©Journal of Sports Science and Medicine (2015) 14, 172-178 http://www.jssm.org
Research article
Effects of Oral Sodium Supplementation on Indices of Thermoregulation in Trained, Endurance Athletes Elizabeth L. Earhart, Edward P. Weiss , Rabia Rahman and Patrick V. Kelly Saint Louis University, Department of Nutrition and Dietetics, Saint Louis, MO, USA
Abstract Guidelines recommend the consumption of sodium during exercise to replace losses in sweat; however, the effects of sodium on thermoregulation are less clear. To determine the effects of high-dose sodium supplementation on indices of thermoregulation and related outcomes, 11 endurance athletes participated in a double-blind, randomized-sequence, crossover study in which they underwent 2-hrs of endurance exercise at 60% heart rate reserve with 1800 mg of sodium supplementation (SS) during one trial and placebo (PL) during the other trial. A progressive intensity time-to-exhaustion test was performed after the 2-hr steady state exercise as an assessment of exercise performance. Sweat rate was calculated from changes in body weight, accounting for fluid intake and urinary losses. Ratings of perceived exertion (RPE) and heat stress were assessed using verbal numeric scales. Cardiovascular drift was determined from the rise in HR during the 2-hr steady state exercise test. Skin temperature was measured with an infrared thermometer. Dehydration occurred in both SS and PL trials, as evidenced by substantial weight loss (2.03 ± 0.43% and 2.27 ± 0.70%, respectively; p = 0.261 between trials). Sweat rate was 1015.53 ± 239.10 ml·hr-1 during the SS trial and 1053.60±278.24 ml/hr during the PL trial, with no difference between trials (p = 0.459). Heat stress ratings indicated moderate heat stress (“warm/hot” ratings) but were not different between trials (p = 0.825). Time to exhaustion during the SS trial was 6.88 ± 3.88 minutes and during the PL trial averaged 6.96 ± 3.61 minutes, but did not differ between trials (p = 0.919). Cardiovascular drift, skin temperature, and RPE did not differ between trials (all p > 0.05). High-dose sodium supplementation does not appear to impact thermoregulation, cardiovascular drift, or physical performance in trained, endurance athletes. However, in light of the possibility that high sodium intakes might have other adverse effects, such as hypertension, it is our recommendation that athletes interpret professional recommendations for sodium needs during exercise with caution. Key words: Salt, sweat, hydration, heat stress, heart rate, electrolyte.
Introduction The physiological effects of decreased plasma volume and low serum sodium concentration have been studied extensively in endurance performance. It is clear that appropriate hydration plays a role in thermoregulation and cardiovascular changes during exercise (von Duvillard et al., 2004). Although increases in sodium intake are encouraged to offset sodium losses in the form of sweat (ACSM exercise and fluid replacement guidelines recommend 20-30 mEq sodium·L-1 (~460-690 mg·L-1) fluid
during exercise), high serum sodium concentrations reduce sweat rates and thus, could impair thermoregulation (Cosgrove and Black, 2013; Sawka et al., 2007; Shibasaki et al., 2009). Based on recommendations and perceptions, many endurance athletes consume salt supplements while exercising, despite the possibility that salt may impair thermoregulation during prolonged endurance exercise. During endurance exercise, sweat rates of athletes typically average 1.0-1.5 L of fluid per hour. The amount of sodium lost during exercise averages 0.8 grams per liter of sweat, but can vary with genetics, diet, heat acclimatization and hydration status (Sawka et al., 2007). One proposed mechanism by which sodium alters sweat rate is due to its impact on active cutaneous vasodilation through changes in plasma osmolality. As core body temperature increases during exercise, skin blood flow and sweat rate increase. Subsequently, heat dissipation occurs. Increased plasma osmolality, either by ingestion of sodium or dehydration, raises the core body temperature threshold at which sweating occurs, delaying cutaneous vasodilation and lowering sweat rate (Wendt et al., 2007). Previous studies have addressed the effects of pre-exercise sodium loading on sweat rate and core body temperature, however the