The impact of dehydration on run performance

Alan Couzens, M.S. (Sports Science)

Oct 16th, 2015

Can anyone guess what might have inspired this post? :-) Yes, it was a hot year in Kona this year. It’s always hot but the big island took it up a notch this time around.

From my perspective, Kona was a great year for my team, with a number of performances towards the very front of their respective age groups & a couple even nudging the podium, but in absolute terms, even very good performances were on the ‘slow’ side, especially on the run. Triathlete magazine did a brief analysis of the impact of the heat on the top 10 males and females here & concluded that times for the top guys and gals were ~3% slower than last year. Considering this sample represented the folks who dealt with the heat the best, it probably understates the real impact on the field.

My athletes who executed well were, on the whole ~10% slower for the marathon than the paces I was projecting for them based on key workouts leading in. While this may have carried a little disappointment for the athlete, it is an unavoidable reality of racing in an environment as harsh and unpredictable as Kona. When it comes to that race, the only way to execute a consistently good result is to race with the conditions that the island gives you on that day.

In the case of October 10, 2015, the island chose to throw out conditions that would almost certainly lead to dehydration on the run. At 1pm, when the speedy age groupers were heading out on the run, the temperature was 89 degrees and humidity was ~80%!

In a previous post, I provided a ‘ready reckoner’ to give a breakdown of some of the heat transfer mechanisms at play to maintain a safe body temperature under extreme climatic and racing conditions. If we plug in a competitive age-group athlete of 75kg running 7min miles (3:03 marathon pace), we see that typical sweat rates would need to be ~2L/hr to get rid of the heat created at such an output in last Saturday's conditions. Considering max gastric emptying rates are closer to 1.5L/hr (Coyle et al., 1978) & even less when the gut is being jostled around by a competitive run pace, we’re left with one inevitable conclusion – progressive dehydration!

A number of studies have investigated the impact of dehydration on endurance performance. The first significant one of these being Craig and Cummings in 1966. Here is the relationship they observed between dehydration and performance (in this case, VO2max)…

They found that even relatively modest reductions in bodyweight of 3%, i.e. ~2kg resulted in an impairment in VO2max of ~20%. This linear relationship between dehydration and a drop in performance has been shown time and again since this study.

Of particularly practical note for those tracking HR, a study by Montain and Coyle (1992) found that HR increased a linear 8bpm for every liter of unreplaced fluid.

If we compare this back to average sweat vs ingestion rates in a race like the one we had a week ago, even at ingestion of 1.3-1.5L/hr – a very high rate for the run, we would expect a 2kg/2L (3%BW) deficit over the course of the run resulting in heart rates approx. 16bpm (~10-15%) higher for a given pace or, more realistically, paces 10-15% lower for the athlete’s ‘normal’ race heart rate. In athletes who executed well, this is precisely what we saw.

You can plug in your own numbers in the following calculator (based on the study referenced above) to get an indication of the pace reduction you can expect for a given level of dehydration.

Bodyweight Losskg
Normal Pace (min/mi)
Race Heart Rate
Heat Corrected Pace (min/mi)

So, what can we take from this?

1. In hot races, maximize your ability for gastric absorption, i.e.

2. In some races, a certain level of dehydration is unavoidable. Adjust your pace according to the conditions AND BE OK WITH THIS. You may have wanted to run a 3hr marathon today but mother nature had other plans for that day. It doesn’t change the fitness that you’re bringing into the race or your ability to do so in a cooler event.

Above all else, race smart!



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