Decreased ability of muscles to respond to anabolic stimuli is part of the underlying mechanism for muscle loss with aging. Previous studies suggest that substantial amounts of essential amino acids (EAA), whey protein and leucine are beneficial for stimulation of acute muscle protein synthesis in older adults. However, these studies supplied only proteins, and no bolus studies have been done with dairy products or supplements that contained also fat and carbohydrates besides proteins. The aim of this study was to evaluate whether a specifically designed nutritional supplement in older adults stimulates muscle protein synthesis acutely to a greater extent than a conventional dairy product. Moreover, the combined effect with resistance exercise was studied by using a unilateral resistance exercise protocol.

Sarcopenia, the age-related decrease in muscle mass and function can result in adverse health outcomes and subsequent loss of independence. Inadequate nutrition is an important contributor to the aetiology of sarcopenia, and dietary strategies are studied to prevent or delay this geriatric syndrome.

The requirement of leucine and essential amino acids (EAA) to stimulate muscle protein synthesis increases with age. To target muscle anabolism it is suggested that higher postprandial blood levels of leucine and EAA are needed in older people. The aim was to evaluate the impact of oral nutritional supplements with distinct protein source and energy density, resembling mixed meals, on serum amino acid profiles and on gastrointestinal behaviour. Four iso-nitrogenous protein (21 g) supplements were studied containing leucine-enriched whey protein with 150/320 kcal (W150/W320) or casein protein with 150/320 kcal (C150/C320); all products contained carbohydrates (10 or 32 g) and fat (3 or 12 g). Postprandial serum AA profiles were evaluated in twelve healthy, older subjects who participated in a randomized, controlled, single blind, cross-over study. Gastrointestinal behaviour was studied in vitro by looking at gastric coagulation and cumulative intestinal protein digestion over time. The peak serum leucine concentration was twofold higher for W150 vs. C150 (521 ± 15 vs. 260 ± 15 μmol/L, p < 0.001), higher for W320 vs. C320 (406 ± 15 vs. 228 ± 15 μmol/L, p < 0.001), and higher for low-caloric vs. high-caloric products (p < 0.001 for pooled analyses; p < 0.001 for interaction protein source*caloric density). Similar effects were observed for the peak concentrations of EAA and total AA (TAA). In vitro gastric coagulation was observed only for the casein protein supplements. Intestinal digestion for 90 min resulted in higher levels of free TAA, EAA, and leucine for W150 vs. C150, for W150 vs. W320, and for C150 vs. C320 (p < 0.0125). A low caloric leucine-enriched whey protein nutritional supplement provides a higher rise in serum levels of TAA, EAA and leucine compared to casein protein or high caloric products in healthy, elderly subjects. These differences appear to be mediated in part by the gastrointestinal behaviour of these products. ClinicalTrials.gov: NCT02013466.