It's nearly always a side effect of weight loss, and it's a good thing. It's confusing how people act like it's bad.
If you're obese, your legs are enormously strong with crazy amounts of muscle. Have you ever seen someone obese do leg presses at the gym? They can handle tons of weight.
When you lose weight, you don't need all that extra muscle. If you're carrying around 180 lbs instead of 300 lbs on your frame, all that extra muscle goes away because there's no point in keeping it. This is a good thing.
Even if you go from 200 lbs to 180 lbs, there's a level of muscle you don't need anymore.
Losing muscle as you lose weight is natural and good. Now, obviously you don't want to lose so much that you become weak for your size, but that won't happen if you continue to be physically active. Your body is exceptionally good at maintaining the exact right level of muscle to meet the regular stresses it undergoes. (Provided you are eating enough protein, but that's easy.)
That is absurd. Muscle mass is a huge predictor of mortality and anyone who isn't actively strength training and maintaining higher levels of muscle would see a health benefit by doing so. The idea that an overweight person has "too much muscle" is nonsense.
The side affect of muscle loss from these glp-1 agonist drugs is a serious downside that everyone should be aware of and try to mitigate if they choose to take them.
> The idea that an overweight person has "too much muscle" is nonsense.
I think you've misunderstood GP. He's not saying they have too much muscle when they're overweight. He's saying that take that exact same amount of muscle, subtract a ton of fat from their upper bodies, then they have "too much muscle" as typically needed for their body mass. I don't agree with their phrasing, but the point isn't "nonsense."
> then they have "too much muscle" as typically needed for their body mass
It's a leap to suggest that it's "good" to lose this muscle mass. If you're obese then it's good to lose fat. It's even better to do so while maintaining muscle mass.
Yeah, but whether you do resistance training while losing weight so that you have proportionally extra muscle mass than you had when you were overweight has nothing to do with the GLP-1 drug.
Every time GLP-1 drugs come up, the convo splinters off into topics that have nothing to do with anything unique to the drug. Now we're just talking about general weight loss and that it's good to exercise. Which is a trivial claim.
The problem is it is bad. People aren't just losing a proportional amount of muscle to fat mass to keep a good ratio - rapid weight loss is more catabolic towards lean body mass than it is with slower weight loss.
And most obese people are below optimal levels of total lean body mass overall even before this - their leg muscles being larger than an untrained person of the same height and an average weight does not mean that their leg muscles are of optimal size, much less the rest of their body.
Sarcopenia is a real risk for any obese person who is rapidly losing weight and GLP-1s are no exception.
I understand the point. Mine is that an ordinary untrained person will see health benefits and a statistically longer life from more muscle. An overweight person would have more muscle than their non-overweight counterparts (mostly localized to the legs, not upper body), and that is the one and only positive of being overweight. Willfully throwing that away will harm your health, full stop. The muscle loss problem with these drugs is talked about a lot because it is in fact a problem. Not because the medical field is mistaken in thinking it's a bad thing.
There’s negative cardiovascular effects from excess muscle just as with excess fat.
Normally that’s offset by the health effects of the exercise required to gain and maintain them as well as the lack of medical conditions that prevent exercise etc. But a fat person losing weight should inherently lose muscle mass long term assuming no changes to lifestyle.
This is one of those statements that is technically true but not particularly relevant. Obese people are almost never at a level of lean body mass that they would overall be at a good amount of muscle even for a healthy total weight, and rapid weight loss has consistently been shown in studies to reduce more lean body mass than slower weight loss.
Getting to the same level of additional weight from muscles as there is from fat is also incredibly difficult. The average 5'8 person not utilizing AAS would take years of dedicated training, dialed in diet and recovery, etc., to get to 200lb at 15% bodyfat. People get there much faster and much easier putting on fat.
What’s relevant is subjective. I’ll agree it’s far from as important as obesity, but the point is some of this adaptation is good.
As to loss of muscle mass from rapid weight loss, that’s very true but slightly overstated as regaining muscle can occur ~10x as fast as it takes to grow it in the first place. Someone without a significant calorie deficit barring nutritional deficiency or other impediment will regain whatever muscle mass is required for their lifestyle quite quickly. However, people don’t train with weighted vests as among other things it targets the wrong muscle groups.
The problem here is that the overwhelming majority of people that need GLP-1s have lifestyles that are not conducive to health and have less muscle mass than would be healthy to begin with.
Which is what has me so confounded by this claims - these people already have less muscle than they should for optimal health. Losing any is a significant issue. And without lifestyle intervention, they're not going to regain any of that lost muscle.
