The purpose of this study is to investigate the acute, separate, and combined ergogenic effects of dietary nitrate (beetroot juice, BJ) and caffeine (CAF) supplementation on discipline-specific athletic performance. The project employs a double-blind, randomized, placebo-controlled, four-arm crossover design. It evaluates two highly distinct, heterogeneous populations of athletes: (A) adult high-intensity functional training (HIFT/CrossFit/Hyrox) competitors performing the standardized Hyrox Physical Fitness Test (P'F'T), and (B) elite junior competitive swimmers (National Team members) performing a progressive 8 × 100 m swimming test. The study will assess physical performance, lactate kinetics, subjective exertion, biochemical indices, psychological metrics, and gastrointestinal tolerance.
Aged 18-45 years.
Minimum of 2 years of systematic HIFT/CrossFit/Hyrox training experience.
Performing >4 training sessions per week.
Ability to perform the HYROX Physical Fitness Test (P'F'T) and has participated in at least one official HYROX event.
Occurrence of acute musculoskeletal injury or trauma during the study period.
Any medical or health contraindications (including severe menstrual disorders, planned pregnancy, or breastfeeding).
Failure or refusal to strictly adhere to the dietary, supplementation, and exercise testing protocols.
Crossover
Caffeine
Dietary supplementAnhydrous caffeine administered orally in a hard gelatine capsule. To address the unique physiological profiles of the two cohorts, a highly specific, differentiated dosing protocol is implemented: Cohort A (adult functional fitness athletes) will receive a high-performance dose of 6 mg/kg body mass, while Cohort B (elite junior swimmers) will receive a safe, youth-adapted dose of 3 mg/kg body mass. In all experimental trials, the capsule is ingested exactly 60 minutes prior to the onset of the discipline-specific exercise test to match peak plasma caffeine concentration.
Dietary Nitrate (Beetroot Juice)
Dietary supplementConcentrated beetroot juice (BJ) administered as a single, acute organic shot of 70 mL (Beet It Sport, James White Drinks Ltd., Ipswich, UK). Each active shot contains approximately 400 mg (equivalent to \~6.4 mmol) of inorganic nitrate. In contrast to chronic loading designs, this study evaluates purely acute effects; the shot is ingested exactly 3 hours (180 minutes) prior to the physical performance test to align with peak systemic concentration of plasma nitrite.
Placebo (Maltodextrin Capsule)
Dietary supplementPlacebo capsule containing inert maltodextrin powder. The capsule is identical in size, shape, weight, color (opaque), and texture to the active caffeine capsule to ensure robust blinding. It is administered at the exact same time point (60 minutes pre-exercise) according to the identical body-mass-adjusted dosing scheme (simulating either 3 mg/kg or 6 mg/kg).
Placebo (Nitrate-Depleted Beetroot Juice)
Dietary supplementNitrate-depleted beetroot juice (Beet It, James White Drinks Ltd., Ipswich, UK) administered as a 70 mL shot. This placebo is identical in taste, smell, appearance, packaging, and viscosity to the active concentrated shot, but has been selectively filtered to contain negligible nitrate levels (\<0.4 mmol / \~25 mg of nitrates). It is ingested exactly 3 hours prior to the exercise test.
Exercise Performance Time (minutes and seconds, Cohort A - Hyrox P'F'T)
Total time (in minutes and seconds) required to complete the standardized HYROX Physical Fitness Test (P'F'T) consisting of 6 specific functional exercises.
Swimming Performance (minutes and seconds, Cohort B - 8x100m Step-Test)
Swimming performance will be assessed using the completion time (seconds and milliseconds) for each 100-m repetition of the standardized 8 × 100-m step test, including the maximal-effort repetition performed during the fifth stage.
Blood Lactate Kinetics (mmol/L)
Blood lactate concentration, including peak concentration, accumulation kinetics, and clearance rate, will be assessed from fingertip capillary blood collected before supplementation (baseline), pre-exercise (3 hours after beetroot juice supplementation and 30 minutes after caffeine supplementation), and post-exercise (3 and 7 minutes after exercise) at each experimental visit.
Heart Rate (HR, bpm)
Real-time heart rate (beats per minute) will be continuously monitored throughout each exercise test using a telemetry-based heart rate monitoring system.
Rating of Perceived Exertion (RPE, points)
Subjective rating of perceived exertion will be assessed using the validated Borg Scale (6-20).
Blood Pressure (mmHg)
Systolic and diastolic blood pressure will be measured using an automated oscillometric blood pressure monitor.
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