Sodium Loading Aids Fluid Balance and Reduces ...

1 Copyright @ 2007 by the American College of Sports Medicine. Unauthorized reproduction of this article is Loading aids Fluid Balance andReduces Physiological Strain of TrainedMen Exercising in the HeatSTACY T. SIMS1,3, LINDA van VLIET1, JAMES D. COTTER1, and NANCY J. REHRER1,21 School of Physical Education,2 Department of Human Nutrition, and3 University of Otago, Dunedin, NEW ZEALANDABSTRACTSIMS, S. T., L. van VLIET, J. D. COTTER, and N. J. REHRER. Sodium Loading aids Fluid Balance and Reduces PhysiologicalStrain of Trained Men Exercising in the Sci. Sports Exerc., Vol. 39, No. 1, pp. 123 130, :This study wasconducted to determine whether preexercise ingestion of a highly concentrated Sodium beverage would increase plasma volume (PV)and reduce the physiological strain of moderately trained males running in the :Eight endurance-trained (V O2max:58mLIkgj1 Iminj1(SD 5); 36 yr (SD 11)) runners completed this double-blind, crossover experiment.

2 Runners ingested a high- Sodium (High Na+: 164 mmol Na+ILj1) or low- Sodium (Low Na+: 10 mmol Na+ILj1) beverage (10 mLIkgj1) before running to exhaustion at70% V O2maxin warm conditions (32-C, 50% RH,Va$ mIsj1). Beverages (~757 mL) were ingested in seven portions across 60min beginning 105 min before exercise. Trials were separated by 1 3 wk. Heart rate and core and skin temperatures were measuredthroughout exercise. Urine and venous blood were sampled before and after drinking and :High Na+increased PVbefore exercise ( (SD )), calculated from Hct and [Hb]), whereas Low Na+did not ( (SD );P= ), and involvedgreater time to exercise termination in the six who stopped because of an ethical end point (core temperature : min (SD 6)vs min (SD 4);P= ) and those who were exhausted ( min (SD 22) vs min (SD 21);P= ; High Na+vs Low Na+,respectively).

3 At equivalent times before exercise termination, High Na+also resulted in lower core temperature ( vs ;P= ) and perceived exertion (P= ) and a tendency for lower heart rate (164 vs 174 bpm;P= ).Conclusions:Preexerciseingestion of a high- Sodium beverage increased plasma volume before exercise and involved less thermoregulatory and perceivedstrain during exercise and increased exercise capacity in warm Words:CITRATE, RUNNING, HYPERVOLEMIA,HYPERHYDRATIONP lasma volume (PV) is fundamental to cardiovascularfunction and may impact on work capacity andendurance exercise performance (23,32). Expansionof PV (hypervolemia) has been suggested to improvethermoregulation and exercise performance in the heat;however, this remains equivocal. The expansion of bloodvolume from expanded body water (hyperhydration) mayreduce the cardiovascular and heat strain observed withexercise, thereby improving exercise performance.

4 Manystudies have examined methods for inducing hypervolemia,including infusion of crystalloid or colloid solutions or pre-exercise overdrinking of plain water or water electrolytesolutions (1,25). Infusion is unrealistic for most circum-stances, whereas overdrinking has caused only transientexpansions of body water, with much of the Fluid overloadrapidly excreted, at least partly because of its use of solutions containing glycerol has induced modesthyperhydration and has resulted in limited and mixed resultsin terms of thermoregulatory and performance advantages,and has sometimes shown side effects (20,28). Studiesin which plasma hyperosmolality was attenuated duringexercise-induced heat stress have shown improved heatdissipation (6,7,24), but many have not addressed exerciseperformance (12,24).Recently, Sodium Loading with a Sodium concen-trated beverage composed of Sodium citrate and sodiumchloride (164 mmol Na+ILj1) with moderate osmolality(253 mOsmIkgj1) has been shown to be effective in induc-ing hyperhydration and hypervolemia at rest (14,25,30).

5 Compared with no Fluid and with salt tablets with water( mmol Na+ILj1) this Sodium - Loading protocol elic-ited the greatest plasma volume expansion at rest (4,8).Greenleaf et al. (14,15) also observed PV expansionand enhanced cycling performance in men, in temperateconditions, after Sodium Loading . It remains unknown,however, whether the same effect would be evident intrained males, who might be more inclined to use such anapproach but who are already hypervolemic (11) and whoshow less cardiovascular benefit in exercise from artifi-cially induced hypervolemia (9). Also, one would expectAddress for correspondence: Nancy J. Rehrer, , School of PhysicalEducation, University of Otago, PO Box 56, Dunedin, New Zealand;E-mail: for publication March for publication July & SCIENCE IN SPORTS & EXERCISE Copyright 2007 by the American College of Sports MedicineDOI: @ 2007 by the American College of Sports Medicine.

