Hydration and Electrolytes: When Plain Water Isn't Enough During Exercise

"Drink water" is insufficient advice for marathon runners — and understanding why explains electrolytes for everyone

In the 1996 Hawaiian Ironman triathlon, multiple athletes were hospitalised with hyponatraemia — dangerously low blood sodium. They had followed the advice to "drink plenty of water." They drank so much plain water that they diluted the sodium in their blood, causing swelling of brain cells and a medical emergency.

The hyponatraemia incidents of the 1990s and 2000s forced a revision of athletic hydration advice. The current understanding is more nuanced than "drink more water" and has implications for exercise hydration well below the Ironman level.

What electrolytes actually do

Electrolytes are minerals that carry an electrical charge when dissolved in water. The key electrolytes in human physiology:

Sodium (Na⁺): the primary extracellular electrolyte. Regulates blood volume and blood pressure, nerve and muscle function, and fluid balance. Normal blood sodium: 135–145 mEq/L.

Potassium (K⁺): the primary intracellular electrolyte. Works with sodium in the sodium-potassium pump that maintains cell membrane potential. Critical for heart rhythm and muscle contraction.

Chloride (Cl⁻): primarily works with sodium to maintain fluid balance and produce stomach acid.

Magnesium (Mg²⁺): involved in over 300 enzyme reactions, muscle and nerve function, blood glucose control.

Calcium (Ca²⁺): muscle contraction, nerve transmission, bone structure.

Sweat contains primarily sodium and chloride, with smaller amounts of potassium, calcium, and magnesium. Sweat sodium concentrations vary substantially between individuals (100–2,000 mg/L) — some people are "salty sweaters" who lose much more sodium per litre of sweat than others.

When plain water is insufficient during exercise

For exercise under ~60 minutes at moderate intensity: plain water is adequate. Fluid loss is modest, and electrolyte losses don't significantly deplete body stores.

For exercise over 60–90 minutes with significant sweating: sodium replacement becomes important for two reasons:

1. Blood sodium maintenance: as you sweat, blood volume decreases and sodium concentration should increase slightly (concentrating effect). Drinking large volumes of plain water dilutes the sodium, potentially causing hyponatraemia.

2. Thirst suppression: sodium in drinks stimulates ongoing thirst. Plain water can satisfy thirst before adequate fluid replacement occurs (particularly in cooler temperatures where thirst is blunted), causing a paradox: you feel "not thirsty" but are still dehydrated relative to sweat losses.

Hyponatraemia in endurance events: the risk profile

Exercise-associated hyponatraemia (EAH) occurs when blood sodium falls below 135 mEq/L during or after exercise. Risk factors:

• Drinking plain water in large volumes (rather than electrolyte-containing drinks)
• Slow pace (more time on course = more opportunity to over-drink)
• Smaller body size (lower total body water; less dilution buffer)
• High ambient temperature (more sweating, more drinking)
• Extremely long events (ultra-marathons, Ironman)
• Female sex (possibly related to hormonal effects on antidiuretic hormone regulation)

Symptoms: headache, nausea, confusion, seizures (severe cases), cerebral oedema (life-threatening).

The paradox: hyponatraemia presents similarly to dehydration (headache, disorientation). Treating perceived dehydration by drinking more plain water when the actual cause is hyponatraemia makes it worse. The key distinguishing feature: EAH patients typically have unchanged or gained body weight during exercise (fluid intake exceeding losses), while dehydration produces weight loss.

Electrolyte replacement during exercise

For moderate exercise (60–120 minutes):

• Sports drinks (containing 400–1,100 mg sodium/litre) or water with sodium supplementation are adequate
• Commercial sports drinks: Gatorade (~450mg sodium/L), Lucozade Sport (~390mg sodium/L), Precision Hydration (~1,000mg sodium/L for higher-sweat athletes)

For prolonged exercise (>2 hours):

• Higher sodium intake: 500–1,500 mg/hour (more for heavy or salty sweaters)
• Electrolyte tabs/capsules with water (e.g., SaltStick, Precision Hydration tablets)
• Food sources: pretzels, salted crackers provide sodium alongside carbohydrate

The "drink to thirst" guideline: current consensus from sports medicine (ACSM, IMMDA) recommends drinking to thirst rather than scheduled drinking during exercise. Thirst is a reliable signal of hydration need for most athletes in most conditions. The exception: events where cognitive impairment from heat or exhaustion may blunt thirst sensation.

Daily hydration and electrolytes

For non-athletes in everyday conditions:

• Dietary sodium from food covers most electrolyte needs
• Plain water and other beverages cover fluid needs
• Additional electrolyte supplementation is not needed unless: heavy sweating occurs, dietary sodium is very restricted, or specific deficiency is identified

When electrolyte supplementation matters outside sport:

• Prolonged diarrhoea or vomiting (significant electrolyte loss)
• Very hot environments with heavy sweating (occupational or climate)
• Diuretic medication (which increases electrolyte excretion)
• Keto or very low-carb diets (insulin reduction increases kidney sodium excretion, causing rapid initial fluid and sodium loss)

How to use the Water Intake Calculator on sadiqbd.com

1. Enter weight and activity level
2. Read baseline fluid needs — the calculator provides a starting point
3. Adjust for exercise: add approximately 500ml per 30 minutes of intense exercise, and consider electrolyte replacement for sessions over 60 minutes
4. Use urine colour as a real-time guide — pale yellow = adequately hydrated

Frequently Asked Questions

Should recreational gym-goers use sports drinks? For most gym sessions under 60 minutes, no — plain water is sufficient. Sports drinks provide unnecessary calories for most recreational exercise. For long sessions, group fitness classes exceeding 90 minutes, or training in hot conditions, electrolytes become more relevant.

Can you drink too much water? Yes — as the hyponatraemia discussion demonstrates. Severe over-drinking is dangerous. For everyday life, the kidneys handle excess fluid efficiently, but water intoxication from extreme volumes is a real (if rare) medical emergency.

Is the Water Intake Calculator free? Yes — completely free, no sign-up required.

Electrolytes turn water from a simple volume replacement into a precisely calibrated fluid management system. Understanding when plain water suffices and when sodium replacement is needed makes exercise hydration evidence-based rather than guesswork.

Try the Water Intake Calculator free at sadiqbd.com — find your personalised daily hydration target and adjust it for exercise and climate.