Exercise is fueled by muscle glycogen. And no that doesn’t get restored during exercise by the liver.
that’s what i’m saying
I have to question what you say there as muscle loss is such an important and unwanted feature of the drugs.
Muscle loss might be mitigated by increasing protein intake and resistance training but that advice is extrapolated from other weight loss techniques such as calorie restriction or bariatric surgery.
There is trial evidence clearly demonstrating muscle loss on GLP1 agonists but as far as I know effective mitigation is not evidence based, only theorized.
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Yep. Hepatic gluconeogenesis and glycogenolysis are critical to performing endurance exercise.
This reference is quite to the point:
The accelerated metabolic demands of the working muscle cannot be met without a robust response from the liver. If not for the hepatic response, sustained exercise would be impossible.
It’s the same reason drinking a lot of alcohol decreases aerobic performance. Ethanol oxidation depletes hepatic NAD+ which impairs the Cori cycle, which is quite an important source of muscle fuel during aerobic exercise.
Exercise is fueled by muscle glycogen… but also by circulating fatty acids, hepatic glycogenolysis and gluconeogenesis, and recycling of lactate created by muscle by the liver back into pyruvate (Cori cycle). Among other processes.
See above. At low intensities and short durations, fatty acid metabolism is the major fuel. At high intensities and short durations, muscle glycogen is the major fuel. Even here though, plasma glucose/lactate from the liver and elsewhere still contribute significantly.
However, as you start getting into longer durations of aerobic exercise (ex:longer than 60-120min), the role of muscle glycogen decreases further, with the liver taking over a larger role:
During 2 h of exercise at 65% VO2max plasma-derived substrate oxidation progressively increased over time, whereas muscle glycogen and triglyceride oxidation decreased.
We don’t completely understand the mechanism of muscle loss or sarcopenia (potentially slightly different things) from GLP1ras. So an assertion that it is caused by inadequate protein intake or exercise is disingenuous.
I agree there’s certainly construct validity that doing these things would help prevent or slow it though.
Doesn’t it seem obvious though that it’s due to the negative energy balance?
Not at all!
I mean, rapid weight loss will be part of it for sure. We see big drops in muscle mass after bariatric surgery for this reason.
But there is a signal in the literature that GLP1ras may be doing something directly to muscle cells as well.
This is all very new and there is not much science on this yet.
Here’s an example from some local researchers.. This is looking at cardiac muscle rather than skeletal muscle mind you, but even in vitro where they just expose cultured cells to the drug and you’ve ruled out the effects of calorie restriction and weight loss, there turns out to be a direct effect on the muscle cells themselves:
Moreover, cultured human cardiomyocytes (AC16 cells) treated with semaglutide (10 nmol/L, 24 hours) showed reduced cardiomyocyte area, demonstrating a direct cardiomyocyte autonomous effect of semaglutide, that is independent of potential changes in workload that may have occurred in vivo… Together these data indicate that the reduction in cardiac size induced by semaglutide occurs independent of weight loss.
Anyways, stay tuned. There will be a flurry of research on this over the next few years, and we can hopefully say with a bit more certainty what’s happening then.
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It is a refill mechanism and if you disagree with that, it’s ok with me. Hepatic gluconeogenesis is not an “important source of muscle fuel during higher intensity exercise”. The liver’s 24hour production of glucose alone tells you this. Discussion over from my part, no further replies needed nor forthcoming.
Ok! If you ever want to read or learn more about the role of the liver in fueling exercise, I’d love to talk more, or give you further resources!
For anyone else who’s curious, I’d direct you towards the references I’ve already provided above which talk about this.
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All good, but still muscle glycogen doesn’t get restored during intense exercise
Agreed.
I don’t think the commenter you originally replied to meant that was happening during exercise though.
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That’s exactly what happens. These drugs help to eat less eventually (not for everyone), for a while at least, until you reached max dose and the effect weakens at some point.
Subsequent effects are then caused by weight loss due to a negative energy balance.
Apart from that there is nothing magic about it.
Which also explains how to counter the atrophy: protein & weight lifting.
