Was reading a cycling mag at a bookstore this weekend (don’t remember the name) which had an interview with pro Nicholas Roche. He was talking about race weight etc. and said that he has been on so many diets over the last 10 years that it’s damaged his metabolism (aka the body’s chemical processes).
He didn’t go into any further detail but can this actually happen?
Thanks!
i listened to a general fitness podcast, Muscle for Life, talking about this awhile ago,
https://www.muscleforlife.com/metabolic-damage/
He brought up a few studies, my favorite of which, was the Minnesota Starvation Experiment where people lost an average of 25% of their body weight over 24 weeks. The conclusions of the study was that most metabolic processes returned to normal relatively quickly. But none of these studies were looking at advanced athletes. So far as i know the research hasn’t been done. it wouldn’t surprise me if something could happen with prolonged restriction, our bodies desire to survive and always being prepared for the coming long winter and short harvest sometimes result in some backasswords built in safety mechanisms. Also, event if its not anything strictly biological, the minnesota starvation experiment recorded all sorts of crazy psychological conditions under starvation conditions, and any sort of psychological effect long term dieting has on an athlete can both impact performance directly, and indirectly by effecting the metabolic process in a roundabout way like the placebo effect.
My understanding from having watched my fitness and bodybuilding friends “diet” for competitions years ago is that nothing is really damaged. However, because the body is adaptive, the big changes in weight or weight swings end up changing the way our brains regulate our weight. As a result of all the dieting, their metabolism slows down. 
I think I got that right.
edit
relevant paper: NCBI Paper: Adaptive thermogenesis in humans
There was a really good study a few years ago regarding competitors from the Biggest Loser - I believe the summary was that the competitors metabolism dropped something like 25% as a result of the caloric restriction, such that it made the likelihood of the post show weight gain materially higher as they needed to basically continue starving themselves just to maintain the weight…
6 Years after The Biggest Loser, Metabolism Is Slower and Weight Is Back Up
Not 100% on point, but close enough and interesting: studies of people who were in utero during the Dutch Hunger Winter have shown life-long metabolic changes. The linked NYT article below doesn’t discuss whether adults going through severe starvation have similar long term impacts.
The biggest loser study is really bad science, when I get home I will find the study that explains exactly why.
I have often wondered this myself. I went 37 years of life eating whatever I felt like and was one of those lucky people that stayed the same weight. Burned calories super easy. Then when I found endurance training I have changed my diet multiple times in search for a performance or recovery advantage and I truly struggle with food now. I feel like I’ve screwed up a what was once a good natural metabolism…
If the restriction is big enough and long enough, your metabolism will slow to limit the damage. In transient situations it’ll usually return to normal. Go long enough or restrictive enough and you can, indeed damage your metabolism.
If you’re trying to lose weight/body fat, doing it slowly and intelligently will usually get you to your goal without causing any harm. Then when you reach your goal, remember you can’t keep doing the stuff that helped you lose weight. Namely, you usually need to add some calories back in.
For folks that are trying to train hard and/or have a lot of stress their lives, severely restricting calories (and especially carbs) can cause your hormone levels to fall off, and even thyroid problems.
Finally, remember that a large part of your metabolism is your muscle mass. Do whatever you can to not lose muscle.
-Hugh