We may not be able to explain whether we are thin or fat based just on our metabolism, according to a new study being presented Sunday, June 15, at The Endocrine Society’s 90th Annual Meeting in San Francisco.

“The causes of obesity are complicated and likely cannot be solely explained on differences in rates of metabolism,” said Dr. Daniel Bessesen, an endocrinologist at the University of Colorado Health Sciences Center and a study co-author.

To better understand the causes of obesity, Bessesen and his colleagues looked at thin people who say they have trouble gaining weight. They tested the theory that thin people can overeat without gaining weight because they have a higher metabolic rate—they burn more calories—than do people who have a problem with weight gain. The authors studied 26 naturally thin people, whom they called “obesity resistant,” and 23 people who had at least one obese close relative (parent, sibling, or child) and were thus “obesity prone.”

In both groups the investigators tested metabolic rates at two separate times: once after the subjects ate a normal diet and once after 3 days of overeating 40 percent more food than their body needed. The research team monitored metabolic rates by having these individuals stay for 24 hours in a room calorimeter. This special room controls air going in and coming out and allows measurement of oxygen and carbon dioxide levels. Burning calories requires a certain amount of oxygen. Therefore, a calorimeter is an accurate way of measuring daily energy expenditure, or calories burned, Bessesen said. It also measures how much fat the subjects burned in a day.

Both groups had higher metabolic rates at rest after they overate for 3 days than when they ate a normal diet, the authors reported. However, the increase was not greater in the thin subjects. This finding does not support the idea that the increased metabolic rate after overeating protects thin individuals from weight gain.

“This suggests that differences in hunger, fullness, food intake and physical activity may be more important factors in why some people are thin,” Bessesen said.

The National Institutes of Health provided funding for this study.

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Founded in 1916, The Endocrine Society is the world's oldest, largest, and most active organization devoted to research on hormones and the clinical practice of endocrinology. Today, The Endocrine Society's membership consists of over 14,000 scientists, physicians, educators, nurses and students in more than 80 countries. Together, these members represent all basic, applied, and clinical interests in endocrinology. The Endocrine Society is based in Chevy Chase, Md. To learn more about the Society, and the field of endocrinology, visit our web site at www.endo-society.org.