Hydration and Heat in a Pre-Season Trial
A male 16-year-old basketball player weighs 72.0 kg before a 90-minute pre-season trial in 31 degrees Celsius. He drinks 600 mL of water during the game and does not urinate. He weighs 70.6 kg immediately afterwards. He reports a headache, dizziness on standing and dark urine. His coach offers a sports drink and asks whether he should play the second trial 60 minutes later. His baseline sweat rate from a cooler session was 0.9 L per hour.
1. Calculate his sweat rate and percentage body-mass loss for the trial. Interpret the numbers against recognised thresholds.
Read the worked answer
Sweat loss = pre-mass - post-mass + fluid consumed.
- Mass change: 72.0 - 70.6 = 1.4 kg (equivalent to 1.4 L of sweat)
- Plus fluid consumed: 0.6 L
- Total sweat loss = 1.4 + 0.6 = 2.0 L over 1.5 hours
- Sweat rate = 2.0 / 1.5 = 1.33 L per hour
Percentage body-mass loss = 1.4 / 72.0 x 100 = 1.94%, approximately 2%.
Interpretation: a 2% body-mass loss is the widely used threshold at which performance and thermoregulation begin to decline. His sweat rate of 1.33 L/h is well above his cooler-session baseline of 0.9 L/h, showing heat substantially increases his fluid needs. He replaced only 0.6 L of a 2.0 L loss, roughly 30% of losses, which is inadequate. Combined with dark urine, headache and orthostatic dizziness, the picture is consistent with dehydration and possible heat strain, not merely thirst.
2. Should he play the second trial 60 minutes later? Justify your decision using the evidence and recommend an immediate plan.
Read the worked answer
No. He should not play the second trial.
Reasoning: he is approximately 2% down in body mass with symptoms (headache, dizziness on standing, dark urine) that suggest dehydration and heat strain. The 60-minute window is insufficient to replace 1.4 L of deficit safely; gastric emptying and renal handling limit rehydration to roughly 1-1.5 L per hour at best, and rapid large-volume drinking can cause gastrointestinal distress. Playing again in 31 degrees while still dehydrated would compound cardiovascular strain, impair thermoregulation and raise the risk of heat illness.
Immediate plan:
- Move him to shade or air conditioning and remove excess clothing.
- Begin oral rehydration with a sodium-containing sports drink, roughly 1.5 L over the next 2-3 hours (about 150% of the deficit), sipped steadily rather than gulped.
- Monitor symptoms: headache, dizziness and urine colour should improve. Persistent symptoms, confusion, vomiting or absence of urine warrant medical review.
- Re-weigh before any return to play. He should be within roughly 1% of his pre-trial mass and symptom-free before resuming.
- Educate for future sessions: drink to thirst plus planned volumes, and use his measured sweat rate (1.3 L/h in heat) to set a target of roughly 500-800 mL per hour during play.
The decision rests on symptoms plus measured deficit, not on his desire to play.