Thursday, February 21, 2008

Conventional Wisdom: Yo-Yo Dieting Slows Your Metabolism

We've all heard that repeated cycles of dieting and weight regain can result in a permanent lowering of the metabolism and a "rebound effect," where dieters end up weighing more than ever. But is there a scientific basis for this claim?

Most people who try to lose weight encounter plateaus as a result of a metabolic slowdown. This phenomenon has been well characterized in scientific studies (1-4). Leibel et al. (4) conducted a particularly convincing study in which 18 obese and 23 never-obese subjects were admitted to the hospital so their food intake and activity could be strictly controlled. These subjects were then over- or under-fed until they had reached a 10% increase or decrease in their body weight. In both the obese and never-obese groups, a 10% increase in body weight resulted in a 16% increase in energy expenditure. Correspondingly, the 10% weight loss groups decreased their energy expenditure by 15%. The authors' interpretation of this study was that these changes in energy expenditure represent an attempt by the body to return to its "setpoint" weight.

So, changes in body weight do seem to result in changes in energy expenditure. But do repeated changes in body weight permanently affect your metabolism?

A number of researchers set out to answer this question using animal studies in the 1980s and 1990s. Ernsberger et al. (5) looked at rats with a specific genetic mutation making them prone to obesity and mimicked yo-yo dieting by putting half of the rats through a process of "weight cycling." The weight cycled rats were fed ad lib for 4-6 weeks, and then switched to a very low calorie diet (16.7% of baseline calories) for 12 days. This cycling of ad lib and restricted feeding was done three times. The researchers found that the cycled rats had a higher final body weight than controls which were fed ad lib throughout the study. They also found that the cycled rats lost slightly less weight each successive time they were put on the very low calorie diet. Similarly, Lim et al. (6) found that weight cycled rats had a lower resting metabolic rate than rats that were consistently food restricted (both groups were restricted to 70% of the food control rats ate), but that exercise on a treadmill eliminated that difference. These studies seemed to validate the hypothesis that yo-yo dieting had lasting effects on metabolism. However, Lu et al. (7) found that weight cycling of rats facilitated the development of insulin resistance, but didn't have any effect on body weight.

In humans, the story appears to be somewhat more consistent. While one paper (8) found that adolescent male wrestlers who engaged in yo-yo dieting had lower energy expenditure than those that didn't, subsequent studies of humans have repeatedly failed to see any permanent effects of yo-yo dieting on weight or metabolism in either obese or lean individuals (9-16).

So why the differing results in the human and animal studies? I can think of a few possible explanations. One possibility is that humans and rats may simply differ physiologically with respect to the effects of repeated weight cycling. Another possibility is that since the rat studies tended to involve a greater degree of restriction, there may be effects of weight cycling that don't kick in until the person or animal is in a severely hypocaloric state. This might also explain why the one human paper that did show an effect of yo-yo dieting was in adolescent male wrestlers...adolescent males have tremendous caloric needs so severe dieting may have more of an impact on this group. A third possibility is that rats exposed to involuntary food restriction with no knowledge of when their food access will return to normal may differ physiologically from people who voluntarily restrict their food intake, knowing that food is available to them if they need it.

The upshot is that while weight loss can decrease your metabolic rate, this change appears to be reversible with a return to "normal" weight.

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