Sodium Loading for Endurance: The Pre-Race Protocol

In the last 20 years, the dominant hydration advice in endurance sport has flipped. The old fear — dehydration — has been largely replaced by a more lethal one: exercise-associated hyponatremia (EAH), drinking so much plain water that blood sodium drops dangerously low. The solution isn't drinking less. It's loading sodium correctly.

Why Sodium Matters More Than Water Alone

Sweat contains 500–2000 mg of sodium per liter, depending on genetics, acclimatization, and heat. Over a 4-hour Ironman bike leg or marathon, that's 2–8 grams lost. Replacing fluid without replacing sodium dilutes your blood — the recipe for cramps, nausea, and in extreme cases, EAH-induced seizures.

The Pre-Race Loading Protocol

• 2–3 days out: Increase dietary sodium by ~30%. Add salt liberally to meals. Skip the "low-sodium" advice that applies to sedentary populations.
• Night before: Salty dinner. Include a sports drink with dinner instead of plain water.
• Race morning: Sip 16–20 oz of fluid with 500–800 mg sodium 90 minutes before start.
• 30 min pre-start: Final 8–12 oz with another 300–500 mg sodium.

During the Race

For events over 90 minutes, target 300–700 mg sodium per liter of fluid. Heavy sweaters, hot races, and salty-shirt athletes (visible white residue post-workout) need the upper end. Combine sports drink + salt capsules to tune precisely.

Heavy Sweater? Get Tested

Sweat sodium concentration varies 4x between athletes. A simple sweat patch test (~$75) tells you whether you're a 400 mg/L bleeder or a 1500 mg/L gusher — and exactly how much sodium to carry per hour.

Symptoms You Got It Wrong

• Too little sodium: Cramps in the second half, swollen fingers, sloshy stomach, headache, nausea.
• Too much sodium (rare): Excessive thirst, GI distress, salty mouth. Easy to correct — drink plain water.

The Bottom Line

The single biggest hydration upgrade most endurance athletes can make isn't drinking more — it's salting more. Load 500–800 mg of sodium an hour before the gun, 300–700 mg/L during, and tune to your sweat rate. Cramps and bonks usually trace back to electrolytes, not water.