Does a long diet slow down your metabolism?

Short answer

Yes. But the calories your body saves on its own are small, around 100 kcal a day, and most of it disappeared once people stopped losing and held their weight. The big drop comes from simply weighing less.

Does a long diet slow down your metabolism?
'Long diet → broken metabolism → no more fat loss' — does this formula hold?
A common claim: diet long enough and your body switches to 'saving mode,' your metabolism breaks, and you hit a plateau. We checked whether metabolism really drops, by how much, and whether it comes back.
Metabolism really does drop
But more than half is just a smaller body
When 32 people ate half the calories they needed for 3 weeks, they lost 6 kg and the calories they burned at rest fell by 266 kcal a day. Take out the part explained by the weight they lost, and the calories the body saved on its own came to 72 kcal a day.
It looks bigger while you are still dieting
Holding weight cut it by more than half
71 people ate just 1,000 kcal a day for 8 weeks and lost 14 kg on average. Right after the diet, the body was saving 92 kcal a day. After 4 weeks of holding their weight, it was 38 kcal. When 45 of them were measured 1 year later, it was 7 kcal — no difference left.
Does it stick around for years?
After 1-2 years there was no meaningful gap
Right after 171 people lost 12 kg on average, their bodies were saving 54 kcal a day. In the 46 followed for 2 years, it was 18 kcal at 1 year and 19 kcal at 2 years — no meaningful difference left. The drop was also unrelated to how much weight people regained.
So where does 'my metabolism is broken' come from?
An extreme weight-loss case
14 contestants on a TV weight-loss show lost 58 kg on average over 30 weeks, and 6 years later their bodies were still saving 499 kcal a day. But the drop at the end of the show did not predict how much weight they regained, and those who kept more weight off 6 years later had the bigger drop.
What about 21 weeks of contest prep?
It dipped, then came back up after the show
Over about 21 weeks of dieting for a contest, 19 people burned 95 kcal a day less at rest. At 12 weeks after the show, they were burning 143 kcal more than when they started. Another study of 23 people prepping for a contest also saw it return toward its starting level 23 weeks after the show.
Taking breaks meant a smaller drop
2 weeks of dieting, 2 weeks at maintenance, repeated
51 people either dieted for 16 weeks straight or alternated 2 weeks of dieting with 2 weeks at maintenance. Of the 36 who finished, the break group lost 14.1 kg and the straight group 9.1 kg. The calories the body saved were 86 kcal a day with breaks — half of the 179 kcal in the straight group.
Bottom line: it drops, but a little, and it comes back
The 'broken metabolism' mostly fixes itself once you stop losing
During a diet, the body saved around 100 kcal a day on its own. For someone eating 2,000 kcal a day, that is about 5%. Most of a plateau comes from burning fewer calories because you weigh less, and a stretch at maintenance erased most of the extra savings.