Habitual Protein Intake and Muscle Protein Synthesis

Overview

Protein intake stimulates muscle protein synthesis. From the standpoint of maintaining skeletal muscle mass with aging, it is important to optimize the adaptive response to food intake. However, a paucity of information is available describing the effects of habitual dietary protein intake (i.e. either high or low amounts of dietary protein consumed on a regular basis), on the subsequent meal-induced stimulation of muscle protein synthesis. An adaptation to a diet of several days or weeks may involve splanchnic and/or skeletal muscle adaptations that may further enhance, or decrease, the amino acid sensitivity of muscle protein synthesis after protein ingestion. The aim of this study is to investigate the effect of a habitual (14 days) high protein diet when compared with low protein diet on digestion and absorption kinetics and the subsequent muscle protein synthetic response to dietary protein ingestion.

During the adult life skeletal muscle mass remains fairly constant until the fourth or fifth decade. Then, the slow process of sarcopenia (the age-related loss of muscle mass) is believed to begin. The maintenance of skeletal muscle mass is regulated by a balance between the opposing processes of muscle protein synthesis and muscle protein breakdown. Food intake, dietary protein in particular, stimulates muscle protein synthesis and allows net muscle protein accretion throughout the day, which allows the normal maintenance of muscle mass in healthy individuals. Many studies have described the postprandial muscle protein synthetic response to protein intake and/or physical activity, and these acute findings have led to recommendations for protein intake for both athletes wishing to gain muscle mass as well as patients and elderly individuals to help them maintaining muscle mass. However, translating the acute findings from a single meal to long-term recommendations is perhaps premature, since scientists know very little with regard to how previous consumed meals affect the anabolic responsiveness to subsequent food intake. A characteristic of the adaptation to habitual high or low protein intake is thought to be associated with a change in the amplitude of diurnal cycle of whole body proteins. If this speculation is accurate, it implies that the muscle protein synthetic responses to feeding (differences between fasting and feeding muscle protein synthesis rates) are adapting to differing habitual protein intake, which may reduce (or enhance) the anabolic responsiveness to protein intake. To gain a more complete scientific understanding, it is necessary to examine whether an adaptation does in fact occur after habitual high or low amounts of protein intake with regard to the anabolic response to subsequent protein intake. In the present investigation, we wish to investigate the impact of the habitual consumption of either high or low protein diets for 14 days on the anabolic responsiveness to a protein meal in healthy elderly. Previous work has determined that whole body adaptations to protein intake occur after \>10 days. Collectively, our findings will be valuable to maximize the skeletal muscle adaptive response to food intake and, ultimately, to develop nutritional strategies for maintenance or enhancement of skeletal muscle mass in elderly men.