Rehydration during Endurance Exercise: Challenges, Research, Options, Methods

Abstract

During endurance exercise, two problems arise from disturbed fluid-electrolyte balance: dehydration and overhydration. The former involves water and sodium losses in sweat and urine that are incompletely replaced, whereas the latter involves excessive consumption and retention of dilute fluids. When experienced at low levels, both dehydration and overhydration have minor or no performance effects and symptoms of illness, but when experienced at moderate-to-severe levels they degrade exercise performance and/or may lead to hydration-related illnesses including hyponatremia (low serum sodium concentration). Therefore, the present review article presents (a) relevant research observations and consensus statements of professional organizations, (b) 5 rehydration methods in which pre-race planning ranges from no advanced action to determination of sweat rate during a field simulation, and (c) 9 rehydration recommendations that are relevant to endurance activities. With this information, each athlete can select the rehydration method that best allows her/him to achieve a hydration middle ground between dehydration and overhydration, to optimize physical performance, and reduce the risk of illness.

Keywords: dehydration; drinking; hyponatremia; marathon; overhydration; sodium; sweat; thirst; triathlon.

Figure 1

Signs and symptoms of dehydration in men who walked in the desert without drinking. The symbol which appears to the left of each sign or symptom identifies the approximate water deficit of its first report. Based on information from [12].

Figure 2

The relationship between body weight change (%) and serum Na⁺ after 4.0–13.3 h of exercise (n = 2135) as modified from [20]. Solid circles (●) represent asymptomatic marathon runners and Ironman triathletes. Open circles (○) depict athletes with severe symptoms including hyponatremic encephalopathy (central nervous system dysfunction due to brain swelling). Horizontal zone abbreviations: >RR, serum Na⁺ concentration above resting normal; RR, the laboratory reference range for healthy adults (green horizontal boundaries); H_B, biochemical hyponatremia which involves few or no symptoms; H_SYM, symptomatic hyponatremia. Symbols A–M were overlaid by the present author (see details below in Section 3.2.1). Gray highlighted symbols depict individuals with fluid intake rates of ≤700 mL/h and body mass losses of 0.1 to 2.6%. Yellow highlighted squares indicate exertional hyponatremia cases (each n = 1) with fluid intake rates ranging from 733–2061 L/h and body mass increases of +0.1 to +5.0%. Reprinted via the PNAS Open Access option from [20].

Figure 3

The effects of dehydration on exercise performance. Fractions represent the number of statistically significant (p < 0.05) observations out of the number at each level of body mass loss. Across all dehydration levels, 68% of comparisons indicated impairment. Reprinted from [32] via the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/) accessed on 2 March 2021.

Figure 4

Influences on thirst and drinking behavior during endurance exercise. All factors in this diagram are perceived, monitored, and/or regulated by the brain.

Figure 5

Frequency distribution of the plasma osmolality (P_osm) threshold for the onset of thirst. The horizontal gray bar delineates the laboratory reference range of P_osm values (285–295 mOsm/kg) for healthy adults. Reprinted under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) accessed on 2 March 2021. Modified from [97].

Figure 6

Factors that influence the relationship between body weight gain/loss and serum Na⁺ during endurance exercise. The largest effects on whole body fluid–electrolyte balance are the volume of sweat lost and the volume of fluid consumed. Notes: ^a, water and low-sodium fluids promote dilution of body fluids; ^b, physical training and heat acclimatization increase sweat rate and decrease sweat sodium concentration; ^c, urine production decreases during exercise; ^d, increased dietary sodium encourages water retention but affects only a minor increase of serum Na⁺.