You’ve nailed your long runs, tracked your heart rate zones, and dialled in your fueling. But come race day, cramps strike, your stomach rebels, or your pace fades in the heat. Often, the culprit isn’t training — it’s sodium.

Sodium is the primary electrolyte lost in sweat, and every athlete’s sweat profile is unique. Some lose less than 500mg per litre of sweat. Others lose over 2,000mg. That difference can make or break performance, especially in hot, humid, or long-duration events.

In Threshold #136 | Electrolytes Explained, we explored the fundamentals of how electrolytes fuel endurance. This article goes deeper — into sodium concentration, sweat testing, and how to personalise your hydration plan.

So how do you crack your sweat code and hydrate like a pro?

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The Main Feature

Leg 1: The Science of Sodium in Endurance Performance

Sodium is the gatekeeper of fluid balance and nerve-muscle communication. It helps retain water in the bloodstream, regulates blood pressure, and ensures electrical signals fire properly in working muscles. When sodium drops too low — either from sweat loss or over-drinking plain water — performance nosedives.

The condition athletes fear most is hyponatremia, when sodium concentration in the blood falls dangerously low. It often results not from dehydration but from overhydration with insufficient sodium. Symptoms include dizziness, nausea, confusion, and in severe cases, collapse.

On the flip side, chronic sodium under-replacement leads to early fatigue, cramping, and heat intolerance. This is especially relevant in long races, where sweat loss can reach 3–5 litres, translating into multiple grams of sodium lost.

The key point: sodium needs aren’t one-size-fits-all. Genetics, diet, sweat rate, and environment determine your losses — and your replacement plan must reflect that.

T1: Mental Preparation

Leg 2: Cracking Your Personal Sweat Code

Step 1: Estimate Sweat Rate

• Weigh yourself pre- and post-session (nude, dry).

• Sweat rate = (pre-weight – post-weight + fluid consumed) ÷ duration.

• Example: Lose 1.2kg in 90 minutes and drank 0.5L = ~1.13L/hour sweat rate.

Step 2: Measure Sodium Concentration

• Lab or at-home sweat testing kits can determine sodium concentration per litre.

• Broad categories:

  • Low: < 800mg/L

  • Moderate: 800–1,200mg/L

  • High: 1,200–2,000mg/L+

Step 3: Build a Replacement Strategy
Multiply sweat rate by sodium concentration to estimate hourly sodium loss. Example: 1.2L/hour × 1,200mg/L = 1,440mg sodium/hour.

Step 4: Practical Application

• Aim to replace ~60–70% of sodium lost during training/racing. Full replacement isn’t always necessary, but chronic under-replacement will impair performance.

• Spread intake across the session — capsules, electrolyte mixes, or sodium-rich sports drinks.

Pro Tip: Track subjective data too — perceived thirst, cramp frequency, GI comfort. These often reveal hydration success more clearly than numbers alone.

Leg 3: Implementing Sodium Strategy Across Training and Racing

Daily Training:

• For sessions under 60 minutes in cool conditions, water is often enough.

• For longer sessions or hot climates, include 300–600mg sodium per hour as a baseline.

Race Prep:

• In the final weeks before an event, test your sodium strategy in race-like conditions. Practise with the same drink mix, capsule timing, and fluid volume you’ll use on the day.

Hot/Ultra Events:

• High sweat rate athletes in hot conditions may require 800–1,200mg sodium per hour — or more. In ultras, sodium intake should be planned alongside carbohydrate fueling to avoid GI distress.

Recovery:

• Post-session, replace ~150% of fluid lost, paired with 1,000–1,500mg sodium, to accelerate rehydration. Salty meals (broth, eggs, cured meat) are effective real-food options.

Remember: sodium is a system, not a single session hack. By integrating testing, training, and race-day practice, you can eliminate cramping, stabilize performance, and stay sharp even in extreme conditions.

Conclusion

Sodium is the lynchpin of hydration and performance. Cracking your sweat code means moving beyond generic advice and into personalised strategy. By testing, tracking, and refining, you ensure every drop of sweat lost is matched by a plan — keeping your engine cool, your muscles firing, and your performance consistent.

Coaches Corner

Workout of the Week: Heat + Hydration Simulation Run

Goal: Practise sodium replacement under race-like stress.

Warm-Up (10 min):

• Easy jog, dynamic stretches.

Main Set (60–75 min):

• Run in Zone 2–3 in warm conditions or layered clothing.

• Measure fluid intake precisely (e.g., 500ml bottles).

• Ingest sodium according to your estimated sweat code (start with 600–800mg/hour).

• Weigh yourself before and after to validate your sweat rate.

Cool Down (10 min):

• Easy jog/walk.

• Note thirst, GI comfort, and cramp signs in a log.

Why it works: This session not only trains aerobic capacity but gives direct, practical feedback on whether your sodium plan matches your physiology.