| dc.description.abstract | Dietary protein is essential to support the turnover and reconditioning of the tissues in the body. Of particular interest is skeletal muscle tissue, owing to its critical role in locomotion, significant contribution to whole-body protein turnover, myriad roles in substrate metabolism and malleability in response to diet and exercise. There is a substantial evidence base to demonstrate that protein ingestion increases muscle protein synthesis rates. The extent to which a protein can increase muscle protein synthesis is dependent upon its amino acid composition, digestibility, and ability to incite rapid and large increases in plasma amino acid concentrations. It is by these three factors that we judge the quality of a protein. There is now a wealth of evidence to demonstrate that protein quality can vary across various protein sources. However, little consideration has been given to how protein form (that is, the variation in which we consume the same protein) can modulate protein quality. Indeed, the majority of research that underpins our understanding of protein nutrition and human protein metabolism has been performed using isolated protein sources. However, the protein in our diet is ingested as wholefoods, as part of whole meals that have been cooked, processed, and prepared in a variety of ways. These factors have largely been ignored with respect to their impact on postprandial protein handling, yet all are relevant to virtually every meal that we consume. Therefore, this thesis set out to investigate how modulating protein form alters various aspects of postprandial protein metabolism.
I directly assess the role of the wholefood matrix of mycoprotein in its capacity to stimulate muscle protein synthesis. Comparing leucine matched boluses of mycoprotein (MYC; 31.5 g protein, 2.5 g leucine) and protein concentrated from mycoprotein (PCM; 28.0g protein, 2.5 g leucine), I demonstrate that the wholefood matrix of mycoprotein does not contribute to its anabolic potential, reporting equivalent (P>0.05) stimulation of muscle protein synthesis across conditions in both rested (MYC, Δ0.031±0.007%·h−1 and PCM, Δ0.020±0.008%·h−1) and exercised (MYC, Δ0.057±0.011%·h−1 and PCM, Δ0.058±0.012%·h−1) muscle. Interestingly, the equivalent stimulation of protein synthesis was observed despite plasma essential amino acid and leucine concentrations increasing more rapidly (both 60 vs 90 min; P<0.0001) and to greater magnitudes (1367 vs 1346 μmol·L−1 and 298 vs 283 μmol·L−1, respectively; P<0.0001) following consumption of PCM. These data contribute to the growing narrative that postprandial leucinemia is not a strong determinant of muscle protein synthesis when ingesting protein in wholefoods.
I compared post-exercise muscle protein synthesis rates following ingestion of mycoprotein (MYC), pea protein (PEA) and a mycoprotein/pea protein (39/61%) blend (BLEND). Plasma essential amino acid and leucine concentrations increased following ingestion of all protein sources, but more rapidly in BLEND (30 and 30 min, respectively) and PEA (45 and 45 min, respectively) compared with MYC (90 and 90, respectively) (time x condition interaction; P<0.0001). However, despite these differing plasma amino acid responses, I report equivalent postprandial, post-exercise muscle protein synthesis rates across all three conditions (MYC, 0.076±0.004%·h−1; PEA, 0.087±0.01%·h−1; BLEND, 0.085±0.01%·h−1). These data suggest that the methionine deficiency in pea protein does not limit its capacity to support post-exercise muscle protein synthesis rates over a 4 h postprandial period. Further, all three non-animal derived protein sources have utility in supporting post-exercise muscle reconditioning.
I investigated the role of high moisture extrusion (HME) on postprandial amino acid availability by comparing a dry blend (CON) of mycoprotein/pea protein (39/61%) and an identical blend that had undergone HME (EXT). Expecting that HME would cause protein structural changes that would impact digestion (and subsequent in vivo bioavailability), I employed a standardised in vitro model of simulated digestion to assess how these structural changes would impact protein digestibility. Protein ingestion increased plasma total, essential and branched-chain amino acid concentrations (time effect; P<0.0001), but more rapidly and to a greater magnitude in the CON compared with the EXT condition (condition x time interaction; P<0.0001), resulting in greater plasma availability of essential and branched-chain amino acids during the early postprandial period (0-150 min). These data were supported by those generated in vitro, which showed a greater protein availability with the CON compared with EXT at the outset. This resulted in greater protein digestion in the gastric phase, therefore explaining the rapid availability of protein following CON ingestion in vivo. Conversely, HME appeared to cause aggregate formation in the pea protein, resulting in lower protein availability and digestibility following the ingestion of CON. These data demonstrate that common industrial processing methods of dietary protein can drastically modulate the digestibility and bioavailability of the protein.
In summary, this thesis demonstrates that variations in protein form can modulate the factors that underpin protein quality, while also adding a layer of complexity to our understanding of how these factors regulate postprandial protein metabolism. Contained within this thesis, is a novel body of work employing multidisciplinary approaches to provide a comprehensive assessment of an often overlook concept; how dietary protein form modulates postprandial protein metabolism. | en_GB |
