I’ve lost a lot of weight with carb concentrated dieting, and now that my weight has equilibrated, I calculate that my daily calorie intake is lower than that of my “reference man” with a BMI of 22 — but my own BMI is still higher than that. In other words, I’m achieving true calorie restriction, but I’m not as lean as I would expect to be. What’s the problem?

Studies show that, when people lose a lot of weight, their daily energy expenditure tends to decline.   In fact, when their weight equilibrates, their daily energy expenditure tends to be lower than that of people of comparable size and body composition who haven’t lost weight.  Only a small portion of this difference is attributable to a lower resting metabolic rate – most of the reduction is in so-called “non-resting energy expenditure” associated with physical activity.  And it’s not because people who have lost weight are lazier.  Apparently, an adaptation occurs whereby it takes less food energy (ATP) to fuel a given amount of muscular effort; in other words, your body becomes more “energy efficient”.  This may account for the fact that you may not get as lean as other people with comparable activity levels who eat more calories than you do. 

The increased energy efficiency of people who are post-obese likely represents a homeostatic adaptation conserved throughout evolution that enables animals that are underfed and lean to perform the physical activities necessary for survival and procreation while minimizing their use of scarce food energy reserves.   Conceivably, it may reflect increased nitric oxide production in skeletal muscle.  Recent studies show that diets high in nitrate can improve the energy efficiency of skeletal muscle by boosting nitric oxide production in muscle; how this effect is mediated is still unclear.      

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