Q5 Dehydration

Question 1

Mechanisms

Answer 1

Dehydration effects performance through the following mechanisms:

Increased blood viscosity and decreased blood volume increases strain on the heart, so increases heart rate and perceived exhaustion

Dehydration effects aerobic output by decreasing cardiac output. This is due to the decreased blood volume, less blood enters the heart during diastole and so less is available to be pumped during systole so overall there is reduced cardiac output

Dehydration also reduces rate of fluid absorption from the stomach so once dehydration begins to occur it becomes harder to reverse

Dehydration causes increases in core temperature, which has also shown to increases rates of glycogen breakdown

Question 2

Dose/ Timing

Answer 2

Current debate in the literature about the drinking strategies of athletes during exercise: planned drinking or drinking to thirst

In a study of cyclists performance over 1 hour decreases by 2.5% when drinking at a high rate compared to no drinking at all

(High rates: 0.15-0.34ml/kg body mass/min)

However performance was increased by up to 3.2% in cycling events of more than 2 hours

Drinking during cycling also improved perceptions of fatigue

Stimulating pharyngeal receptors with 100mL fluid improved endurance performance vs mouth rinse or no ingestion

No evidence to support the inclusion of calcium or sodium chloride in drinks consumed hours before exercise

Drinking plain water after exercise reduces the drive to drink before the athletes is fully rehydrated
- But in events of less than 60 minutes water sufficient as a rehydration tool, although in extreme heat/sweating a sports drink may be required

The amount of fluid ingested should be roughly 1-1.5L per kg of body weight lost (150%)

The inclusion of 20-50mmol/L of salt in the drink increases palatability and stimulates thirst so the athlete will naturally drink more
- This also stimulates renal water reabsorption and the restoration of plasma volume

Including a carbohydrate content of 6-12% can improve the rehydration status of the athletes as it promotes water retention

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