Does one use up less energy after losing weight?

One question I often get asked is “Can weight regain after a diet be explained by a lowered metabolic rate?”

The short answer is: “No, not systematically”. In other words: “Usually not, though it can happen in some rare cases”.

And yet, how often we’ve heard it repeated: “after you lose weight, your body goes into economy mode, it stores up everything instead of using up energy…”

What is true is that the energy expenditure of the body is lower after weight loss than before. But it is not abnormally low. The central concept here is Resting Energy Expenditure, which is the quantity of energy (= the number of calories) our body needs to just keep functioning. And the main factor that determines this resting energy expenditure is… body weight (1).

This is fairly easy to understand if you think of a car or a truck, which burns more fuel when it is heavily loaded than when it is not; in the same way, a heavy body needs more calories than a light one. Whenever a person loses some their body weight, it will be like unloading the truck, less fuel (=fewer calories) will be used to keep moving.

[Click here to find out how resting energy expenditure is measured].

So, yes, your energy expenditure decreases after weight loss.

The catch is that for a long time, it was thought that it went down much lower than what would have been normal for the new body weight. This putative phenomenon was dubbed “adaptive thermogenesis” or “energy gap” and was used to explain difficulties in maintaining weight loss. This theory has been shaky for a good many years now (2), and an elegant study has undermined what was left of it.

In this study, published in the American Journal of Clinical Nutrition, the investigators asked whether resting energy expenditure was abnormally low in successful weight-loss maintainers (WLMs).

The investigators used two approaches. First, they compared the resting energy expenditure of the WLMs with that of two control groups: one group of people who had never been obese and whose normal weight was similar to the new weight of the WLMs, and one group of overweight or obese people whose weight was similar to the initial weight of the WLMs (their weight before weight loss). If the theory of adaptive thermogenesis were correct, resting energy expenditure should be lower in WLMs than in people who had the same weight but had never been overweight. This, however, was not the case. After taking into account the differences in body composition, resting energy expenditure was similar. In other words, every “kilogram of person” burns about the same number of calories, whatever the person’s weight history.

Next, the investigators used the baseline data from the different groups to generate equations predicting resting energy expenditure. On average, metabolism after weight loss was well predicted by the equation developed from the data of the normal weight controls. This too indicates energy expenditure was not excessively low after weight loss. Not systematically, anyway: there was a good deal of variation in individual results, and that is an important point to note, because it means that, though in most people resting energy expenditure does not decrease abnormally after weight loss, it can happen to some.

It does seem that in a few people, metabolism is lower than it should be. What we don’t know is whether this is a consequence of the loss of weight, of if, on the contrary, such people have always had a low metabolism, which would have caused them to become overweight in the first place.

(1) Particularly the proportion of lean body mass, but going into that in detail would not be relevant for this post.
(2) Particularly based on results from modern medical imaging methods. The HOMAWLO project gave a synthesis of the basis of and objections to the “adaptive thermogenesis” theory.