High-intensity functional training (HIFT), a new version of high-intensity interval training, has gained interest in recent years. HIFT is based on the CrossFit training template and includes multijoint movement patterns via both endurance and strengthening exercises. Research has shown positive effects of HIFT on body composition, cardiorespiratory fitness, and muscle performance of young individuals. The effectiveness of HIFT in higher ages and its protein requirements are less well documented. Protein requirements have been widely investigated in resistance training, where it has been found that protein supplementation may have additive beneficial effects on muscle strength and lean body mass. However, there is a research gap regarding the adaptations to HIFT when combined with protein supplementation.

Thus, the aim of this research is to compare the effects of egg white supplementation, whey protein supplementation (as a positive comparator), and maltodextrin (a carbohydrate as placebo) on body composition, physical performance, and plasma amino acid profile in young and middle-aged trained individuals of both sexes who perform HIFT. Participants will take all three supplements for 6 weeks each, with 2 weeks of washout (no supplementation) in between, in random and counterbalanced order. Researchers will not know the supplementation status of the participants. Participants will receive 0.6 g of protein or placebo per kg body weight daily on top of isoenergetic dietary plans to avoid differences in energy intake that might compromise the validity of the study. The dietary plans will be individualized and will provided 1.0 g protein/kg body weight/day. The HIFT protocol will include multimodal patterns of movement, combining endurance and strengthening exercises with the use of equipment such as Total Resistance eXercise (TRX), Bosu, kettlebells, and barbells.

Participants will undergo measurements of muscle strength, muscle endurance, aerobic capacity, and body composition at the beginning and end of the study, as well as during the two washout periods. Also, plasma amino acids, hematology, biochemistry, and hormones will be measured. Comparison of all these outcome measures between supplements will reveal whether protein supplementation is useful in HIFT.

Inclusion Criteria:

• Regular training (mixed endurance and resistance training 3 to 6 times a week, 50 min each session, for the past 4 months), as assessed by gym records or questionnaires
• Clearance from a pathologist or cardiologist to perform maximal exercise
• Mixed isoenergetic diet for the past 4 months

Exclusion Criteria:

• Smoking (even one cigarette or nicotine-containing device over the past 6 months)
• Any injuries to the musculoskeletal system that could interfere with the execution of training
• Chronic disease
• Egg allergy
• Milk allergy
• Pregnancy, lactation, or planning a pregnancy within the duration of the study
• Regular use of prescription medicine or supplements that might affect muscle function or recovery over the past month
• Intermittent or religious fasting
• Any vegetarian, ketogenic or protein diet

Participants are instructed not to disclose the supplement they are taking to their trainers, investigator, or outcome assessor.