MY WHOOP HAD to be lying.
As a dietitian, I was aware of the common side effects of starting a GLP-1 medication: nausea, fatigue, indigestion, heartburn, gut struggles. I’d heard about all these issues from my clients. But my Whoop numbers were something different.
Right now, 85 to 90 percent of the people I work with as a dietitian are either starting a GLP-1 or plateauing on one. That’s higher than the 11 percent in the general population who are either currently taking a weekly shot or a daily pill. But my clients—from people who have long struggled to lose weight to high-performing athletes who want an edge—come to me already looking to change their body composition.
The thing was, I couldn’t really relate to my clients’ issues because I’d never taken a GLP-1 myself. So, earlier this year, I started on one.
I had just finished my first Hyrox, was training four to five days a week, and put in two and a half years of consistent work to see my VO2 max climb from the mid-thirties to the mid-forties.
Then, after 30 days of taking the medication, I stared at my Whoop data, pissed.
My heart rate variability (HRV), an indicator of how resilient your heart is, was down a significant percentage and the VO2 max I’d fought so hard to raise now sat a full point lower.
None of my clients on GLP-1s had mentioned that their performance metrics dropped. And while it’s been well-reported that GLP-1s can decrease muscle mass, that didn’t explain my metrics plunge.
I put on my human physiology hat: Conventional science indicates that with lower body mass, less work is required to oxygenate the muscle. So with weight loss, I’d assume that my VO2 max would improve. (VO2 max is calculated in ml/kg, so when the kg goes down, the volume goes up even mathematically.) Mine plummeted.
Sure, I’d lost two or three pounds on the medication, which we’re told improves overall health, but also at what cost?
I wanted an answer.
Enter The Grey Zone
THE FIRST PLACE I went searching was the Annual Conference of Sports Dietitians, which is basically like a nutrition Disney World for a guy like me.
That’s where hundreds of the world’s smartest sports performance minds—some of them in the highest ranks of the NBA, NFL, UFC, and NCAA—go to swap expertise and learn about the science shaping the Next Big Things in athletics.
This year’s event multi-day event in New Orleans held seminars on creatine and brain health, metabolic management, and continuous glucose monitors. There was nothing on GLP-1s and athletic performance, though, so between sessions, I asked around—and was surprised by what I heard.
“I’ve seen all ends of the spectrum with GLP-1s, some good, some negative,” James Burns, RD, CSSD, founder of Chain Effect, a sports wellness practice in Raleigh, North Carolina, told me. “With athletes curious about taking them, it’s such a nuanced conversation.”
Sometimes an athlete has been doing everything right—training, sleeping, eating—and can’t seem to unlock that next level. Then a low dose of Zepbound turns the key, Burns says. But then there are other times, like with me, when a GLP-1 leads to weight loss—but athletic performance markers dip.
Many other dietitians I talked to at the conference admitted that they didn’t have an informed answer as to why athletes respond to these drugs in wildly different ways. (Just look at the discussion around Serena Williams right now.) And as frustrating as that was, it’s fair. These are the people who separate themselves from your typical influencers and wellness grifters precisely because they follow the science before they open their mouths.
But there is growing concern around possible effects—positive and negative—that athletes on GLP-1s may experience. Rachel Suba, RD, CSSD, a Division I collegiate sports dietitian, told me at the conference that her program added a GLP-1 screening question to its standard intake, just to flag potential issues early. “College athletes aren’t very good at disclosing what they’re taking,” she said.
What I learned from the conference was that there’s still so much we don’t know about GLP-1s and performance. But so many athletes and fitness-focused people are now taking the drugs, to the extent that use is outpacing clinical guidance.
Some emerging research, however, has made some fascinating connections.
What Early Data Reports
BECAUSE I’M A Whoop guy, I guess it was inevitable that I’d talk to Greg Grosicki, PhD. He’s the staff research scientist at the data wearables company, and he recently authored a peer-reviewed study in the American Journal of Physiology that (mostly) validated the performance loss I saw during my GLP-1 experiment.
After tracking 66 active adults (average age: 42) on a GLP-1 for 12 weeks, Grosicki found that three things happened simultaneously. Their body mass dropped, their resting heart rate increased, and their heart rate variability decreased.
The first effect is obvious; this is what GLP-1 drugs do. But the other two factors are more curious. When you’re on a GLP-1, Grosicki’s research indicates that your heart rate doesn’t fully downshift during recovery the way it should.
His study even had receipts: Study participants, on average, saw their overall resting heart rate increase by 3.2 beats per minute, and they lost 6.2 milliseconds from normal HRV. Those numbers sound small, but for an athlete, that’s the kind of shift you’d see after taking two weeks off of vigorous training—the difference between feeling dialed-in and feeling like you’re starting all over again. “Someone with a resting heart rate of 50 bpm, a high HRV, and years of training experience is often highly attuned to subtle changes in their physiology,” Grosicki told me. And his study showed that HRV reductions were statistically significant as early as four weeks.
I noticed this effect in both my runs and my interval sessions. A run that normally sat at 140 beats per minute was suddenly pushing 160 at a slower pace. I was stopping to walk during interval runs, something I don’t usually do. After seven hours of sleep, I’d wake to an HRV in the 30s, when Whoop’s estimate usually had me in the 100s prior to my experiment. The numbers were telling me something was off.
