Electrolyte balance and sports drinks
Sports drinks give fluid, carbohydrate and sodium together. An isotonic 6-8% drink suits most training over an hour; water is enough for shorter sessions.
Electrolyte balance in exercise
Sodium, potassium and chloride keep water in the right body compartments and carry nerve and muscle signals. Sweat is mostly water with sodium and chloride; how much sodium is lost varies widely between people, and 'salty sweaters' show white streaks on their clothes. Replacing heavy losses with plain water alone can lower blood sodium and bring on cramps. A little sodium in a drink keeps thirst going, helps glucose and water absorption in the gut (they are absorbed together), and reduces urine loss, so more fluid is kept. Potassium losses are small and are usually covered by food such as bananas and tender coconut water.
Types of sports drinks
| Type | Concentration | Best use | Examples |
|---|---|---|---|
| Hypotonic | Weaker than body fluids; under about 4% carbohydrate | Fast fluid with little energy: short sessions, athletes watching weight | Diluted sports drink; water with a pinch of salt |
| Isotonic | About equal to body fluids; 6-8% carbohydrate | Fluid and energy together: most training and competition over an hour | Commercial sports drinks; homemade salted nimbu pani with sugar |
| Hypertonic | Stronger than body fluids; over about 8-10% carbohydrate | Energy for refuelling after exercise; empties slowly during it | Fruit juice, soft drinks |
Making a homemade isotonic drink
A coach mixes 1 litre of water, 60 g sugar, a quarter teaspoon of salt (about 1.5 g) and the juice of one lemon. Find the carbohydrate %, the sodium in mmol/L, the energy per litre, and the carbohydrate an athlete gets by drinking 750 mL an hour.
- Carbohydrate % = 60 g ÷ 1000 mL × 100 = 6 g per 100 mL = 6%.
- Salt is sodium chloride, about 58.5 g per mole. Moles of salt = 1.5 ÷ 58.5 = 0.0256 mol = 25.6 mmol, so sodium ≈ 26 mmol/L.
- Energy = 60 g × 4 kcal = 240 kcal per litre (the small sugar in lemon juice is ignored).
- Carbohydrate per hour = 0.75 L × 60 g/L = 45 g.
- Check: 25.6 × 58.5 = 1498 mg ≈ 1.5 g salt.
About 6% carbohydrate and 26 mmol/L sodium (rounded to the nearest whole number): isotonic and within the ACSM range. It gives 240 kcal per litre and 45 g carbohydrate per hour at 750 mL/h, inside the 30-60 g target.
Sports drinks and other drinks compared
| Drink | Carbohydrate | Sodium | Verdict |
|---|---|---|---|
| Plain water | None | None | Enough for under an hour in cool conditions |
| Sports drink | 6-8% | Added | Good for sessions over an hour or in heat |
| Tender coconut water | A little natural sugar | Low; rich in potassium | Good for recovery with a salty snack; not enough sodium alone for heavy sweaters |
| WHO ORS | Low: glucose 75 mmol/L (about 1.35%) | High: 75 mmol/L | Made for diarrhoea; useful for heavy salt loss, but too little carbohydrate to fuel exercise |
| Fruit juice and soft drinks | About 10% or more | Very low | Hypertonic; dilute juice for use during exercise; fizzy drinks cause bloating |
| Energy drinks | High sugar | Low | Not sports drinks: often high in caffeine; not for rehydration |
| Salted buttermilk (neer mor) | Low (milk sugar) | Moderate | Good recovery fluid |
A school athletics coach can make an isotonic drink cheaply: 1 litre of water, 60 g sugar, a quarter teaspoon of salt and a lemon gives about 6% carbohydrate and 26 mmol/L sodium, close to commercial sports drinks. Energy drinks are not a substitute and should not be given to young athletes for hydration.
2 marks: “What is an isotonic sports drink?” A drink with about the same concentration as body fluids, usually 6-8% carbohydrate with sodium, giving fluid and energy quickly.
5 marks: “Explain the role of sports drinks in maintaining electrolyte balance.” Electrolytes lost in sweat, why sodium matters, ACSM composition, hypo-, iso- and hypertonic drinks, when to use them, and a homemade recipe.
Hypo = hydrate fast; Iso = fuel + fluid; Hyper = refuel later.