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Exercise Fluid Balance – Pints Vs. Pounds

“A pint is a pound the world around.” This common expression is true when talking about a pint of water, which weighs one pound.  It is true also for many other liquids that are mostly water, including sweat.  For this reason, runners and other athletes are given the simple advice to drink one pint of water for every pound of body weight lost during exercise, thus replacing sweat losses in 1:1 fashion and sustaining hydration.  But water loss is not the only loss reflected by the scale.

Exercise requires that stored chemical energy be converted into energy of motion, but in the process, a tremendous amount of heat is generated.  Sweat secreted onto the skin removes body heat upon evaporation and creates a measurable loss of body mass which can be equated with water loss where mass = volume (i.e., 1 g = 1 mL).

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However, oxidative byproducts of energy metabolism also include carbon dioxide and water, which are lost through respiration.  It is important to understand that the de novo formation of metabolic water represents a transformation of molecular mass, the production of which is closely coupled to the loss of respiratory water.  This results in a loss of mass with no net change in body water.  As a consequence, the sum of oxidative byproducts of metabolism can be used to correct whole body mass losses and provide a more accurate estimate of body water losses.

In a recent paper published in Experimental Physiology, researchers from the U.S. Army explain how the combination of respiratory water and carbon dioxide losses can be estimated neatly as 0.20 g per kcal of energy expenditure (i.e., 0.2 kg per 1000 kcal).  This simple relationship affords the necessary correction to any measured change in body mass so that mass and water volume can be equated.

An average sized runner may expend 3000 kcal during a full length 42 km marathon and would stand to lose 0.60 kg (1.3 lbs) of metabolic mass.  Sweat losses of 2.0 to 4.0 kg (4.4 to 8.8 lbs) could also accumulate during the event.  Failure to account for metabolism would over-estimate water losses and drinking needs by 15 to 30%.  In this example, a pound is not a pint (16 oz.) – it is between 11.2 and 13.6 oz.  The effect is substantially smaller for exercise lasting less than an hour (e.g., ≤ 10 km race) but magnified greatly during ultra-endurance exercise (e.g., ≥ 50 km race).

Knowledge of this correction is fundamental for sports scientists and sports nutritionists.  Since energy expenditure can be non-invasively estimated with reasonable accuracy, the formula: (Δ mass – 0.20g*kcal)/mass*100, can be used to assess the percentage of body mass loss from water and by extension, the adequacy of exercise fluid intake practices.

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These findings are described in the article entitled Myths and methodologies: Making sense of exercise mass and water balance, published in the journal Experimental Physiology. This work was led by Samuel N. Cheuvront, Ph.D., RD and Scott J. Montain, Ph.D., at the U.S. Army Research Institute of Environmental Medicine in Natick, MA. The author’s views are not official U.S. Army or DoD policy.

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