How Important is Hydration?

Thirsty Yet?

So what we set out to do was to physically isolate hydration by itself without the major confound of drinking and thirst. Conditions #2 and 4 above are unique in that they have not been studied at all in any prior experiment.

What were our results?

The blinding of hydration was successful. Interviewing the participants after all of their trials, they guessed the conditions correctly 61% of the time, but with only 54% confidence (where 50% was a blind guess).

The hydration and thirst manipulations also achieved their goals. In the hydrated trials, body weight was kept within -0.5% of baseline after 90 min and -0.4% after the TT. In the dehydrated trials, participants finished 90 min at -2.1% body weight, and finished the TT at -3.2% body weight.

The mouth rinse protocol also produced major differences in thirst throughout the experiment according to our perceptual scales.

No differences were seen across any of the 4 conditions in heart rate and core temperature during the 90 min of steady-state riding at 50% VO2max.

During the 20 km TT, both heart rate and core temperatures were higher in the dehydrated trials. This matches classical expectations like in Montain and Coyle. However, it is also important to note that none of the cyclists reached critical or dangerous levels of heart rate or core temperature.

Most important were the performance results. There were NO differences across any of the 4 conditions in average power output, pacing strategy, or 20 km TT completion time.

More: How Much Does Dehydration Affect Performance?


I did >20 media interviews the past week about this study, so I've had good practice in distilling the key messages and the important limitations and caveats of this study.

The primary message is that we showed that neither physical dehydration of 2-3% body weight, nor the associated thirst, actually impaired performance of about 2 h in the heat. So if you want to drink, do so, but it's not going to really influence your performance within this range of exercise and dehydration.

What I have not tested are the effects of such repeated dehydration on day-to-day recovery, glycogen replacement, etc.

I also did not test the effects of electrolyte or carbohydrate replacement on performance.

This is NOT a blanket statement to ignore drinking and fluid replacement during daily training and living, as water is absolutely our most important nutrient. This is a very specific application to a one-off event where you're aiming for peak performance.

I also have not tested whether being dehydrated (lower body weight) is an ergogenic aid in itself. The only study relevant to that was by Ebert et al. (2007), which found that being dehydrated after 2 h (-2.5% body weight) also cost you in lower power output during a final hill climb.

More: Can a Secret Drink Mix Solve Your Hydration Issues?

Dowsett's Hour

Is there proof in this scientific pudding? In a fortuitous bit of timing, my paper came out a week after Alex Dowsett's breaking of the Hour Record in the Manchester velodrome.

Dowsett's team deliberately set the velodrome temperature as high as it could go, to about 28-29°C (higher the temperature, the lower the air density and thus air resistance). This obviously could be negated by greater potential dehydration. The fact that this gamble worked certainly provides some indirect but real-life backing to the science in my view.

The full paper is available for free download.

Full ACSM Position Stand: Exercise and Fluid Replacement.


Cheung SS, McGarr GW, Mallette MM, Wallace PJ, Watson CL, Kim IM, Greenway MJ. Separate and combined effects of dehydration and thirst sensation on exercise performance in the heat.. Scand J Med Sci Sports 2015: 25 Suppl 1: 104-111.

Ebert TR, Martin DT, Bullock N, Mujika I, Quod MJ, Farthing LA, Burke LM, Withers RT. Influence of hydration status on thermoregulation and cycling hill climbing. Med Sci Sports Exerc 2007: 39: 323-329.

Montain SJ, Coyle EF. Influence of graded dehydration on hyperthermia and cardiovascular drift during exercise. J Appl Physiol 1992: 73: 1340-1350.

Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American college of sports medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc 2007: 39: 377-390.

More: Pre-Race Nutrition Guide for Cyclists

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