The goal of this clinical trial is to learn whether creatine supplementation changes how the body adapts to repeated exercise in the heat in healthy recreationally active adults and varsity athletes. The study will also examine whether creatine affects fluid balance, blood volume changes and exercise performance.

The main questions it aims to answer are:

Does creatine supplementation increase plasma volume and total body water during repeated heat exposure more than placebo? Does creatine supplementation improve exercise performance after heat acclimation more than placebo? Does creatine supplementation influence cardiovascular responses to heat stress?

Researchers will compare a creatine supplementation group to a placebo group to determine whether creatine changes the magnitude of heat acclimation adaptations.

Participants will:

Complete baseline measurements of body composition, hydration, blood markers, and exercise performance.

Consume either creatine monohydrate or a placebo supplement daily Complete repeated supervised exercise sessions in a controlled heated environment Undergo repeated assessments of body water, temperature, heart rate, and exercise performance throughout the study

Heat acclimation (HA) is a physiological adaptation process that occurs following repeated exposure to hot environmental conditions. HA is associated with several beneficial cardiovascular and thermoregulatory adaptations, including plasma volume expansion, improved heat tolerance, lower physiological strain during exercise, and improved exercise performance in hot and temperate environments. However, substantial variability exists in the magnitude and rate of adaptation between individuals.

Creatine monohydrate supplementation is widely used to improve high-intensity exercise performance and is known to increase intracellular water retention within skeletal muscle. Because HA also influences fluid regulation and plasma volume expansion, creatine supplementation may alter fluid distribution and cardiovascular responses during repeated heat exposure. Despite this theoretical interaction, limited research has investigated the combined effects of creatine supplementation and HA on fluid balance and exercise performance.

The purpose of this randomized, double-blind, placebo-controlled trial is to examine whether creatine monohydrate supplementation influences body-fluid distribution, plasma volume expansion, and exercise performance during repeated heat exposure in healthy recreationally active males and varsity athletes.

Participants will complete baseline testing consisting of body composition assessment, body water measurement, capillary blood sampling, resting temperature assessment, and cardiorespiratory and musculoskeletal performance testing using cycle ergometry. Participants will then be randomly assigned to receive either creatine monohydrate or a placebo supplement under double-blind conditions.

The supplementation protocol will include a 7-day loading phase followed by a maintenance phase throughout the remainder of the study. Following supplementation, participants will complete 7 consecutive days of supervised heat exposure consisting of moderate-intensity cycling in a heated environment designed to elevate body temperature and induce heat acclimation adaptations.

Throughout the study, researchers will assess changes in plasma volume, total body water, resting temperature, heart rate responses, and exercise performance. Physiological monitoring will occur during all heat exposure sessions to support participant safety.

The findings from this study may improve understanding of how creatine supplementation influences fluid regulation and adaptation to heat stress. Results may have implications for athletes, physically active individuals, and occupational populations exposed to hot environmental conditions.

Inclusion Criteria:

• Healthy biological males between 18 and 27 years of age
• Recreationally active males and varsity athletes
• Able to safely participate in moderate-intensity exercise and heat exposure
• Willing to maintain habitual diet and physical activity throughout the study
• Willing to refrain from additional dietary supplementation during study participation
• Able to attend all required laboratory visits and heat exposure sessions
• Able to provide informed consent

Exclusion Criteria:

• History of cardiovascular, metabolic, respiratory, renal, neurological, or heat-related illness
• Contraindications to exercise testing or heat exposure identified during health screening
• Current musculoskeletal injury limiting exercise participation
• Current use of creatine supplementation or inability to complete the required washout period
• Current use of medications or supplements known to affect hydration, thermoregulation, cardiovascular function, or exercise performance
• Known allergy or intolerance to creatine monohydrate, maltodextrin, or orange juice
• Inability to comply with study procedures or supplementation protocol
• Resting tympanic temperature ≥38.0°C prior to a heat exposure session
• Participation in another research study that may interfere with this protocol

This study uses a double-blind design. Participants and investigators involved in data collection and testing procedures will be blinded to supplement allocation. Creatine monohydrate and placebo supplements will be prepared in identical containers and mixed with orange juice to maintain blinding. Group allocation codes will remain concealed until completion of data collection and primary analyses.