The goal of this clinical trial is to learn if different forms of caffeine ingestion can influence fat oxidation during submaximal cycling exercise. The study will recruit healthy adults, both men and women, aged 18 to 50, who regularly perform aerobic exercise. The main questions it aims to answer are:

• Does caffeine increase fat oxidation during one hour of cycling at the Fatmax intensity?
• Do different forms of caffeine ingestion (capsule, coffee, chewing gum, or mouth rinse) differ in their effect on fat oxidation?
• Do these caffeine forms alter total energy expenditure, cardiorespiratory response, or perceived exertion during exercise?
• Are there differences in the occurrence or severity of side effects between caffeine administration methods?

Researchers will compare each caffeine form to its corresponding placebo to determine their effects on fat oxidation and other physiological outcomes.

Participants will:

• Attend a pre-experimental session to determine their Fatmax intensity and VO₂max using an incremental cycling test
• Perform eight experimental sessions, each consisting of one hour of cycling at Fatmax intensity
• Receive caffeine or placebo via capsules, coffee, chewing gum, or mouth rinse according to a randomized, double-blind, crossover design
• Be monitored for fat and carbohydrate oxidation, total energy expenditure, heart rate, and perceived exertion during exercise
• Report any side effects immediately after the session and 24 hours later using a structured questionnaire

All sessions will be conducted under controlled laboratory conditions with standardized protocols to ensure safety and reliable measurements.

Inclusion Criteria:

• Healthy men and women aged 18-50 years
• Regular aerobic exercise at least 1 hour per day, 2 days per week
• Provide written informed consent

Exclusion Criteria:

• Smoking
• Use of medication or ergogenic supplements during the study period
• Caffeine intolerance or allergy
• Cardiopulmonary or musculoskeletal disorders
• Use of oral contraceptives in female participants

De-identified individual participant data underlying the results reported in publications may be made available upon reasonable request. These data may include anonymized metabolic, ventilatory, cardiovascular and perceptual variables collected during the study (e.g., fat oxidation rate, energy expenditure, VO₂, VCO₂, heart rate, capillary blood lactate concentration, and ratings of perceived exertion). Any data sharing will be subject to ethical approval, compliance with applicable data protection regulations, and approval by the study investigators.

De-identified individual participant data may be shared after the publication of the main study findings and will remain available for up to three years following publication.

Access to any de-identified individual participant data will be considered on a case-by-case basis. Requests will require a scientifically sound proposal and may be subject to additional ethical review and data use agreements, in accordance with institutional and legal requirements. The study investigators reserve the right to decline requests.