**5. Conclusions**

This is the first systematic review and meta-analysis to investigate the effects of protein supplementation on arduous concurrent training adaptations. Based on the limited number of studies identified, more work in this area is clearly warranted, particularly given the importance of developing aerobic fitness and muscle strength concurrently in many exercising populations. The findings of this review sugges<sup>t</sup> that protein supplementation may be an effective strategy at supporting lean-mass accretion and muscle strength adaptations in healthy adults, whilst considering the impact that training programme duration, total energy and protein intake of participants has on outcome measures. From the existing literature, it is reasonable to recommend that individuals aim for total daily protein intakes between 1.7 and 2.2 <sup>g</sup>·kg−1·day−<sup>1</sup> whilst ingesting 20–40 g of protein immediately post-exercise to maximise MPS and support muscle strength and FFM adaptations. However, the disassociation between MPS and chronic physiological adaptations should be acknowledged. Based on the novel data included in this review, subsequent research may consider investigating the potential benefits of higher total daily protein intakes during arduous concurrent training on adaptation and performance outcomes.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/nu13051416/s1, Figure S1: Funnel plot of the comparison of the effect of protein supplementation vs. placebo on muscle strength, aerobic endurance, FFM and fat-mass adaptations. All studies are included. Figure S2: Forest plot showing the effect sizes of the studies which assessed the effects of protein supplementation on muscle strength adaptations. Figure S3: Forest plot showing the effect sizes of the studies which assessed the effects of protein supplementation on aerobic endurance adaptations. Figure S4: Forest plot showing the effect sizes of the studies which assessed the effects of protein supplementation on fat-free mass (FFM) changes in response to concurrent training. Figure S5: Forest plot showing the effect sizes of the studies which assessed the effects of protein supplementation on fat-mass changes in response to concurrent training.

**Author Contributions:** Conceptualization, S.C. and J.D.R.; methodology, S.C., J.D.R. and H.C.C.; formal analysis, S.C. and H.C.C.; investigation, S.C. and H.C.C.; data curation, S.C.; writing—original draft preparation, S.C.; writing—review and editing, H.C.C., J.D.R., A.J.R., L.S. and R.I.; supervision, J.D.R., A.J.R. and L.S; project administration, S.C.; funding acquisition, J.D.R. and L.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by UK Ministry of Defence, gran<sup>t</sup> number not applicable.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** See Supplementary Materials. Raw data available on request.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