GLP-1 drugs are phenomenal and a huge win for health outcomes. But that doesn't mean we shouldn't be telling people the truth about their downsides and what they need to do to counteract them.
Let’s ignore GLP-1, and assume an individual slowly dieted to that exact same weight. What’s going to happen over the following year(s) is their lifestyle causes identical muscle loss.
In that context is this a long term downside of GLP-1 or their lifestyle?
Treating lean muscle mass as an inherent good makes sense when it’s adapted to a person’s current lifestyle. It doesn’t make sense when it’s a temporary situation.
False dichotomy - much of the muscle loss happens because of the rapid weight loss. If you diet more slowly and are not at very high levels of relative musculature you’re not going to lose nearly as much even in the long run, at least prior to advanced age. The newly skinny person might lose some muscle mass on their legs if they dieted down slowly, but it is significantly less likely that they will lose all of the other body-wide muscle mass that they do with rapid weight loss.
Your body will hold on to a surprising amount of muscle. Even people that take PEDs can drop back down to natural testosterone levels and keep ~80% of their gains in the long term. (Actual gains, not the temporary glycogen and intra-muscular water retention that you see with some compounds on cycle.) It takes significant effort for them, but they’re trying to hold on to supraphysiological levels, vs. someone who is trying to hold on to what is already on the lower end of the physiological range.
No, you’re looking at the short term effects on a largely irrelevant metric (volume) while ignoring the underlying mechanisms at play.
> Your body will hold on to a surprising amount of muscle.
It’s only surprising if you ignore what going on at the cellular level. Gaining fat or muscle eventually involves gaining new cells and structures like capillaries not just increased the volume of existing cells. This is a really slow process in muscle, but those new cells stick around and can rapidly adapt to stimulus as long as you have a sufficient diet.
So yes in rapid weight loss your individual muscles cells become smaller and less capable but that’s very quick to recover. Longer term, you’ll hit the exact same homeostasis point based on stimulus and long term diet.
PS: If this seems like voodoo magic it’s simply a very old evolutionary response to starvation that predates hominid development. Being able to fairly rapidly lower energy expenditure over say winter and then recover is a major advantage.
> Gaining fat or muscle eventually involves gaining new cells and structures like capillaries not just increased the volume of existing cells. This is a really slow process in muscle, but those new cells stick around and can rapidly adapt to stimulus as long as you have a sufficient diet.
It's an open question if hyperplasia even occurs in adult humans, and if so, under what conditions. MSC proliferation and differentiation is a thing, but none of this is actually particularly relevant to the discussion at hand.
> So yes in rapid weight loss your individual muscles cells become smaller and less capable but that’s very quick to recover. Longer term, you’ll hit the exact same homeostasis point based on stimulus and long term diet.
I'm not talking about hypothetical situations where people treat this more like a cut and then turn their lives around when it comes to resistance training and protein intake, because we know that in large part they don't. We know that a good portion of people on GLP-1 medications become sarcopenic and stay that way. And this whole thread basically started because someone was claiming that the muscle loss was good!
I'm not saying GLP-1s are bad or rapid weight loss is bad - I'm just saying you need to take steps to avoid muscle loss. And a huge amount of people on GLP-1s don't know about or understand these risks.
> I'm just saying you need to take steps to avoid muscle loss.
Do you have any evidence that long term lifestyle isn’t going to result in similar levels of strength?
> if hyperplasia even occurs in adult humans
It definitely occurs in other mammal muscle and some human tissue, and there’s studies supporting it occurring in human muscle. Myofibre splitting for example definitely occurs. https://pubmed.ncbi.nlm.nih.gov/16625366/
I’ll admit there’s controversy here, but I think the default position should be human biology is similar to other mammals unless someone demonstrates otherwise. Otherwise our unwillingness to preform experiments on humans and the expense of primate experimentation is going to create a scientific bias.
I was looking for this comment! Not a doctor, but as I understand it muscle is roughly equivalent to fat as far as your heart’s workload is concerned. I thought I also read that muscle movement helps with blood return.
Your complaint doesn't make sense to me. The negative health impact of being obese isn't equalized by having a little more muscle mass.
Also, if this is your stance, then GLP-1 is a red herring because you have the same issue with weight loss in general. Weight loss, without increasing resistance training, leads to muscle loss.