6 Unauthorized reproduction of this article is a situation in which Sodium Loading would be par-ticularly beneficial would be for orthostatically stressfulexercise performed in the heat, because this involves botha very high demand for cutaneous perfusion and a poolingof blood. Therefore, the aims of this project were to deter-mine whether ingestion of a highly concentrated Sodium -citrate beverage would induce hypervolemia in trainedindividuals and enhance running capacity in the healthy, nonsmoking, male, mod-erately trained runners, maximal oxygen consumption(V O2max) (SD ), age 36 yr (SD 11),body mass kg (SD ), height cm (SD ),completed this institutionally approved study, which wasconducted in the University of Otago EnvironmentalChamber Laboratory for Human Performance.

7 Each subjectgave written informed double-blind, placebo-controlled, crossover design was employed during thewinter months of the southern hemisphere (June toSeptember; average daily maximum outside temperatureof 7 10-C, respectively) to control for heat accli-matization. Participants completed two treadmill trials toexhaustion in the heat (32-C, 50% RH,Va$ mIsj1)at70% of their temperate-environment V treadmill V O2maxtest was conducted on aseparate day, 1 2 wk before experimental testing, to assessfitness and to set the intensity for the experimental then underwent a familiarization session tomimic the actual trial(s). Experimental trials wererandomized, separated by 1 3 wk, and involved a run-to-exhaustion performance test after ingestion of 10 mLIkgj1body mass (~757 mL) of a control, low- Sodium bever-age (Low Na+, 10 mmol Na+ILj1, ,42 mOsmIkgj1), or a high- Sodium beverage (High Na+,164 mmol Na+ILj1, g of Sodium -citrate with gNaCl, 253 mOsmIkgj1) (15,16), chilled overnight at was blind to the researcher conducting trials andto the participant.

8 The allocation of the first beverageassignment was random for the first participant andalternated standardize training effects onPV, each participant maintained a training diary ofduration, mode, and intensity of activity, which wasreplicated for consistency preceding each trial. Further,each participant completed a 40-min treadmill run at50% V O2max48 h before each testing day and thenrefrained from training until the experimental trial. Thesame meal, of each participant`s choice (no alcohol orcaffeine permitted) was consumed the evening beforeeach testing session, and participants were required todrink 750 mL of water throughout that evening. Theywere asked to refrain from smoking or drinking alcoholand to avoid tea, coffee, cola (or other caffeinatedbeverages) on the day before and the day of the test.

9 Onthe day of testing, participants reported to the lab, fullyhydrated, after a standardized breakfast (1680 kJ, 13 gprotein, 10 g fat, 63 g CHO, 265 mg Na+), which wasconsumed between and 2 h before additional 500 mL of water was given to participantsbetween breakfast and the start of the testing specific gravity was measured in an initial baselinesample to verify adequate hydration status before each test( ). arrival at 0900 h, participants voidedbefore nude body mass was recorded (T10 g). Eachparticipant was then seated for catheter placement (22-gauge Teflon intravenous catheter) in a suitable vein of theleft arm. To ensure steady-state plasma volume andconstituents, participants stood for 20 min before baselineblood samples were withdrawal of the baseline bloodsample, the participant began ingesting the beverage,which was measured (659 870 mL) in seven equalportions, one every 10 min (Fig.)

10 1). During this drinkingperiod, they were required to walk approximately 1 minevery 20 min to limit venous pooling; however, they wererequired to stand in one place for 20 min before bloodsampling (15). To avoid any perceptive taste variations inthe beverages, the participants were instructed that thestudy was designed to investigate varying concentrations ofsodium in a preexercise beverage; additionally, eachbeverage was chilled overnight at 4-Candremainedrefrigerated until being partitioned into individual 1 Experimental protocol. S, sampling (V O2,V CO2, perceived exertion, heart rate, and skin and rectal temperatures); B, blood sampling;BW/U, measurement of body mass (kg), urine osmolality, plasma and urine [Na+], and urine specific gravity; D, drink bolus; EXH, volitionalexhaustion orTrec= Journal of the American College of Sports MedicineCopyright @ 2007 by the American College of Sports Medicine.