A toxic side of weight loss: pollutants may slow body's metabolism.

Weight loss isn't only frustrating, it's also complicated. Scientists expect a person's metabolism to slow as he or she loses weight, but there's sometimes more of a drop than the equations predict. Researchers call this excessive slowdown "adaptive thermogenesis," although they don't fully understand why the body's internal furnace sometimes changes efficiency in what seems to be an effort to minimize weight loss.

Now, researchers propose that the largest contributor to adaptive thermogenesis is increased concentrations of pollutants in the blood, rather than changes that weight loss seems to trigger in the dieter's hormones. Many toxic industrial chemicals, such us organochlorines, are stored in fat cells and escape into the bloodstream when those cells shrink during weight loss.

"Pollution seems to be a new factor affecting the control of thermogenesis in some obese individuals experiencing body-weight loss," says Angelo Tremblay of Laval University in Quebec City, Quebec, in the July International Journal of Obesity.

Tremblay and his colleagues put 15 obese people on a 15-week reduced-calorie diet, and the volunteers lost an average of 23 pounds. There was no difference in weight loss between the 11 participants who took the weight-loss drug fenfluramine and the 4 who received a placebo. The participants' average body mass index dropped from 35.4 to 31.8; anything over 30 is considered obese.

The researchers collected blood samples from the participants before and after weight loss and compared the concentrations of certain natural hormones and synthetic organochlorines, including 14 polychlorinated biphenyls and 11 pesticides. After 15 weeks of dieting, the blood concentration of leptin--a hormone that helps regulate body weight--had dropped by 33 percent. Concentrations of the other natural hormones had stayed constant, and the combined concentrations of the pollutants had increased an average of 23 percent.

Tremblay's team also found that the changes in the dieters' metabolic rates showed adaptive thermogenesis. The researchers checked the mathematical correlation between the metabolic slowdown recorded for each participant and changes in blood concentrations of organochlorines and leptin. They found that the measured adaptive thermogenesis was more strongly linked with the pollutants than with the natural hormone.

"Modern pollutants probably complicate the regulation of energy and even exceed the impact of leptin," says Tremblay. He admits, however, that the mechanism remains unknown.

He speculates that the pollutants interfere with both the thyroid gland, which helps regulate the body's metabolism, and individual cells' mitochondria, which convert fuel into energy. Even if the body burns only 100 fewer calories per day, that can add up to significant weight gain over a year.

"I suspect [these pollutants] may be mitochondrial poisons, but we need stronger molecular evidence," says environmental toxicologist Glenn Miller of the University of Nevada in Reno.

Rudolph Leibel, an obesity geneticist at Columbia University, points out that the new study offers merely a correlation, rather than proof that high blood concentrations of pollutants underlie adaptive thermogenesis.

Leibel also questions whether industrial pollutants could be solely responsible for what appears to be a natural process in the body. "There is some evidence that the body does seem to make metabolic adjustments," he says. "The wiring for the defense of body fat has been around a lot longer than pesticides."