Yea what you're replying to is just pure fat-logic that isn't really backed by science. You will lose muscle from calorie restriction - that isn't really in doubt by anyone. However, when cutting weight, you can do a routine that maintains/builds muscle as you cut, to reduce the effect. A body with more lean muscle mass will be able to keep weight off for longer - this has been known, settled, and accepted in weight loss and fitness science for decades now. I've never heard anyone, anywhere posit that muscle loss is good - and would love to see a source, so I can laugh at it.
One insidious thing with these GLP1 drugs, is that they also seemingly affect muscles like the heart. I would not be willing to take one unless the risk of me carrying my weight far outweighed (no pun intended) the risk of the side effects. However, a lot of people seem to be treating it as some kind of miracle fad diet drug, which is concerning.
It also has other side effects like reduced elasticity on skin, etc. I suspect we'll see longer term issues in the next decade from these drugs, and I'm glad alternatives are being explored.
> Yea what you're replying to is just pure fat-logic
I can't even imagine what that's supposed to mean.
> However, when cutting weight, you can do a routine that maintains/builds muscle as you cut, to reduce the effect.
I literally talked about staying physically active.
The point is, you're going to have however much muscle your workouts and physical activity build/maintain. And you're going to lose whatever extra muscle isn't needed in your workouts. And that's fine, because you probably want well-balanced strength rather than legs that can carry around 300 lbs all day long.
None of this has anything to do with weight loss, except that simply walking around and daily movement becomes less of a workout as you lose weight because you're moving less mass. But it's not the weight loss directly that makes you lose muscle (assuming you're eating protein), it's the reduced physical strain because you weigh less so you're not needing those muscles. Do you get the distinction?
You don't need to work out even more to "reduce the effect" as you say. There's no effect. There's just working out to have whatever muscles you want. Weight loss will never lead to losing the level of muscle you need for your workout.
If you're "cutting" as part of bulking and cutting, then obviously.
But if you're going from obese to healthy, then your goal isn't to retain all your leg muscle, that's absurd. Your goal is to get to a healthy weight with overall balanced healthy muscle -- not disproportionately large legs.
Nothing is "strictly superior". What is best depends on what your goals are. Bodybuilding and not being obese any more involve wildly different measures of success.
If we didn't see tons of people reaching their "target weight" on GLP-1 drugs while having unhealthily low levels of lean body mass, you might have a point.
But that is what we do see. I'd argue that they're still in a better place than they were before, and we know that muscle that has been lost recently grows back very quickly when exposed to stimulus and adequate protein and rest, so I absolutely am a full believer in GLP-1 medications.
But if everyone on GLP-1 meds were keeping up with their protein and resistance training, even leg day, there would be very very few that had huge legs. That one portion of their body might be, on average, more muscular than a totally untrained individual, but it's not like being fat is the equivalent of a low bf% bodybuilder when it comes to lower body muscle mass. A formerly 300lb person dropping down to 180 with average genetics is almost certainly going to have smaller legs than someone who has been lifting for a year at the same height/weight.
The goal for anyone obese is to lose as much fat as sustainably possible.
For any given rate of weight loss, losing a higher proportion of body fat is always better.
This notion of "balanced" healthy muscle is one you've made up that no one else thinks of. I've been on fitness forums for well over a decade and have literally never seen a single case of this.
> The goal for anyone obese is to lose as much fat as sustainably possible.
Correct.
> For any given rate of weight loss, losing a higher proportion of body fat is always better.
That is in direct contradiction to your previous sentence. No, losing the higher absolute amount of body fat is better. While being sustainable healthy.
> I've been on fitness forums for well over a decade and have literally never seen a single case of this.
You may be on the wrong forums then. Most forums don't think all your days should be leg days.
> That is in direct contradiction to your previous sentence. No, losing the higher absolute amount of body fat is better. While being sustainable healthy.
This isn't in any way a contradiction if you're mathematically literate. If the highest rate of weight loss is some rate R, and losing weight without preserving muscle has you losing 0.5R muscle and 0.5R fat, losing R fat and 0 muscle is literally going to lose you more fat at the same rate. This is middle school math, and is literally what anyone who knows what they're talking about suggests, and is validated in the research.
> You may be on the wrong forums then. Most forums don't think all your days should be leg days.
Oh good more shit you've made up. No where did I suggest this. If you're going to engage in bad faith, go away.
Any well-rounded strength program will preserve muscle mass in a cut. You only need to work every muscle twice per week. No leg emphasis needed.
No insults, please. I'm sure that if you have good arguments, you won't need to resort to that kind of attitude. And the fact that you're resorting to swearing later in your comment doesn't help you.
