Sunday, July 12, 2015

How Much Water To Drink After Exercise?

Picture by Daniel Orth from Flickr
I was reading a paper on the 24-hour hydration status in runners after a dehydrating run and testing what measurements are reliable to detect this when I thought of a very funny thing that happened at the Olympics.

As Team Singapore's Physiotherapist at the Beijing 2008 Olympics, other than treating the athletes, I was sometimes roped in to help test our Singaporean athletes' hydration status (since we only had one nutritionist and no exercise physiologist at the games).

Director of High Performance and other senior management at Singapore Sports Council (now SSI) decided that in order to perform well and/ or win medals, our athletes needed to be amply hydrated at all times.

Some of our athletes complained about this rather strongly as they were asked to drink more water when their hydration status showed that they needed more fluids. These athletes complained that after consuming more water they had to get up in the middle of the night to pee and afterwards could not fall asleep again for a long time.

You know what they did? They simply added water to their urine samples to pass the hydration test. Our staff got their testing done, the athletes got their much deserved sleep. Team Singapore won the medals they came to win. Win win situation for all parties. No names will be mentioned to protect the athletes involved of course.

Anyway, back to the study.

The group of runners in the study ran in hot conditions until they lost about three percent of their starting weight on three occasions. The three rehydration protocol was either 3.2, 4.2 or 5.2 litres of fluid over the next 24 hours.

Rehydration in the first 24 hours were assessed by measuring body weight of the runners, urine colour, urine specific gravity (a measure of urine concentration) and reported thirst sensation.

The most sensitive marker for fluid consumption during the first 12 hours was urine specific gravity. (Atago - which measures devices to measure specific urine specificity was used and they also sponsored this study).

Thirst emerged as a very sensitive and reliable marker by the end of the 24 hour period. The problem is that differences in thirst did not appear after the run but only emerged the next morning so you may go to bed not knowing you've not drunk water.

When the runners drank a lot, they urinated more, when the drank less, they urinated less. You may be surprised with this but when food and fluid intake were tightly controlled, the runners' body weight was unable to detect low fluid intake. The researchers concluded that body weight wasn't a particularly helpful way of tracking how much you're drinking.

There were only subtle changes in urine colour when measured on the researchers 8-point scale. So you have to pay close attention to the shade of your urine colour should you choose to to use this cue.

4.2 litres corresponded to replacing about 100 percent of the sweat losses within the first 12 hours of their run and 200 percent within 24 hours. This was sufficient to return the runners to normal by the time they were ready to run again the next day.

Turns out it corresponded most closely to how much the runners would have normally drunk too.


O'Neal EK, Stevenson MC et al (2015). Hydration Assessment Technique Over 24-h Post-run With Low, Moderate, And High Fluid Replacement. ACSM Annual Meeting. May 28, 2015. Abstract here.

With the then Singapore President at the 2008 Olympics

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