Betaine Addition to Water Loading Prior to Running Exercise

Time required to complete a 5-km treadmill running time trial, expressed in seconds. Participants will be instructed to complete the distance as quickly as possible and will be allowed to adjust treadmill speed throughout the trial. Lower completion time indicates better performance.

Whole-body sweat rate will be estimated from the change in body mass from immediately before exercise to completion of the 5-km time trial, adjusted for fluid intake and urine output, if applicable, and expressed as L/h. Body mass will be measured using the calibrated electronic scale integrated with the BOD POD system (COSMED, Italy) and expressed in kilograms (kg). Changes in body mass will also be calculated and reported as a percentage of pre-exercise body mass (%).

Sodium concentration will be measured in sweat samples collected from standardized sites on the forearm and thigh during exercise. Sweat samples will be collected between minutes 30 and 60 of the 60-minute running preload. Sodium concentration will be analyzed using a HORIBA ion-selective electrode analyzer and reported in mmol/L.

Potassium concentration will be measured in sweat samples collected from standardized sites on the forearm and thigh during exercise. Sweat samples will be collected between minutes 30 and 60 of the 60-minute running preload. Sweat potassium concentration will be analyzed using a HORIBA ion-selective electrode analyzer and reported in mmol/L.

Blood lactate concentration will be measured using a portable Lactate Scout 4 analyzer. Measurements will be obtained immediately after completion of the 60-minute running preload and immediately after completion of the subsequent 5-km time trial. Blood lactate concentration will be reported in mmol/L.

Heart rate will be monitored continuously using a Garmin chest-strap heart rate monitor with integrated electrodes throughout the 60-minute running preload and the subsequent 5-km time trial. Outcome variables will include mean heart rate during the 60-minute preload, mean heart rate during the 5-km time trial, and maximal heart rate achieved during the 5-km time trial. Heart rate will be reported in beats per minute (bpm).

Core body temperature will be monitored continuously using a CORE body temperature sensor attached to the Garmin chest strap throughout the 60-minute running preload and the subsequent 5-km time trial. Outcome variables will include mean core body temperature during the exercise protocol and the highest core body temperature recorded during the exercise protocol. Temperature will be reported in degrees Celsius (°C).

Running economy will be assessed by breath-by-breath indirect calorimetry using a QUARK RMR metabolic system during the final 10 minutes (minutes 50-60) of the 60-minute running preload. Running economy will be quantified as mean oxygen consumption (VO₂) relative to body mass at the standardized running velocity and expressed as mL·kg-¹·min-¹. Lower oxygen consumption at the same running velocity indicates better running economy.

Respiratory exchange ratio (RER) will be assessed by breath-by-breath indirect calorimetry using a QUARK RMR metabolic system during the final 10 minutes (minutes 50-60) of the 60-minute running preload. RER will be calculated as the ratio of carbon dioxide production (VCO₂) to oxygen consumption (VO₂) and expressed as a unitless value. The mean RER over the measurement period will be used for analysis.

Urine specific gravity (USG) will be measured using a URIT-31 urine analyzer from urine samples collected immediately before the 60-minute running preload and after completion of the subsequent 5-km time trial. USG will be reported as a dimensionless value. Urine osmolality will also be estimated from USG using a predefined conversion equation and reported in mOsm/kg.

Rating of perceived exertion will be assessed using the Borg 6-20 Rating of Perceived Exertion scale, ranging from 6 ("no exertion at all") to 20 ("maximal exertion"). RPE will be recorded at 15-minute intervals during the 60-minute running preload and immediately after completion of the 5-km time trial.

Perceived thirst will be assessed using a 9-point categorical thirst sensation scale ranging from 1 ("not thirsty at all") to 9 ("very, very thirsty"). Participants will be asked to rate their current sensation of thirst at each measurement time point.

Thermal sensation will be assessed using a 17-point thermal sensation scale ranging from 0 to 8 in 0.5-point increments, with 0 representing "unbearably cold", 4 representing "neutral", and 8 representing "unbearably hot". Participants will rate their whole-body thermal sensation at each measurement time point.