> If the highest rate of weight loss is some rate R
That's not a valid precondition. Nobody chooses a rate to lose weight at, and then magically chooses some proportion of fat vs muscle. What actually happens is that you lose fat through eating less calories, and you naturally lose muscle from using your leg muscles less by putting less weight on them, because you weigh less.
> No where did I suggest this.
You did. You literally said:
> This notion of "balanced" healthy muscle is one you've made up
You are arguing that one should preserve all of the leg muscle for a 300 lb body, even once you're 180 lbs. Again, that's absurd. That's going to take a far disproportionate amount of time at the gym on your legs to maintain. You say "No leg emphasis needed." That's completely and utterly false.
You are talking about bulking and cutting for strength training. But you already have a balanced body, I'm assuming. This conversation, on the other hand, is about obese people losing weight. Their muscles are disproportionately in their legs. Do you get that now? Do you understand the concept of a balanced distribution of muscle and strength, e.g. between arms and legs? Or since you're so concerned about having a conversation at a "middle school" level, do you need it explained in even simpler terms?
>you naturally lose muscle from using your leg muscles less by putting less weight on them, because you weigh less.
That is one cause of muscle loss, but there is a more important factor: when running a calorie deficit, the body removes protein from muscle so that it can turn the protein into glucose for energy. The human body preferentially gets fuel (calories to use to fight infections, digest food, and do the other things needed just to stay alive) from muscle (as opposed to getting it from fat) during times of not-enough-food because it takes more energy (fuel) to keep muscle cells alive than it takes to keep fat cells alive.
We evolved in an environment in which the body had no way to know whether the time of not-enough-food would last so long that it would lead to death. In that scenario, the policy that maximizes the probability of survival is to disassemble most of the muscles first and only then to start to disassemble the fat in the fat cells.
Corollary: most people on weight-loss diets should regularly exercise their muscles especially if they are elderly (because the elderly find it hard or impossible to regain the muscle once it is lost) and should consume a decent amount of protein.
Having a large amount of muscle mass doesn't make you live longer (as far as I know): the case for striving to preserve muscle mass during the aging process is that it gives a higher quality of life.
> No insults, please. I'm sure that if you have good arguments, you won't need to resort to that kind of attitude. And the fact that you're resorting to swearing later in your comment doesn't help you.
I have zero patience for intellectual dishonesty and made up claims as you've repeatedly done.
> That's not a valid precondition. Nobody chooses a rate to lose weight at, and then magically chooses some proportion of fat vs muscle. What actually happens is that you lose fat through eating less calories, and you naturally lose muscle from using your leg muscles less by putting less weight on them, because you weigh less.
Do you not know how thought experiments work? If you lose weight without strength training, you lose less fat and more muscle than if you lost weight while strength training. The numbers were chosen to illustrate that point, not to say those were precisely the numbers you'll see.
And of course it's entirely wrong that you lose muscle in your legs specifically because of the reduction in weight. You lose muscle everywhere you don't use it, which means a smaller proportion of your weight is lost as fat, which means it takes you longer to reach your target body composition.
> You are arguing that one should preserve all of the leg muscle for a 300 lb body, even once you're 180 lbs. Again, that's absurd. That's going to take a far disproportionate amount of time at the gym on your legs to maintain. You say "No leg emphasis needed." That's completely and utterly false.
This is yet more evidence of your complete ignorance of this topic and even what I've been suggesting. Your obsessive fear of having muscular legs blinds you to everything I've said.
Maintaining muscle mass takes far less work than building it. You literally only have to hit your muscles twice per week to do it, which could take anywhere as little as 1-3 hours per week of work if you know what you're doing.
You are objectively and empirically wrong about this and are only demonstrating how much you don't know about this topic.
> You are talking about bulking and cutting for strength training. But you already have a balanced body, I'm assuming. This conversation, on the other hand, is about obese people losing weight. Their muscles are disproportionately in their legs. Do you get that now? Do you understand the concept of a balanced distribution of muscle and strength, e.g. between arms and legs? Or since you're so concerned about having a conversation at a "middle school" level, do you need it explained in even simpler terms?
So much for no insults. Rules for thee and not for me seems like your motto.
If an obese person losing weight is worried about an imbalance, then the solution is not to let their legs atrophy to nothing but to increase the volume of the work for the muscles they fear are lagging so they can experience more growth and catch up, just like anyone else who has ever lifted weights.
These are extremely basic concepts in the field.
