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9 min read<\/p>\n9 min read<\/p>\n
MY <\/strong>WHOOP<\/strong><\/a> HAD<\/strong> to be lying.<\/p>\n As a dietitian<\/a>, I was aware of the common side effects<\/a> of starting a GLP-1 medication: nausea, fatigue, indigestion, heartburn, gut struggles. I\u2019d heard about all these issues from my clients. But my Whoop numbers were something different.<\/p>\n 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\u2019s higher than the 11 percent<\/a> in the general population who are either currently taking a weekly shot or a daily pill. But my clients\u2014from people who have long struggled to lose weight to high-performing athletes who want an edge\u2014come to me already looking to change their body composition.<\/p>\n The thing was, I couldn\u2019t really relate to my clients\u2019 issues because I\u2019d never taken a GLP-1 myself. So, earlier this year, I started on one.<\/p>\n I had just finished my first Hyrox<\/a>, was training four to five days a week, and put in two and a half years of consistent work to see my VO2<\/a> max climb from the mid-thirties to the mid-forties.<\/p>\n Then, after 30 days of taking the medication, I stared at my Whoop data, pissed.<\/p>\n My heart rate variability<\/a> (HRV), an indicator of how resilient your heart is, was down a significant percentage and the VO2 max I\u2019d fought so hard to raise now sat a full point lower.<\/p>\n None of my clients on GLP-1s had mentioned that their performance metrics dropped. And while it\u2019s been well-reported that GLP-1s can decrease muscle mass<\/a>, that didn\u2019t explain my metrics plunge.<\/p>\n 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\u2019d 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.<\/p>\n Sure, I\u2019d lost two or three pounds on the medication, which we\u2019re told improves overall health, but also at what cost?<\/p>\n I wanted an answer.<\/p>\n Enter The Grey Zone<\/strong><\/p>\n THE FIRST PLACE<\/strong> I went searching was the Annual Conference of Sports Dietitians<\/a>, which is basically like a nutrition Disney World<\/a> for a guy like me.<\/p>\n That\u2019s where hundreds of the world\u2019s smartest sports performance minds\u2014some of them in the highest ranks of the NBA, NFL, UFC, and NCAA\u2014go to swap expertise and learn about the science shaping the Next Big Things in athletics.<\/p>\n This year\u2019s event multi-day event in New Orleans held seminars on creatine and brain health<\/a>, metabolic management, and continuous glucose monitors<\/a>. There was nothing on GLP-1s and athletic performance, though, so between sessions, I asked around\u2014and was surprised by what I heard.<\/p>\n “I’ve seen all ends of the spectrum with GLP-1s, some good, some negative,\u201d James Burns, RD, CSSD<\/a>, founder of Chain Effect, a sports wellness practice in Raleigh, North Carolina, told me. \u201cWith athletes curious about taking them, it’s such a nuanced conversation.\u201d<\/p>\n Sometimes an athlete has been doing everything right\u2014training, sleeping, eating\u2014and can\u2019t 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\u2014but athletic performance markers dip.<\/p>\n Many other dietitians I talked to at the conference admitted that they didn\u2019t 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.<\/p>\n But there is growing concern around possible effects\u2014positive and negative\u2014that athletes on GLP-1s may experience. Rachel Suba, RD, CSSD<\/a>, 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.<\/p>\n What I learned from the conference was that there\u2019s still so much we don\u2019t 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.<\/p>\n Some emerging research, however, has made some fascinating connections.<\/p>\n What Early Data Reports<\/p>\n BECAUSE I\u2019M A <\/strong>Whoop guy, I guess it was inevitable that I\u2019d talk to Greg Grosicki, PhD<\/a>. He\u2019s the staff research scientist at the data wearables company, and he recently authored a peer-reviewed study<\/a> in the American Journal of Physiology that (mostly) validated the performance loss I saw during my GLP-1 experiment.<\/p>\n 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.<\/p>\n The first effect is obvious; this is what GLP-1 drugs do. But the other two factors are more curious. When you\u2019re on a GLP-1, Grosicki\u2019s research indicates that your heart rate doesn\u2019t fully downshift during recovery the way it should.<\/p>\n 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\u2019s the kind of shift you\u2019d see after taking two weeks off of vigorous training\u2014the difference between feeling dialed-in and feeling like you\u2019re 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.<\/p>\n I noticed this effect in both my runs and my interval sessions<\/a>. 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\u2019t 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.<\/p>\n Sure, my experiment was n=1, and Whoop data isn\u2019t controlled lab data, but Grosicki\u2019s research backs up that there\u2019s something happening with GLP-1s and performance metrics.<\/p>\n Grosicki made it clear to me that GLP-1s are not a magic bullet for athletes looking to lean out and boost gains. \u201cThe goal should not simply be weight loss,\u201d he says. \u201cThe goal should be maximizing fat loss while preserving skeletal muscle, maintaining adequate nutrition, and continuing to train,” he says.<\/p>\n And that\u2019s where things get really complex.<\/p>\n The Other Potential Dangers<\/p>\n MY METRICS WERE <\/strong>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\u2019d 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.\u201d<\/p>\n But these issues, he said, \u201cThey resolved once proper nutrition was on board.\u201d He went as far to say that anyone taking a GLP-1 should be required to work with a registered dietitian.<\/p>\n Without a nutrition plan, there\u2019s 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 \u201cRelative Energy Deficiency in Sport.\u201d Basically, this is when your exercise output consistently outpaces caloric intake. If you\u2019re 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<\/a>, and hormone crashes happen.<\/p>\n 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<\/a> can show up in your output, your recovery, and your focus long before injury enters the picture.<\/p>\n Charles Stull, MS, RDN<\/a>, 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\u2019t seen athletes use GLP-1s, he knows that it\u2019s 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.<\/p>\n 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.”<\/p>\n What Athletes on GLP-1s Should Do<\/p>\n THINK OF A<\/strong> GLP-1 like a dial with two controls. After a dose, the medication turns down food noise<\/a>, 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.<\/p>\n 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).<\/p>\n 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.<\/p>\n 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 \u00bd<\/strong> 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.<\/p>\n 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.<\/p>\n
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