results were conflicting. One study showed no significant differences in sweat rate or core temperature during cycling with an isotonic sodium load vs. a hypotonic placebo in male cyclists (Coles and Luetkemeier, 2005). Two other studies with nearly identical protocols found that a concentrated sodium beverage delayed the rise in core body temperature, and lowered sweat rate when compared to a low sodium beverage in female cyclists, but the same effect was not seen with male cyclists (Sims et al., 2007a; 2007b). The primary purpose of this research study was to determine the effects of high-dose salt consumption during long-duration endurance exercise on indices of thermoregulation in trained, endurance athletes. Earlier research studies focused on largely on pre-exercise sodium intake and used lower doses of sodium supplementation (~300-700 mg·hr-1). Additionally, the method of sodium delivery was typically through oral fluid ingestion or intravenous injections in a laboratory setting, as opposed to tablets or capsules, which more closely resemble realworld practices. Because thermoregulation impacts other areas of endurance performance, a secondary purpose of the study was to examine the effects of salt supplementation on cardiovascular drift, perceived rating of exertion and time to exhaustion. It was hypothesized that highdose salt supplementation during 2 hours of endurance exercise would decrease sweat rate, increase perceived
Received: 01 August 2014 / Accepted: 28 October / Published (online): 01 March 2015
Earhart et al.
heat stress, decrease the magnitude of cardiovascular drift, decrease the rating of perceived exertion, and increase time to exhaustion.
Methods Participants The experimental protocol was approved by the Saint Louis University Institutional Review Board, and informed consent was obtained from all participants. Participants could include males and females, aged 18-60 years. Participants met the qualifications for the study if they were considered at low risk for medical complications based on the American College of Sports Medicine’s Risk Classification (ACSM, 2010). Screening indicated participants’ current use of salt tablets for training. Participants were also required to be “trained, endurance athletes” based on current training status. Specifically,they must have been running greater than or equal to 30 miles (~48 km) each week, road-cycling greater than or equal to 150 miles (~241 km) each week, off-road cycling greater than or equal to 70 miles (~113 km) per week, or an equivalent combination of these or another form of endurance exercise (i.e. swimming). The participants must have been training for a minimum of 6 months prior to the study and must have been able to perform 2-hour bouts of endurance exercise at least once every 1-2 weeks to ensure that they could tolerate the test protocol. Study design The study was a randomized, crossover design and was operationalized into measures of sweat rate, heat stress, skin temperature, cardiovascular drift (heart rate), rating of perceived exertion, and time to exhaustion. Testing occurred on two separate occasions, with the participants completing one placebo (PL) test and one experimental test with sodium supplementation (SS). The number of days between tests ranged from 7 to 26 days (13.5 ± 6.45 days). The requirement for time between tests was a minimum of 7 days (Sims et al., 2007b). The PL and SS trials were double blinded and randomized to avoid bias and sequencing effects. During the SS trial, participants received 5 capsules containing sodium chloride. Each capsule contained 360 mg sodium, 540 mg chloride. One capsule was given immediately prior to starting the test and every 25 minutes thereafter (at minute 25, 50, 75, and 100). Participants received a total of 900 mg/hour of sodium. This rate was chosen to supply roughly half of the average amount of sodium lost in 11.5 L of sweat in one hour and a greater amount than that used in previous studies that showed no effect (Volpe, 2007, Janik et al., 2009). The PL trial received an equal number of capsules containing cornstarch on the same ingestion schedule. Participants received 0.4-0.8 L·hr-1 hr of water based on body weight (0.4 L·hr-1 for body weights of 50-2% body weight loss during exercise) or over-hydration during distance events (ACSM, 2010).
