In the present study, older men ingested intrinsically labeled dietary protein to assess the subsequent incorporation of the dietary protein derived amino acids into mixed muscle protein. We compared the post-prandial muscle protein synthetic response after ingesting a single meal-like (20 g) amount of dietary protein with or without 40 g carbohydrate in healthy older men. Carbohydrate co-ingestion strongly stimulated post-prandial insulin release. However, carbohydrate co-ingestion did not augment the post-prandial use of the ingested protein derived amino acids for muscle protein synthesis during a 6 h post-prandial period in healthy older men.
Previous work suggests that co-ingestion of other macronutrients with protein may modulate the post-prandial muscle protein synthetic response [11–15, 39, 40]. In the present study, we assessed whether post-prandial muscle protein accretion following the ingestion of a meal-like amount of casein protein could be augmented by adding 40 g carbohydrate. Therefore, we applied specifically produced intrinsically L-[1-13C] phenylalanine labeled dietary protein which allowed us to assess the metabolic fate of the ingested protein towards its incorporation into muscle protein [33, 41]. Importantly, this allows us to investigate the impact of carbohydrate co-ingestion in a more physiological (i.e. bolus feeding) setting. Carbohydrate co-ingestion strongly elevated plasma insulin concentrations with peak values of 138±18 vs. 34±5 mU . L-1, in the PRO-CHO and PRO experiment, respectively. After 90 min, plasma insulin concentrations no longer differed between experimental conditions (Figure 1). In agreement with previous work [42, 43], the greater insulin response seemed to modulate the plasma amino acid kinetics (Figure 2). Carbohydrate co-ingestion attenuated the post-prandial rise in plasma amino acid concentrations, mainly the branched chain amino acids (Figure 2). Plasma [1-13C] phenylalanine enrichments were also significantly lower in the PRO-CHO vs. PRO experiment, during the first 90 min after protein ingestion (Figure 3). The attenuated rise in plasma phenylalanine concentration and [1-13C] phenylalanine enrichment following carbohydrate co-ingestion coincided with the greater rise in circulating insulin levels, implying that (muscle) tissue amino acid uptake was accelerated during the early post-prandial phase in the PRO-CHO experiment.
An insulin induced stimulation of amino acid uptake could allow a more rapid provision of amino acids available for muscle protein synthesis. In accordance, we observed a tendency for a greater incorporation rate of dietary protein derived L-[1-13C] phenylalanine into skeletal muscle protein during the first 2 hours after protein ingestion when carbohydrate was co-ingested (P=0.13; Figure 4).
The suggested impact of carbohydrate ingestion on muscle protein accretion during the early post-prandial phase was not maintained. Incorporation of dietary protein derived L-[1-13C] phenylalanine into skeletal muscle protein during the 6 h following protein ingestion did not differ between experimental conditions and averaged 0.0213±0.0024 and 0.0185±0.0010 MPE in PRO-CHO and PRO, respectively (P=0.30; Figure 4). From these data we can conclude that carbohydrate co-ingestion may accelerate, but does not modify the metabolic fate of dietary protein derived amino acids for muscle protein synthesis over a 6 h post-prandial period in older men.
Our results suggest that the post-prandial insulin response following ingestion of 20 g casein only was already sufficient to maximize insulin-mediated stimulation of muscle protein synthesis. This finding is consistent with Greenhaff et al. who demonstrated that under clamp conditions insulin concentrations of merely 5–15 mU.L-1 are already sufficient to allow maximal stimulation of muscle protein synthesis rate during hyperaminoacidemic conditions in healthy young men. However, the latter has been questioned for the older population, as insulin resistance in senescent muscle has been associated with impaired insulin stimulated endothelial-dependent vasodilation [13–15]. Therefore, it can be hypothesized that higher post-prandial insulin concentrations are required to maximize the muscle protein synthetic response to food intake in the older population. Though the present data indicate that a greater post-prandial insulin response in the older population may accelerate the use of dietary protein derived amino acids for muscle protein synthesis, this does not modulate the total 6 h post-prandial muscle protein synthetic response. Discrepancies between the present results and previously published work [11–15, 39] may be attributed to differences in study design with the other studies using a continuous supply of extra amino acids and/or carbohydrate rather than a single bolus. Nevertheless, this does not mean that the post-prandial muscle protein synthetic response cannot be further increased in the elderly population. Recently, we showed that by ingesting more protein (35 over 20 g), post-prandial dietary protein derived amino acid incorporation rates can be further improved . Though we can only speculate on the impact of carbohydrate co-ingestion and/or greater insulin release on post-prandial muscle protein synthesis following ingestion of greater amounts of dietary protein, these combined findings may be used to suggest that more protein dense meals should be consumed by elderly and/or more clinically compromised populations.
The observation that additional carbohydrate ingestion is not required to optimize post-prandial muscle protein synthesis is of important clinical relevance. With a proposed blunted muscle protein synthetic response in the elderly population, the need for proper dietary intervention is well recognized. Therefore, protein supplementation is presently being used in clinical practice to prevent or attenuate muscle loss in the older population. However, in the light of preventing excess energy intake and hyperglycemic blood glucose excursions it has been questioned what food products or specifically designed nutritional supplements would be preferred to maximize post-prandial muscle protein accretion. The present study shows that large amounts of carbohydrate are definitely not required to maximize post-prandial muscle protein accretion to the ingestion of a single 20 g bolus of dietary protein, suggesting that products high in protein and low in carbohydrate are as effective to stimulate muscle protein synthesis without increasing energy intake.
We conclude that carbohydrate ingestion may accelerate, but does not further augment post-prandial incorporation of dietary protein derived amino acids into muscle protein in healthy elderly men. This implies that ingestion of carbohydrate is not required to optimize post-prandial muscle protein accretion in the older population. This has important clinical relevance for the development of clinical nutrition for the elderly population.