Protein supplementation, particularly after exercise, can play a significant role in enhancing fitness gains, especially in the context of concurrent training [1, 2]. Here's a breakdown of its key roles:

Muscle Protein Synthesis and Repair: Protein intake after exercise stimulates muscle protein synthesis by providing essential amino acids, which are crucial for muscle repair and growth [3, 4]. This process is activated via the mechanistic target of rapamycin (mTOR) pathway, which is essential for muscle repair and growth [4].
Offsetting Catabolic Effects: During concurrent training, which combines endurance and resistance exercises, protein supplementation can help to counteract the catabolic effects of endurance training. This allows for the promotion of muscle hypertrophy and strength gains through subsequent resistance training [4].
Supporting Muscle Recovery: Both endurance and resistance training can lead to muscle micro-trauma. Post-exercise protein provides the substrates necessary for muscle repair and promotes faster recovery, allowing for more effective subsequent training sessions and enhancing overall training adaptations [4].
Hormonal Influence: Protein ingestion has been shown to elevate the release of anabolic hormones such as insulin and growth hormone, which facilitate muscle protein synthesis and hypertrophy. This can counteract the potential catabolic effects of endurance training, promoting a net anabolic state that supports resistance training adaptations [4].
Enhancing Muscle Quality: Protein supplementation may also enhance muscle quality by promoting myofibrillar protein synthesis, increasing the density of contractile proteins within muscle fibers, which leads to more efficient force production per unit of muscle mass [4].
Cardiovascular Adaptations: Protein intake supports repair and remodeling processes that are essential for cardiovascular and mitochondrial adaptations [5]. It is necessary for the synthesis of new contractile proteins and enzymes that facilitate increased cardiac output and stroke volume [5]. Protein ingestion after endurance training can also support the formation of new capillaries and may enhance mitochondrial adaptations by promoting mitochondrial biogenesis and function [5].
Specific Findings from the Study:

A study involving untrained healthy adults performing low-volume concurrent training (LOW-HIIT and LOW-RT) found that those who received 40g of whey-based protein after each session (PRO group) showed significantly greater improvements in leg muscle strength compared to those receiving a placebo (PLA group) [1, 2].
The PRO group also showed improved 1-RM in all tested muscle groups, whereas the PLA group only improved in chest and upper back [1, 6, 7].
While both groups improved their VO2max, the improvement was larger in the PRO group, although not statistically significant [8-11].
The study concluded that protein supplementation after low-volume concurrent training particularly improves leg muscle strength in untrained individuals [1, 2, 12].
The study also notes that the observed difference in VO2 max improvement between the PRO and PLA groups, while not statistically significant, can be considered clinically meaningful, with a 1 mL/kg/min improvement being associated with a 9% reduction in cardiovascular disease related mortality [9, 11].
Important Considerations:

The majority of previous studies on protein supplementation and concurrent training have focused on athletes or trained individuals using higher-volume exercise programs [13, 14].
This study is the first to evaluate the impact of post-exercise protein supplementation on low-volume concurrent training in untrained individuals [15, 16].
While the study observed significant benefits for leg muscle strength with protein supplementation, it is possible that other muscle groups and cardiorespiratory fitness may require larger samples and/or longer interventions to show statistically significant differences [17, 18].
The study used a fixed dose of 40g of protein, which is a common practice [19, 20]. Some may argue for a dose matched to body weight [19, 20].
The study did not find significant increases in muscle mass, suggesting that 8 weeks of low-volume training may not be enough to induce substantial muscle hypertrophy [3, 4].
The study also acknowledges the absence of biochemical markers like mTOR, which would have allowed for a more comprehensive understanding of the molecular mechanisms driving the observed physiological changes [19, 20].
In summary, protein plays a crucial role in supporting adaptations to exercise, especially concurrent training, by aiding in muscle protein synthesis, recovery, and hormonal balance. While the study showed significant improvements in leg muscle strength with post-exercise protein supplementation, further research is needed to fully understand its impact on other muscle groups and cardiorespiratory fitness [17, 18].