Monday, April 30, 2012

Does lean mass preserve resting metabolic rate? By Jerry Brainum

One of the major hazards of dieting, or more specifically,greatly reducing caloric intake, is a loss of lean mass.Since lean mass, mainly muscle, is the primary arbiter of resting metabolic rate, losing muscle from too stringent a diet can result in a lowered resting metabolic rate (RMR). Having a lower RMR, in turn, makes it more likley that the lost weight, and body fat will return when the diet ends. It's no wonder then, that figures show a 97% recidivism rate of people who have lost weight through dieting. In simple terms, the majority of people who lose weight from dieting alone usually regain all the lost weight, and often gain more fat than they had at the start of the diet because of the lowered resting metabolic rate that results from dieting alone.
   The obvious answer to this dilemma is to do something that will help you to retain muscle while dieting. The simple solution is to exercise while on a diet. Since weight-training is the best way to develop lean mass, lifting weights would appear to be a mandatory part of any weight-loss program. Yet, a new study suggests that even rigorous exercise will not offset the effect of dieting on the RMR. But there are a few caveats that need to be explained about this finding.
     The subjects of the study, seven men and nine women, ages 20 to 56, were all contestants on the popular TV show, The Biggest Loser. This show revolves around a competition to lose weight over a  30-week period, with those who lose the most weight being the victors in the competition. Contestants on the program are trained by professional trainers, and they workout an average of 90 minutes a day, 6 days a week. The exercise programs consist of combinations of weight-training and aerobics, with the weight-training usually being a form of circuit training, or doing several exercises in a sequence with minimal rest.  The contestants on the show, however, are encouraged to exercise an additional three hours each day. While their diets are not monitored, they are advised to consume a calorie-restricted diet containing 70% of their estimated baseline daily caloric requirements.
    In the new study, most of the weight loss in the contestants consisted of fat, with 17% coming from lean mass stores.But that lean mass also includes water and bone, so it's unclear just how much muscle the contestants lost on this regime. The contestants, despite the exercise, still showed metabolic adaptations that showed up by the sixth week, and this effect doubled by the end of the competition.Those who lost the most weight also showed the greatest level of metabolic adaptation.
     Specifically, levels of thyroid-stimulating hormone (TSH) increased by 67%, while levels of the active form of thyroid hormone, T3, dropped by 49%. T4 levels, which function as a storage form of thyroid hormone, weren't affected.These changes in active thyroid hormone accounted for the significant drop in RMR shown by the contestants. While it was previously believed that exercise would prevent this effect, it apparently did not in this group of people. The ramifications of this are that with the resultant lower RMR., the contestants, despite a large loss of bodyweight and fat, are far more likely to regain all the lost weight, and then some. In fact, this is precisely what has happened to most of the past contestants on The Biggest Loser. The salient question that arises here is: why didn't the extensive exercise prevent the drop in RMR, especially since the contestants didn't seem to lose much muscle?
    This aspect wasn't discussed by the study authors. However, I suspect that it was the speed of body composition changes that led to the significant drop in RMR shown by these people. Although they appeared to maintain lean mass through exercise, the body still may have construed the rapid loss of weight as a stress effect. This could have led to a resultant body response of lowering metabolism via the thyroid axis as means of dealing with the perceived rapid weight loss. Consider that the contestants were all obese at the start, and lost nearly half or more of their bodyfat stores. Doing so too rapidly is not a natural effect, so the body responds by turning on mechanisms to slow the weight loss.I suspect that if these people had taken longer to lose the weight, the RMR drop would not have occurred. Also, if the 17% loss of lean mass were mostly muscle, this would also account for the drop in RMR. Circuit weight-training is a good way to change body composition, but not the best way to maintain or build muscle mass.So the style of training may also have affected the outcome. The contestants may have been doing too much exercise, which is construed by the body as a stressor, leading to reduced metabolism as a way of dealing with the stress.The final aspect to consider is that rates of weight-loss on the Biggest Loser do not reflect a modest reduction in daily caloric intake. Some contestants, in their zeal to win, are probably starving themselves. This alone could result in a significant drop in RMR, and in fact, is the likley cause of what happened here, since the diets are not closely monitored.

Johannsen DL, et al. Metabolic slowing with massive weight loss despite preservation of fat-free mass.J Clin Endocrin Metab 2012: in press.

©,2012, Jerry Brainum.Any reprinting in any type of media, including electronic and foreign is expressly prohibited.

See Jerry's book at