Metabolic adaptation on a diet

A plateau on a diet is often explained by a «slowed metabolism», and there is a grain of truth in this. The editors examined what metabolic adaptation is, what mechanisms lie behind it, how pronounced it is in ordinary people and athletes, and what can actually be done.

What metabolic adaptation is

Anyone who has dieted for a long time knows the situation: in the first weeks weight comes off steadily, and then, on the same food, progress slows or stops. Partly this is explained by simple arithmetic — a smaller body spends less energy. But studies show that expenditure falls more than can be explained by loss of mass alone. This «excess» drop is called metabolic adaptation, or adaptive thermogenesis.

The classic work on this topic was published by Leibel, Rosenbaum and Hirsch (1995) in the New England Journal of Medicine. Participants who reduced their body weight showed a drop in total energy expenditure greater than predicted for their new mass and body composition, while those who gained weight showed, on the contrary, an increase. The body seems to «defend» the previous mass.

Even earlier, the Minnesota Starvation Experiment led by Ancel Keys described how prolonged substantial calorie restriction lowers the basal metabolic rate and spontaneous activity and heightens an obsession with food. This work remains a foundational source on the psychophysiology of prolonged deficit.

For athletes this topic is especially relevant: they often push their body-fat percentage below the levels «comfortable» for the body, and it is precisely then that adaptation shows most strongly. A detailed review of this question in the context of sport was offered by Trexler, Smith-Ryan and Norton (2014).

What energy expenditure consists of

To understand where exactly adaptation «hides», it is worth breaking daily energy expenditure into components. The basal metabolic rate is the energy for sustaining life at rest; it makes up the largest share in most people. The thermic effect of food is the cost of digestion. Then comes the energy of planned training and so-called NEAT: all unplanned activity, from walking to gesturing.

With a prolonged deficit, all components fall. The basal metabolic rate drops because of loss of mass and, additionally, because of hormonal changes. The thermic effect of food decreases simply because the person eats less. Training «costs» less, because a lighter body needs less energy for the same work, and the muscles become more efficient.

The most variable component is NEAT. People on a diet unconsciously move less: they walk less, sit more often, get up less. This change can be substantial and at the same time imperceptible to the person, so a pedometer is one of the most useful monitoring tools during a cut.

Rosenbaum and Leibel (2010) in their review stress that adaptive thermogenesis after weight reduction persists as long as the mass remains reduced. That is, it is not a temporary «glitch» but a new state of regulation to be worked with.

Hormonal and neural mechanisms

A key role in metabolic adaptation is played by leptin — a hormone produced by adipose tissue. Its level falls with a decrease in fat stores and, even faster, with calorie restriction. To the brain this is a signal of energy deficit: appetite grows, energy expenditure and sympathetic nervous system activity decline.

• Leptin— falls, heightens hunger and contributes to reduced energy expenditure;
• Ghrelin— the «hunger hormone», in many people rises on a diet;
• Thyroid hormones— active T3 may decline with a prolonged deficit;
• Sex hormones— in men testosterone may fall, in women the cycle may be disrupted;
• Cortisol— may rise against a backdrop of stress, sleep loss and intense training.

These changes make evolutionary sense: under conditions of food scarcity, a body that saves energy and searches harder for food had a better chance of survival. The problem is that under modern conditions the same mechanism gets in the way of holding onto the achieved result.

It is important to distinguish adaptation from pathology. A decline in T3 or testosterone on a diet is mostly a physiological reaction that recovers when normal calorie intake returns. Self-«treatment» of these changes with hormonal drugs is dangerous and has no medical basis.

In female and male athletes with prolonged low energy availability, the syndrome of relative energy deficiency in sport (RED-S) has been described, affecting bones, hormones, immunity and the psyche. Metabolic adaptation is one of its components.

How strong the effect can be

The magnitude of adaptation is highly individual and depends on the duration and depth of the deficit, the initial body-fat percentage and genetics. In most people on moderate diets it is relatively small — on the order of a few percent of daily expenditure. In lean athletes on prolonged preparation and in people with extreme weight loss it can be much larger.

The best-known example is the study by Fothergill and colleagues (2016) of contestants from the TV show «The Biggest Loser». Six years after the competition their basal metabolic rate remained substantially below that predicted for their mass — on average by several hundred kilocalories a day, even in those who had partly regained weight.

At the same time, these data should not be transferred directly to an ordinary athlete. The show's contestants lost a very large mass in a short time under extreme conditions. For a person on a moderate cut, adaptation is usually far more modest and partly recedes after the diet ends.

The widespread notion of a «damaged metabolism», in which a person cannot lose weight even on a very low calorie intake, is not supported by science. More often the cause of a stall is inaccurate food tracking, reduced NEAT and water retention, not a «broken» metabolism.

What can be done in practice

It is impossible to avoid metabolic adaptation entirely, but it can be softened. The first tool is a moderate deficit and a slow pace: the less aggressive the diet, the weaker the compensatory reactions and the better the muscle is preserved — muscle being the main «consumer» of energy at rest.

The second is high protein and strength training. They preserve lean mass and support energy expenditure. The third is control of NEAT: the daily step count is a simple, objective measure that helps notice an unconscious drop in activity in time.

The fourth tool is diet breaks. In the MATADOR study (Byrne et al., 2018), men with obesity who alternated two weeks of deficit with two weeks of maintenance calories lost more fat and held the result better than the continuous-diet group. The data for athletes are less clear-cut, but the very idea of planned breaks looks rational.

The fifth is sleep and stress. Sleep loss heightens hunger and lowers spontaneous activity, so without normal sleep any dietary strategy works worse.

Finally, ending the diet should be planned. A gradual increase in calorie intake after a cut allows the hormonal background and energy expenditure to be partly restored without a sharp gain of fat.

Editorial conclusions

Metabolic adaptation is a real physiological phenomenon: with a prolonged deficit, energy expenditure falls more than the loss of mass predicts. The main mechanisms are a decline in leptin, thyroid hormones, sympathetic activity and NEAT.

For most people on moderate diets the effect is small, and the myth of a «broken metabolism» is exaggerated. A moderate deficit, high protein, strength training, step control and diet breaks help soften the adaptation.

Any hormonal disturbances on a diet require evaluation by a doctor, not self-treatment.

We also recommend our articles on the reverse diet after a cut, the basic principles of nutrition during a cut and RED-S in athletes.

References

1. Leibel RL, Rosenbaum M, Hirsch J. Changes in energy expenditure resulting from altered body weight. N Engl J Med. 1995;332(10):621–628.
2. Rosenbaum M, Leibel RL. Adaptive thermogenesis in humans. Int J Obes (Lond). 2010;34 Suppl 1:S47–S55.
3. Trexler ET, Smith-Ryan AE, Norton LE. Metabolic adaptation to weight loss: implications for the athlete. J Int Soc Sports Nutr. 2014;11:7.
4. Fothergill E, Guo J, Howard L, et al. Persistent metabolic adaptation 6 years after "The Biggest Loser" competition. Obesity (Silver Spring). 2016;24(8):1612–1619.
5. Byrne NM, Sainsbury A, King NA, Hills AP, Wood RE. Intermittent energy restriction improves weight loss efficiency in obese men: the MATADOR study. Int J Obes (Lond). 2018;42(2):129–138.
6. Keys A, Brožek J, Henschel A, Mickelsen O, Taylor HL. The Biology of Human Starvation. Minneapolis: University of Minnesota Press; 1950.