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

uring endurance exercise, two problems arise from disturbed uid–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 uids. 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 eld 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: thirst; drinking; sweat; sodium; hyponatremia; overhydration; dehydration; marathon;triathlon 1. Introduction The essential components of central nervous system maintenance of body water volume and concentration include perceptions, behavior, nervous system responses, and the release of hormones (vasopressin, AVP; angiotensin II; atrial natriuretic peptide;apelin) [1–4]. Per- turbations of whole-body water volume and concentration are monitored by the brain, the resulting thirst and oropharyngeal sensations modulate drinking, and neuroendocrine responses regulate water and electrolyte excretion or retention by the kidneys [1]. Dur-

body water volume and concentration include perceptions, behavior, nervous system responses, and the release of hormones (vasopressin, AVP; angiotensin II; atrial natriuretic peptide;apelin) [1–4]. Per- turbations of whole-body water volume and concentration are monitored by the brain, the resulting thirst and oropharyngeal sensations modulate drinking, and neuroendocrine responses regulate water and electrolyte excretion or retention by the kidneys [1]. Dur- ing typical daily activities that do not include exercise, these complex interactions act to maintain total body water volume and serum concentration within 1–3% of baseline each day [5–7]. However, the relative in uence of these processes varies with different life activ- ities [8]. Table responses and thirst are the primary homeostatic regulators. During prolonged endurance exercise at low intensities (5–24 h duration), renal responses and thirst have minor-to-large effects on water regulation. As the duration of exercise increases, sweat losses become a major factor in whole-body water balance [9], regardless of the volume of uid consumed. This review article considers endurance exercise from the perspectives of body water and electrolyte balance, the negative effects that substantial uid–electrolyte disturbances (i.e., both water loss and gain) have on competitive performance and health, and ways that endurance athletes can minimize performance decrements and mitigate the risk of exercise-associated illness. This is important because an endurance athlete can lose as much as 11–12% (7.8–8.5 kg) of body weight in the form of water, during a 12.3-h Ironman triathlon in a cool environment (3.8-km swim, 180-km bike, 42.2-km run) [10]. This also is important because day-long walking or hiking in a desert environment can result in extreme body mass losses of 14–18% when uids are unavailable or restricted (Figure). Nutrients2021,13, 887.

Nutrients2021,13, 887 2 of 21 Conversely, excessive uid intake (i.e., water retention) can result in a body mass gain of more than 10% (7.8 kg) in 12.7 h while competing in an ultraendurance triathlon (ambient temperature, Tamb, 20.5 C) [11]. These vastly different changes of body mass represent the primary problem and focus of this review paper: how to maintain a rehydration middle ground during prolonged exercise that reliably reduces the risk of illness by avoiding overhydration, and maintains exercise performance by avoiding signi cant dehydration. Table 1. The relative effects of thirst, drinking, and physiological responses on uid-electrolyte balance during ordinary daily activities and endurance exercise. Activity Thirst & Drinking Behavior Sweat Gland Secretion of Hypotonic Fluid Kidney Regulation of Water & Electrolytes Neuroendocrine Homeostatic Responses a Effects on Water & Electrolyte Balance Sedentary daily activities (16 h) Basal b Negligible Basal b Basal b CNS responses are suf cient to maintain water and electrolyte homeostasis Brief exercise (5–30 min) at moderate-to-high intensity Minor Minor-to-moderate Minor Minor, brief Water and electrolyte losses are minor Endurance exercise (0.5–5 h) at low-to-high intensity Minor-to-large Moderate-to-large Minor-to-moderate Minor-to-large, prolonged Moderate-to-large turnover c due to sweating and drinking Ultraendurance exercise (5–24 h) at low-to-moderate intensity Moderate-to-large Large Moderate-to-large Large, prolonged Water and electrolyte losses in sweat and urine exceed 24 h dietary intake CNS, central nervous system (i.e., brain and spinal cord); TBW, total body water. a , CNS effects involving nerves and hormones that regulate whole-body water volume and concentration, blood volume/pressure/osmolality, and thirst (see [9] for a review of this topic). b , a standard low level maintenance of whole-body uid-electrolyte balance with small turnover (intake versus loss) and minor perturbations. c , turnover refers to the sum of gains and losses of water and electrolytes. 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 identi es the approximate water de cit of its rst report. Based on information from [12]. 2. Problem: Water and Salt Losses during Endurance Exercise Most ultraendurance

losses of water and electrolytes. 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 identi es the approximate water de cit of its rst report. Based on information from [12]. 2. Problem: Water and Salt Losses during Endurance Exercise Most ultraendurance competitors do not meet their uid needs during competi- tion [13], due primarily to three factors that interact to in uence sweat volume and body mass during prolonged exercise [14]. The rst of these factors is exercise intensity. Table presents whole-body water balance measurements of 32 cyclists who completed a 164-km

