This post is a bit of a "rush to publish" w/o any scientific literature or blog linking, etc. I'll hopefully get to put the finishing touches on further installments addressing some studies (some James Krieger has discussed in posts on his blog and in comments here, and some others) and such.
But the discussion came up again in the comments HERE , and I figured this might be as good a time as any to put at least a few general comments together.
We hear this term a lot. The "damaged metabolism". What does that mean really? Near as I can figure, there are a group of people who believe that "metabolism" refers mostly "glucose metabolism" ... and this is assessed not by the metabolism of glucose (glycolysis) but rather by the ability to properly clear glucose from circulation. This in and of itself is a misnomer. Actual defects in glucose metabolism -- glycolysis -- are quite rare. Indeed they are likely fatal if significant because we need glucose and the ability to "burn" it for ATP/fuel production to survive. Indeed most cases of "glucose intolerance" are linked to defects or problems in what is known as non-oxidative glucose disposal. Not the metabolism of glucose for fuel but a defect in the metabolic pathway for storage as glycogen. There is evidence that a pre-disposition towards this problem is genetic, though environment (diet and inactivity) clearly contribute in just about every case. Further, changes in diet and activity can dramatically improve this defect.
So ... damaged? Perhaps, but well within our ability to influence through diet and, more importantly, activity. And just about any activity, contrary to what you might have heard! Measures of peripheral insulin resistance are generally measures of glucose clearance from circulation. And IR can be improved by exercise varying from the now much maligned "chronic cardio"/endurance exercise to HIIT, to more classic resistance training. While some strict application of one type may be better than another, if you have a damaged glucose metabolism, what to do? Get up off that computer chair and move. Just move. You'll do your metabolism -- glucose that is -- wonders.
OK, but can you do permanent damage? Well, yes. If you've progressed to being an insulin dependent diabetic (even through no lifestyle fault of your own as in T1 and autoimmune types) there's likely a permanent damage to your glucose metabolism. I'll be taking on the debate of whether reducing carb consumption to a bare minimum or ingesting reasonable carbs and supplementing with insulin is preferable here. But, that said, if you're recently diagnosed or merely obese and very insulin resistant, your metabolism is deranged, impaired, out-of-whack, etc. -- it is showing the signs of temporary damage -- but you can take your body to the shop and fix it. Perhaps like tuning up an old engine it's not like having a brand new car, but it's still more than capable of doing the job. From the gastric bypass results, to that crash diet cure, to intensive early insulin therapy -- all have shown a high success rate for not just glycemic control, but actual "cure" for the diabetic state where a high percentage can enjoy normal or close-enough-to-normal glucose tolerance. That, my friends, would be a "refurbished metabolism" in my book.
So ... this notion of some permanently damaged "glucose metabolism" is, IMO, largely bunk. Yes, if your pancreas has indeed suffered irreparable damage, then yes, it is permanent. But if not, it can be restored, perhaps through diet only, and perhaps with temporary pharmaceutical intervention.
Then there's the other "damaged metabolism" that is often referred to. Does dieting -- caloric restriction -- damage one's metabolism? I think there's no doubt that it does. In study after study when it's been measured, both basal and total energy expenditure tends to go down with prolonged caloric restriction. This is metabolic adaptation. A survival mechanism. But for the individual, this is complicated in the "anecdote" by the fact that once you weigh less, you will actually need less.
There is a body of animal and human evidence that shows that caloric restriction, even mild forms, results in an efficiency that preserves lean mass but favors saving fat for a rainy day over building lean mass. The body becomes more efficient in the face of scarcity just as we do in our daily lives. Reduced obese have lower energy expenditures than never obese weight-matched peers. This seems so depressing on its face.
Of course the LC schtick on all this is that because you get to eat high calorie, you avoid the fate of those hapless calorie restrictors. Ummm ... no. If you truly are eating more on LC and losing weight you have a "damaged digestive system" -- as in a fat absorption problem, and should probably see your doctor. Otherwise, if you're losing weight you are doing so by having established a caloric deficit and your BMR and/or TDEE will also go down accordingly. Perhaps in the short run there is an increase in NEAT (non-exercise activity). But carbohydrate restriction is the SAME metabolically as starvation for the body. I think this is the reason so many long term low carbers (excepting those who were never obese or significantly overweight to begin with) struggle so. If calorie restriction tanks one's metabolism, then carbohydrate restriction seems to do this doubly so over the long haul.
So ... what of the damage to your metabolism of dieting? My personal experience leads me to believe that this damage is not permanent -- or at least not completely so. There's no way to know what my TDEE or BMR would be today had I not gone through all the cycles and such in my life, but it's not zero. And it does appear to have rebounded considerably (although that is my impression since I never had it measured or kept much in the way of meticulous records). Still, I don't struggle against an appetite to prevent regain. (My issue is with losing more.) And even if our "dieted" bodies are more efficient in energy utilization to move, walking a mile still burns more calories than not doing it. So what if Frank burns 120 cals and Sally burns 100 and I only burn 80. I still have the ability to burn the 80 -- and in doing so do something about that other "damaged metabolism" I addressed up on the top of the page.
When you lose weight you lose some lean mass. Getting sufficient protein seems to minimize this, but it is not a total stop gap. And, frankly, you don't really want that b/c you just don't need the organ capacity to sustain the smaller organism you've become. But just because you've lost it doesn't mean you can't enact strategies to help build muscle. Here is where a high carb, moderate-high protein diet seems to outperform a high fat, moderate-high protein diet.
In closing I'll leave you with this. And perhaps it's Pollyanna wishful thinking, but it beats defeatist negativism in my book. There's almost no such thing as the hopelessly damaged metabolism. In almost all cases, one can repair and rebuild to live fully another day. < /corny >