Blood Sodium vs Body Sodium — Why the Test Your Doctor Orders Won't Help You Race

When athletes ask their GP about cramping or hydration issues, they're sometimes sent for a blood test that includes serum sodium. The result comes back within normal range. The doctor says everything looks fine. The athlete goes back to cramping in every race.

The problem isn't with the test result. The problem is that blood sodium is the wrong thing to measure for this purpose.

What a blood sodium test actually tells you

A serum sodium blood test measures the concentration of sodium in your blood plasma at the moment the sample was taken. The normal range is typically 135–145mmol/L.

Your body works extraordinarily hard to keep this number stable. Sodium regulation is so critical to survival that your kidneys, hormonal systems, and vascular responses are all continuously adjusting to maintain blood sodium within a narrow band. Getting this wrong — either too high or too low — has severe consequences.

The problem with using blood sodium to assess sports hydration

Because your body defends blood sodium concentration so aggressively, this number stays normal even when your total body sodium is significantly depleted.

Here's what happens: when you lose sodium through sweat faster than you're replacing it, your body pulls sodium from other compartments — specifically from the interstitial fluid that surrounds your muscles and organs — to maintain blood concentration. The blood test looks normal. But the fluid around your muscles now has less sodium in it than your muscles need to contract and relax properly.

This explains a finding from the Kona Ironman research: athletes presenting at the medical tent with severe muscle cramping had virtually identical blood sodium levels to non-cramping athletes. Blood sodium isn't the right measure of what's happening at the muscle.

What the sweat test tells you instead

A precision sweat test doesn't measure what's in your blood — it measures what you're losing through sweat. This is the actionable number.

By knowing your sweat sodium concentration and combining it with your sweat rate, you can calculate your total sodium losses over any given session and ensure your replacement strategy keeps pace with those losses.

It's a fundamentally different measurement for a fundamentally different question. The blood test asks: is my blood sodium at a safe level right now? The sweat test asks: how much sodium is leaving my body per hour, and am I replacing it?

For performance and injury prevention purposes, the second question is the one that matters.