Does your metabolism slow down?
"I heard dieting slows your metabolism for good. Is that true?" Partly, and much less than the sentence implies.
What the term means
After weight loss the body uses less energy, for two separate reasons. The first is simply that there is less of it: a smaller body has a lower resting expenditure and cheaper movement. That is not an adaptation; it is arithmetic, and any calculator captures it by recomputing from the new weight. The second is adaptive thermogenesis — expenditure falling below what the new weight and composition would predict. That is the part people mean when they say "slowed metabolism", and it is the part the research is about.
What the reviews show
A systematic review of the measurement studies finds that the adaptation is real, that it varies a great deal between individuals and between studies, and that in the better-controlled studies it is often small — on the order of a few percent of expenditure rather than the hundreds of kilocalories the folklore suggests. It appears during active weight loss and tends to shrink with time at a stable weight, though the studies are not long enough to say "it disappears" with confidence. The review deliberately does not offer a single number, and neither does this page: the honest summary is "some, usually modest, and not the same for everyone".
The famous extreme case
The study most often quoted measured fourteen contestants from a television weight-loss competition six years after it, and found a persistent, large adaptation. It is a real finding about those fourteen people — who lost enormous amounts of weight very fast, with extreme exercise, on camera. It is not a description of what happens to someone losing half a percent of body weight a week with adequate protein, and a later analysis of the same data argued that the adaptation was proportional to how much weight had been kept off rather than a fixed penalty. It is cited here because you will meet it, not because it is the norm.
What makes a difference
The factors that show up consistently are the ones this site keeps returning to: a moderate pace rather than a crash, enough protein, and resistance training to keep lean tissue — since lean tissue is what sets resting expenditure. None of them makes adaptation zero. All of them keep it in the small range.
What this page does not claim
It does not give a figure for "how much your metabolism slows", because the evidence does not support one. It does not claim adaptation is zero. And it does not endorse "metabolic damage" or "starvation mode" as explanations for a stalled diet — the plateau page has the explanations that are usually right.
In the app
The app assumes no adaptation up front — an honest prior, since the evidence gives no reliable number — and corrects through the trend of your actual weight instead of guessing at a penalty.