Have I permanently damaged my metabolism from years of dieting?

Metabolic damage is a term that spread through dieting and bodybuilding forums, not through the research. It does rest on something real: adaptive thermogenesis. When you diet for a long time the body lowers its energy expenditure through shifted hormones (leptin among them), less spontaneous movement and more efficient mitochondria (Rosenbaum and Leibel, 2010; Trexler and colleagues, 2014). The point is that this is a response to being in a reduced-weight state, not a broken machine.

How large and how lasting can the brake get? The clearest example comes from The Biggest Loser (Fothergill and colleagues, 2016). Fourteen contestants were followed for six years after an extreme intervention. By the end they had lost 58 kg on average, and resting metabolism sat 610 kcal per day below baseline. Six years later they had regained 41 kg, yet resting metabolism was still around 700 kcal below baseline, and the part due to metabolic adaptation specifically was about 500 kcal per day. So the adaptation can persist for a long time, especially in those who manage to keep a large loss: the people who held onto the most weight loss also carried the most adaptation.

But persisting is not the same as permanent damage. The adaptation is proportional to the ongoing effort to hold a lower weight (Fothergill and colleagues, 2016) and is driven by the body trying to restore its fat stores (Rosenbaum and Leibel, 2010). As weight returns toward the body's defended level, the extra brake eases too. There is no support for ordinary dieting breaking your metabolism so that it never recovers. Your metabolism still scales with your body size. It is dampened while you are reduced, not broken.

The term feels true when you are struggling. After years of diets and a scale that will not move, "my metabolism is ruined" is an explanation that puts the fault outside yourself, and it is easier to carry than the thought that intake crept up or that the body defends its weight. It also sells. Reverse-dieting coaches, programs that promise to reset your metabolism and supplements that claim to fix your burn all rest on the premise that something is broken and needs their product. The more permanent the damage sounds, the more justified the expensive fix feels. And a dramatic story, the body that broke, spreads more easily in a feed than the quieter truth of a proportional and largely reversible adaptation.

You don't need to repair anything. Keep moderate deficits rather than extreme ones, and build in stretches at maintenance where you eat in balance. That lets the adaptation ease and is easier to sustain over time. If you want to see what your body actually burns, calculate your energy needs from your current size rather than trusting the feeling of a broken metabolism. And if years of dieting have made food a source of anxiety or control, that is worth raising with a health professional.