Impact of Mild Dehydration

The negative impact of dehydration on sports performance can be significant even at low levels of dehydration (1.5% bodyweight loss – Moquin et al 2000) and affects a wide range of key performance factors from power output to skill execution. While most people are broadly aware of the importance of maintaining good hydration, the far-reaching impact of dehydration on sports performance is often underestimated.

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During moderate to intense exertion, typical athlete sweat rates range from 1-3 litres per hour. This means that for most people, when exerting for an hour or more, fluid loss is likely to be near or over 2% of body mass loss (BML) leading to physiological responses from the body to combat dehydration.

As the body sweats to cool down during sport, the resulting fluid loss decreases blood plasma (the fluid part of the blood), causing the blood to thicken. As the blood thickens, its ability to carry oxygen decreases, putting extra pressure on the heart to work harder to pump oxygen and glucose (fuel) to the muscles. This in turn has a flow-on effect of increasing the body’s core temperature which further impairs the body’s ability to maintain water balance. Performance decline is inevitable as the body needs to work harder to achieve the same output and uses fuel less efficiently, which increases the rate of fatigue (Armstrong et al. 1985; Craig and Cummings 1966; Maughan 1991; Sawka and Pandolf 1990).


Dehydration of over 2% BML has been shown to reduce lactate threshold, lower absolute power output and significantly decrease time to exhaustion (Moquin et al 2000). Additional evidence includes the following studies.

  • Cycling time trial performance was 13% slower at 2-3% BML (Sprenger et al 2015)
  • Cycling sprint to exhaustion time was 29% lower at 2.5% BML (Ebert et al 2007) and has even been reported to reduce by 30% at only 1.8% BML (Walsh et al 1994).
  • Maximal aerobic power (VO2max) was 5% lower at 3% BML (Pinchan et all 1988).
  • BML as low as 1.6% can slow running performance by 7% (Armstrong et al 1985)
  • Even when drinking to thirst, athletes can experience over 2% BML resulting in a 7% decrease in cycling power output (Adams et al 2018)

What is the potential impact of dehydration and BML in a marathon at your race pace?




“Calculations are based on being fully hydrated for the first half of a marathon and incurring the impairment during the second half.
If you were 2% dehydrated for the the second half of a marathon, you would run the equivalent of an additional 1.3 miles.
If you were 3% dehydrated for the the second half of a marathon, you would run the equivalent of an additional 3.2 miles.”