Milk protein supplementation and the regulation of lean tissue mass in healthy community dwelling 50-70y old women and men

Abstract

Population demographics demonstrate that the number of older individuals will triple to approximately 1.5 billion by 2050. Age-related sarcopenia is associated with a progressive loss of lean tissue mass (LTM), strength and function. Based on population specific data from the University of Limerick Body Composition (ULBC) study, the rate of decline in LTM is approximately 0.35-0.45% (0.14-0.26 kg) per annum after the age of 50 years. Interest has developed in the retention of LTM with advancing age to maintain functional and metabolic benefits that an appropriate LTM afford. Older populations display a blunted muscle protein synthetic response to the main anabolic stimuli, namely nutrient ingestion and mechanical loading. In the absence of mechanical loading, a potential dietary strategy to overcome anabolic resistance is the distribution of daily protein intake to equal amounts (25-30 g) over the three main eating occasions of the day. A 20-30 g, quality protein intake is deemed to provide an extracellular essential amino acid (EAA) concentration to stimulate muscle protein synthesis (MPS). Specific to the EAAs, leucine commands greatest importance acting as both a substrate and intracellular regulator of MPS. Augmentation of the nutrient effect is provided by mechanical loading, i.e. resistance exercise, an interaction that needs to be evaluated in the elderly in situ with optimal timing and quality of nutrient intake. Following in vitro evaluation of a milk-based protein nutrient formulation this thesis investigated the bioactive properties of oral ingestion of the nutrient formulation in vivo in elderly subjects. The nutrient formulation invoked a postprandial increase in circulating peak plasma EAA (1,848 ± 83 μM) and leucine (321 ± 22 μM) concentration. Parallel studies by in vivo skeletal muscle microdialysis (MD) confirmed that the extracellular tissue concentration of these key amino acids was similarly advanced. A 24 week RCT in a group of healthy 50-70y old community dwelling women and men was then conducted to explore the nutrient effect on the maintenance of LTM, measured by dual energy x-ray absorptiometry (DXA). For this RCT, isocaloric amounts of milk-based protein formulation (FORM) (0.33 g.kg-1.d-1) or maltodextrin (MALT) was added to the two smaller main meals of the day i.e. breakfast and lunch. Potential augmentation of the nutrient supplement was investigated by engagement in whole body, resistance-band exercise adapted specifically for this population (FORM + Progressive Resistance exercise Training (PRT)). The main outcome of the RCT was a differential effect of the composition of the dietary supplement on whole body lean tissue mass. A statistically significant between treatment, net effect amounting to +0.60 kg LTM was observed (-0.16 ± 0.87 kg in MALT vs. 0.44 ± 1.06 kg in FORM; P = 0.004) after 24 weeks. In a separate group, the treatment effect attributed to supplementation with milk-based protein augmented the net change in LTM by engagement in PRT (0.80 ± 1.02 kg), but not to a statistically significant extent (P = 0.60). In conclusion, a milk-based dietary supplement designed to correct habitual, sub-optimal, meal based protein intake in healthy, protein sufficient 50–70y old adults resulted in a positive effect on whole body lean tissue mass. The mechanism of action of this effect is informed by the investigation of the in vivo bioactivity of the milk-protein based supplement reported in this thesis