173
They were instructed to consume all of the provided water during the course of the 2-hour test; although most participants consumed a portion of the provided water with the capsules, there was no specific requirement for this. Allowing the subjects to drink the water ad libitum during the trials was done to mimic a real-world training setting in which participants would consume as much water as they determined to be sufficient when taking the capsules. Diet & exercise control Participants were required to keep a log of their food and beverage intake for 48 hours prior to the first test. They were instructed to consume the same food and beverages for the 48 hours before the second test. Participants were educated by a registered dietitian to follow a sodiumrestricted diet (
Research article
Effects of Oral Sodium Supplementation on Indices of Thermoregulation in Trained, Endurance Athletes Elizabeth L. Earhart, Edward P. Weiss , Rabia Rahman and Patrick V. Kelly Saint Louis University, Department of Nutrition and Dietetics, Saint Louis, MO, USA
Abstract Guidelines recommend the consumption of sodium during exercise to replace losses in sweat; however, the effects of sodium on thermoregulation are less clear. To determine the effects of high-dose sodium supplementation on indices of thermoregulation and related outcomes, 11 endurance athletes participated in a double-blind, randomized-sequence, crossover study in which they underwent 2-hrs of endurance exercise at 60% heart rate reserve with 1800 mg of sodium supplementation (SS) during one trial and placebo (PL) during the other trial. A progressive intensity time-to-exhaustion test was performed after the 2-hr steady state exercise as an assessment of exercise performance. Sweat rate was calculated from changes in body weight, accounting for fluid intake and urinary losses. Ratings of perceived exertion (RPE) and heat stress were assessed using verbal numeric scales. Cardiovascular drift was determined from the rise in HR during the 2-hr steady state exercise test. Skin temperature was measured with an infrared thermometer. Dehydration occurred in both SS and PL trials, as evidenced by substantial weight loss (2.03 ± 0.43% and 2.27 ± 0.70%, respectively; p = 0.261 between trials). Sweat rate was 1015.53 ± 239.10 ml·hr-1 during the SS trial and 1053.60±278.24 ml/hr during the PL trial, with no difference between trials (p = 0.459). Heat stress ratings indicated moderate heat stress (“warm/hot” ratings) but were not different between trials (p = 0.825). Time to exhaustion during the SS trial was 6.88 ± 3.88 minutes and during the PL trial averaged 6.96 ± 3.61 minutes, but did not differ between trials (p = 0.919). Cardiovascular drift, skin temperature, and RPE did not differ between trials (all p > 0.05). High-dose sodium supplementation does not appear to impact thermoregulation, cardiovascular drift, or physical performance in trained, endurance athletes. However, in light of the possibility that high sodium intakes might have other adverse effects, such as hypertension, it is our recommendation that athletes interpret professional recommendations for sodium needs during exercise with caution. Key words: Salt, sweat, hydration, heat stress, heart rate, electrolyte.
Introduction The physiological effects of decreased plasma volume and low serum sodium concentration have been studied extensively in endurance performance. It is clear that appropriate hydration plays a role in thermoregulation and cardiovascular changes during exercise (von Duvillard et al., 2004). Although increases in sodium intake are encouraged to offset sodium losses in the form of sweat (ACSM exercise and fluid replacement guidelines recommend 20-30 mEq sodium·L-1 (~460-690 mg·L-1) fluid
during exercise), high serum sodium concentrations reduce sweat rates and thus, could impair thermoregulation (Cosgrove and Black, 2013; Sawka et al., 2007; Shibasaki et al., 2009). Based on recommendations and perceptions, many endurance athletes consume salt supplements while exercising, despite the possibility that salt may impair thermoregulation during prolonged endurance exercise. During endurance exercise, sweat rates of athletes typically average 1.0-1.5 L of fluid per hour. The amount of sodium lost during exercise averages 0.8 grams per liter of sweat, but can vary with genetics, diet, heat acclimatization and hydration status (Sawka et al., 2007). One proposed mechanism by which sodium alters sweat rate is due to its impact on active cutaneous vasodilation through changes in plasma osmolality. As core body temperature increases during exercise, skin blood flow and sweat rate increase. Subsequently, heat dissipation occurs. Increased plasma osmolality, either by ingestion of sodium or dehydration, raises the core body temperature threshold at which sweating occurs, delaying cutaneous vasodilation and lowering sweat rate (Wendt et al., 2007). Previous studies have addressed the effects of pre-exercise sodium loading on sweat rate and core body temperature, however the results were conflicting. One study showed no significant differences in sweat rate or core temperature during cycling with an isotonic sodium load vs. a hypotonic placebo in male cyclists (Coles and Luetkemeier, 2005). Two other studies with nearly identical protocols found that a concentrated sodium beverage delayed the rise in core body temperature, and lowered sweat rate when compared to a low sodium beverage in female cyclists, but the same effect was not seen with male cyclists (Sims et al., 2007a; 2007b). The primary purpose of this research study was to determine the effects of high-dose salt consumption during long-duration endurance exercise on indices of thermoregulation in trained, endurance athletes. Earlier research studies focused on largely on pre-exercise sodium intake and used lower doses of sodium supplementation (~300-700 mg·hr-1). Additionally, the method of sodium delivery was typically through oral fluid ingestion or intravenous injections in a laboratory setting, as opposed to tablets or capsules, which more closely resemble realworld practices. Because thermoregulation impacts other areas of endurance performance, a secondary purpose of the study was to examine the effects of salt supplementation on cardiovascular drift, perceived rating of exertion and time to exhaustion. It was hypothesized that highdose salt supplementation during 2 hours of endurance exercise would decrease sweat rate, increase perceived
Received: 01 August 2014 / Accepted: 28 October / Published (online): 01 March 2015
Earhart et al.