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I am suspicious of this graphic on muscle glycogen being the primary driver of energy, from personal experience with using a CGM. I.e. early in MTB rides I would continuously set off the CGM alarm at the top of a climb as blood glucose levels would spike, despite taking on no external carbohydrates. The only thing that would cause that would be the liver dumping glucose into the system to meet the demand call from the working muscles. CGM readings generally correlated with the effort level on un-fueled rides. Until of course my glycogen stores were depleted and I would bonk–on longer un-fueled rides. At the time I was using the CGM, it was purely for personal knowledge of how different foods impacted my glucose levels. I also had some fun playing and learning about how training and fueling impacted blood glucose.
This is very interesting to me, and thank you for sharing! I just started with Trizepatide (Zebound) last Thursday and I am completely blown away by how much is eliminated the “food noise”. Weight management has been a life-long battle for me, and my biggest issue was that my appetite just NEVER turned off. I could eat and that would make me hungry.
I am only 4 days in on the drug, but I feel like I am missing a bit of energy–could be poor sleep the last few nights, but wondering if this is something you also noticed?
Was there a particular training plan that you were following on TR while you were cutting weight?
I am currently just spinning for 60 to 75 minutes at FatMax wattage, which is ~150-165w for me. I.e. EZPZ mid-to-high Z1 (3 zone model) efforts. I am planning to keep it this way, while also picking up some weight training to try and maintain muscle mass. I’ll track that via DEXA scan every 4 months–first one on Thursday. Your results are VERY encouraging, long run! Keep up the good work!
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You lost weight and kept the same FTP, that’s a massive improvement! You should be proud!
I also suspect that Retatrutide is going to really help in this situation. It has a third antagonist that activates the glucagon receptor.
Semaglutide - GLP-1
Trizepatide - GLP-1, GIP
Retatrutide - GLP-1, GIP, Glucagon
Here’s the ChatGPT explanation of how the glucagon antagonist works:
Activating the glucagon receptor works by triggering a series of metabolic responses. Here’s a concise breakdown:
- Receptor Type:
- It’s a G-protein coupled receptor (GPCR) mainly found in the liver.
- Signal Cascade:
- Activation increases cyclic AMP (cAMP) via adenylate cyclase.
- Elevated cAMP activates protein kinase A (PKA).
- Metabolic Effects:
- Increases glycogen breakdown (glycogenolysis).
- Stimulates new glucose production (gluconeogenesis).
- Promotes fat breakdown (lipolysis).
- In a Therapeutic Context:
- Controlled activation can boost energy expenditure.
- When combined with GLP-1 and GIP receptor agonism (as in some triple agonists), it helps balance blood sugar levels while promoting weight loss.
This mechanism helps your body mobilize energy stores and regulate blood sugar in a balanced way when used in combination therapies.
The cool part is the part of the peptide that makes you kinda full and nauseous (mainly GLP-1). It can be reduced, and you can eat a bit more but burn a bit more fat. Of course, that’s dose-dependent.
It’s in stage three clinical trials, but I suspect the pros are micro-dosing Retatrutide to gain an edge. There are many bodybuilders taking it openly and swearing by the body comp changes without losing massive amounts of muscle like they would on Sema/Triz.
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don’t know anything about weight loss, but I think when your FTP recovered at a steady ~280 watt and you are looking to improve, you will have to make some changes at some place. Ceilings can easily be broken, but you will need to , for example:
- increase volume
- increase intensity (tricky…)
- improve fueling
- improve recovery
- …
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So anecdotally people are resistance training for hypertrophy and presumably eating adequate protein and still losing muscle on GLP1 agonists? Any idea if that’s the case with microdosing?
Asking because mitigating the muscle loss in the general population is of massive importance if we don’t want to replace an epidemic of obesity with one of frailty.
This 100% matches my experience. I can’t remember missing energy in the early days, but could potentially be that you are just taking less fuel on board overall?
Not really. I completely jumped off in first few weeks from any structured training. Then I started one custom plan, then cancelled it for a different one, then cancelled that for another… I’ve been better since start of January sticking to a custom Gravel plan (4 week base, now on 4 week build), but that’s only coincided with a couple of kilos drop so not my main phase of weight loss.
Strong recommendation on adding weight training though. I was very lucky that I didn’t need to add muscle whilst also trying to cut weight, just had to try and cling on to what I already have. Wish I’d done DEXA scans like you’re planning!
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GLP1’s like semaglutide will directly cause weight loss independent of calorie intake. IE, if you take two sets of people on the same amount of calories & macros, the one taking semaglutide will still lose more weight (including muscle).
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