| dc.description.abstract | Dietary protein is essential to support whole-body and tissue specific turnover, maintenance, and adaptation over time. Regarding skeletal muscle tissue, the protein synthetic response following dietary protein ingestion is thought largely to be dictated by the postprandial plasma aminoacidaemic response. Although this is well established, the evidence is almost exclusively extracted from studies utilising animal-derived proteins, and the role of (alternative) dietary protein source has received comparatively little attention. This is particularly striking given we are facing a global food transition involving the inevitable decrease in animal-based food production and the increased utilisation of more sustainably produced non-animal-derived protein sources that is required to meet increasing future protein demands for our growing (and ageing) global population. To fill this knowledge gap and to assess the impact of alternative dietary protein sources on muscle anabolism, this thesis aimed to comprehensively assess the potential, capacity, and therefore quality, of novel alternative protein sources to support skeletal muscle remodelling in various populations.
Firstly, the current state of evidence (at the outset of this PhD) on the environmental sustainability credentials and anabolic potential across a range of alternative protein sources derived from plants, fungi, and algae was extensively reviewed, and examined using a novel hierarchical framework, which was designed to comprehensively assess in vivo protein quality (Chapter 3). This approach revealed a lack of evidence on the anabolic properties of most alternative protein sources to support muscle anabolism across young and older populations. To address this, a series of experimental studies were conducted in different populations to identify and characterise the anabolic credentials of various alternative protein sources. Firstly, utilising a scoping investigation to provide maximum information, Chapter 4 examined plasma amino acid (AA) and insulin responses following the ingestion of 30 g protein from novel plant- (pea, lupin) and microalgae-derived (chlorella and spirulina) protein sources, in comparison to protein matched boluses of high-quality animal- (milk) and non-animal-derived (mycoprotein) comparators, in both young and older adults. Protein ingestion elicited divergent circulating AA and insulin responses, with superior responses observed for pea, spirulina, and mycoprotein, and inferior responses for chlorella, implying divergent digestive behaviour and anabolic potential across alternative protein sources. Accordingly, pea and spirulina were identified as promising high-quality protein sources, followed by lupin, and lower anabolic credentials were attributed to chlorella. Moreover, across protein sources, systemic aminoacidaemic and insulinemic responses were inferior in older adults, implying that advancing age requires special consideration in future work examining alternative proteins.
Subsequently, utilising a less broad, but more in depth approach, the capacity of these novel alternative proteins to support skeletal muscle anabolism was explored in multiple Chapters. First, in Chapter 5, postprandial myofibrillar protein synthesis (MyoPS) rates in resting and exercised tissue were assessed after the ingestion of 25 g protein from microalgae with high (spirulina) and lower (chlorella) anabolic credentials, in comparison to an isonitrogenous dose of a high-quality non-animal-derived comparator (mycoprotein), in healthy young adults. In line with the previous findings, postprandial plasma aminoacidaemic responses were superior for spirulina and mycoprotein when compared to chlorella. However, postprandial MyoPS rates increased in both rested and exercised tissue in all groups, with no differences between protein sources, and to greater extent in exercised compared with rested muscle. Therefore, despite remarkably divergent plasma AA responses, these data are the first to identify microalgae as a novel and viable alternative protein source to support acute skeletal muscle remodelling. Though this work provided foundational information on the anabolic credentials of alternative protein, questions remained whether such protein could be used to generate optimal anabolic responses. Accordingly, Chapter 6 examined the utility of plant-based protein blends to optimise resistance exercise induced increases in MyoPS. Following a bout of bilateral lower-body resistance exercise, resistance-trained adults consumed 32 g protein from a novel plant-protein blend containing isolates from pea, brown rice, and canola, or an isonitrogenous dose of whey protein isolate. Despite plant-blend protein ingestion elicited an inferior plasma aminoacidaemic response, postprandial, post-exercise MyoPS rates increased rapidly and remained elevated throughout the entire postprandial period, with no differences between protein sources at all timepoints. Consequently, these data provide high translational value for resistance-trained athletes by demonstrating the utility of plant-protein blends to optimise postexercise skeletal muscle remodelling.
These findings and implications of the acute experimental Chapters were subsequently implemented into a longer-term trial investigating the feasibility and utility of a vegan diet to support skeletal muscle remodelling in healthy older adults. In Chapter 7, daily, free-living MyoPS rates were assessed during a 3-day habitual period, and after the adoption of a 7-day high-quality vegan diet providing the Recommended Daily Allowance (RDA) (0.8 g·kg-1 bm·day-1) of protein, in comparison to an isonitrogenous omnivorous diet, in rested and exercised muscle. Despite the amount of dietary protein intake decreased during the dietary intervention period, daily MyoPS rates in rested tissue remained unaltered when compared to the habitual period. Moreover, daily MyoPS rates increased by ~15% in the exercised leg, and were ~26% higher in exercised compared to rested tissue, and no differences between diets were observed at all timepoints. Therefore, these data provide direct translational value by demonstrating that carefully considered vegan diets do not compromise daily skeletal muscle remodelling in older adults, even at modest amounts of dietary protein intake. Moreover, these findings demonstrate that ageing muscle retains the capacity to rapidly and robustly respond to resistance exercise, thereby underscoring the importance of active ageing.
Finally, Chapter 8 examined the impact of consuming high amounts of alternative protein on athlete health beyond muscle hypertrophy. Circulating cardiometabolic health markers and micronutrient statuses were examined before and after healthy young adults adopted a 10-week high-protein (~2 g·kg-1 bm·day-1), hypercaloric (~10% energy surplus) mycoprotein-rich (~40% of total protein intake) vegan diet or an isonitrogenous omnivorous diet whilst undergoing high-volume whole-body resistance exercise training. Serum insulin concentrations and homeostatic model assessment of insulin resistance (HOMA-IR) increased in the omnivorous group only (both P<0.05), whilst the plasma lipidome and micronutrient statuses remained largely unaltered in both groups. As such, high-protein vegan diets can be adopted during high-volume resistance exercise training without compromising cardiometabolic health and micronutrient status in young athletes.
In conclusion, the present thesis comprehensively examined the feasibility and utility of a range of novel alternative protein sources to support acute and longer-term skeletal muscle remodelling, and other health implications, in young, older, trained, and untrained individuals. The findings from the present work illustrate that there are numerous alternative protein sources available that possess attractive sustainability and anabolic credentials. In addition, this thesis provides substantial evidence that individual protein sources are accompanied by unique, but distinctly divergent nutritional compositions and food structures, which substantially modulate postprandial protein handling. However, this does not necessarily reflect or compromise the acute or prolonged skeletal muscle anabolic response, and thereby the in vivo quality of alternative protein to support skeletal muscle anabolism. Accordingly, these are fundamental findings that not only significantly expand the existing data portfolio on the use of alternative proteins to support skeletal muscle remodelling across the lifespan, they also serve as the foundation for future work assessing the anabolic properties of other alternative proteins, and for refined nutritional guidelines that are required to establish a healthy, sustainable and secure food future. | en_GB |
