Metabolic adaptation

Metabolic adaptation – known technically as adaptive thermogenesis – describes how your body becomes more economical during a sustained energy shortage: total energy expenditure falls further than the reduced body mass alone would lead you to expect. Typically this amounts to 5 to 15% of expenditure, or 100 to 250 kcal a day – more with extreme diets. The effect isn't a myth, but it's no reason to panic either: it slows weight loss down but doesn't prevent it.

How it works: falling leptin levels signal a shortage to the brain. It responds with less thyroid hormone (especially T3), a dampened sympathetic nervous system (the activating part of the nervous system), a lower body temperature and more efficient muscles that use less energy for the same movement. At the same time, everyday activity drops without you noticing – you sit more and gesture less. And hunger increases. For our ancestors in times of famine, all of this was a lifesaver.

The most famous evidence is the Minnesota Starvation Experiment of 1944: with calorie intake halved for 24 weeks, the participants' basal metabolic rate fell by around 40% – far more than the weight loss could explain. A more recent example is the study of contestants from the US show “The Biggest Loser” (Fothergill 2016): six years after the extreme weight-loss competition, their resting metabolic rate was still around 500 kcal below the expected value. Figures like these apply to extreme cases; with a moderate deficit, the adaptation remains much smaller and largely reverses once the diet ends.

What keeps the adaptation small: a moderate deficit of 300 to 500 kcal instead of a crash diet; enough protein and strength training to preserve muscle; deliberately high everyday activity, for example through step goals; enough sleep; and planned maintenance phases of one to two weeks every few months, during which hormones can recover. In the Australian MATADOR study (2018), men who switched between a deficit and maintenance calories every two weeks lost more fat and kept their weight off better afterwards than the group in a continuous deficit. What's wrong is the idea of “starvation mode”, in which you gain weight despite a deficit. That doesn't happen – energy expenditure falls, but never below intake.

For children and teenagers, this is another reason why diets aren't for them: their bodies need energy to grow and react particularly sensitively to shortages.