Novel Nutritional Strategies to Preserve Skeletal Muscle Health in Ageing

Background: Sarcopenia is a condition characterised by diminished skeletal muscle mass and function, and it is most common amongst older adults. One of the critical mechanisms involved in sarcopenic muscle loss is anabolic resistance, whereby older adults experience a blunted muscle protein synthetic response to dietary protein ingestion and resistance exercise compared to younger adults. Strategies to combat sarcopenia are urgent given the world growing elderly population. While whey protein amino acid bioavailability and derived muscle protein synthesis response in older persons is well understood, the bioavailability of plant proteins and their combinations are poorly characterised. Further, plant-based proteins are generally considered less effective for stimulating muscle protein synthesis than animal-based proteins due to their reduced digestibility and suboptimal indispensable amino acid profile, yet they have a lower environmental footprint. Aim: This thesis explored three nutritional strategies to improve protein bioavailability to potentially benefit muscle protein synthesis and accretion in older adults. Strategies investigated included the addition of long-chain polyunsaturated fatty acids (LC PUFA) to leucine-enriched protein as a way of modulating plasma lipids and their mediators, oxylipins (Study 1), probiotic supplementation to enhance plant protein digestibility (Study 2), and optimising plant protein fibre products’ amino acid profile to reach the quality of whey protein, known as the gold standard for muscle protein synthesis stimulation (Study 3).Methods: The main methods employed included plasma oxylipins and amino acid quantification, fractional synthetic rate analysis in skeletal muscle, and 16s rRNA gene sequencing for microbiome analysis.
Results: Our findings show that plant protein ingestion results in lower and slower plasma indispensable amino acids and leucine peaks in circulation than whey protein in healthy older adults, even when matched for leucine content. Probiotic supplementation did not improve plant-protein plasma AA appearance or muscle protein synthesis measured over two days in healthy older adults. Furthermore, correlations existed between baseline n-6 and n-9 PUFA-derived oxylipin concentration and markers of skeletal muscle biology, although these were no longer present after 24 weeks of supplementation with LC n-3 PUFA in community-dwelling older adults at risk of sarcopenia. Discussion and conclusion: Contrary to our hypotheses, none of the strategies studied enhanced protein quality nor benefited muscle mass or strength. This thesis does not support the use of these strategies, in their current form, to ameliorate sarcopenia in older adults. Instead, future research will need to further optimise plant-based protein and/or microbiome approaches relating to ageing and nutrition to better understand their effect on short- and long-term muscle health in older adults, especially those malnourished and/or anabolic resistant, and to develop ways of incorporating alternative protein sources in foods designed for older adults’ consumption.