Sunday, 27 May 2012

Are you drinking enough?

Hello friends, welcome back!  I apologize for the long layoff since a unique article has been posted on the blog.  Today's hits close to home as I have spoiled many-a-races via thinking I am too tough to drink sufficient fluids.  

Believe it or not...that millisecond you lose when you slow down to drink is actually a millisecond well spent.

This is especially true when your legs completely fall off due to profound dehydration rendering you unable to do anything but crawl to the finish line (and by you I mean me..and by crawl I mean drop out).

How much to drink? 

The only issue is that the amount you need to drink is completely variable depending on factors such as how hot it is outside and even your genetics.

Humans typically sweat at a rate of 1-2 litres per hour during sustained exercise.  But even that range doesn't apply to everybody-  Alberto Salazar (an ok runner, completing a bunch of sub- 2:10 marathons among other things!) has been quoted to sweat up to 4L/hour.

The American College of Sports Medicine recommends, in general, 5-8 ounces of fluids, every 10-15 mins (depending on the ambient temp and humidity), for events lasting more than 40-60 mins.

Those numbers are great, but there are two issues; (1) they include a large range of fluid recommendations and (2) how do you know if you fall in the upper end, lower end, or totally off the entire spectrum of normal (like our friend, Alberto)?


A 2010 study published in the British Journal of Sports Medicine tries to answer this question of how to know when you are dehydrated, and understanding when you need to drink.

The BEST way to see if somebody is dehydrated is to (a) take a blood sample and measure the plasma osmolality (is it too concentrated?), or (b) take a urine sample and find the specific gravity (is it too concentrated?).

Unfortunately, most of us don't have the fancy equipment handy required to make these measures.  The second best method is to just go by body weight!  The more water you lose, the less you weigh.  In this study, researchers stated that if you lose MORE than 3% of your body weight, then you are clinically dehydrated.

However, during a race situation (or even our day to day lives), we don't always have the time to weigh ourselves before, during and after a bout of physical activity.  So how reliable are the other signs of dehydration?  This is what they looked at:
  • reduced skin turgur
  • sunken eyes
  • mucous membrane dryness
  • dryness of the axillia (armpit dryness)
  • inability to spit
  • heart rate
  • blood pressure
  • respiratory rate
  • thirst
So lots of things can change as you become dehydrated.  But which of these are reliable measures of how dehydrated you really are??


In the study, 606 runners were evaluated during a marathon taking place in New Zealand.  This is what they saw:
  • reduced skin turgor had the strongest link to percent of weight loss
  • sunken eyes and thirst were also linked to percent weight loss
  • inability to spit and dry mucous membranes were not linked to weight loss
  • combining the individual signs of dehydration did not improve dehydration evaluation
But, MOST importantly:
  • the above clinical signs, in general, were not able to reliably predict the 3% weight loss cutoff point
What does this mean?

In essence, this study shows that these objective measures of dehydration simply do not tell you if you are dehydrated.  Looking to see if your eyes are sunken, if your face is less puffy, or if you are thirsty is undoubtedly linked to your body requiring more fluids.  However, the best way to know for sure (and if you don't want to test your blood and urine) is to weigh yourself pre and post activity.

Practical Application

Fluid is important to make you perform at your best!  The hotter it is outside, the more you sweat, and the more you need to drink.  So how should you approach your fluid status?  Experiment.  Start with that 5-8 ounces every 10-15 minutes on a training day that mimics your race climate.

Weigh yourself before and after; if you are losing over 3% of your body weight, you need to drink more...and if you don't, you will end up looking like AND performing like this guy:

Me not following my own advice

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