The progressive loss of skeletal muscle mass with aging, or sarcopenia, has a major impact on our healthcare system due to increased morbidity and greater need for hospitalization and/or institutionalization. The age-related loss of skeletal muscle mass is facilitated by a combination of factors, which include a less than optimal diet and a sedentary lifestyle. These factors contribute to a disruption in the regulation of skeletal muscle protein turnover, leading to an imbalance between muscle protein synthesis (MPS) and degradation. One way to overcome this problem is to improve dietary intake of the elderly. It has been well established that nutrient intake greatly affects protein turnover in skeletal muscle tissue.

Ingestion of dietary protein stimulates MPS rates and inhibits muscle protein breakdown rates, resulting in an overall positive net protein balance in both the young and elderly. However, it is not clear what the impact is of co-ingestion of other macronutrients on digestion and absorption kinetics or MPS rates in the healthy young or the elderly. We have recently conducted a study to examine the impact of carbohydrate co-ingestion on postprandial MPS in the healthy young and old. Indeed, preliminary results show that carbohydrate co-ingestion stimulates protein synthesis.

Interestingly, very little is known about the impact of fat co-ingestion with protein on the stimulation of post-prandial MPS rates. What is noteworthy is that Elliot et al. investigated the effect of whole milk ingestion on net muscle protein balance after resistance exercise using an arteriovenous balance approach. Ingestion of whole milk (containing 50 en% fats) stimulated the post-exercise net uptake of phenylalanine and threonine to a greater extent than ingestion of fat-free milk (containing 6 en% fat). Although, amino acid uptake is indicative of 'muscle anabolism', it is not a direct measure of MPS so no firm conclusions can be deduce from this work. Furthermore, milk also contains a certain amount of carbohydrates (fat-free milk 55 en% and whole milk 30 en%), which does not allow for direct assessment of fat co-ingestion per se.

Certainly, other studies have investigated the effect of long term fatty acid intake, using direct incorporation methods, on the MPS rates. For example, long term omega-3 polyunsaturated fatty acid (n-3 PUFA) supplementation increased feeding-mediated MPS rates in young, middle-aged, and older adults. The mechanism(s) underpinning the enhanced effect of n-3 PUFA supplementation on post-prandial MPS rates to dietary protein are not well defined. It has been speculated that the enhanced feeding-effect of n-3 PUFA on postprandial MPS rates is due to remodeling of the sarcolemma to include a greater n-3 PUFA content, and ultimately enhances insulin's action on muscle protein metabolism. This is clearly a long term effect, but what about the acute effects of fat co-ingestion on postprandial MPS rates? Katsanos et al. found that elevated plasma fatty acid concentrations did not interfere with the post-prandial stimulation of MPS. However, subjects ingested a single bolus of essential amino acids while receiving fatty acid infusion, which clearly does not reflect a 'real world' setting. In the end, there is reason to believe that the presence of fat in a meal further stimulates the muscle anabolic response to meal ingestion. However, fat intake may also modulate gastric emptying and dietary protein digestion and absorption kinetics. To date, the acute (not long-term supplementation) impact of fat in a meal on post-prandial muscle protein anabolism and digestion and absorption kinetics remains completely unexplored, and thus we can only speculate on the impact that fat co-ingestion has on postprandial MPS rates.

In the present study we will investigate the effect of a single meal-like amount of protein with or without fat on postprandial MPS in healthy elderly men. Furthermore, we will assess digestion and absorption kinetics. The use of intrinsically labeled casein will allows us determine de novo MPS from amino acids that come available through the test beverage.