**1. Introduction**

Concurrent training is defined as the combination of resistance and endurance training as part of a periodised physical training model [1]. The simultaneous development of strength, power and endurance is required by many athletic and exercising populations to meet the physical demands of their chosen sporting discipline (e.g., soccer, rugby, hockey) or exercise activity (e.g., circuits, cross-fit training) [2–6]. Similarly, recruits undergoing arduous military training routinely engage in concurrent training so as to meet the training and operational demands of military life [7–13]. Military recruit training programmes are designed to transform civilians into trained soldiers, therefore, physical training is necessarily arduous, involving a combination of aerobic training, strength and conditioning, obstacle courses, swimming, circuit training and loaded marching [14,15]. Despite the requirement of concurrent training in athletic and military recruit populations, and the positive effects protein supplementation may have on training outcomes, the majority of systematic reviews and meta-analyses have focused mainly on the effects of protein

**Citation:** Chapman, S.; Chung, H.C.; Rawcliffe, A.J.; Izard, R.; Smith, L.; Roberts, J.D. Does Protein Supplementation Support Adaptations to Arduous Concurrent Exercise Training? A Systematic Review and Meta-Analysis with Military Based Applications. *Nutrients* **2021**, *13*, 1416. https:// doi.org/10.3390/nu13051416

Academic Editor: Shanon L. Casperson

Received: 19 March 2021 Accepted: 20 April 2021 Published: 23 April 2021

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supplementation when either resistance or endurance training are studied in isolation with no specific population in particular being studied [16–19].

In untrained individuals, a bout of endurance exercise upregulates muscle protein synthesis (MPS) in mitochondrial and myofibrillar proteins, whereas, a bout of resistance training elicits an increase primarily in myofibrillar protein synthesis [20]. Moreover, a period of chronic (10 weeks) endurance or resistance training refines the MPS response following exercise in proteins specific to each mode of training. Resultantly, chronic endurance training improves the oxidative capacity of muscle, which can increase whole-body oxygen uptake, leading to a more fatigue-resistant muscle, whereas resistance training develops muscle strength [21]. Both modes of exercise have been shown to increase the phosphorylation of protein in the protein kinase B-mammalian target of rapamycin-p70 ribosomal protein S6 kinase (Akt-mTOR-p70S6K) pathway, leading to an increase of MPS [20]. Indeed, studies have suggested an interference effect when both modes of training are conducted concurrently within the same training programme [22–28], however, others have disputed this interaction [29–31]. Mechanistically, endurance exercise stimulates a rise in adenosine monophosphate-activated protein kinase (AMPK) [32], which may inhibit mTOR through the activation of the tuberous sclerosis complex (TSC) [2]. This has the potential to reduce the post-exercise MPS response, and subsequently attenuate muscle strength adaptations [27] when individuals undertake concurrent endurance training [28].

MPS has been shown to be maximised when protein is consumed in 20–40 g doses immediately-post resistance training [33]. Studies have also shown that concurrent resistance and aerobic training stimulates myofibrillar protein synthesis to a similar degree compared to when resistance training is performed in isolation [34]. This response is further augmented when 25 g of protein is ingested in the immediate post-exercise period [35]. Elevated levels of amino acids in the blood upregulate the localisation and activation of mTOR by deactivating the TSC [27]. The concept of "nutrient sensing" has also been suggested, whereby other proteins such as VPS34 may be key at stimulating the mTOR pathway and myofibrillar protein synthesis in response to elevated blood amino acid concentrations [27,36,37]. As such, an elevated protein intake during arduous concurrent training may be an effective strategy for attenuating the interference effect of endurance exercise [27,28], by maximising mTOR activity and the MPS response to resistance training [33,38–43], thus supporting muscle strength adaptations. Moreover, individuals undertaking arduous concurrent training with limited recovery time between exercise sessions (i.e., military recruit training) may benefit further from strategies which elevate the amount of energy and protein in the diet to support muscle adaptations [44]. In addition to muscle endurance, military recruits are required to pass strength-based tests during basic training [7,8]. Therefore, strategies which support the development of muscle strength and/or attenuate the interference effect are likely to be advantageous, particularly when considering strength is a key determinant of occupational performance [7].

To our knowledge, no study has systematically evaluated the literature to establish the effects of protein supplementation on training adaptations during arduous concurrent training. Therefore, the aim of this systematic review and meta-analysis was to evaluate the literature on protein supplementation and its effects on adaptations to arduous concurrent exercise training in healthy individuals with potential applications to recruits undergoing military training.
