February 8, 2026 · 4 min read

Cyclist Hydration — Electrolytes, Salt and Isotonic Drinks in Training

Dehydration of 2% of body weight (1.4 kg at 70 kg) lowers output power by 7–10% and worsens perceived effort (RPE) by several points on the Borg scale — meaning the same wattage starts to feel subjectively harder. Drinking plain water without electrolytes doesn't solve the problem: diluted sweat isn't just water, it's sodium, potassium, magnesium and chloride too. Below is a concrete protocol for calculating and replacing losses for cyclists.

How to calculate your own sweat rate

Sweat rate varies between people by a factor of 3–5×: from 0.5 l/h to over 2.5 l/h during intense effort in the heat. You can measure your own value yourself:

1. Weigh yourself before the ride (no clothes, after using the bathroom)
2. Ride for 60 minutes in typical conditions — either with no drinking, or with a precisely measured amount of fluid
3. Weigh yourself after the ride (no clothes, without wiping off sweat)
4. Calculate: sweat rate [l/h] = (weight before − weight after) + fluid drunk [l]

Example: -0.9 kg of weight + 0.3 l of fluid drunk = 1.2 l/h. That's your reference point for that temperature and intensity.

How much salt does a cyclist lose in one session?

Sweat contains, on average, 400–1100 mg of sodium per litre (the range depends on genetics, acclimatisation and diet). At a sweat rate of 1.2 l/h over a 2-hour session: 2.4 l × 750 mg/l = ~1800 mg of sodium. That's nearly the entire daily recommended limit (1500–2300 mg/day) lost in a single workout. Replacing it with water alone dilutes interstitial sodium → exercise-associated hyponatremia — rare, but genuinely dangerous during very long activities (>4–5 h).

When to drink water, and when an isotonic drink?

Water vs isotonic — when to choose which?

• Water: sessions under 60–75 minutes in moderate temperature, low intensity (zone 1–2). Replacing electrolytes from a post-workout meal is enough
• Isotonic: sessions over 75–90 minutes, high intensity (>zone 3), temperature >25°C, riding fasted or with limited glycogen
• Hypertonic (energy drink, gel): replaces energy, but absorbs water more slowly — it isn't a hydrating drink, use it alongside water

How to make a homemade isotonic drink better than the shop-bought kind

Commercial isotonic drinks often contain too much sugar, dyes, and too little sodium. A simple recipe at an osmolality of ~280 mOsm/kg (isotonic value):

• 500 ml of water
• 30 g of sucrose or glucose (a 6% carbohydrate concentration — the upper limit for fast absorption)
• 0.5 g of table salt (~200 mg of sodium) or 1 g of Himalayan salt
• Optional: lemon juice for flavour + a pinch of potassium chloride (KCl, available at pharmacies as a dietary salt)

A drink like this performs comparably to premium products in cycling studies, at a fraction of the cost.

Sodium: how much and when to supplement?

Are electrolyte tablets necessary?

For sessions up to 90 minutes with an isotonic drink: electrolyte tablets aren't necessary — adequate sodium in the drink is enough. Tablets make sense for sessions >2 h, multi-day training camps, and ultra events (gran fondos, cyclosportives >4 h). A standard dose: 300–600 mg of sodium/h from all sources combined (drinks + gels + tablets).

Dietary salt and cycling performance

Cyclists training intensely have a higher sodium requirement than the general population — cardiology recommendations (max 2300 mg/day) are too restrictive for people sweating 10–15 h a week. Heat acclimatisation over 10–14 days reduces sweat sodium loss by 20–30%, but doesn't eliminate the need to replace it during long sessions.

Magnesium and potassium — the often-overlooked electrolytes

Magnesium is involved in over 300 enzymatic reactions related to ATP metabolism. Its deficiency shows up as muscle cramps, sleep disturbances and general exhaustion. Cyclists with a high weekly TSS often have suboptimal magnesium levels despite a balanced diet — supplementing 200–400 mg/day (preferred forms: citrate, glycinate) before bed improves sleep quality and reduces night-time cramps.

Potassium regulates the electrical gradient across muscle membranes. Potassium loss through sweat is smaller than sodium's, but rises for sessions >3 h. Bananas, potatoes and cocoa are the best dietary sources — potassium supplementation needs care and medical consultation for anyone with kidney disease.

Monitoring body weight before and after every intense workout is the simplest way to track your hydration balance — a difference >1% of body weight signals inadequate fluid replacement during the session.

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