenuous exercise was prohibited so that the researchers could narrow the explanations for differences in the number of calories burned rather than deposited in fat. One possibility was innate variations in basal metabolism--calories burned by normal body processes such as digestion. Another candidate was individual differences in the low exertion movements, which Levine and Jensen called nonexercise activity thermogenesis (NEAT).
To ferret out the culprit, the researchers began a clever process of elimination. They first recorded basal metabolism--the minimal numbers of calories needed to stay alive-- by a method known as indirect calorimetry, which measures the amount of oxygen consumed and carbon dioxide produced. They next measured overall energy expenditure by giving volunteers water laced with hydrogen and oxygen isotopes and measuring how much was excreted in urine. From this value the researchers subtracted the basal metabolism value and the calories that had been converted to fat (estimated with special x-rays that reveal fat deposits). What's left over, the researchers reasoned, are calories burned during involuntary everyday movement.
Apparently, twitching and fidgeting can be hard work. The amount of weight gained by volunteers varied by 10-fold, with the most amassed by those with the least involuntary movement. The four females fared the worst, gaining up to 4.23 kilograms, while one man actually burned off 700 of the extra 1000 calories consumed, gaining as little as 0.36 kilograms.
Energy expenditure variations "have never been measured in such an elegant way before," says endocrinologist Eric Ravussin, director of Obesity Research at Eli Lilly in Indianapolis. And the bottom line could be good news for holiday feasters regretting their indulgences. "You don't have to get on an exercise bike and grunt your guts out for 30 minutes to burn a lot of calories," says Jensen.