Week of July 17, 2004; Vol. 166, No. 3 , p. 35

A Toxic Side of Weight Loss: Pollutants may slow body's metabolism

Carrie Lock, Science News

http://www.sciencenews.org/articles/20040717/fob2.asp

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 as 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."
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References:

Tremblay, A., et al. 2004. Thermogenesis and weight loss in obese individuals: A primary association with organochlorine pollution. International Journal of Obesity 28(July):936-939. Abstract available at http://dx.doi.org/10.1038/sj.ijo.0802527.

Sources:

Rudolph Leibel
Columbia University Medical Center
1150 St. Nicholas Avenue
New York, NY 10032

Glenn Miller
Natural Resources and Environmental Science
University of Nevada, Reno
1000 Valley Road/MS 186
Reno, NV 89512

Angelo Tremblay
Department of Social and Preventative Medicine
Division of Kinesiology
Laval University
Quebec, QC G1K 7P4
Canada

From Science News, Vol. 166, No. 3, July 17, 2004, p. 35.

Shucks, I was going to post this. Good find!

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."
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I agree with Leibel... If these pollutants are environmental, then we would infer that weight loss is more difficult now than it was 500,000 years ago. Hmmmm... I'm doubting that modern day pollutants are major player in lowered metabolism. Interesting article though. I look forward to more studies being done.

But obesity is a choice! Therefore we must be choosing to slow down our metabolisms.

How do we know that weightloss now isn't indeed more difficult than it was 500,000 years ago?

I know weightloss is more difficult for me now than it was 20 years ago...

You know I keep hearing about this terrific slowdown of metabolism that occurs in response to body fat loss, but I have yet to experience it first hand.

Yes, I do notice my metabolism does slow down during dieting, that is to say restricting energy from my body... but fat loss itself has not seemed to significantly affect my metabolism. In other words, when I eat more total calories and more frequent feedings, I notice my metabolism goes through the roof, but when I eat less, my metabolism slows down terrifically to adapt.

In my journal I speak about my experiences from raising my calories from 1150 weekly average to 1500 average, as I attempt to segue into maintenance. The results? I actually grew muscle mass (without trying to, so obviously I had lost a lot of "natural" muscle during weight loss), and I actually *lost* fat at a faster rate than I had been losing. My previous deficit on 1150 calories was only 500 per day... so technically 1500 calories should be enough to lose extremely slowly. This was not my experience. Eating more raised my usage of and need for energy, producing more muscle mass and less fat mass.

Now it should be mentioned that my metabolism might taper off to a lower level once my body rebuilds its muscle (which it is now doing now that I am feeding it a reasonable amount), but so far my experiences demonstrate that slow metabolism much more strongly correlates with changes in consumption rates than it does changes in body fat levels (at least when the individual still has a reasonable amount of superfluous body fat left, anyway).

However, it should be mentioned I have no control to compare my metabolism to. I've never been thin (until recently) so I have no idea if my metabolism is indeed slower than it "used to be" at this weight. I know my metabolism is significantly slower than my sister's... however she runs on occasion, weighs like 150 lbs, is 5'7, and has a heavy build of muscle. I am less active, weigh 25 lbs less, am 2 inches shorter, and have low muscle mass, so it is expected my requirements for energy should be significantly lower.
My other sister, who is an inch shorter than me and about the same weight is perhaps a better comparison. I think our rate of consumption is roughly equal. Whereas my other sister eats all the time, I notice she eats very little in comparison, more akin to how I eat. If she does have a faster metabolism than me, it probably isn't by very much at all. She has never been very overweight or lost very much weight, nor is she dieting.

So my environment tells me my metabolism is pretty normal. In my experiences, the amount of food I'm eating adversely affects metabolism significantly (aka ATTEMPTING to lose fat), but being thinner/less fat itself does not seem to be as significant a factor, if one at all.

You forgot the age factor. You are still pretty young. This is pretty empirical, but it seems harder to loose weight as you get older.

You're right. We have no way of knowing.
I was just thinking that animal metabolism has evolved to slow down when food is scarce. So from that idea I assumed that the same mechanism of metabolic slowdown that affects us now also affected us then.