GLP-1 for Athletes: My Journey to Race Weight

Thomas Prommer Hybrid Athlete & Technology Executive Follow on Strava

Published: March 27, 2026

Updated: April 9, 2026

Why I Started GLP-1

In early 2026, I was deep into Ironman training for Challenge Roth (July 2026) and running multiple marathons per year. My training weeks regularly hit 15-20 hours, and the hunger that comes with that volume is relentless. The cycle was always the same: train hard, get ravenous, overeat, undo the caloric deficit. Ironman training made it worse, not better.

I have a lifelong sensitivity to sugar — a childhood pattern of sugar binges and emotional eating that surfaces whenever stress is high. This is not a clinical obesity story. I started at 94.5kg, and my goal was reaching a competitive race weight where power-to-weight actually matters on the bike and the run. Willpower-based approaches had failed me for years. Every training block, I would get leaner for a while, then the appetite would win.

The conversation with my doctor was straightforward. He prescribed tirzepatide (Mounjaro) 5mg KwikPen at 2.5mg per week — a low dose, targeted intervention. Not a long-term plan. A tool to break the hunger cycle during the heaviest training block, establish new eating patterns, and get to race weight before the season starts. I injected the first dose 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

This was deliberately designed as a short protocol — not long-term maintenance, but a targeted 5-week intervention to break the hunger cycle and establish new eating patterns before Ironman season. The idea was simple: use the medication window to build habits that stick after stopping.

Monitoring: Weekly body measurements including weight, waist, belly, chest, hip, and thigh circumferences. Body fat estimated via circumference measurements and a linear regression model. Blood work done before starting and after completing the protocol.

Weight Timeline

Weight fluctuates 2-3kg day to day depending on hydration, training load, and meal timing. The trend matters more than any single number. I hit 89.0kg on Mar 22 at full dose (30 units). After dropping to 20 units two weeks ago, I am back up to 91.96kg as of April 8. The cravings come back harder at the lower dose, especially in the last couple days before the next injection. More on that in the sugar cravings update.

Body Composition

I do not have DEXA scan access here, so body composition tracking relies on weekly circumference measurements fed into a linear regression model for body fat estimation. Not perfect, but consistent and trackable over time.

InBody scan baseline (Jan 26, 2026): 94.1kg total weight, 50.9kg skeletal muscle mass, BMR 2,271 kcal.

Body fat estimates: 13.0% (Feb 22) down to 11.3% (Mar 22).

The pattern is clear: fat loss concentrated in the trunk and midsection, while limb measurements stayed stable. That is a good sign for muscle preservation. I maintained full body resistance training throughout the protocol with no noticeable change in working weights. Subjectively, I just look and feel leaner.

Race Results — Baseline & Upcoming

Important context: all marathon results below are pre-GLP-1. These are my baseline performances at various weights. The first races where I will compete at my 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. Over the 5-week protocol, my FTP went from 261W to 281W — a gain of 20 watts (+7.7%). Combined with the weight loss, my W/kg improved from 2.76 to 3.04, a 10% jump. That is a meaningful difference on hilly Ironman courses.

I want to be honest about what I can and cannot attribute to GLP-1 here. The FTP gains could easily be from structured training — I was in a dedicated build phase with intervals and threshold work. But the weight loss and body composition changes are clearly driven by the medication. The appetite suppression broke the cycle where training-induced hunger would lead to overeating.

What clearly improved: body composition, power-to-weight ratio, subjective feeling of being leaner and lighter on the run.

What stayed the same: training volume (no missed sessions), energy levels during training, sleep quality, recovery between sessions.

What I cannot isolate: how much of the FTP gain is GLP-1 weight loss versus training adaptation. Probably both, and I am comfortable with that ambiguity.

Side Effects

My first injection was during the day (Feb 20), and I had some nausea shortly after. That was the only time. For every subsequent injection — Fridays at 19:30, right before bed — I experienced zero side effects. The simple fix: inject in the evening, sleep through any potential GI effects, wake up fine.

No energy dips during training. No GI issues during long rides or runs. No disruption to sleep or recovery. The appetite suppression worked as intended — cravings were noticeably reduced, hunger was manageable, and the urge to overeat after big training days was simply gone for most of the week.

The one pattern I noticed: appetite suppression is strongest on days 1-5 after injection. By days 6-7, hunger starts creeping back before the next dose. Manageable, but noticeable.

I got lucky with side effects. Many athletes report more significant GI issues, especially during dose titration. My low dose (2.5mg tirzepatide) and evening timing likely helped. Your experience may be very different — 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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