Hydration and Electrolytes

Water makes up roughly 60% of adult body weight and is involved in every physiological process — digestion, temperature regulation, waste removal, and joint lubrication. Despite the famous "8 glasses a day" rule, actual needs vary significantly by body size, climate, activity, and diet.

Group	Total water target (all sources)	Notes
Adult men	~3.7 L/day	US National Academies AI
Adult women	~2.7 L/day	Increases during pregnancy (3.0 L) and lactation (3.8 L)
Active adults	Add 0.5–1.0 L per hour of exercise	More in heat or high altitude
Older adults (65+)	At least 1.7 L/day minimum	Thirst sensation declines with age

About 20% of daily water comes from food — fruits, vegetables, soups, and dairy all contribute significantly. Urine colour is the most practical guide: pale yellow (like lemonade) indicates good hydration; dark yellow or amber signals you need more fluid.

Tip: Coffee and tea count toward your fluid intake. Despite mild diuretic effects, research confirms they do not cause net dehydration at typical consumption levels.

Dehydration begins when you lose about 1–2% of body water. At this level, performance and cognitive function already decline — often before thirst registers strongly.

Severity	Body water loss	Symptoms
Mild	1–2%	Thirst, reduced concentration, slight headache, darker urine
Moderate	3–5%	Headache, fatigue, dry mouth, dizziness, muscle cramps
Severe	6–10%	Rapid heartbeat, sunken eyes, poor skin turgor, confusion
Life-threatening	>10%	Organ failure — requires immediate medical attention

Specific populations are at elevated risk: infants (high surface area-to-volume ratio), elderly adults (reduced thirst and kidney function), and outdoor workers or athletes in hot conditions. A 70 kg person loses roughly 1–1.5 L/hour of sweat during vigorous exercise in warm weather.

Warning: By the time you feel thirsty during exercise, you may already be mildly dehydrated. Sip 150–250 mL every 15–20 minutes during sustained activity rather than waiting for thirst.

Electrolytes are minerals that carry an electrical charge when dissolved in water. They regulate fluid distribution across cell membranes, support nerve impulses, enable muscle contractions, and maintain blood pH between 7.35 and 7.45.

Electrolyte	Daily adequate intake	Primary role	Deficiency symptom
Sodium (Na⁺)	1,500 mg (AI); <2,300 mg limit	Fluid volume, nerve signals	Muscle cramps, confusion
Potassium (K⁺)	2,600–3,400 mg	Heart rhythm, blood pressure	Weakness, arrhythmia
Chloride (Cl⁻)	2,300 mg	Fluid balance (pairs with sodium)	Rare in healthy adults
Magnesium (Mg²⁺)	310–420 mg	Muscle relaxation, enzyme function	Cramps, insomnia, fatigue
Calcium (Ca²⁺)	1,000–1,200 mg	Muscle contraction, bone	Tetany, osteoporosis
Phosphate (PO₄³⁻)	700 mg	ATP energy production, bone	Rare; seen in malnutrition

Sweat is hypotonic (less concentrated than blood), so you lose proportionally more water than electrolytes during moderate exercise. Electrolyte replacement becomes critical mainly during prolonged or intense activity.

For most people, a balanced diet provides adequate electrolytes without supplements or sports drinks. The exceptions are endurance athletes, people in very hot environments, and those on diuretic medications.

Electrolyte	Best food sources	Amount
Sodium	Table salt, bread, soup, cheese	1 tsp salt = 2,300 mg sodium
Potassium	Banana, sweet potato, avocado, spinach	1 medium banana = 422 mg
Magnesium	Pumpkin seeds, dark chocolate, leafy greens	28 g pumpkin seeds = 168 mg
Calcium	Dairy, fortified plant milks, sardines, kale	1 cup milk = ~300 mg

Sports drinks (e.g., Gatorade) typically contain 110–200 mg sodium and 30–75 mg potassium per 240 mL, plus 14–18 g carbohydrates. They are useful for exercise lasting >60–90 minutes but add unnecessary calories and sugar for shorter sessions or rest days.

Electrolyte tablets or powders (e.g., LMNT, Nuun) offer electrolytes without significant calories — a good option for people who sweat heavily but don't need extra carbohydrates.

Tip: Coconut water contains roughly 600 mg potassium and 40 mg sodium per 240 mL — good for mild post-exercise rehydration but lower in sodium than commercial sports drinks for heavy sweaters.

Hyponatremia — dangerously low blood sodium (<135 mmol/L) — is caused by drinking excessive plain water that dilutes sodium faster than the kidneys can excrete it. It is rare in everyday life but a documented risk for endurance athletes, particularly slow-pace marathon runners who drink large volumes of plain water.

• Early symptoms: nausea, headache, confusion, bloating.
• Severe symptoms: seizures, coma — requires emergency treatment.
• Risk rises when drinking >750–1,000 mL of plain water per hour for several consecutive hours.

Scenario	Hydration recommendation
Casual exercise (<60 min)	Water only; 400–600 mL before, sip during
Endurance event (1–3 hours)	Sports drink or water + salty snack; 500–750 mL/hour
Ultra-endurance (>3 hours)	Individualised plan; sodium replacement is essential
Hot weather, heavy sweating	Add sodium to fluids; monitor urine colour

Warning: Drinking to thirst — rather than to a schedule — is the safest strategy for most endurance athletes. Over-drinking plain water is more dangerous than mild dehydration during most recreational exercise.