Hydration & Electrolytes for Marathon des Sables and Other Desert Ultramarathons

Introduction

A race like the Marathon des Sables isn’t just a test of endurance—it’s a trial by fire, sand, heat, and isolation. Over multiple days, you’ll face temperatures that can soar above 40-50°C (104-122°F), constantly shifting terrain, and self-sufficiency that puts enormous strain on your body. Hydration for Marathon des Sables and other desert or hot races isn’t just about performance — it is mission-critical for survival and success.

Neglect them, and dehydration leads to fatigue, impaired thermoregulation, kidney stress, and risk of heat illnesses. Overdo fluids without matching electrolyte replacement, and you risk hyponatremia (low blood sodium), which can be as dangerous. The solution lies in a science-driven hydration strategy: understanding your sweat rate, your sweat sodium (and other electrolytes) loss, and building a plan around that. That’s where i-Thrive Surrey comes in: sweat electro-lyte testing, DEXA body composition, VO₂ and RMR (resting metabolic rate) assessment—all combine to create an optimal plan so you don’t just endure the desert… you thrive.

Why Hydration Matters in the Desert

Hydration for Marathon Des Sables and other hot weather races becuase sweat loss can be huge : In extreme heat, athletes often lose 1-3+ litres of sweat per hour depending on intensity, acclimation, humidity, clothing, etc. That’s water and electrolytes leaving your body.

• • Body weight loss of 2-5% of your mass impairs performance, reduces power output, increases cardiovascular strain.

• • Thermoregulation fails without water: your core temperature rises, heat dissipation is reduced, risk of heat illness increases.

• • Kidney & fluid homeostasis suffer: insufficient hydration + electrolyte loss causes drop in plasma volume, reduced circulatory efficiency.

But more isn’t always better: drinking too much water (without sufficient electrolyte / sodium) can dilute blood sodium, cause hyponatremia—leading to nausea, swelling, confusion, in severe cases seizures or worse.

Electrolyte Balance – The Key to Performance

Sodium loss dominates: Sodium (Na⁺) is lost in large quantities in sweat; chloride follows. Potassium, magnesium, calcium matter too but in smaller loss volumes.

Inter-individual variation is big: Some people lose 500 mg of sodium per litre sweat, others 1,500-2,000 mg per litre. Research shows wide spread depending on genetics, acclimation, sweat gland efficiency. Sweat & Sodium for Endurance Runners

Electrolyte loss symptoms: cramps, muscle weakness, dizziness, fatigue, cognitive slowdown, nausea, risk of electrolyte imbalance syndromes.

Some Evidence & Research Insights

It helps to build your plan on recent results.  Don’t leave hydration for Marathon des Sables to chance.  

1. 1. Personalised sodium replacement in the heat
      A 2023 randomized controlled trial had athletes do ultra-endurance running in hot conditions, comparing full replacement of sweat sodium vs placebo, with fluid intake ad libitum. Key findings:
      
      • • Sodium replacement increased plasma sodium concentration at mid-exercise & post-exercise compared to placebo.
      • • It didn’t significantly alter net fluid balance or thermophysiological strain (heart rate, temperature, overall perceived effort) during exercise. i.e. replacing sodium helps with plasma sodium, but doesn’t on its own change all markers of strain. (McCubbin & Costa, 2023)

1. 1. Sweat electrolyte measurement and heat training adaptation
      A late-2024 review (Applied Sciences) emphasizes the importance of how sweat electrolytes are measured (whole-body washdown vs regional patches) and notes that heat acclimation reduces sweat electrolyte concentration over time. That is, with repeated exposure to heat, the body becomes more efficient: retaining more sodium, chloride, so less is lost per litre of sweat. (Keefe, et al., 2024)

1. 1. Field studies / ultramarathon in hot ambient environment
      There are observational studies in hot multi-stage ultras showing that runners often fail to match their sodium losses, leading to drops in plasma sodium if fluids are sodium-free; also that pre-race nutrition & electrolyte strategy is often under-planned. (Costa, et.al, 2013)

How to Know Your Sweat Rate & Sodium Loss

To build a reliable plan, you need test data specific to you, especially in heat.

• • Sweat Rate: Do a field test (long run in heat similar to what you’ll face in race). Weigh yourself before and after (minus clothing, equipment) with minimal clothing. Account for any fluid you took in. Weight loss = fluid loss. Divide by duration → L/hr sweat rate.

• • Sweat Sodium / Electrolyte Testing:
    
    • • Methods like regional sweat patches, whole-body washdowns (gold standard) are used in research.
    • • Lab or field sweat test (e.g. via precision hydration services) to measure mg sodium per litre of sweat. i-Thrive is a Precision Fuel & Hydration Sweat Test Centre – Learn More about our Sweat Composition Test Here.