The aim of the present study is to evaluate the effectiveness of a milk permeate-based sports drink on hydration, carbohydrate oxidation, exercise performance, vigilance, and reaction time in the heat. We anticipate that the milk permeate, with 44 g per liter and high sodium and potassium, will lead to better hydration, energy delivery, and physical and cognitive performance.

EXPERIMENTAL PROTOCOL Each subject will complete five trials (one familiarization and four experimental), consisting of computer-based vigilance and reaction time tests (VIG/RT) administered before and after approximately 2 hours of cycling in a warm environment (90°F, 25-35% relative humidity). Each trial will be identical, with the only difference being the type of drink consumed during each trial. The beverages are commercially available products and are not considered investigational products under FDA regulations.

The first trial will be a familiarization trial, during which participants will complete the VIG/RT tests and cycling protocol exactly as performed in the experimental trials, but no biological samples will be collected. The remaining trials will be completed in randomized order, and the drink consumed will be:

A) Placebo (flavored water) B) GoodSport® (milk-permeate, lactose-free sports drink) C) Gatorade® (traditional sports drink) D) Low-Carb GoodSport® During the experiments, core body temperature, heart rate, skin temperature, urine samples, and expiratory gases will be collected. Blood samples will be collected using an intravenous catheter placed in an antecubital vein (see protocol figure on the next page). During the familiarization trial, no samples will be collected.

INFORMED CONSENT AND BASELINE ASSESSMENT During the initial visit, subjects will read the consent form, discuss any concerns, and have questions addressed. After signing the consent form, participants will have 20 minutes to complete the medical health history, PAR-Q questionnaire, and Ingestible Core Body Temperature Sensor Contraindication Form in person on paper.

If the volunteer is deemed eligible, height, weight, and body composition will be assessed on site using a bioelectrical impedance analyzer, and the procedure will take approximately five minutes.

Participants will then complete a standard graded exercise test (GXT) performed on a cycle ergometer. Performance data (e.g., power output) obtained from this test will be used to determine the resistance applied to the cycle ergometer during the experimental trials (description provided below). During this test, the investigators will also assess maximal oxygen uptake (VO₂max) via indirect calorimetry to evaluate aerobic capacity.

Once testing is completed, the experimental trials will be scheduled. Participants will receive a food diary form to record all food and fluid intake on the day before each trial, and dietary restrictions or allergies will be documented. Participants will be asked to replicate this intake for subsequent visits. Frozen meals will be provided for consumption as dinner before the first experimental trial. Additional meals will be provided after each experimental visit.

EXPERIMENTAL PROCESSES Subjects will ingest an individually calibrated telemetric temperature pill (e-Celsius®, France) on the evening preceding each trial to avoid interaction with fluid ingestion during testing. Subjects will report to the laboratory after at least 10 hours of fasting from food and beverages other than water, and a urine sample will be collected to verify adequate hydration status.

The urine sample will be analyzed for urine specific gravity (USG). If USG is below 1.020, indicating adequate hydration, the protocol will proceed. If USG is greater than 1.029, the trial will be canceled and rescheduled. If USG is between 1.020 and 1.029, the volunteer will consume 16 oz of water, and another urine sample will be collected within 30 minutes. The trial will be canceled and rescheduled unless USG is below 1.020 at the subsequent sample or unless the volunteer is unable to provide an additional sample.

At the start of each experimental trial, subjects will consume a light breakfast, after which body composition will be assessed using a bioelectrical impedance scale. Subjects will then complete the vigilance and reaction time tests. After completion of the tests, an intravenous catheter will be placed in an antecubital vein, and a baseline blood sample will be collected. Subjects will then void the bladder, and body weight will be measured. Once these procedures are completed, subjects will be positioned on the cycle ergometer.

Cycling Endurance Exercise Performance Each trial includes 120 minutes of cycling at 70% of VO₂max, followed by a 5-km time trial. A relative beverage volume of 15 mL/kg/h will be provided. The total beverage volume will be divided into aliquots and administered every 10 minutes during the 120-minute exercise period.

At the end of the 120-minute exercise bout, subjects will consume an additional beverage bolus before the 5-km time trial (see Figure 1 for drink timing), and another bolus will be consumed immediately before the post-exercise vigilance and reaction time (VIG/RT) tests. After completion of the tests, body composition will again be assessed using a bioelectrical impedance scale.

At regular intervals throughout the exercise protocol, body weight, blood samples, respiratory gases, and perceptual measures will be collected. See the study schematic provided below for specific measurement time points.

Inclusion Criteria:

• Healthy and injury free (muscle, skeletal) for greater than 2 months
• Active cyclists who train at least 3 times per week
• Body mass index (BMI) range of 18.5-30 kg/m2
• VO₂max ≥ 51 mL/kg/min for male participants
• VO₂max ≥ 45 mL/kg/min for female participants

Exclusion Criteria:

• Taking medicine influences the cardiovascular system
• non-steroidal anti-inflammatory drugs during the last week
• Cancer Diagnosis during the last 5 years
• Current Pregnancy
• History of renal disease, hypertension, obstructive gastrointestinal tract disease, disorders or impairment of the gag reflex, previous gastrointestinal surgery
• History of heat stroke
• Use of testosterone therapy
• Adults who are unable to consent (impaired decision-making capacity)
• Adults having Felinization of the esophagus
• Adults who will undergo Nuclear Magnetic Resonance (NMR) or MRI scanning within 3 days of swallowing a temperature sensitive pills