And lastly, a topic I should have mentioned earlier, anyone who follows your advice, which is clearly the advice of someone who neither knows what they're talking about nor the advice of someone with a healthy relationship with body image, will inevitably be increasing the risk and extent of lose skin if they avoid strength training. If you don't bother preserving muscle, not only will it take longer to reach your target body composition, you will have end up with less total tissue filling your skin and, short of surgery, you would have to build muscle to fill it up again to reduce or resolve the issue.
Your advice is objectively terrible, rooted in ignorance of basic fundamentals of the space, and will lead people to worse outcomes.
You should in no way be offering advice on this topic to anyone. You will only increase the chances that they give up as their journey takes longer and results in worse outcomes in terms of both health and aesthetics.
Don't you also need a bigger heart when you gain weight and not need as big of one at less weight? Liposuction and amputations can also result in muscle loss in the heart from it having less work to do.
You don't want to have too much hypertrophy in the heart for sure. My understanding though is that it's very hard (almost impossible?) for it to be a problem without exogenous hormones, or some other condition that allows you to accrue an abnormal amount of muscle mass (e.g. myostatin defficiency).
Edit: I mean someone with a healthy fat percentage body composition. Of course having to pump blood to a 300lb-140kg body is problematic for the heart, be it a mostly fat or mostly muscle body composition. My point is it's just much easier to be fat enough for it to be a problem than muscular enough without exogenous hormones or an abnormal condition.
Telmisartan, an ARB generally used for BP management, can actually reverse LVH to a significant degree over time, though. Popular for bodybuilders on large quantities of AAS for that reason.
> One insidious thing with these GLP1 drugs, is that they also seemingly affect muscles like the heart.
Okay so I don't know where I picked this up - it was a decade or more ago - but I always thought the problem was losing weight too fast is what causes bad muscle loss such as from the heart, or from leg/other muscles beyond what's no longer needed from the weight loss. Something like, you're starving yourself so your body starts drawing energy not just from your fat but from anywhere it can.
The idea that people on GLP-1s shouldn't be trying to preserve (or indeed build) as much muscle mass as they can is absurd. It is more important than ever to perform resistance training and make sure you are getting adequate protein.
But this isn't anything special about GLP-1s - the same is true for any sort of rapid weight loss approach, be it diet, GLP-1, lap band, whatever.
And we see improved cardiovascular outcomes independent of weight loss for people on GLP-1 drugs in widespread clinical trials, vs. one mechanistic in vitro study showing loss of heart muscle cells.
As long as you're not using exogenous hormones, muscle mass can only be achieved with exercise that builds or preserves muscle. I'd say that's a pretty good predictor against frailty, which is strongly associated with mortality among the elderly.
True, but frailty in the old age has a reason that cannot fully be mitigated with exercise: depletion of stem cells. The same mechanism will make our blood vessels thin and prone to bursting etc.
Exactly my point! you have a chain of 3 associations right there. One is nearly tautological and another has backwards causality. Correlation =/= causation.
Something being a good a good predicative indicator does not mean it is an effective intervention.
There is no backwards causality in the implication that building or preserving muscle that would otherwise be lost can prevent or delay frailty, nor is it backwards to imply that frailty can lead to death (from falls, disease, etc.). I really don't understand what you're trying to say.
Muscle actually is an intervention for one of the biggest risk factors in metabolic syndrome - insulin resistance.
Muscles use glycogen. They use more glycogen when you're doing the sort of thing that builds muscle, so of course it's even better there, but someone who just genetically puts on more muscle at the same level of activity as someone who puts on less will still use more blood sugar for their muscles, and thus be less likely to increase their insulin resistance.
There's a significant number of posts in this larger thread saying that it is basically just the benefits of exercise that is providing the positive health outcome and not just having the muscle, and I interpreted intervention in a broader manner based on that context.
My point is simply that muscle in and of itself has positive benefits, even if you didn't need to do another healthy activity to maintain or gain it.
"The idea that an overweight person has "too much muscle" is nonsense."
Our bodies like to have a balance of everything.
"Muscle mass is a huge predictor of mortality"
In biology, most such predictors work only up to a point. Massively muscular people don't live to be 120. Bodies don't work in a straightforward fashion, and there are other effects to consider. For example, activation of the mTOR pathway, associated with tissue growth, is associated with shorter lifespan, and mTOR inhibitors like rapamycin seem to be modestly prolonging lives of many species.
