GLP-1s and Endurance Training: Better Power-to-Weight or Compromised Performance?

[457?]

Pretty much what the title says—if you only want the questions, jump down to the TLDR. This thread touches on something similar, though its angle is a bit different.

Lately I've been mulling over how GLP-1 medications fit together with endurance training and sports. In my case that's mainly soccer, cycling, running, and triathlon, but I'm curious about any activity with an endurance element. Understandably, most GLP-1 discussion centers on weight loss. For endurance-type activities, though, dropping weight only helps if we hold onto enough muscle, power, and aerobic capacity to genuinely perform better.

Take my own goal: I want to climb better on the bike. So the scale number matters less to me than squeezing out the best power-to-weight ratio, building endurance, and bouncing back well between sessions.

That leaves what looks like a real trade-off:

Possible upsides:

• A lighter body can raise relative power output (watts/kg), especially going uphill.
  
• Shedding extra body fat may make running more economical.
  
• Better metabolic health might help some athletes indirectly.

Possible downsides:

• Suppressed appetite can make eating enough to fuel hard training difficult. A HUGE con that I've run into when I wasn't prepared; it can be absolutely brutal.
  
• Too few carbohydrates may undercut glycogen availability and high-intensity performance. For me it's most obvious around explosive efforts or hills.
  
• Fast weight loss can mean losing lean mass. I hit this in the aftermath of reta—overall I was "healthier" but less "fit" by comparison when it came to my race times. Humbling to finish the same race, feel better, and still see the clock 20+ minutes slower than the year before.
  
• GI side effects could complicate fueling during long workouts.

What I keep wondering is whether the weight-loss gains outweigh the possible performance costs, and how much that varies from person to person.

Two studies worth a look:

1. A randomized trial in the New England Journal of Medicine looked at exercise, liraglutide, and the two combined following initial weight loss. Pairing exercise with the medication led to greater total weight loss than either alone. DOI: 10.1056/NEJMoa2028198

2. A 2024 follow-up analysis reported that people who did exercise plus liraglutide kept better weight-related results after treatment stopped than those on liraglutide alone. PubMed: 38544798

Those results are encouraging, but neither study directly settles whether GLP-1 medications boost performance in athletes who are already trained.

TLDR: What I'd most like to know:

1. While on a GLP-1, has anyone tracked real performance numbers—cycling watts, FTP, VO2max, running pace, race times, and so on?
   
2. Did you see relative performance improve from weight loss even when absolute strength or power stayed the same?
   
3. Has appetite suppression gotten in the way of fueling longer workouts or recovering afterward?
   
4. Did you have to deliberately raise carbohydrate or protein intake to keep performance up?
   
5. Has anyone noticed shifts in exercise tolerance, perceived exertion, or recovery that seem to happen independently of weight loss?

I'd especially like to hear from those with objective before-and-after measurements rather than just scale changes. There may be a genuine chance to improve body composition and performance at the same time, but my hunch is that the approach has to look quite different from just chasing maximum weight loss. At the risk of pinging too many people and setting off alarm bells: @gusterbuster, @Wakesetter5477, @StrawberrySeltzer51, @Bruce Campbell @MapleMisfit, @Jack Rabbit Slims, @rixsternaz, @Foggy-Hollow, all of you have touched on related insights, so I'd really value your input.

 

[gusterbuster]

457? said:

Pretty much what the title says—if you only want the questions, jump down to the TLDR. This thread touches on something similar, though its angle is a bit different.

Lately I've been mulling over how GLP-1 medications fit together with endurance training and sports. In my case that's mainly soccer, cycling, running, and triathlon, but I'm curious about any activity with an endurance element. Understandably, most GLP-1 discussion centers on weight loss. For endurance-type activities, though, dropping weight only helps if we hold onto enough muscle, power, and aerobic capacity to genuinely perform better.

Take my own goal: I want to climb better on the bike. So the scale number matters less to me than squeezing out the best power-to-weight ratio, building endurance, and bouncing back well between sessions.

That leaves what looks like a real trade-off:

Possible upsides:

• A lighter body can raise relative power output (watts/kg), especially going uphill.
  
• Shedding extra body fat may make running more economical.
  
