VARIABILITY IN THE SWEATING RATE AND SWEAT SODIUM
VARIABILITY IN THE SWEATING RATE AND SWEAT SODIUM
CONCENTRATION OF ULTRA
CONCENTRATION OF ULTRA-
-ENDURANCE TRIATHLETES DURING EXERISE
ENDURANCE TRIATHLETES DURING EXERISE
Matthew D. Pahnke, Joel D. Trinity, Jacob J. Baty, Jeffrey J. Zachwieja†, FACSM, John R. Stofan†, W. Douglas
Hiller‡, and Edward F. Coyle, FACSM
The Human Performance Laboratory, Department of Kinesiology and Health Education, The University of Texas at Austin; Gatorade Sports Science Institute
Exercise Physiology Laboratory, Barrington, IL†; North Hawaii Community Hospital, Waimea, HI‡
Determine the variability in sweat sodium concentration and
sweating rate in Ironman-distance triathletes during exercise in
a warm environment.
There is a large amount of variability in the sweating rate
(CV~30%) and sweat sodium concentration (CV~40%) of
heat-acclimatized ultra-endurance triathletes during exercise
in a warm environment.
Acknowledgements: We wish to thank the subjects for thei r enthusiastic participation. Special thanks
to Simon S. Schenk and Laura Dierenfield for their support.
This study was funded by the Gatorade Sports Science Institute
• A total of 71 heat acclimatized men (n=48) and women (n=23).
• All subjects were registered to compete in the 2003 Ironman World
Championship in Kailua-Kona, HI three to seven days after the exercise
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in normal human males. Exp. Phys. 85(6):869-875, 2000.
4. Jackson A.S. and M.L. Pollock. Generalized equations for predicting body density of men. Br. J.
Nutr. 40:497-504, 1978.
5. Jackson, A.S., M. L. Pollock, and A. Ward. Generalized equationsfor predicting body density of
women. Med Sci Sports Exerc 12:175-82, 1980.
During competition Ironman-distance triathletes can lose
large amounts of fluid and sodium through sweating. To
date no studies have investigated the variabilit y in
sweating rate and sweat sodium concentration in heat-
acclimatized Ironman triathletes during exercise.
PURPOSE: Determine the variability in sweating rate
and sweat sodium concentration in Ironman-distance
triathletes during exercise. METHODS: Three to seven
days prior to competing in the Ironman World
Championship in Hawaii, 71 heat acclimatized subjects
(48 male, 23 female, 42.1±1.4y) cycled a stationary
ergometer for 30 min at 70-75% of maximum heart rate,
following an 8-min warm-up. Sweat rate was calculated
from the change in body weight. An absorbent patch (10
x 12cm) was placed on the right forearm and scapula for
sweat collection during the 30 min trial. Sweat sodium
concentrations were subsequently determined using
chemical analysis (Nova 5, Waltham, MA). Trials were
performed in a warm, outdoor environment (26.4±0.2°C
WBGT) and subjects were fan cooled (3.5±0.4 m·s-1).
RESULTS: Absolute sweat rate was 1.4±0.1 L·hr-1 with
a coefficient of variation (CV) of 30.8%. Between subject
variability was similar when adjusted for body weight
(19.4±1.7 ml·kg-1·hr-1, CV = 28.5%). Sweat sodium
concentrations for the arm and back were 39.6±2.0
mEq·L-1 (CV=42.6%) and 47.2±2.3 mEq·L-1
(CV=41.2%), respectively. Estimated regional sodium
loss (whole body sweating rate x regional concentrations)
was 56.5±3.9 mEq·hr-1 and 66.8±4.5 for the arm and
back, and exhibited high variability (CVarm = 58.7%;
CVback = 56.2%). CONCLUSION: These results
demonstrate that there is large variation in both the
sweating rate and sweat sodium concentration of ultra-
• An Ironman-distance race encompasses a 3.86 km swim, a 180.2
km bike, and a 42.2 km run and can take up to 17 hours to
• Sweating rate in humans can exceed 1.8 L·hr-1 during exercise in the
• Sweat sodium concentration in untrained heat acclimated
individuals ranges from less than 10 to over 60 mEq·L-1(2).
