Sweat Rate vs. Sodium Loss Rate: Why Both Matter for Hydration & Performance

November 06, 20263 min read

Most endurance athletes in South Africa know hydration is critical, especially under the baking sun of a summer race. But fewer understand exactly what they need to replace. To dial in your hydration, you need to understand three distinct measurements:

Sweat rate: How much fluid you lose over time, measured in litres per hour (L/hr).
Sweat sodium concentration: The amount of sodium in your sweat, measured in milligrams per litre (mg/L).
Sodium loss rate: Your total sodium loss over time, measured in milligrams per hour (mg/hr).

When you combine your sweat rate (L/hr) with your sweat sodium concentration (mg/L), you find your true sodium loss rate (mg/hr). This helps you build a hydration strategy based on your actual physiology, rather than generic advice.

Two Core Principles for Better Hydration

1. Use Sweat Rate for Planning Calculating your sweat rate across varying local conditions—from crisp Highveld winter mornings to humid Durban summers—helps you forecast your hydration needs. This prevents you from running out of bottles, lugging around unnecessary weight, or panicking at the start line. It is about logistics, not rigid dogma.

2. Thirst is the Boss When it comes to the actual act of drinking, trust your body's built-in system: thirst. If you finish a ride or run dehydrated even after drinking to thirst, the solution usually isn't forcing more water—it is replacing lost sodium.

How to Measure Your Sweat Rate (Step-by-Step)

Weigh yourself nude (or in minimal clothing) in kilograms before your session.
Record exactly how much fluid you consume during the workout in grams or millilitres.
Weigh yourself again post-workout in the exact same condition.
Calculate your sweat rate: [(Pre-exercise weight in kg - Post-exercise weight in kg) + (Fluid consumed in litres)] / Time in hours.

Tracking Dehydration To check your dehydration level, divide your post-exercise weight by your pre-exercise weight, and multiply by 100.

A drop of 0 to 2% is an ideal target.
If you lose more than 3% of your starting body weight, you need to incrementally increase your sodium intake during the next session to help drive your thirst response.

Putting It Together: Matching Fluid and Sodium Once you know your numbers, you can target your intake. Here are common scenarios to use as starting points:

Low Sweat Rate (<0.8 L/hr) + Low Sodium Concentration (<800 mg/L): Target 400–600 mL/hr of fluid and 250–500 mg/hr of sodium. Think: cool Cape winters or shorter training sessions.
High Sweat Rate (1.2–1.5 L/hr) + Low Sodium Concentration (<800 mg/L): Target 750–1,200 mL/hr of fluid and 300–600 mg/hr of sodium. Think: heavy sweaters who don't leave white marks on their kit.
Low Sweat Rate (<0.8 L/hr) + High Sodium Concentration (>1,000 mg/L): Target 500–800 mL/hr of fluid and 800–1,500 mg/hr of sodium. Think: athletes who leave salt crusts but don't sweat buckets.
High Sweat Rate (1.2–1.5 L/hr) + High Sodium Concentration (>1,000 mg/L): Target 1,000–1,500 mL/hr of fluid and 1,000–2,000 mg/hr of sodium. Think: long, gruelling days at the Cape Epic or ultra-marathons in peak summer heat.

During exercise, match your intake to your individual losses, not a generic benchmark.