GLP-1 for Athletes: My Journey to Race Weight

Thomas Prommer Hybrid Athlete & Technology Executive Follow on Strava

Published: March 27, 2026

Updated: April 16, 2026

Why This Question Is Suddenly Everywhere

GLP-1 medications are on the verge of going mass. Morgan Stanley projects the number of Americans on GLP-1s will quadruple over the next decade — from roughly 13 million at end of 2025 to 55 million by 2035, about 15% of the US population. A KFF survey already found 12% of US adults (~30 million people) saying they were currently taking a GLP-1 drug. The gap between those numbers comes mostly from the booming compounded market — people getting prescriptions through Hims, telehealth clinics, and online pharmacies that don't show up in pharma sales data.

The next wave will be larger. Lower prices from generics and compounders, new pill-based formulations that require no weekly injection, and expanded Medicare coverage are all converging in 2026–2027. This is no longer a niche question for diabetics or the severely obese. It's becoming a mainstream tool — and endurance athletes are ahead of the curve, already asking the practical questions that matter: Can I train on this? Will it ruin my race fueling? Will I lose muscle? That's exactly what I documented.

Why I Started GLP-1

In early 2026 I was deep in Ironman prep for Challenge Roth (July 2026) and running multiple marathons a year. Training weeks regularly hit 15–20 hours, and the hunger that comes with that volume is relentless. The loop was always the same: train hard, get ravenous, overeat, undo the deficit. Ironman training did not solve it. It made it worse.

I also have a lifelong sensitivity to sugar. Childhood sugar binges, emotional eating when stress is high, the kind of pattern that doesn't go away, it just waits. This is not a clinical obesity story. I started around 96 kg and wanted to get to a real race weight, where watts per kilo actually changes what I can do on the bike and what the last 10 km of a marathon feels like. Willpower had failed me for years. Every training block I'd get leaner for a while, then the appetite would win.

The doctor conversation was short. He prescribed tirzepatide (Mounjaro) 5 mg KwikPen at 2.5 mg per week — the starter dose. Not long-term maintenance. A tool to break the hunger loop during the heaviest training block and get me to race weight before the season started. First injection went in on February 20, 2026.

My Protocol

• Medication: Tirzepatide (Mounjaro) 5mg KwikPen
• Dose: 2.5mg per week (30 units) for weeks 1-6, reduced to ~1.67mg (20 units) from week 7
• Injection day: Fridays at 19:30 — evening before bed, to sleep through any potential side effects
• Start date: February 20, 2026
• Initial protocol end: March 27, 2026 (5 weeks)
• Continuation: Reduced to 20 units from late March — ongoing as of April 8

The plan was always 5 weeks, not indefinite use. Break the hunger loop in the heaviest training block, build habits that would carry after stopping, get into Ironman season at race weight. That was the theory.

Monitoring: weekly circumferences (weight, waist, belly, chest, hips, thighs), body fat and lean mass estimated from those numbers via a linear regression model, blood work before and after the protocol.

Weight Timeline

All measurements taken at the same time of day, tracked in my athlete tracking system. Numbers below are the raw measurement data.

Weight swings 2–3 kg day to day with hydration, training load and meal timing, so the trend matters more than any single reading. Lowest weigh-in was 89.0 kg on Mar 22 at the full 30-unit dose. Two weeks after cutting to 20 units I was back at 93.6 kg on April 6, and this week (week 9) hunger is noticeably sharper than it was at full dose. The suppression is weaker, the window before the next injection feels longer, and for the first time on this protocol I've had a few small binges. Honest bit: alcohol made it worse. Evenings with a drink or two, the brakes came off and portions went sideways in a way that simply did not happen at 30 units. The old childhood pattern — emotional eating, sugar sensitivity — is still there. The lower dose just stopped covering for it. That's what I'm weighing right now: return to the full dose, or accept a slower rate of progress and work on the habits directly. More on the appetite-alcohol-binge dynamic in the sugar cravings update.

Body Composition

No DEXA scan access locally, so body-comp tracking is weekly circumference measurements run through a linear regression model for body fat and lean mass. Not perfect. Consistent enough to trend.

Body fat estimates: 13.0% (Feb 22) → 11.3% (Mar 22, low point) → 11.6% (Apr 6, reduced dose).

Lean mass estimates: 83.3 kg (Feb 22) → 78.9 kg (Mar 22). This is the number I keep staring at. About 4 kg of estimated lean mass came off alongside the fat, even with full resistance work and high protein intake. So much for "muscle preservation happens on its own." It's active work, and I was doing all of it, and I still lost lean mass.

Pattern: trunk loss dominates. Waist down 2 cm, belly button down 3.5 cm, chest basically flat. Limbs moved 1–1.5 cm at the low point and most of that came back as weight rebounded, which fits glycogen and water swings better than actual leg-tissue loss. Subjectively I still look and feel leaner, and the waist is still ticking down even as weight goes back up.

Race Results — Baseline & Upcoming

Context: every marathon result below is pre-GLP-1. Baseline performances at various weights. The first races I'll run at the new weight are still ahead.

Upcoming — first races at new weight:

• IM 70.3 Da Nang (May 10, 2026) — my first triathlon race at the lower weight
• Challenge Roth (July 6, 2026) — full Ironman, the primary target for this entire journey

This page will be updated with race data, split analysis, and power numbers as these events are completed.

For detailed split analysis and Garmin data from past races, see my full race results breakdown.

What Actually Changed

The number I care about most on the bike is watts per kilogram. Across the 5 weeks my FTP went from 261W to 281W, a 20-watt gain (+7.7%). Combined with the weight loss, my W/kg went from 2.76 to 3.04. A 10% jump. On a hilly Ironman course, that's real.

I want to be careful with attribution here. The FTP gains could easily be down to structured training — I was in a dedicated build phase with intervals and threshold work, and that alone adds 10-15 watts in a 5-week block if you do it right. The weight loss and body-comp changes are clearly the medication. The appetite suppression broke the loop where training hunger would always blow up the deficit.

The clear wins were body comp, power-to-weight, and how I actually feel running. Training volume held (no missed sessions), in-session energy held, sleep and recovery held. The one thing I can't separate cleanly is how much of the FTP gain was the weight loss and how much was a well-structured build phase. Probably both, in some ratio I'll never know exactly, and I'm fine with that.

Side Effects

First injection was during the day (Feb 20). I was nauseous for a couple of hours afterwards. That was the only time. Every injection after that went in Friday at 19:30, right before bed, and the side-effect profile was zero. The trick was simple: inject in the evening, sleep through the window where GI stuff might show up, wake up normal.

No energy dips in training, no GI issues on long rides or runs, no hit to sleep or recovery. The appetite suppression did exactly what it was supposed to do: cravings dropped, hunger was manageable, and the post-big-session urge to eat the kitchen was gone for most of the week.

One pattern stood out. Suppression is strongest days 1–5 post-injection. By days 6–7, hunger starts creeping back before the next dose. Manageable, but noticeable.

I also got lucky. Plenty of athletes report serious GI issues, especially during dose titration. The starter dose (2.5 mg tirzepatide) plus evening timing probably helped a lot. Your experience may look nothing like mine. This is a sample size of one.

For the full protocol on managing side effects around training, see my side effects and dose timing guide.

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