• There is a considerable amount of variability in the sweating
characteristics of moderately trained individuals when exercising at
a low intensity in a temperate environment (3).
• The variability in the sweating rate and sweat sodium concentration
of heat-acclimatized ultra-endurance athletes during exercise in a
warm environment is curr ently unknown.
141.0-157.7145.7±0.3Resting Serum [Na] (mEq·L-1)
10.6-33.516.6±1.1Body Fat-Female (%)
3.9-25.110.7±0.6Body Fat-Male (%)
Table 1. Subject Characteristics
• All trials were completed in an outdoor field laboratory in Kailua-Kona, HI, in
warm conditions (26.4±0.2°C WBGT).
• Blood was sampled prior to exercise. Serum [Na] was measured by chemical
analysis (Nova 5/5, Waltham, MA).
• Skin-fold thickness was measured prior to exercise using Harpenden calipers
(Model HSK-BI, British Indicators, UK). Percent body fat was calculated using
the Jackson and Pollock 3-site method (4, 5).
• Body mass was measured (A & D Medical, Life Source ProFit Scales, Model
UC-321, Milpitas, CA) before and after the exercise trial.
• Subjects cycled at 70-75% of heart rate maximum for 30 minutes on a stationary
ergometer following an 8 minute warm-up.
• All subjects were fan cooled (3.5±0.4 m·s-1) during the trial.
• Sweating rate was calculated as the change in body mass pre to post exercise.
• A breathable waterproof sweat patch (10 x 12cm) was utilized to capture sweat
from the right forearm and upper back during the trial.
• Chemical analysis (Nova 5 , Waltham, MA) of the sweat was completed to
determine sweat sodium concentration.
• To determine the variability in sweating rate, sweat sodium concentration, and
sodium loss means, standard deviations, and coefficients of variation
(CV=SD/mean) were calculated.
• Regression and correlation analysis were performed to describe the relation
between age, gender, sweating rate, sweat sodium concentration, and sodium
54.00.9±0.1Mean Na Loss - Arm,Back (mEq·hr-1·kg-1)
54.50.9±0.1Na Loss - Back (mEq·hr-1·kg-1)
57.50.8±0.1Na Loss - Arm (mEq·hr-1 ·kg-1)
56.061.6±4.1Mean Na Loss - Arm,Back (mEq·hr-1)
56.266.8±4.5Na Loss - Back (mEq· hr-1)
58.756.5±3.9Na Loss - Arm (mEq·hr-1)
40.043.4±2.1Mean Sweat [Na] - Arm,Back (mEq·L-1)
41.247.2±2.3Sweat [Na] - Back (mEq·L-1)
42.639.6±2.0Sweat [Na] - Arm (mEq·L-1)
28.519.4±1.7Sweating Rate (ml·kg-1·hr-1)
30.81.4±0.1Sweating Rate (L·hr-1)
Table 2. Sweating Rate and Sweat Sodium Concentration
0 0.8 1.2 1.6 2.0 2.4
Sweat Rate (L·hr
Figure 1a. Sweat rate (L·hr-1) during moderate intensity exercise.
0 5 10 15 20 25 30
Sweat Rate (ml·kg
Figure 1b. Sweat rate (ml·kg-1·hr-1) during moderate intensity exercise.
Figure 2. Sweat sodium concentration during moderate intensity exercise.
([Na] values are mean values of the forearm and back)
Sweat [Na] (mEq·L-1·hr-1)
Figure 3. Estimated sodium loss during moderate intensity exercise. (Na loss is
calculated as whole body sweat rate x mean sweat [Na]).
Na loss (mEq·hr
• Sweating rate (ml·kg-1 ·hr-1) is not significantly correlated with age (r=0.05,p≥0.05,
two-tailed) or gender (r=0.14, p≥0.05, two-tailed).
• Sweat sodium concentration is not correlated with age (r=0.04, p≥0.05, two-tailed)
or gender (r=0.03, p≥0.05, two-tailed).
• There is a high correlation between regional sweat [Na] of the arm and back (r=0.83,
• There is a correlation between sweating rate (ml·kg-1·hr-1) and sweat [Na] (r=0.26,