5 Ways Dehydration Wrecks Your Race (and How to Avoid It)

At Ironman 70.3 Santa Cruz, I learned a painful lesson: dehydration doesn't just make you thirsty — it can shut your gut down. Here are five ways not drinking enough fluids can ruin your race:

1. Blood Volume Drops

When you don't replace sweat losses, plasma volume falls. Less blood is available for both working muscles and digestion. Your body prioritizes survival — sending blood to the heart, brain, and muscles instead of the gut.

2. Core Temperature Spikes

With less plasma, your cooling system is impaired. As heat stress rises, your heart rate increases, and fatigue sets in sooner. A hotter core = even less blood flow to your stomach and intestines.

3. Gut Blood Flow Tanks

Dehydration accelerates splanchnic hypoperfusion — reduced blood flow to the GI tract. That means poor digestion, slower absorption, and a higher risk of cramps or nausea mid-race.

4. The "Leaky Gut" Effect

Low blood flow and heat stress damage the gut lining. This increases permeability, letting inflammatory substances leak into your bloodstream — triggering stomach pain, bloating, and nausea.

5. Stomach Shut-Down

With little blood to the gut, gastric emptying slows or stops. Water and carbs sit in your stomach, causing sloshing, cramping, or vomiting. At this point, even plain water can be hard to drink.

The Hidden Cost: Recovery

Dehydration also slows recovery. Less blood flow means slower clearance of waste products and reduced nutrient delivery to muscles — leading to more soreness and longer rebound times.

Takeaway

Practice your hydration just like your intervals. Aim for 500–750 ml/hour, depending on sweat rate, and pair that with 60–90g carbs/hour on the bike. Nail fluids early so your gut can still absorb fuel when you hit the run.