01/12/2026
OPTIMAL HYDRATION CHEAT SHEET
Why Sweat-Based Hydration is Wrong
- Typical sweat sodium: ~800-1,200 mg/L
- Low sweat sodium: ~ 200-600 mg/L often signals low salt status in the body and that someone needs more salt, not less (counterintuitive).
- Don't dose sodium just on sweat losses...it's "myopic" and ignores full-body fluid shifts.
- Sweat pulls water mostly from extracellular fluids:
63% from plasma
14% from interstitial fluid
23% from inside cells
- Plasma volume drops (not cellular dehydration) cause fatigue, elevated heart rate, and performance dips, which can be dangerous.
What Happens During Exercise
- In just 5-15 minutes of vigorous activity, plasma volume drops ~10%.
Reasons:
1. Blood redistribution: Blood flows to working muscles and skin, reducing supply to the heart. This lowers stroke volume, spikes heart rate, increases perceived exertion & fatigue.
2. Arteriole dilation: Increases pressure in capillaries, pushing water out into interstitial fluid (so interstitial fluid increases while plasma drops).
3. Muscle metabolites: Lactate, inorganic phosphates, and creatine build up, pulling more fluid into cells from plasma/interstitial spaces (dropping plasma volume even more).
- Plasma sodium is high (~3,200mg/L), ~ 3X higher than sweat sodium concentration, so you need higher sodium intake to restore plasma volume effectively.
Optimal Pre-Hydration (Before Competition)
- Goal: Boost plasma volume by up to 10% to offset drops.
- Ideal concentration: 3,700-4,200mg sodium per liter of fluid (hypertonic, normal saline is 3,540mg/L).
Timing & Method:
- Start 90-105 minutes before competition.
- Drink slowly over ~30 minutes (allows 60 minutes for absorption and blood volume expansion).
- Use refrigerated salt solutions for pre-cooling, especially in heat or for long sessions.
- Not for everyday use...save for competitions.
- For daily training, a milder preload is fine (1,150-2,300 mg/L 30-45 minutes prior to exercise).
Optimal Hydration During Exercise
- Recommended concentration: ~2,300mg sodium per liter to maintain/slightly boost plasma volume.
- May need periodic "reloads" (e.g., another full liter 30-60 minutes in) for high intensity prolonged sessions (4+ hours in heat).
- Minimum effective dose: 1,150mg/L...shown to improve tennis performance when taken ~ 30 minutes before + during.
- For everyday workouts: 1,150mg/L about 30 minutes before to feel good and perform better.
Key Takeaways
- Hydrate based on plasma volume needs, not sweat losses—aim for sodium levels that match or exceed plasma (3,200+ mg/L) for real benefits.
- This prevents elevated heart rate and core body temperature, dehydration, fatigue, cramps, and low energy
- Key books for more learning The Salt Fix and Win.
Shout out to for the infographic! :)