• Better metabolic health might help some athletes indirectly.

Possible downsides:

• Suppressed appetite can make eating enough to fuel hard training difficult. A HUGE con that I've run into when I wasn't prepared; it can be absolutely brutal.
  
• Too few carbohydrates may undercut glycogen availability and high-intensity performance. For me it's most obvious around explosive efforts or hills.
  
• Fast weight loss can mean losing lean mass. I hit this in the aftermath of reta—overall I was "healthier" but less "fit" by comparison when it came to my race times. Humbling to finish the same race, feel better, and still see the clock 20+ minutes slower than the year before.
  
• GI side effects could complicate fueling during long workouts.

What I keep wondering is whether the weight-loss gains outweigh the possible performance costs, and how much that varies from person to person.

Two studies worth a look:

1. A randomized trial in the New England Journal of Medicine looked at exercise, liraglutide, and the two combined following initial weight loss. Pairing exercise with the medication led to greater total weight loss than either alone. DOI: 10.1056/NEJMoa2028198

2. A 2024 follow-up analysis reported that people who did exercise plus liraglutide kept better weight-related results after treatment stopped than those on liraglutide alone. PubMed: 38544798

Those results are encouraging, but neither study directly settles whether GLP-1 medications boost performance in athletes who are already trained.

TLDR: What I'd most like to know:

1. While on a GLP-1, has anyone tracked real performance numbers—cycling watts, FTP, VO2max, running pace, race times, and so on?
   
2. Did you see relative performance improve from weight loss even when absolute strength or power stayed the same?
   
3. Has appetite suppression gotten in the way of fueling longer workouts or recovering afterward?
   
4. Did you have to deliberately raise carbohydrate or protein intake to keep performance up?
   
5. Has anyone noticed shifts in exercise tolerance, perceived exertion, or recovery that seem to happen independently of weight loss?

I'd especially like to hear from those with objective before-and-after measurements rather than just scale changes. There may be a genuine chance to improve body composition and performance at the same time, but my hunch is that the approach has to look quite different from just chasing maximum weight loss. At the risk of pinging too many people and setting off alarm bells: @gusterbuster, @Wakesetter5477, @StrawberrySeltzer51, @Bruce Campbell @MapleMisfit, @Jack Rabbit Slims, @rixsternaz, @Foggy-Hollow, all of you have touched on related insights, so I'd really value your input.

1. I'm mainly a mountain biker and don't use a power meter, so watts and VO2max are unknown to me. Still, I've been beating my Strava times regularly and sit in the top 10 on every local trail, with one 2nd fastest. My performance is at a level I've never reached before. To be honest, I'm on TRT now, which probably contributes, and I also developed asthma later in life from exposure to crap during Army service. A big part of it, I believe, comes from fueling more consistently instead of the old binge-and-diet cycle where I spent most of my time in diet mode.

2. Power has gone up, and I've weighed various amounts over the past decade. My guess is that most of the gains come from steadier fueling now that GLPs have greatly cut down the prior binge/restrict cycles.

3. Never. Honestly, GLPs have never kept me from eating at least my minimum calories. They just made it simpler to stay on my meal plan. And I've run decent doses, like 2.5mg sema and 8mg reta weekly.

4. Not really. 2 uncrustables before a ride work like jet fuel for me. On big riding days the only real change is diet, plus carbs/electrolytes in my hydration pack.

5. I actually feel like perceived exertion is lower while on a GLP. That probably traces back to more consistent fueling across the week. Recovery seems unchanged.

My honest view is that most people overthink how GLPs affect performance. Any downside is probably from unintentional under fueling.

 

[457?]

If your Strava times keep improving, it's tough to claim performance is being compromised—though how much TRT contributes is probably debatable.

Fueling being essential is something I'd agree with. It's interesting that you've never run into problems getting the calories you need. I've been on much lower doses and still had issues. I wonder whether pairing something with a GLP might make that less likely; haven't tried it yet.

Either way, thanks for sharing your input!

 

[Vash]

Reta threw my heart rate completely out of whack — rides that used to sit in zone 2 were suddenly zone 4 efforts. Appetite suppression never hit me on either Reta or Tirz, so getting carbs in was never a problem.

