*2.1. Eligibility Criteria*

This review sought peer-reviewed papers with human participants published in English with the following Population, Intervention, Comparison and Outcome (PICO) criteria being implemented to identify eligible studies [46]. The PICO was designed with

the aim of the findings of this review being applied to military recruits undertaking military training. Military recruits are typically aged between 16–35 years [47], and are required to meet aerobic fitness and muscular strength test standards such as maximal strength and muscular endurance tests [8]. Studies that did not meet all the PICO were excluded from this review.

Population: (a) stated as healthy active male or females; (b) aged between 16–35 years.

Intervention: (c) include both endurance/aerobic training and resistance/weight training, circuit training, cross-fit training, military training but not high-intensity interval training (HIIT); (d) daily protein supplementation included but not with vitamins and/or antioxidants or through an increased intake of whole-food protein sources in the diet; (e) studies assessing body composition and/or performance were required to have ≥two sessions per week and be ≥four weeks in duration; (f) studies assessing muscle recovery were required to be <one week in duration; (g) training sessions performed at moderate or vigorous intensity (e.g., jogging, running, cycling, weight training) [17].

Comparison: (h) changes in outcome measures across repeated timepoints; (i) participants grouped by supplement condition.

.

Outcome: (j) change in primary variable(s): maximal oxygen uptake ( V O2max), time trial (TT), one-repetition maximum (1RM), fat-free mass (FFM), fat mass, musculoskeletal injury (MSKI) incidence, muscle function/soreness/damage.

### *2.2. Search Strategy and Study Selection*

The final electronic database searches were completed in February 2021 in three databases (PubMed, Web of science and SPORTDiscus) using the terms "protein" or "protein supplementation", "training", and "concurrent training"" either alone or concurrently. The reference lists of all papers that met the inclusion criteria were interrogated to identify additional studies not found in the electronic search, until no further studies could be identified [48]. First, the study title and abstract were screened by one reviewer (SC) followed by the full text by the same reviewer. The characteristics that were extracted from each study included: author, participant sample, total protein intake (g·kg−1·day−1), training intervention, protein timing and dose. The study selection process is outlined in Figure 1.