I think the downside is that you don't just lose muscle in your legs and core, but rather you lose muscle mass all over. Yes, this is the expected result of calorie restriction but the issue is that when people lose a lot of weight without taking action to build and maintain muscle the amount of muscle loss they experience can cause other problems. Someone relying on a GLP-1 drug is often someone who isn't very active at all.
Well, you lose weight all over. You lose fat in your arms and your shoulders don't need to be quite as strong. You lose weight on your face and your neck muscles don't need to be quite as strong. You lose more in your legs, but you're supposed to lose muscle mass all over.
> Someone relying on a GLP-1 drug is often someone who isn't very active at all.
Sure, but that's a completely separate issue. It doesn't have anything to do with weight loss. Being in good strong physical shape is great, but nobody should expect weight loss to magically result in strength. That's like thinking you can stop going to the gym but won't lose any of the muscle you'd previously built up. It's got nothing to do with weight loss though. It's got to do with the fact that you're not working out.
The problem is that fat, sick people lose a lot of weight and muscle mass and then become skinny and frail. It just puts you at risk of other medical issues. In older individuals this can cause decreased mobility and raise the risk of falls. Its well documented in medical literature, that's why people worry about it.
But my point is that this is no different from being skinny and frail in the first place. It has nothing to do with losing weight.
Yes, that puts you at risk. That's why you should exercise, even if it just means daily walks.
The point is, you're not winding up with too little muscle because you lost weight. If you're winding up with too little muscle, it's because you're not being physically active enough. If you're physically active, you won't lose the muscle that you still need even as you lose weight.
> The point is, you're not winding up with too little muscle because you lost weight
Imagine someone eating only candy and pastries, but because they eat so huge amounts, they actually get a half-decent amount of protein. Then they decrease portion size with the help of appetite suppresants.
If you were physically active enough and careful about your diet you likely wouldn't be on a GPL-1 drug. That's the point. People end up in bad shape for a lot of reasons, including injury. The search for a mechanism to burn fat without losing muscle is to help support people who are on GLP-1 drug because for whatever reason they were insufficiently active.
I was describing how you lose fat all over. And as part of overall fat reduction, you lose fat in your arms, because your arms are one part of all over.
This is silly. I'm a big proponent of GLP-1s (and on one!) but we can and do see people on these reaching sarcopenic levels of lean body mass. The overwhelming majority of people on these drugs should absolutely be doing everything they can to preserve their muscle mass.
(The overwhelming majority of people losing weight in general should be doing everything they can to preserve their muscle mass.)
Having slightly larger quads/hamstrings/calves than the average person doesn't mean they have an optimal amount of muscle mass in their legs to begin with, much less on the rest of their body.
Leg presses are also an absolutely terrible exercise to measure strength and by proxy muscle mass. The angle reduces the weight - generally only 71% - and they are one of the most "cheated" exercises in existence, with people doing what could charitably be called half reps (and is often more like quarter reps.)
> Losing muscle as you lose weight is natural and good.
It's natural but that doesn't make it good. In particular, muscle loss can cause heart problems because the heart is a muscle. The body's catabolic processes don't distinguish between different kinds of muscle.
I don’t think this is true. My understanding is that the heart is a very unique muscle because it doesn’t grow. If your heart does grow, either by genetic defect or anabolic abuse, it works much worse. So you’ll live a shorter life.
Heart muscle can shrink in extreme calorie deficits which can lead to heart issues, therefore it must grow when leaving such calorie deficits, even if that growth has an upper bound.
I would think any drastic change to body composition would have its risks, and should be mitigated through healthy diet and exercise. I suspect the detractors are quick to overemphasize such risks because users of weight loss drugs aren't 'earning' it. My counterargument would be that it's better to be underexercised and normal weight than to be underexercised and overweight. And I suspect being normal weight makes exercise more pleasant.
If you're obese, your legs are enormously strong with crazy amounts of muscle. Have you ever seen someone obese do leg presses at the gym? They can handle tons of weight.
When you lose weight, you don't need all that extra muscle. If you're carrying around 180 lbs instead of 300 lbs on your frame, all that extra muscle goes away because there's no point in keeping it. This is a good thing.
Even if you go from 200 lbs to 180 lbs, there's a level of muscle you don't need anymore.
Losing muscle as you lose weight is natural and good. Now, obviously you don't want to lose so much that you become weak for your size, but that won't happen if you continue to be physically active. Your body is exceptionally good at maintaining the exact right level of muscle to meet the regular stresses it undergoes. (Provided you are eating enough protein, but that's easy.)