Sure, my experiment was n=1, and Whoop data isn’t controlled lab data, but Grosicki’s research backs up that there’s something happening with GLP-1s and performance metrics.
Grosicki made it clear to me that GLP-1s are not a magic bullet for athletes looking to lean out and boost gains. “The goal should not simply be weight loss,” he says. “The goal should be maximizing fat loss while preserving skeletal muscle, maintaining adequate nutrition, and continuing to train,” he says.
And that’s where things get really complex.
The Other Potential Dangers
MY METRICS WERE one thing, but my sluggishness during exercise also felt concerning. James Burns, the Raleigh-based dietitian I spoke with at the conference, told me he’d seen this play out with his clients on GLP-1s too. “The symptoms I was seeing, like sleep tanking, increased moodiness, brain fog, perceived effort for their normal workload going up, those are yellow to red flags for low energy availability.”
But these issues, he said, “They resolved once proper nutrition was on board.” He went as far to say that anyone taking a GLP-1 should be required to work with a registered dietitian.
Without a nutrition plan, there’s a serious risk for athletes on GLP-1s. Because these medications suppress appetite, athletes who take them are susceptible to a condition called RED-S, or “Relative Energy Deficiency in Sport.” Basically, this is when your exercise output consistently outpaces caloric intake. If you’re suffering from RED-S, your body starts borrowing against itself to keep you going, a debt that can quietly build up until injury, bone loss, and hormone crashes happen.
But you don’t have to reach that point to feel the drag on your performance. With suboptimal energy availability, even a modest calorie deficit can show up in your output, your recovery, and your focus long before injury enters the picture.
Charles Stull, MS, RDN, senior director of performance nutrition at the UFC Performance Institute, has seen this drag in fighters who cut weight aggressively, then have roughly 24 hours to restore performance capacity. And while Stull hasn’t seen athletes use GLP-1s, he knows that it’s counterintuitive to suppress the hunger cues that drive that process of recovery. “Anytime you’re manipulating how much someone gets in, it’s really hard to out-supplement and out-diet a deficit that’s artificially created through an appetite suppressant,” he says.
If you’ve ever run a long training session or pushed through an all-day tournament, you know that signal. Your body gets loud about what it needs. But now, imagine that signal turned down. Your recovery window narrows in ways you won’t always notice until your performance does. “Maybe the short-term outcome is weight loss,” Stull says, “but restoration and recovery is the other 50 percent of the equation of being an athlete.”
What Athletes on GLP-1s Should Do
THINK OF A GLP-1 like a dial with two controls. After a dose, the medication turns down food noise, the constant background hum of hunger, while simultaneously turning up your fullness, so even a small amount of food leaves you surprisingly full. For fat loss, that’s the whole mechanism.
For maintaining performance, that creates a quiet problem. Your body needs adequate fuel to support training, and a suppressed appetite will work against everything you’re building in the gym, often without you noticing (until maybe your data does).
With my clients on a GLP-1, I now recommend eating on a schedule: three meals, one snack, consistent timing. On training days, pre-workout fuel is non-negotiable, and so is the window right after.
Because GLP-1s suppress the hunger signal that primes you to refuel, endurance athletes must eat with intention. A 3:1 carb-to-protein ratio is a solid place to start: roughly 90 grams of carbs to 30 grams of protein, which you might find in a grain bowl (1 cup to 1 ½ cups of rice, a layer of legumes, colorful vegetables, and a protein). You don’t have to go that exact route, but you have to eat something.
We also now know that people on GLP-1s tend to be deficient in two specific micronutrients: vitamin D and vitamin B12, both of which affect energy, mood, and recovery.
If your energy is flat and your output isn’t matching your effort, those are the first two places to look before you change your program. Start with food. Salmon twice a week is a good anchor, and a daily diet that includes eggs, dairy, and meat covers most of what you need for both. If that’s not closing the gap, work with a dietitian to have your levels assessed and tested before you add a supplement to your program.
Then watch your wearables. The data will tell you when your nutrition deficit has become a performance problem before your body does. “The scale and the VO2 max may look better while the engine itself is not necessarily getting stronger,” says Grosicki, of Whoop.
And lastly, if you’re on a GLP-1 and you’re training, don’t think that you have to fight the medication by pushing through workouts when your body is saying “stop.” The goal is to work with the meds. Let the GLP-1 do its job, and let your nutrition do yours.
Now, what happens if you stay on a GLP-1 past 30 days is still an open question. After I stopped taking mine, by numbers took time to bounce back. At 30 days post-experiment, my HRV was still at 79 ms, down from a baseline of 118. My VO2 max hadn’t moved off 45. My resting heart rate was still elevated. At 60 days, the numbers were back to where they’d started: HRV at 114, VO2 max at 46, resting heart rate at 57. In my case, the engine came back. It took two months.
What I can’t answer—and what no one can yet—is what happens if you stay on a GLP-1 for six months or more. Do the metrics stabilize? Does HRV find a new baseline while the medication is still active? Grosicki’s study ran 12 weeks. The data on what happens after that is still being written. Which means right now, if you’re an athlete on a GLP-1, you are part of the data being collected.
Dezi Abeyta, RDN, is a Men’s Health Nutrition Adviser, author of Lose Your Gut Guide, and founder of Foodtalk Nutrition LLC.
