Sweat Rate & Fluid Loss

"It is important to note that there is much debate about the implications of fluid loss from sweat rates. Many health care professionals and physiologist disagree on if replacing fluid loss is necessary during a workout or what is required for a race even in events up to a marathon."

Since we are discussing hydration and fluid loss in the realm of distance runners its likely that the five percent sweat loss statistic is higher than the general population. This is especially true during warmer months on the calendar or during longer or more challenging workout sessions. This means we should work to understand even more closely how to recognize fluid loss from sweat and know the related implications.

Monitoring sweat loss

Athletes should recognize days where there is a risk for greater than normal sweat rates and attempt to track fluid loss. Knowledge of how an individual responds to heat, humidity, indoor workouts or other factors should help athletes and coaches understand when potential for risk-factor level sweat loss occur. Athletes can learn basics on how to figure out sweat rates and then relate that information to staying hydrated during and/or after workouts.

Sweat Loss Implications

Basic homeostatic functions along with performance can be affected by precipitous fluid loss. It only takes a small amount of fluid loss in a workout over 30 minutes before negative repercussions take effect. Fluid loss can be measured by percentage of overall body weight lost during workouts.

  • 1% -  weight loss- heart rate increase at same work rate
  • 2% - reaching physiological definition of dehydration
  • 3% - aerobic capacity begins to significantly drop
  • Only a limited amount fluid loss can be replaced during a continuous workout or run
  • Despite disagreement on hydration during exercise it is almost universally agreed upon that pre & post workout/event fluid balance is the most important way of preventing dehydration

Coaches and athletes should understand sweat rate in individuals and decide if in workout hydration is going to be necessary to prevent these types of fluid losses. In most cases only workouts of 60+ minutes or work done in extreme heat require fluid loss replacement during the duration of the bout of continuous running.

Figuring out Sweat Rate

  1. Weigh athlete nude before a workout that lasts around 60 minutes
  2. Weigh athlete again in same fashion after workout
  3. Weight lost is sweat loss per hour (or divide by 60 for minute breakdown)

This method is obviously not a scientifically exact way of gaining numbers on sweat loss but it does shed light on an important topic. This can help athletes and coaches decide the need for fluid replacement during runs based on if fluid loss during a workout or competition will have an opportunity to negatively affect performance.

References

Clark, Nancy. (2014). Nancy Clark's Sport Nutrition Guidebook (5th Ed.). Champaign, IL: Human Kinetics.

Eberle, S.G. (2014). Endurance Sports Nutrition (3rd Ed.). Champaign, IL: Human Kinetics

Maughan, R. J., & Shirreffs, S. M. (2008). Development of individual hydration strategies for athletes. International journal of sport nutrition & exercise metabolism, 18(5).

Noakes, T.D. (1985). Temperature Regulation in Exercise, The Lore of Running (4th ed., pp 197-230). South Africa: Oxford University Press.

Peterson, J. A. (2004). Take Ten: Take-and-Save: 10 Nice-To-Know Facts About the Body's Need for Water and    Hydration. ACSM's Health & Fitness Journal,8(4), 44.

Riebl, S. K., & Davy, B. M. (2013). The Hydration Equation: Update on Water Balance and Cognitive Performance. ACSM's Health & Fitness Journal, 17(6), 21-28.

Tam, N., & Noakes, T. D. (2013). The quantification of body fluid allostasis during exercise. Sports Medicine, 43(12), 1289-1299.