What is metabolic adaptation?

In one line

Metabolic adaptation is when a body that has lost weight uses less energy than its smaller size alone would predict.

What is metabolic adaptation?
The saying 'dieting lowers your metabolism'
After a long diet you often hear 'my body adapted and my metabolism dropped'. Does losing weight really make the body use less energy? If so, what exactly is happening?
In one line
Using less than body size predicts
Metabolic adaptation is when, after weight loss from dieting, the body uses less energy than its smaller size alone would predict. In research it is also called 'adaptive thermogenesis'.
Background
Metabolism and the energy you use each day
All the chemical reactions by which the body gets energy from food and uses it are together called 'metabolism'. The energy the body uses each day splits into what it uses at rest, what it uses to digest food, and what it uses to move. The biggest share is the energy used at rest.
A smaller body
Naturally uses less
When weight drops, the energy needed to keep the body going also drops, in line with the fat and the non-fat parts such as muscle and bone (lean body mass) that were lost. This is the normal drop from having a smaller body, and it is not metabolic adaptation.
Metabolic adaptation is
The drop beyond the prediction
Researchers use the remaining fat and lean body mass to predict 'how much energy this body should use' and compare it with what they actually measure. If the measured value is lower than predicted, the gap is called metabolic adaptation. For example, if the prediction is 1,500 kcal and the body actually uses 1,400 kcal, the adaptation is 100 kcal a day.
Where the drop comes from
At rest while losing, in movement while maintaining
While people were losing weight, metabolic adaptation showed up mainly in the energy used at rest. While they kept the lower weight, most of the shortfall below the prediction came from the energy used to move.
Less energy for the same movement
Muscles become more efficient
The less energy something needs to do the same work, the higher its 'efficiency'. People who kept their weight 10% lower showed about 20-27% higher muscle efficiency during cycling. In other words, they used less energy for the same movement.
Why it happens
Leptin, a signal sent by fat
A hormone is a substance made in one part of the body that travels in the blood to send a signal elsewhere. Fat cells make a hormone called 'leptin' that tells the brain how much fat there is. When fat shrinks and leptin falls, the body shifts toward saving energy.
Its size depends on timing
Larger while losing, smaller after 4 weeks of holding
Right after losing weight on a 1,000 kcal a day diet, energy used at rest was about 90 kcal a day below the prediction. Measured again after 4 weeks of holding weight steady, the gap had shrunk to less than half.
After a very large loss
About 500 kcal a day even 6 years later
Contestants on a TV weight-loss competition who lost a very large amount of weight within months used about 280 kcal a day less at rest than predicted at the end of the show. 6 years later, after much of the weight had come back, the gap was even larger, at about 500 kcal.
A common misunderstanding
Does metabolic adaptation stop weight loss even in a deficit?
No. Metabolic adaptation lowers the calories you use and so shrinks the 'calorie deficit', the state of eating fewer calories than you use, by that amount. So weight drops less than calculated, but as long as a deficit remains, weight still falls. In a study of a 1,000 kcal a day diet, people still lost 14 kg on average during the very period when adaptation appeared.
Summary
Less than predicted, but weight fell while a deficit remained
Metabolic adaptation is when a body that has lost weight uses less energy than its smaller size would predict. It makes the calorie deficit smaller, but as long as a deficit remained, weight went down. Related terms: basal metabolic rate, calorie deficit, leptin.