New data challenge popular set point theory of obesity

OB/GYN News, August 1, 2003 by Bruce Jancin

DENVER -- It's high time to toss out the long-popular set point theory of obesity, according to speakers at an international conference of the Academy for Eating Disorders.

The set point theory holds that obesity entails a metabolic defect that functions as a homeostatic mechanism. This defect is supposed to result in a slowdown of resting metabolic rate in an overweight or obese individual who has lost weight. The resultant reduction in energy expenditure is said to be responsible for the often-observed scenario in which the individual regains the weight that was lost and thereby returns to his or her set point.

The set point theory enjoys widespread popularity among the overweight public, in whom it fosters a fatalism regarding the condition of obesity. The theory also holds sway among many physicians who work with obese patients and/or those with eating disorders.

But scientific support for the set point theory is limited to old data derived from outdated methods of physiologic measurement.

Recent studies using far more sophisticated and accurate methods of metabolic assessment consistently challenge the theory, said Cheryl L. Rock, Ph.D., professor of family and preventive medicine at the University of California, San Diego.

One new method is the doubly labeled water technique, a form of indirect calorimetry based on the elimination of deuterium and oxygen-18 from urine. It allows researchers to measure total energy expenditure over a period of days, using only periodic sampling of urine to measure turnover of hydrogen and oxygen into water and carbon dioxide.

Such modern methods have shown that although a metabolic slowdown can occur during active weight loss because the patient has a marked caloric deficit, the slowdown ends once the patient reaches the target weight and shifts into weight-maintenance mode.

"There really is not a lot of data here in 2003 that support the existence of some defect in energy expenditure or energy utilization. The set point theory is on pretty thin ice now in the nutrition science community," Dr. Rock said at the meeting, which was sponsored by the University of New Mexico.

Instead, differences in physical activity are emerging as the key issue in determining who will regain lost body weight. "Rather than something internal going wrong metabolically, the weight return seems to be largely a function of [a lack of] voluntary physical activity of the type we call lifestyle activity: just moving around more, not being a couch potato," she explained.

She cited a recent influential prospective study led by the late Dr. Roland L. Weinsier of the University of Alabama, Birmingham. It included 47 premenopausal sedentary women followed for 1 year. Of the 47 women, 27 were classified as weight maintainers because they gained no more than 3% of their initial body weight 'during the 1-year period; this group had a mean weight loss of 0.5 kg. The other 20 women gained more than 10% of their initial body weight and were classified as gainers; this group had a mean increase of 9.5 kg.

Four weeks of metabolic testing during energy-balanced, diet-controlled conditions at baseline and after 1 year showed that gainers had a lower energy expenditure during normal activities, a lower level of physical activity, and less muscle strength, at baseline and at follow-up. About 77% of the weight gain in the gainer group could be explained by their lower energy expenditure. Energy expenditure during sleep, 24hour utilization of nutritional substrates, and exercise economy didn't differ between the two groups (Am. J. Clin. Nutr. 75[3]:499-504, 2002).

BRUCE JANCIN

Denver Bureau