Is it true that weight loss can result in a “slow metabolism”? Original paper

Your body burns calories just to maintain its weight. Even maintaining your fat stores requires energy. Therefore, the heavier you are, the more calories your body needs.

It follows that if you lose weight, your body needs to burn fewer calories; this is normal. In some cases, though, the decrease in caloric expenditure is greater than can be explained by the simple loss of body weight; people who have lost a lot of weight quickly often have a resting metabolic rate slower than normal (they have a “slow metabolism”) as a result of adaptive thermogenesis (AT).

AT is a phenomenon by which your body reduces its heat production to save energy and thus keep you from starving when food is scarce. From a survival perspective, it makes sense; but when you’re trying to lose weight, it can be a problem. Along with a lack of adherence to dietary and exercise recommendations, AT may play a role in the high rates of weight regain.

This systematic review of 33 studies (2,528 people) examined the effect of AT on resting energy expenditure (REE), total daily energy expenditure (TDEE), and sleeping energy expenditure (SEE) during or after a weight loss induced by diet, exercise, bariatric surgery, or pharmacological therapy.

Of the 33 included studies, 8 were randomized controlled trials, 12 nonrandomized trials, 3 retrospective observational studies, and 10 prospective observational studies.

• AT reduced REE in 23 of 29 studies.
• AT reduced TDEE in 4 of 5 studies.
• AT reduced SEE in 2 of 2 studies.

The studies differed greatly in design and diet (caloric deficit and macronutrient composition). AT was more likely to be smaller or nonsignificant in well-designed studies and studies that assessed AT during weight maintenance than in studies of lower quality and studies that assessed AT during weight loss. The magnitude of weight loss was not associated with AT.

AT might be caused by several factors, such as an increase in mitochondrial efficiency,^[1] a decrease in leptin,^[2] and a decrease in non-exercise activity thermogenesis (NEAT).^[3] Regardless of the cause, avoiding rapid weight loss,^[4] interspersing periods of energy restriction with periods at energy balance,^[5] and simply walking more can mitigate AT.