Nutrients2021,13, 887 3 of 21 event in the state of Texas, USA during the month of August [15]. Cyclists have been grouped on the basis of time to complete 164 km: 9.6, 6.3, and 4.8 h. The total volume of sweat lost by these groups were similar (range of 7000–7200 g), demonstrating that the higher exercise intensities of groups 4.8 and 6.3 stimulated a greater sweat rate per hour (p< 0.01 to 0.0001) than that of group 9.6. Exercise intensity also affected body mass proportionally. The body mass change values for cyclist groups 9.6, 6.3, and 4.8 (columns 2–4, row 11) were 1800, 2300, and 2750 g. The second factor is exercise duration. As shown in Table, slower cyclists may be on the course at least twice as long as faster competitors. Not surprisingly, similar body mass losses occur commonly (Figure) during ultra-running, ultra-cycling, and ultra-triathlon events [16]. Environmental temperature represents a third factor that in uences body water balance. During 42.2 km marathon running, mild ambient conditions of 7, 10–12, and 20 C resulted in mean sweat rates of 0.81, 0.96, and 1.52 L/h, respectively [17]. In addition, researchers measured the sweat rates of athletes in a laboratory building (29 C, 51% relative humidity; running and cycling protocols), a mobile laboratory (29 C, 65% rh), or eld environment (25 C, 55% rh) [18]. The majority of these athletes competed in team/skill sports (n= 1022) and individual endurance sports (n= 255). The highest average sweat rates were observed in the sports of American football (1.51 L/h) and Endurance Sports (1.28 L/h), whereas the lowest occurred in baseball (0.83 L/h) and soccer (0.94 L/h). Table 2. Characteristics of three groups of cyclists who completed a 164 km summer road cycling event in 4.8–9.6 h (modi ed from [15]). No drinking instructions or experimental interventions were involved. Variables Average Exercise Duration (h) a 9.6 6.3 4.8 Pre-event body mass b (kg) 81.90 82.05 82.55 Number of male cyclists 11 11 10 Ground speed (km/h) 17.2 d 26.6 d 34.0 d Rating of perceived exertion at nish c 16 16 16 ad libitum

in 4.8–9.6 h (modi ed from [15]). No drinking instructions or experimental interventions were involved. Variables Average Exercise Duration (h) a 9.6 6.3 4.8 Pre-event body mass b (kg) 81.90 82.05 82.55 Number of male cyclists 11 11 10 Ground speed (km/h) 17.2 d 26.6 d 34.0 d Rating of perceived exertion at nish c 16 16 16 ad libitum total uid intake e (g) +6100 +4500 +3900 f Rate of uid intake (g/h) +635 +715 +810 Sweat secreted g (g) 7700 7150 7000 Sweat rate (g/h) 800 d 1135 1460 Urine excreted g (g) 1300 550 450 d Solid food mass consumed e (g) +423 +355 +350 Body mass change b (g) 1800 2300 2750 Body mass change (%) 2.0 2.9 3.4 Note: values are means or medians; negative values represent reduced mass or loss of uid from the body; air temperature ranged from 24.4 C (08:00 h) to 41.1 C (15:00 h); for the purposes of this table, 1 g = 1 mL and 1 kg = 1 L. a , cyclist groups 9.6 and 6.3 voluntarily stopped at 3 roadside aid stations for research measurements, elimination, drinking, and eating. Group 4.8 rode as part of a 5-h pace team and did not stop during the entire event. b , measured with a calibrated oor scale ( 100 g). c , using a printed 6 (very, very light) to 20 (very, very hard) point perceptual rating scale [19]. d , signi cantly different from all other groups (p= 0.01 to 0.0001). e , based on cyclist diet records and con rmed by interviews. f , signi cantly different from group 9.6 (p= 0.04). g , detailed methods are described in the original publication [15]. Representing extreme points of reference, the following individual sweat rate values have been observed. First, an elite marathon runner (age, 26 y; height, 185 cm; body mass, 66.9 kg) produced 3.7 L of sweat per h during the 1984 Los Angeles Summer Olympics marathon (24–28 C Tamb; time to complete 42.2-km race, 2 h 14.3 min; body mass change, 5.43 kg, 8.1%) [21]. Second,

reference, the fo

The article reviews hydration strategies for endurance athletes to prevent dehydration and overhydration.