Effects of Protein Supplementation During High-Intensity Functional Training on Physical Performance in Recreationally Trained Males and Females: A Randomized Controlled Trial

Background/Objectives: High-intensity functional training (HIFT) combines multijoint aerobic and resistance exercises. Despite its popularity, limited research has investigated dietary or supplementation strategies to enhance adaptations to HIFT. Hence, this study aimed to examine the effects of egg white and whey protein supplementation during HIFT on physical performance in trained individuals. Methods: Thirty recreationally trained volunteers (20 males, 10 females), aged 23–55, underwent 6 weeks of HIFT (three times/week) while receiving 0.6 g/kg/day of egg white protein, whey protein, or maltodextrin (placebo) in a researcher-blinded, randomized, triple-crossover, and counterbalanced design, with 2 weeks of washout between supplements. Participants followed isoenergetic diets providing 1.0 g/kg/day of protein. Before and after each intervention, VO₂max, the maximal strength (1 RM) and force–velocity relationship of shoulder press, the peak torque and strength endurance of knee extensors and flexors, and the strength endurance of core muscles were measured. The training session load was monitored during each intervention period’s first and last weeks. Data were analyzed by three-way ANOVA (supplement × time × sex), with repeated measures on supplement and time. Results: The duration, energy expenditure, training load score, and cardio load of each training session increased from the beginning to the end of each training period by 2–11% (p < 0.05). The 1 RM of shoulder press and strength endurance of core muscles increased by 3–6% (p < 0.001). Protein supplementation did not affect any of these outcomes. Conclusions: Short-term HIFT improved exercise capacity, upper-body strength, and core endurance. However, increasing protein intake from 1.0 to 1.6 g/kg/day did not further enhance performance.