This prospective, single-blind, randomized controlled trial investigates the comparative effects of plant-based protein (soy and rice blend) versus animal-based protein (whey) supplementation on body composition parameters in elite male youth soccer players. Twenty-five players (ages 16-18.7 years) from Udinese Calcio's Primavera (U-19) category are randomized into three groups: Plant Protein Group (n=8), Animal Protein Group (n=9), and Placebo Group (n=8). The intervention consists of post-training protein supplementation (17g protein/dose) administered within 15-30 minutes after each training session over a 4-week microcycle during the competitive season. Body composition is assessed using a multi-compartmental model including anthropometry (ISAK protocol), bioelectrical impedance analysis (BIA), and dual-energy X-ray absorptiometry (DEXA).

BACKGROUND AND RATIONALE Protein supplementation is widely used in elite sports to support muscle protein synthesis and recovery. While animal-based proteins (particularly whey) have traditionally been considered superior due to their complete amino acid profile, plant-based proteins have gained attention for ethical, environmental, and health reasons. However, limited evidence exists comparing their efficacy in elite youth athletes during competitive periods with high training loads. This study addresses this gap by comparing isocaloric, iso-nitrogenous plant-based and animal-based protein supplements on body composition in elite male youth soccer players during a 4-week competitive phase.

STUDY DESIGN Prospective, single-blind, randomized controlled trial with three parallel arms (1:1:1 allocation). Participants are unaware of group assignment and cannot identify supplement content due to identical packaging and delivery methods.

POPULATION Male youth soccer players (ages 16-18.7 years) from Udinese Calcio's Primavera (U-19) category during the 2025-2026 season, maintaining regular training schedules (minimum 4 sessions/week) combining technical-tactical field training and strength conditioning.

INTERVENTION

Participants are randomized to three groups:

• Plant Protein Group (n=8): 330ml soy beverage + 22g plant protein powder = 17.4g total protein
• Animal Protein Group (n=9): 200ml rice beverage + 33g whey protein (22g neutral + 11g vanilla) = 17.3g total protein
• Placebo Group (n=8): 200ml rice beverage + vanilla flavoring = 0.2g total protein

Supplements are administered post-training (15-30 minutes) under direct nutrition staff supervision. Consumption is verified before participants leave the recovery area.

SUPPLEMENT PREPARATION Prepared 8 hours before consumption (09:00h) in individual labeled containers. Liquids measured with precision graduated containers, protein powder added via funnel (11g scoop). Containers manually shaken, refrigerated at 4°C, and re-shaken before consumption (17:00h).

BLINDING Single-blind design: participants unaware of three distinct groups; all containers and delivery identical. Nutrition staff aware of assignments (necessary for preparation and safety).

TRAINING CONTEXT Intervention occurs during competitive season with official matches every Saturday. Training microcycle features undulating workload with high-volume sessions (field + gym) on Tuesdays and Wednesdays. Following baseline assessment, players receive general nutritional guidelines but no strict dietary control (maintains ecological validity).

BODY COMPOSITION ASSESSMENT Multi-compartmental model maximizes precision. Participants maintain 8-hour fast and perform last urination 30 minutes before 07:00h assessment.

Assessment sequence:

1. Bioelectrical Impedance Analysis (BIA 101 BIVA PRO): Verifies stable hydration via Bioelectrical Impedance Vector Analysis (BIVA)

2. Anthropometry (ISAK protocol): International Society for Advancement of Kinanthropometry standardized restricted profile. Same certified evaluator performs all measurements on right body side:

   • 8 skinfolds: triceps, subscapular, biceps, iliac crest, supraspinal, abdominal, front thigh, medial calf
   • 3 circumferences: relaxed arm, mid-thigh, calf
   • Body fat via Faulkner formula: %FAT = [(triceps+subscapular+suprailiac+abdominal) × 0.153 + 5.783]

3. Dual-Energy X-ray Absorptiometry (DEXA): Gold standard providing detailed fat mass, lean mass, bone mineral density, and regional distribution analysis

Assessment timeline: Baseline (Week 0) and Final (Week 4) with identical protocols.

STATISTICAL ANALYSIS Primary analysis: One-way ANOVA (or Kruskal-Wallis if non-normal) comparing mean fat-free mass changes between groups. Post-hoc tests (Bonferroni/Tukey HSD) for pairwise comparisons if significant. Secondary analyses use similar approaches. Intention-to-treat includes all randomized participants; per-protocol includes participants with ≥80% adherence. Baseline values, training load, and dietary intake may be covariates in ANCOVA models. Significance: α=0.05 (two-tailed).

SAMPLE SIZE N=25 (8-9 per group) based on feasibility given specialized population. This pilot study may detect moderate-large effects (Cohen's d≥0.6-0.8) using sensitive multi-compartmental methods.

SAFETY MONITORING Participants instructed to report adverse effects immediately. Regular check-ins during training, standardized adverse event documentation, classification by severity and relationship to intervention. Medical staff available during all sessions.

STUDY OBJECTIVES Primary: Compare effects of plant-based vs animal-based vs placebo protein supplementation on lean body mass changes in elite male youth soccer players over 4 weeks during competitive season.

Secondary: Compare effects on body fat percentage, total body mass, regional body composition, individual skinfold measurements, circumferences, adherence rates, and adverse events.

Inclusion Criteria:

• Active contract with Udinese Calcio's Primavera (U-19) category during the 2025-2026 season
• Regular participation in the team's technical-tactical and strength training program (minimum 4 sessions per week)
• Signed informed consent (parental consent for participants under 18 years)
• Commitment to adhere to measurement protocol requiring 8-hour fast and assessment at 07:00h
• Commitment to adhere to post-training supplementation protocol with administration at 17:00h
• Confirmed presence and availability for both baseline assessment (Week 0) and final assessment (Week 4)

Exclusion Criteria:

• Open injury records or currently in functional readaptation phase at study initiation
• Consumption of ergogenic or nutritional supplementation independently and external to the club-supervised protocol during the 4 weeks prior to study or during the study period
• Previous diagnosis or current symptomatology compatible with eating disorders
• Known allergies or intolerances to soy, rice, dairy-based proteins, or vanilla flavoring
• Current use of medications that may affect body composition or protein metabolism (anabolic steroids, growth hormone, corticosteroids, thyroid hormones)
• Diagnosed chronic diseases that may affect protein metabolism or body composition (diabetes mellitus, chronic kidney disease, liver disease, malabsorption syndromes)

Individual participant data will be available immediately following publication of the primary manuscript and will remain available for at least 5 years. Supporting documents (study protocol and statistical analysis plan) will be available indefinitely.

De-identified individual participant data will be shared with researchers who provide a methodologically sound proposal for academic purposes. Requests should be directed to the corresponding author at antoniomolina@centrosnutrifarma.es. Requestors will need to sign a data access agreement and specify the intended use of the data. Data will be provided in anonymized CSV format via secure file transfer within 30 days of approval.

Participants are blinded to group assignment and the existence of three distinct intervention groups. All supplements are prepared and delivered in identical containers. Nutrition staff administering supplements are not blinded (necessary for proper preparation).