How Dehydration Wrecked My Gut and Slowed My Recovery at Ironman 70.3 Santa Cruz
I came away from Ironman 70.3 Santa Cruz with a shiny new PR on the run — but it wasn't the performance I imagined. From the moment I got off the bike, every sip of fluid during the half-marathon was painful. My stomach cramped, bloated, and felt like a balloon ready to burst. I ended up running the entire 13.1 miles in a state of GI distress, unable to take in calories, and barely able to tolerate water.
Even so, I pushed through for a 1:33 half-marathon PR off the bike. But the big question stuck with me: could I have run even faster if I had fueled correctly?
Looking back, the culprit was clear: dehydration on the bike set me up for disaster on the run.
Let's break it down.
My Race-Day Fueling Recap
Here's what I actually consumed:
Pre-swim: 1 gel (25g carbs)
On the bike:
800 ml of fluid (just over one 26 oz bottle)
70g carbs mixed in the bottle
2 Maurten gels (50g carbs)
1 packet LMNT electrolytes
Total bike intake: ~120g carbs, 800 ml fluid
On the run: ~400 ml of water with electrolytes, no calories
Race duration: 4h 38m (31 min swim, 2h 26m bike, 1h 33m run, 8 min transitions)
Total intake: ~145g carbs and ~1.2 L fluids
So, in nearly 5 hours of racing, I took in about the same fluid volume most experts recommend in 90 minutes.
What I Should Have Taken In
Guidelines for endurance fueling are well established:
Carbs: 60–90g/hour on the bike (higher end for larger athletes); 30–60g/hour on the run
Fluids: 500–750 ml/hour depending on sweat rate, heat, and body size
Here's how my race stacked up:
Bike:
Duration: 2h 26m
Fluids consumed: 800 ml (~330 ml/hour) → short by ~1 L
Carbs consumed: ~50 g/hour → ~40 g/hour short
Run:
Duration: 1h 33m
Fluids: ~260 ml/hour (well below recommended)
Carbs: zero
By the start of the half-marathon, I was both dehydrated and under-fueled — the perfect storm for gut shutdown.
Why Dehydration Wrecks the Gut
GI distress in long-course triathlon is not just about gels and drink mix — it's strongly linked to hydration status. Here's why:
Blood Volume Drops
Dehydration reduces plasma volume, leaving less blood available.
Lower blood volume = reduced cooling capacity.
Core Temperature Rises
Lower blood volume = reduced cooling capacity.
Heat stress further restricts gut blood flow.
Reduced Oxygen & Nutrient Delivery
Muscles and gut cells get less oxygen and nutrients.
This accelerates fatigue and slows nutrient absorption.
Inside the Gut
Splanchnic Hypoperfusion (Low Blood Flow) - Less blood supply → impaired digestion and absorption.
Gut Permeability ("Leaky Gut") - Heat + low blood flow damage the intestinal lining. Inflammatory substances leak into circulation, causing nausea and cramps.
Delayed Gastric Emptying - The stomach stops moving contents forward. Water and carbs sit, leading to bloating, cramping, and sometimes vomiting.
By the time I hit the run:
My gut was inflamed and under-supplied.
Gastric emptying slowed or stopped.
Even plain water triggered cramps and pain because it couldn't leave the stomach.
At that point, the gut is essentially shut down. Recovery requires slowing down, walking, and gradually rehydrating until blood flow normalizes.
The Hidden Cost: Recovery
The pain didn't end at the finish line. My legs were destroyed for a week — quads so sore I could barely climb stairs. That wasn't just muscle fatigue; it was also dehydration.
Why?
Less blood volume = slower clearance of waste products like lactate.
Reduced oxygen/nutrient delivery = slower repair of muscle fibers.
Dehydration increases muscle breakdown and cramping risk.
So, dehydration didn't just limit performance during the race — it delayed recovery long after.
The "What If"
Despite all this, I still ran a PR 1:33 half-marathon. That tells me:
My fitness is there — the training worked.
My fueling and hydration are the limiters.
If I'd nailed hydration, I probably could have:
Avoided stomach distress,
Taken in carbs on the run,
Run faster than my PR,
And bounced back quicker post-race.
Takeaways for You
Don't underestimate hydration. Even perfect carb intake won't help if you're under-drinking.
Know your sweat rate. Test in training to estimate hourly needs.
Practice fueling at race pace. Gut training is just as crucial as VO₂ intervals.
Respect the compounding effect. Falling behind early on the bike sets up problems on the run.
I'm glad this happened in Santa Cruz. It was a painful reminder, but now I know what I need to fix for Marbella: drink more on the bike so I can fuel the run.
References
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American College of Sports Medicine (ACSM) Position Stand: Exercise and Fluid Replacement. Medicine & Science in Sports & Exercise, 39(2), 377–390.
Sawka, M. N., et al. (2007). American College of Sports Medicine position stand: Exercise and fluid replacement. MSSE, 39(2), 377–390.
Costa, R. J. S., et al. (2017). Systematic review: Exercise-induced gastrointestinal syndrome—implications for health and intestinal disease. Alimentary Pharmacology & Therapeutics, 46(3), 246–265.
Rehrer, N. J. (2001). Fluid and electrolyte balance in ultra-endurance sport. Sports Medicine, 31(10), 701–715.
van Wijck, K., et al. (2012). Exercise-induced splanchnic hypoperfusion results in gut dysfunction in healthy men. PLoS One, 7(7), e42179.
de Oliveira, E. P., & Burini, R. C. (2014). Food-dependent, exercise-induced gastrointestinal distress. Journal of the International Society of Sports Nutrition, 11(1), 12.
Cheuvront, S. N., & Kenefick, R. W. (2014). Dehydration: Physiology, assessment, and performance effects. Comprehensive Physiology, 4(1), 257–285.