hydration and dehydration

Answer: The body loses water during exercise through three main mechanisms: (1) Sweating - the most significant source of water loss, as sweat is produced to cool the body through evaporation. During intense exercise in heat, sweat loss can reach 2-3 litres per hour. (2) Breathing - increased breathing rate means more water vapour is lost through exhaled air. (3) Urine production - though the kidneys reduce urine output during exercise to conserve water, some water is still lost. Sweating is the most significant because it accounts for the vast majority of fluid loss during exercise, especially in warm conditions.

Answer: The body loses water during exercise through three main mechanisms: (1) Sweating - the most significant source of water loss, as sweat is produced to cool the body through evaporation. During intense exercise in heat, sweat loss can reach 2-3 litres per hour. (2) Breathing - increased breathing rate means more water vapour is lost through exhaled air. (3) Urine production - though the kidneys reduce urine output during exercise to conserve water, some water is still lost. Sweating is the most significant because it accounts for the vast majority of fluid loss during exercise, especially in warm conditions.

Answer: Four consequences of dehydration on sporting performance: (1) Blood thickens (increased viscosity) - reduced blood volume means the heart must work harder to deliver oxygen to muscles, reducing aerobic performance. (2) Increased heart rate - with less blood returning per beat, the heart must beat faster to maintain cardiac output, meaning the performer reaches maximum heart rate sooner. (3) Overheating - reduced sweat rate impairs the body's cooling mechanism, causing dangerous rises in core temperature. (4) Slower reaction time - dehydration affects brain function and nerve conduction, impairing decision-making and coordination.

Answer: Dehydration causes an increase in heart rate because when body water is lost through sweat, blood volume decreases. With less blood in the circulatory system, less blood returns to the heart with each beat (reduced venous return and stroke volume). To maintain the same cardiac output (the amount of blood pumped per minute), the heart must beat faster to compensate for the lower volume per beat. This is known as cardiovascular drift - heart rate progressively increases during prolonged exercise even if the exercise intensity stays the same. The dehydrated performer reaches their maximum heart rate sooner, reducing performance capacity.

Answer: The athlete should drink 1.5kg × 1.5 litres = 2.25 litres of fluid. The 150% rule is used because not all fluid consumed is retained - the body continues to produce urine after exercise, so some of the rehydration fluid is lost again. By drinking 150% of the weight lost, the athlete accounts for this ongoing loss and ensures full rehydration. The 1.5kg weight loss represents 1.5 litres of fluid lost (1kg ≈ 1 litre of water), so 1.5 × 1.5 = 2.25 litres needed for complete rehydration.

Answer: A marathon runner has much greater hydration needs because the event lasts several hours, during which sweat loss can reach 2-3 litres per hour. They need a comprehensive strategy: pre-hydration in the days before, regular fluid intake during the race (150-250ml every 15-20 minutes), and thorough rehydration after. Sports drinks are essential during the event to replace both fluid and electrolytes. A 100m sprinter's event lasts under 10 seconds, so sweat loss during the race is negligible. Their hydration strategy focuses on pre-event hydration to ensure they start well-hydrated, with no need for during-event drinking. Water is sufficient for their needs. The difference is driven by duration and total sweat loss.

Answer: Sports drinks are recommended for events over 60 minutes because they provide three things water alone cannot: (1) Carbohydrates - to maintain blood glucose levels and provide energy when glycogen stores are being depleted during prolonged exercise. (2) Electrolytes (especially sodium) - to replace those lost in sweat, maintain fluid balance, prevent muscle cramps and improve fluid retention. (3) Better fluid absorption - the sodium and carbohydrate in isotonic drinks enhance water absorption from the gut. For events under 60 minutes, glycogen stores are not significantly depleted and electrolyte loss is minimal, so water is sufficient for maintaining hydration without the need for additional carbohydrates or electrolytes.