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Runners may underestimate their fluid needs on longer summer runs: Optimum Performance

John Muritu (8) holds off Dominic Ondaro (6) to win the 37th running of the Allstate Sugar Bowl Crescent City Classic 10K road race Saturday, April 4, 2015. The race started at the Superdome and finished in City Park with over 20,000 runners and walkers, may of whom where dressed in costumes. (Photo by David Grunfeld, | The Times-Picayune)

It goes without saying that during the summer time in Louisiana our high temperatures and humidity can make extended exercise time outside quite challenging, especially as it pertains to consuming the proper amounts of water and /or sports drinks.

The American College of Sports Medicine’s (ACSM) position stand: Exercise and Fluid Replacement says, “During exercise athletes should start drinking early and at regular intervals in an attempt to consume fluids at a rate sufficient to replace all the water lost through sweating (i.e., body weight loss), or consume the maximal amount that can be tolerated.”

The ACSM also notes that, “It is recommended that individuals drink about 500 ml (about 17 ounces) of fluid about 2 hours before exercise to promote adequate hydration and allow time for excretion of excess ingested water.”

In a 2012 edition of the International Journal of Sports Nutrition and Exercise Metabolism, a research paper – Runners Greatly Underestimate Sweat Losses Before and After a 1-Hr Summer Run – “suggest that inadequate fluid intake during runs or between runs may stem from underestimations of sweat losses and that runners who do assess sweat-loss changes may be making sweat-loss calculation errors or do not accurately translate changes in body weight to physical volumes of water.”

Story by

Mackie Shilstone,

Contributing writer

Researchers from the Department of Health, Physical Education, and Recreation at the University of Northern Alabama recruited 39 experienced runners (19 men and 20 women) 19-60 years of age at a meeting of a regional running club who had, “reported regularly training in the heat and that they could complete a continuous 1-hour outdoor run without difficulty. In the last 24 months, 21 runners had completed an organized race of marathon distance or longer, 10 had completed a half-marathon, and 7 had completed a 10-km race.”

The runners had, among many detailed pre / post run measurements, their urine specific gravity tested; body weight assessed in minimal clothing with the same clothing used pre and post measurements; clothing, shoes, etc. provided for use during the run, along with a post run towel to dry off were weighed pre and post run; and, all “voids” (urine) after the initial weigh-in were collected, measured, and incorporated into sweat-loss measurements.

Then, “in a private room, participants were individually presented with a stack of 237-ml (8-fluid-oz) paper cups and a pitcher containing ~3 L of water. They were informed that each cup would hold 237 ml of water. The runners were then asked to predict the volume of sweat loss they believed they would experience during their 1-hour run by filling the cups with water.”

The running event took place between July and late August over a 5 km (one kilometer equals 0.621371 miles) course, which could be repeated to complete the prescribed time. “Runners were allowed to self-select their running pace and asked to complete a number of laps that would result in a finishing time of approximately 1 hour.”

After the run, “participants returned to a private room …, dried off thoroughly, and changed into their pre-run clothing for a post-run weight measurement. They completed a post-run sweat-loss estimation and were then given free access to consume chilled (~5–6 °C) water in the air-conditioned laboratory for 1 hour. Ad libitum water consumption was measured in the 1-hr recovery period. A questionnaire concerning hydration-monitoring strategies and confidence in estimations was administered during the recovery period.”

It was determined that, “more than 75% of our runners did not consider or were unaware that changes in body mass could be used to determine sweat loss.” Also, “runners who reported frequently monitoring their sweat losses using this technique failed to more accurately estimate their sweat losses than those who did not.”

It’s time for us to play it safe in the heat!

Mackie Shilstone, a regular contributor to | The Times-Picayune, has been involved in the wellness sports performance industry for nearly 40 years. He is currently a fitness consultant to Serena Williams and has trained numerous other professional athletes and consulted a litany of professional sports franchises. He is St. Charles Parish Hospital’s fitness and wellness expert. Contact him at