Fasting plasma phenylalanine enrichments (tracer/tracee ratio) were similar in the moderate (young: 0.112±0.003; elderly: 0.113±0.002) and high protein groups (young: 0.101±0.008; elderly: 0.113±0.002), (p>0.05). Following meal ingestion there was an expected dilution of the labeled plasma phenylalanine pool. Mean post-prandial enrichment values in the moderate protein group (113 g beef) were 0.105±0.002 (young) and 0.105±0.004 (elderly), (p>0.05), while enrichment values in the high protein group (340 g beef) were 0.090±0.008 (young) and 0.092±0.009 (elderly), (p>0.05). As described, a correction factor was applied to account for the transient post-prandial decrease in the precursor enrichment and subsequent underestimation of mixed muscle fractional synthesis rate (20
Protein synthesis following ingestion of 113 g and 340 g of lean beef are presented in . Post-absorptive mixed muscle FSR values were similar in all groups and did not differ with age. Ingestion of 340 g of lean beef increased mixed muscle FSR increased by ~46 % (p = 0.008) in both the young and the elderly. This was consistent with the 50% increase following ingestion of 113 g of lean beef (20
). Dose- and age-specific differences were too small to be considered physiologically relevant, particularly if considered in the context of the myriad of additional factors that would influence protein synthesis in a real world setting.
Figure 2 Mean (± SEM) mixed muscle fractional synthesis rate before and after ingestions of 113 g (A), 12 or 340 g (B) of 90% lean ground beef in young and the elderly. * Significant increase from fasting (young and elderly) following 113g and 340g of (more ...)
There is little debate that the ingestion of high quality protein is of paramount importance in the maintenance of muscle mass and function in the elderly. To this end, our findings are consistent with previous work demonstrating an improved protein synthetic response to intact protein source such as whey protein, milk and beef (4
). However, in circumstances where the total ingested protein content is low (i.e., EAA content less than ~7 g), (12
) or when glucose and amino acids are co-ingested (23
) the protein synthetic response of elders may be blunted compared to their younger counterparts. These findings may have considerable practical significance should they reflect the response to the smaller, mixed nutrient meals commonly consumed by many older adults.
While a blunted protein-anabolic response to a small, mixed nutrient meal may, over-time, contribute to the development of sarcopenia (23
), there is no age-related discrepancy in muscle protein synthesis following ingestion of a higher total amino acid load (6
). In the current study, participants consumed approximately 30 g or 90 g of high-quality protein in a single serving. The key finding was that no further protein synthetic advantage was elicited by the larger meal when compared to the response to a more moderate 30g protein serving (20
). In terms of stimulating muscle growth, it therefore appears likely that under resting/non-exercising conditions, consumption of more than 30 g of protein in a single meal is not justified. Indeed, it may well be the case that a slightly smaller meal would produce a similar protein synthetic response.
The data presented in this study represent a practical extension of previous proof of concept
research that has largely focused on amino acid or whey protein supplementation (12
). Nevertheless, there are several limitations that could influence our results. Perhaps the most obvious is the fact that a single menu item, such as a serving of lean beef, is seldom eaten alone. As noted, there are some data suggesting elders may have a less robust protein synthetic response to the combined ingestion of protein and carbohydrate than their younger counterparts (23
). This has yet to be explored in the context of an actual mixed-nutrient meal, but warrants further investigation. Further, there is the potential of an added protein synthetic response if protein were to be consumed in combination with physical activity (18
In summary, a large 340 g serving of lean beef, increases mixed muscle protein synthesis by approximately 50% in both the young and the elderly. However, a moderate size portion (113 g) represents an equally effective and more energetically efficient means of stimulating muscle protein synthesis than the 3-fold larger serving. We suggest that instead of a single, large protein-rich meal, ingestion of multiple moderate-sized servings of high quality protein-rich foods over the course of a day may represent an effective means of optimizing the potential for muscle growth while permitting greater control over total energy and nutrient intake.