Rehydration and Heat Loss During a Hot, Humid Football Match
A 17-year-old footballer, mass 60.0 kg, plays a 90-minute match in Malaysia at 32 °C and 80% relative humidity. His heart rate rises from 70 beats per minute before kick-off to 168 beats per minute at the final whistle; his skin is flushed and dripping with sweat. He is weighed immediately before and after the match: 60.0 kg before, 58.6 kg after. He drank nothing during the match. Afterwards the coach offers three drinks:
- Drink A: 6% carbohydrate, 40 mmol/L sodium, in 250 mL bottles
- Drink B: plain water, 0 mmol/L sodium
- Drink C: 15% carbohydrate, 5 mmol/L sodium A teammate argues that plain water is best because 'sweat is only water'. The coach wants the player rehydrated quickly and ready for another match the next morning.
1. Explain how the player's body tried to stop his core temperature rising during the match, and why his heart rate increased so much.
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Heat loss (the hypothalamus detects blood above the set point of about 37 °C and triggers two responses):
- Sweating increases. Water on the skin absorbs latent heat as it evaporates, cooling the blood in the skin capillaries. At 80% humidity evaporation is slow, so much of the sweat simply drips off - which is why he is soaked but still overheating.
- Vasodilation of skin arterioles. More warm blood flows near the surface, so more heat is lost by radiation and convection; the skin looks flushed for this reason.
Why heart rate rises: working muscles need extra oxygen and glucose, and the skin needs extra blood flow for cooling at the same time. Cardiac output (heart rate × stroke volume) must rise to serve both demands, so 70 bpm becomes 168 bpm. The two demands compete, which is why a hot, humid day feels harder than a cool one at the same running speed.
Not just recall: the answer must link the 80% humidity figure to reduced evaporation efficiency; a candidate who simply lists 'sweating and vasodilation' without explaining why sweating was ineffective here has not used the data.
2. Calculate the percentage loss of body mass. Then decide which drink the coach should choose and justify the choice with reference to the figures given.
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Calculation: loss = 60.0 − 58.6 = 1.4 kg. Percentage = (1.4 ÷ 60.0) × 100 = 2.3% of body mass. A deficit of about 2% is where endurance and concentration begin to fall, so replacement is needed, not optional.
Choice: Drink A.
- 40 mmol/L sodium replaces the salt lost in sweat; sodium also helps the gut absorb water and reduces urine loss, so more of the drink stays in the body.
- 6% carbohydrate is dilute enough to leave the stomach quickly and supplies glucose for the next match and for glycogen refill.
- Its concentration is close to that of body fluids, so it is not hypertonic.
Why not the others: Drink C at 15% carbohydrate is hypertonic - it draws water into the gut, slows emptying and commonly causes bloating and cramps during or after exercise. Drink B contains no sodium and no carbohydrate: much of it is excreted as dilute urine within an hour, so the 1.4 L deficit is only partly corrected, and none of the lost salt is replaced.
Practical point: the deficit is 1.4 L, but sweat continues after the whistle, so roughly 1.5–2 L should be taken in small amounts over the next few hours rather than in one go.
3. Predict and explain what would happen if the player drank 1.5 L of Drink B very quickly in one sitting instead of Drink A.
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Prediction: he would feel bloated and nauseous, would pass a large volume of pale urine within an hour, and would still start the next match partly dehydrated and with low muscle glycogen.
Reasoning chain:
- 1.5 L is drunk faster than the gut can absorb it, so fluid sits in the stomach - distension causes discomfort and the player stops drinking.
- The fluid contains no sodium, and he has already lost sodium in sweat, so plasma sodium is diluted. The kidneys respond by producing dilute urine to restore the concentration, so much of the 1.5 L is lost rather than retained.
- There is no carbohydrate, so nothing is available to rebuild muscle glycogen overnight, and blood glucose is not supported.
- In extreme cases, replacing heavy sweat losses with large volumes of plain water can cause dangerously low blood sodium.
Better plan: 200–250 mL of Drink A every 15–20 minutes after the match, plus a carbohydrate-containing meal, gives fluid, electrolyte and energy without flooding the stomach.