heat stress, decrease the magnitude of cardiovascular drift, decrease the rating of perceived exertion, and increase time to exhaustion.
Methods Participants The experimental protocol was approved by the Saint Louis University Institutional Review Board, and informed consent was obtained from all participants. Participants could include males and females, aged 18-60 years. Participants met the qualifications for the study if they were considered at low risk for medical complications based on the American College of Sports Medicine’s Risk Classification (ACSM, 2010). Screening indicated participants’ current use of salt tablets for training. Participants were also required to be “trained, endurance athletes” based on current training status. Specifically,they must have been running greater than or equal to 30 miles (~48 km) each week, road-cycling greater than or equal to 150 miles (~241 km) each week, off-road cycling greater than or equal to 70 miles (~113 km) per week, or an equivalent combination of these or another form of endurance exercise (i.e. swimming). The participants must have been training for a minimum of 6 months prior to the study and must have been able to perform 2-hour bouts of endurance exercise at least once every 1-2 weeks to ensure that they could tolerate the test protocol. Study design The study was a randomized, crossover design and was operationalized into measures of sweat rate, heat stress, skin temperature, cardiovascular drift (heart rate), rating of perceived exertion, and time to exhaustion. Testing occurred on two separate occasions, with the participants completing one placebo (PL) test and one experimental test with sodium supplementation (SS). The number of days between tests ranged from 7 to 26 days (13.5 ± 6.45 days). The requirement for time between tests was a minimum of 7 days (Sims et al., 2007b). The PL and SS trials were double blinded and randomized to avoid bias and sequencing effects. During the SS trial, participants received 5 capsules containing sodium chloride. Each capsule contained 360 mg sodium, 540 mg chloride. One capsule was given immediately prior to starting the test and every 25 minutes thereafter (at minute 25, 50, 75, and 100). Participants received a total of 900 mg/hour of sodium. This rate was chosen to supply roughly half of the average amount of sodium lost in 11.5 L of sweat in one hour and a greater amount than that used in previous studies that showed no effect (Volpe, 2007, Janik et al., 2009). The PL trial received an equal number of capsules containing cornstarch on the same ingestion schedule. Participants received 0.4-0.8 L·hr-1 hr of water based on body weight (0.4 L·hr-1 for body weights of 50-2% body weight loss during exercise) or over-hydration during distance events (ACSM, 2010).
173
They were instructed to consume all of the provided water during the course of the 2-hour test; although most participants consumed a portion of the provided water with the capsules, there was no specific requirement for this. Allowing the subjects to drink the water ad libitum during the trials was done to mimic a real-world training setting in which participants would consume as much water as they determined to be sufficient when taking the capsules. Diet & exercise control Participants were required to keep a log of their food and beverage intake for 48 hours prior to the first test. They were instructed to consume the same food and beverages for the 48 hours before the second test. Participants were educated by a registered dietitian to follow a sodiumrestricted diet (
