More important factors may be differences in food intake and activity, and the fact that people who gain weight may not truly realize how much they consume

SAN FRANCISCO | Metabolism alone may not explain why some people are fat or thin, according to a study presented at this year’s annual Endocrine Society meeting here.

It is unclear how some individuals remain thin in the current obesigenic environment that promotes significant weight gain in the majority of people. However, researchers with the University of Colorado Health Sciences Center in Denver say it is not because thin people have a faster metabolism or metabolize their food differently than obese people.

“The causes of obesity are complicated and likely cannot be solely explained on differences in rates of metabolism,” said Dr. Daniel Bessesen, an endocrinologist and professor of medicine at this institution.

To better understand the causes of obesity, he and his colleagues looked at thin people who say they have trouble gaining weight. They tested the theory that these individudals can overeat without gaining weight because they have a higher metabolic rate and thus burn more calories than people who have a problem with weight gain.

The investigators studied 26 naturally thin people, whom they called “obesity-resistant,” and 23 people who had a least one obese close relative and were called “obesity prone.”

The Colorado researchers hypothesized that energy expenditure and fat oxidation would increase following overfeeding in the obesity-resistant group, protecting them from weight gain.

In both groups the investigators tested metabolic rates at two separate times: once after the subjects ate a normal diet and once after three days of eating 40% more food than their body needed.

The obesity-resistant subjects had a body mass index between 19 and 23, no obese first-degree relatives and had self-described difficulty gaining weight. The obesity-prone individuals had BMIs between 23 and 27 and at least one obese first-degree relative. All the subjects underwent two one-week dietary study periods, with four days of a control run-in diet followed by three days of either continued eucaloric feeding or overfeeding.

The researchers monitored metabolic rates by having all the subjects stay for 24 hours in a room calorimeter. This special room controls air going in and coming out, and allows for the measurement of oxygen and carbon dioxide levels. Burning calories requires a certain amount of oxygen. Therefore, a calorimeter provided an accurate way of measuring daily energy expenditure or calories burned, explained Dr. Bessesen. It also measured how much fat the subjects burned in a day.

All the food in the study was provided by a special metabolic kitchen. The researchers determined energy needs from a 24-hour baseline calorimeter stay and dietary composition was identical in all study periods. The food consisted of 20% protein, 30% fat and 50% carbohydrate.

The researchers found both groups had higher metabolic rates at rest after they overate for three days than when they ate a normal diet. However, the increase was not any greater in the thin subjects. “This suggests that differences in hunger, fullness, food intake and physical activity may be more important factors in why some people are thin,” Dr. Bessesen said.

Unaware of intake
He suggested these findings are important because many thin people think they have a “faster metabolism.” However, Dr. Bessesen said his study shows that is simply a myth. He said primary-care physicians often have a significant number of patients coming in for visits and reporting they are eating less but still gaining weight. But it is more likely that these patients are probably not mentally processing how many calories they are actually consuming.

“Overall, we found no evidence that thin people have a higher metabolic rate on a regular diet or that they burn more energy following a period of overfeeding,” Dr. Bessesen said. “The most important take-home message for clinicians is that people who are tending to gain weight may not be getting accurate information on how much they are eating through biologic mechanisms. So self-monitoring might be an important tool for them, such as keeping food diaries and food records, because they may be eating more than they think.”

Dr. Henry Anhalt, a pediatric endocrinologist in Englewood, N.J., described Dr. Bessesen’s study as an important first step. Until now, he said, most studies have focused on why people become obese and what can be done to prevent obesity. Instead, he hopes more studies like this will look at how normal-weight or thin people avoid obesity in today’s fast-food, “super-size me” culture.

“We need to see if there are targeted gene strategies to identify what keeps thin people thin.”