After moving over to Tirz, my heart rate has almost returned to where it was before Reta. I'm also pushing more power and getting back into longer rides.

Based on my own experience, I wouldn't suggest Reta for endurance athletes — or really for anyone. It did a real number on me.

 

[John_C]

I'm not an endurance athlete — my background is more on the lifting side — but the experiences shared here line up with standard sports nutrition guidance: the real bottleneck appears to be under-fuelling rather than the medication on its own. The common strategy is to plan carbs around key sessions rather than relying on hunger cues: something simple before training, then roughly 30 to 60g per hour for any session lasting over 90 minutes, since drinks and gels are easier to take in when appetite is low. Keep protein intake high throughout the day to protect lean mass, and where possible, schedule the hardest sessions away from the first couple of days after your jab, when suppression tends to peak.

 

[457?]

Vash said:

Reta threw my heart rate completely out of whack — rides that used to sit in zone 2 were suddenly zone 4 efforts. Appetite suppression never hit me on either Reta or Tirz, so getting carbs in was never a problem.

After moving over to Tirz, my heart rate has almost returned to where it was before Reta. I'm also pushing more power and getting back into longer rides.

Based on my own experience, I wouldn't suggest Reta for endurance athletes — or really for anyone. It did a real number on me.

On 1.5mg I'm already seeing a mild rise in heart rate. If a bigger dose amplifies that, I can't picture how rough I'd feel. What dose were you running?

Still, it's good to hear Tirz is working out for you. Something to keep in mind.

 

[Vash]

457? said:

On 1.5mg I'm already seeing a mild rise in heart rate. If a bigger dose amplifies that, I can't picture how rough I'd feel. What dose were you running?

Still, it's good to hear Tirz is working out for you. Something to keep in mind.

My dose reached 7.5mg/wk. On some days my resting heart rate stayed above 100bpm from morning to night. I stopped completely, let it wash out, then restarted at 2mg. Even then my heart rate was still elevated and the side effects were rough.

I finished with the trainer this evening. It was the first ride in months where my average stayed below 140bpm.

 

[457?]

Vash said:

My dose reached 7.5mg/wk. On some days my resting heart rate stayed above 100bpm from morning to night. I stopped completely, let it wash out, then restarted at 2mg. Even then my heart rate was still elevated and the side effects were rough.

I finished with the trainer this evening. It was the first ride in months where my average stayed below 140bpm.

That is genuinely concerning. Still, it's great that you've sorted it out. In my case I've been sitting at 76 rather than the mid-60s, and even that shift has been unpleasant for me.

 

[StrawberrySeltzer51]

I wrote a post about this a few days ago — I was having a really hard time. That said, I think the main thing is getting more carbs in. Over the last week I've made an effort to eat more potatoes, because they pack in nutrients and give you a lot of volume for the calories (as long as you don't fry them or load them up with butter).

I'm also on a low dose of tirz — I only just started, which explains the dose. My rhr hasn't changed. I've read that it does happen with Reta, which is a shame since I'm interested in it.

My bike has a power meter, and I intend to keep a closer eye on it than I normally would

 

[fatjacked]

Vash said:

My dose reached 7.5mg/wk. On some days my resting heart rate stayed above 100bpm from morning to night. I stopped completely, let it wash out, then restarted at 2mg. Even then my heart rate was still elevated and the side effects were rough.

I finished with the trainer this evening. It was the first ride in months where my average stayed below 140bpm.

I've experienced a milder form of this. My RHR went from ~55 to ~65, yet during a run that used to be moderate effort (average HR sitting firmly in the 100-120 range for most of it), I'm now at 140+ the entire time. What's strange is that my perceived effort hasn't shifted much — I don't FEEL like I'm in peak zone throughout, but the pattern has held steady for the past several months. I planned to bring it up with my PCP at my next visit.

Back to the thread's main subject: for my preferred vigorous cardio activities, the mobility gains from weight loss have overall outweighed any power drop or muscle loss, though most of this is anecdotal. I went from 265 to 210, and many people have remarked that I move much better on the court. I've certainly lost some raw strength based on 1RM on bench, but not enough to be a problem in competition.