Hydration and Electrolytes for Training: Avoid the Most Common Mistakes

Why hydration is more than just drinking water

Water is essential for athletic performance — but drinking water alone during prolonged or intense training replaces fluid without replacing the electrolytes lost in sweat, particularly sodium. When sodium concentrations in the blood fall because large volumes of water are consumed without electrolyte replacement, the condition is called hyponatremia. In mild cases it causes nausea, bloating, and poor performance. In severe cases it is medically dangerous.

The practical implication: for sessions lasting over 60 minutes, especially in heat, a hydration strategy that addresses both fluid and sodium is meaningfully better than one that addresses only fluid. For shorter, lower-intensity sessions in cool conditions, water alone is usually sufficient. Understanding which situation you are in determines which approach you need.

How to estimate your sweat rate

Sweat rate varies widely between athletes — some people sweat twice as much per hour as others of similar size at similar intensities, and sweat composition (especially sodium concentration) is also highly individual. The simplest way to estimate your sweat rate: weigh yourself in minimal clothing immediately before and after a one-hour training session without drinking anything during it. Every gram of weight lost represents approximately 1ml of fluid loss.

A 70kg runner who is 72kg before and 70.2kg after a one-hour run in moderate conditions has lost roughly 1.8 litres of sweat — a relatively high rate. A 70kg cyclist who drops 0.8kg in the same hour has a lower rate. Both athletes need different fluid strategies. Without this measurement, you are guessing.

Sweat sodium concentration is harder to measure without a lab test, but useful norms exist: most athletes lose between 500–1500mg of sodium per litre of sweat, with endurance athletes often at the higher end. Salty sweaters — those who notice white residue on skin or clothing after training — are typically losing more sodium than average and benefit most from targeted electrolyte replacement.

Sodium: why it is the priority electrolyte

Of all the electrolytes lost in sweat — sodium, potassium, magnesium, calcium — sodium is lost in the largest quantities and has the most immediate performance and safety implications. Sodium maintains blood volume, helps muscles contract, and drives thirst — which is itself a useful hydration signal when sodium is available.

Practical sodium targets during training: for sessions under 60 minutes in moderate conditions, the sodium in a normal diet the rest of the day is sufficient. For sessions of 60–90 minutes in heat, 300–500mg of sodium per hour from an electrolyte drink or supplement is a reasonable starting point. For longer sessions or in very hot conditions, 500–1000mg per hour may be appropriate depending on individual sweat rate and sodium concentration.

Sources of sodium during training: electrolyte tablets dissolved in water, sodium-containing sports drinks, and gels or chews that include sodium. Plain water, fruit juice, and zero-electrolyte drinks provide fluid without sodium — useful for rehydration after short sessions but insufficient for longer efforts.

Electrolytes beyond sodium

Potassium, magnesium, and calcium are also lost in sweat, but in much smaller quantities than sodium. Deficiencies in these electrolytes during a single training session are uncommon in athletes eating a varied diet. The main risk is cumulative — across multiple days of high-volume training with poor dietary variety, total electrolyte intake can fall below what's needed for optimal muscle function and recovery.

The best approach to non-sodium electrolytes is dietary: potassium from bananas, potatoes, and leafy greens; magnesium from nuts, seeds, and whole grains; calcium from dairy or fortified plant alternatives. Supplementing these electrolytes specifically for training is rarely necessary except for athletes with very restricted diets or specific clinical deficiencies confirmed by blood work.

Hydration before training: starting in the right state

Arriving at a training session already dehydrated reduces performance before the session starts. A practical pre-training hydration check: urine colour. Pale yellow to clear before training indicates adequate hydration. Dark yellow or amber suggests catching up is needed.

If you notice you are dark before a session, do not try to rapidly drink a large volume of water immediately before — this can cause GI discomfort and doesn't correct hydration status meaningfully in 10 minutes. Instead, focus on drinking consistently across the day leading up to training, particularly in the 2–3 hours before. 500ml of water 2 hours before a session, with another 200–300ml 20–30 minutes before, covers most athletes in moderate conditions.

Adjusting for weather, altitude, and session type

Sweat rate increases with heat, humidity, and altitude. Training at 30°C requires a meaningfully different hydration strategy than training at 15°C with the same session type and intensity. Athletes who regularly train in variable conditions benefit from building habits around monitoring urine colour, body weight, and thirst rather than following fixed ml-per-hour rules that don't account for environmental context.

Hot weather: increase fluid and sodium intake. The body's cooling mechanism relies on sweat evaporation — suppressing sweat by becoming dehydrated impairs cooling and increases heat stress risk. Cold weather: athletes often underestimate fluid needs in cold because thirst signals are blunted. Respiratory water loss increases in cold, dry air. Indoor training (spin classes, gym sessions) tends to generate higher sweat rates than outdoor training at equivalent intensities due to reduced convective cooling.