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European Journal of Applied Physiology

, Volume 119, Issue 9, pp 2105–2118 | Cite as

Impact of 3-day high and low dietary sodium intake on sodium status in response to exertional-heat stress: a double-blind randomized control trial

  • Alan J. McCubbinEmail author
  • Michelle B. Lopez
  • Gregory R. Cox
  • Joanne N. Caldwell Odgers
  • Ricardo J. S. Costa
Original Article

Abstract

Purpose

To determine the impact of altering dietary sodium intake for 3 days preceding exercise on sweat sodium concentration [Na+], and cardiovascular and thermoregulatory variables.

Methods

Fifteen male endurance athletes (runners n = 8, cyclists n = 7) consumed a low (LNa, 15 mg kg−1 day−1) or high (HNa, 100 mg kg−1 day−1) sodium diet, or their usual free-living diet [UDiet, 46 (37–56) mg kg−1 day−1] for 3 days in a double-blind, randomized cross-over design, collecting excreted urine (UNa) and refraining from exercise. On day 4, they completed 2 h running at 55% \(\dot{V}\)O2max or cycling at 55% maximum aerobic power in Tamb 35 °C. Pre- and post-exercise blood samples were collected, and sweat from five sites using absorbent patches along the exercise protocol.

Results

UNa on days 2–3 pre-exercise [mean (95% CI) LNa 16 (12–19) mg kg−1 day−1, UDiet 46 (37–56) mg kg−1 day−1, HNa 79 (72–85) mg kg−1 day−1; p < 0.001] and pre-exercise aldosterone [LNa 240 (193–286) mg kg−1 day−1, UDiet 170 (116–224) mg kg−1 day−1, HNa 141 (111–171) mg kg−1 day−1; p = 0.001] reflected sodium intake as expected. Pre-exercise total body water was greater following HNa compared to LNa (p < 0.05), but not UDiet. Estimated whole-body sweat [Na+] following UDiet was 10–11% higher than LNa and 10–12% lower than HNa (p < 0.001), and correlated with pre-exercise aldosterone (1st h r =  − 0.568, 2nd h r =  − 0.675; p < 0.01). Rectal temperature rose more quickly in LNa vs HNa (40–70 min; p < 0.05), but was similar at the conclusion of exercise, and no significant differences in heart rate or perceived exertion were observed.

Conclusions

Three day altered sodium intake influenced urinary sodium excretion and sweat [Na+], and the rise in rectal temperature, but had no effect on perceived exertion during moderate-intensity exercise in hot ambient conditions.

Keywords

Salt Sweat Endurance Running Cycling Plasma volume Plasma osmolality 

Abbreviations

CHO

Carbohydrate

CI

Confidence interval

CV

Coefficient of variation

FA

Forearm

FH

Forehead

GIS

Gastrointestinal symptoms

Hb

Haemoglobin

HCT

Haematocrit

HNa

High sodium diet (100 mg kg−1 day−1)

HR

Heart rate

ISE

Ion selective electrode

LNa

Low sodium diet (15 mg kg−1 day−1)

LSR

Local sweat rate

MAP

Maximum aerobic power

MT

Mid-thigh

Na+

Sodium

[Na+]

Sodium concentration

NaCl

Sodium chloride

POsm

Plasma osmolality

Pv

Plasma volume

RPE

Rating of perceived exertion

SD

Standard deviation

SS

Superior scapula

Tamb

Ambient temperature

TBW

Total body water

TCR

Thermal comfort rating

Tre

Rectal temperature

UC

Upper chest

UDiet

Usual free-living diet (mean: 46 mg kg−1 day−1)

UNa

Urinary sodium excretion

USG

Urine specific gravity

\(\dot{V}\)O2max

Maximal oxygen uptake

\(\dot{V}\)O2peak

Peak oxygen uptake

WB

Whole body

WBW

Whole body washdown

Notes

Acknowledgements

The authors would like to thank the participants for their significant time commitment in completing this study, as well as Alexandra Stone, Alice Mika, Ashleigh Gale, Camilo Cayazaya, Rebecca Sue, and Holly Bolton for their assistance in the laboratory during data collection.

Author contributions

AM, GC, JC, and RC were involved in the study design. AM recruited participants. AM and ML collected the data. AM, ML, and RC analyzed all samples. AM and ML analyzed all data. Data interpretation and manuscript preparation were undertaken by AM, JC, GC, and RC. All authors approved the final version of the paper.

Funding

Funding for this work was provided by the Monash University, Be Active, Sleep Eat (BASE) Strategic Grants Scheme, and an Australian Government Research Training Program (RTP) Scholarship.

Compliance with ethical standards

Conflict of interest

The authors have no conflicts of interest, financial or otherwise, to declare.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of Nutrition, Dietetics and FoodMonash UniversityNotting HillAustralia
  2. 2.Department of PhysiologyMonash UniversityClaytonAustralia
  3. 3.Faculty of Health Sciences and MedicineBond UniversityGold CoastAustralia

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