Drinking guidelines: can too much fluid HARM performance?

When does fluid consumption on the move definitely benefit performance, and when might it be counterproductive? Andrew Hamilton looks at some recent evidence

As we approach the winter equinox here in the northern hemisphere, it’s worth remembering that southern hemisphere athletes are about to enter the hottest phase of the year, when staying properly hydrated during training and racing can be a real challenge. And even when conditions are cooler, maintaining adequate levels of hydration is vital, especially as endurance athletes can lose significant amounts of fluid via exhalation, leading to sub-optimum hydration(1,2).

Back in the early 2000s, the conventional advice for endurance athletes was to drink ample fluid during exercise – enough to ensure that any losses in body mass incurred through sweating and respiration were minimal, and certainly less than 2% of body mass (eg 1.5 litres in a 75kg athlete). This basically meant following a pre-prescribed drinking programme. However, this advice has come under increasing attack from a number of sports nutritionists and exercise physiologists who believe that the ‘hydrate at all costs’ approach is at best misguided and at worst, downright dangerous. That’s because the most recent evidence suggests that in real-world conditions (ie outdoors with a degree of wind cooling – not in artificial laboratory conditions), more than 2% dehydration is NOT a problem for endurance athletes. This is contrary to a number of existing guidelines. Likewise, new research throws into doubt the recommendation that you ‘should not rely on your thirst and instead to follow a regular drinking plan’.

The research

Much of the recent debate on hydration has focused on whether to drink freely or to follow a drinking plan in order to prevent a decline in performance. What there’s been very little of however is research into the ideal rates of fluid consumption over different length events to boost performance. But a recent and very comprehensive study by Aussie scientists has attempted to put together some actual drinking volume guidelines for cyclists performing events of different distances(3). In the study, the researchers set themselves three goals:

  • *To determine the magnitude of the effect of drinking fluid on performance during cycling exercise tasks of various durations (compared with no drinking).
  • *To examine the relationship between rates of fluid intake and endurance cycling performance.
  • *To establish fluid intake recommendations based on the above.

To do this, they trawled through previous scientific studies looking at fluid consumption and performance; nine studies that were considered sufficiently rigorous were included. The researchers then used sophisticated statistical analysis to identify the actual performance benefits or otherwise of drinking various fluid amounts during events of different durations.

What they found

There were a number of key findings from the pooled data. These were as follows:

  • *The impact of drinking on performance when cycling in ambient temperatures of 20-33°C was duration dependent – ie the longer the event, the more benefit there was.
  • *During higher intensity events of an hour (or less) drinking at a rate of 0.3mls per kilo of bodyweight per minute (that’s around 1.3 litres over an hour’s duration for a 75kg rider) actually IMPAIRED performance by around 2.5% compared to drinking nothing at all!
  • *For events of 1-2 hour’s duration at a lower intensity, drinking fluid at a rate of 0.15-0.20mls per kilo per minute (around 650-900mls per hour for a 75kg rider) boosted performance by around 2.1% compared with no drinking.
  • *For durations of two hours or more, drinking around 0.14-0.27mls per hour (around 600-1,200mls per hour for a 75kg rider) boosted performance by around 3.2% compared with no drinking. Sports Med. 2017 May 12. doi: 10.1007/s40279-0170739-6. [Epub ahead of print]

Implications for endurance athletes

What’s interesting about this study is a) it suggests that the rate at which cyclists (and presumably other endurance athletes) should consume fluid is dependent on the length of event and b) for harder efforts of an hour or less, consuming fluid will actually harm your performance rather than improve it! Why should this the case? Well taking point b first, when the duration is relatively short, there’s little opportunity to become dehydrated to the point where performance suffers. And if you’re cycling hard, gulping down fluid could produce gastric discomfort, which is known to harm performance. As for the rate of fluid ingestion, consuming higher rates during much longer events (over 2 hours) makes sense because that’s when the effects of progressive dehydration are likely to kick in. Consuming smaller amounts of fluid during events of 1-2 hours’ duration represents the halfway-house approach – enough to take care of possible dehydration during the second hour of cycling while not overdrinking.


  • Unless you’re already dehydrated, there’s no need to consume fluid during a cycling/running event lasting 60 minutes or less.
  • For events of 60-120 minutes, consuming around 650-900mls per hour is a good starting recommendation for a average weight athlete.
  • For events of over two hours, this upper limit should be increased to around 1,200mls per hour.
  • For both these recommendations, use your thirst to guide you as to whether you consume towards the lower or upper limit.
  • Regardless of your drinking strategy, always start your training/competition well hydrated.


  1. Appl Physiol Nutr Metab. 2012 Oct;37(5):931-7.
  2. Arctic Med Res. 1996 Jan;55(1):20-6.
  3. Sports Med. 2017 Nov;47(11):2269-2284.

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