Sodium losses are the most under-managed variable in ultras. This guide explains why ultras need more sodium than other endurance sports, how to test if you're a salty sweater, the pre-loading protocol with SLT+, hourly targets during the race, and how to avoid hyponatraemia.

This guide is part of our Ultra-Trail Snowdonia fuelling master series — read the master guide first for the full UTS protocol, then come back here for the deep dive on this single piece.

Key points

  • Pre-load 500 to 1,000 mg of sodium in the hour before the start, plus another 500 mg in the hour before bed the night before.
  • Five signs: white salt crusts on your race kit, salty taste on your skin, eye-stinging sweat, regular muscle cramps in long sessions, and strong cravings for salty food after exercise.
  • Target 500 to 1,000 mg of sodium per hour during ultras, more if you're a salty sweater (up to 1,500 mg/h) or racing in heat.
  • Hyponatraemia is dangerously low blood sodium, caused by drinking too much plain water without replacing sodium losses.
  • Both have a place.

Why sodium matters more in ultras

Sodium does three things during an ultra: it pulls water into the bloodstream so you can absorb fluid you drink, it powers muscle contraction, and it helps the body retain the fluid it already has. Run out of any of those and the wheels come off - you'll cramp, drop pace, and struggle to absorb anything you put in your mouth.

Most marathoners can get away with 300 to 500 mg of sodium per hour. Ultra runners can't. The combination of multi-hour sweat losses, higher overall fluid intake, and the cumulative time on feet makes sodium the variable that breaks athletes most often.

How to test if you're a salty sweater

The five-sign self-test:

  1. White salt rings on race kit after long sessions in heat.
  2. Sweat tastes obviously salty - lick the back of your hand mid-run.
  3. Eye-stinging sweat that drips down your forehead.
  4. Regular muscle cramps in the back end of long runs, even when fuelled.
  5. Salty food cravings within an hour of finishing.

Two or more signs put you at the heavier end (1,500 to 2,500 mg of sodium per litre of sweat). For exact numbers, a sweat test from a sports lab costs £80 to £200 and gives a definitive reading.

The pre-loading protocol with SLT+

Pre-loading sodium primes the body to retain fluid and starts the race with electrolytes already in the system, not playing catch-up.

  • Night before: 500 mg of sodium with dinner (one SLT+ sachet in 500 ml of water alongside the meal).
  • Morning of: 500 to 1,000 mg in the 60 to 90 minutes before the start. Two SLT+ sachets in 500 ml for salty sweaters.
  • Right before the gun: a final 200 to 300 ml of electrolyte-containing fluid (MIX+ works well — combines carbs and electrolytes).

The pre-load is the easiest gain in ultra prep. It costs nothing in race-day stress, takes 10 minutes, and prevents the early sodium deficit most athletes spend the first two hours digging out of.

STYRKR featured Image01

Sodium targets during the race

Match intake to losses across the full race:

  • Standard sweater (800 to 1,000 mg/L): 500 to 700 mg of sodium per hour.
  • Salty sweater (1,500 to 2,500 mg/L): 700 to 1,500 mg of sodium per hour.
  • Hot conditions or heavy losers: up to 1,500 mg per hour, with extra at aid stations.

The standard split: drink mix delivers 300 to 500 mg per hour at typical concentrations, and an SLT+ or SLT07 tablet adds 500 to 1,000 mg. Heavy sweaters add salted foods (potatoes, broth) at aid stations.

Avoiding hyponatraemia

Hyponatraemia is the opposite problem to dehydration: too much plain water with not enough sodium. Blood sodium drops, the brain swells, and the athlete becomes confused, nauseous and at risk of seizure.

The fix is straightforward: never drink plain water alone for more than 90 minutes during an ultra. Always pair fluid with electrolytes. If you finish a long session without urinating but feel bloated, you're at risk.

Race-day rule: fluid intake should match thirst plus sweat rate, not exceed it. Over-drinking is as dangerous as under-drinking.

Salt tablets vs electrolyte drinks

Quality electrolyte drink mixes (like STYRKR MIX+) deliver sodium plus carbs plus potassium and magnesium in one bottle. They're the simplest way to hit moderate sodium targets while also fuelling.

Salt tablets and high-strength sachets (SLT+, SLT07) deliver concentrated sodium without extra carbs. They're useful when:

  • You need 1,000+ mg per hour and can't fit it all into drink mix.
  • You're getting carbs from gels or real food and want sodium separately.
  • You're pre-loading or topping up at specific points (race start, aid stations).

Most ultra athletes use both: drink mix for steady baseline, tabs for pre-load and heavy-loss top-ups.

Final word

Sodium is the variable most ultra runners under-manage and the easiest to fix. Test if you're a salty sweater, pre-load 500 to 1,500 mg in the hour before the start, target 500 to 1,500 mg per hour during the race, and pair every bottle of plain water with electrolytes.

This protocol pairs with our Ultra-Trail Snowdonia fuelling master guide — sodium is one piece of the broader UTS race-day fuelling system.

Related deep dives in this UTS series

Read the master overview at How to Fuel for UTS Snowdonia: An Ultra Trail Guide.