Metabolic adaptations to extreme weight loss

The Study
Your resting metabolic rate is the number of calories your body burns each day to perform its basic metabolic functioning. The higher your resting metabolic rate, the easier it is to manage your weight. It has been known for some time that resting metabolic rate decreases with weight loss. Prevailing wisdom was that this drop was due to the loss of metabolically active muscle tissue during the weight loss process. This may not be the whole story.

A study conducted by the Pennington research group tested this theory. They measured the resting metabolic rate of 16 Biggest Loser contestants before, during, and after the 30 week competition. In order to reduce the loss of muscle during the weight loss process, the contestants on this show engaged in a lot of resistance training exercise. They were successful in sparing their muscle. The average weight loss after the 30 weeks was 30% of initial body weight. Of this weight loss, 83% was fat and only 17% was muscle. After adjusting for the losses in fat and muscle mass at the end of the 30 weeks, resting metabolism in these contestants decreased by a stunning 504 calories per day. Journal of Clinical Endocrinology and Metabolism 2012; 97:2489-96.

Take Home Message

This study tells us a few important things. The first is that resistance training prevents the loss of muscle mass with weight loss, which is really important. The second is that despite the preservation of muscle, metabolism drops a lot after extreme weight loss. A drop of 504 calories per day is no joke. The question that I don’t have the answer to, is whether this decrease in metabolism happens all the time, or is due to the very extreme nature of the weight loss in this study. The contestants on this show lost an average of 5 lbs. a week. I am wondering if this severe drop in metabolism would occur in those who lose weight more slowly. 

No one really knows why resting metabolism drops so much after weight loss when accounting for lost muscle. The authors of this study believe that the drop in metabolism may be due to changes in leptin and thyroid hormones or perhaps a reduction in the size of very metabolically active organs like the heart, brain, kidney and liver. One thing is certain, when you lose a lot of weight quickly, your body fights very hard to put it back on.

We need to learn more about this phenomenon. Many people who lose weight gain it back within a short period of time, and this drop in resting metabolic rate could be a big reason why.