Keeping older muscle “young” through dietary protein and physical activity

Abstract

Sarcopenia is characterized by decreases in both muscle mass and muscle function. The loss of muscle mass, which can precede decrements in muscle function, is ultimately rooted in an imbalance between the rates of muscle protein synthesis and breakdown that favors a net negative balance (i.e., synthesis < breakdown). A preponderance of evidence highlights a blunted muscle protein synthetic response to dietary protein, commonly referred to as “anabolic resistance,” as a major underlying cause of the insipid loss of muscle with age. Dietary strategies to overcome this decreased dietary amino acid sensitivity include the ingestion of leucine-enriched, rapidly digested proteins and/or greater protein ingestion in each main meal to maximally stimulate muscle anabolism. Anabolic resistance is also a hallmark of a sedentary lifestyle at any age. Given that older adults may be more likely to experience periods of reduced activity (either voluntarily or through acute illness), it is proposed that inactivity is the precipitating factor in the development of anabolic resistance and the subsequent progression from healthy aging to frailty. However, even acute bouts of activity can restore the sensitivity of older muscle to dietary protein. Provided physical activity is incorporated into the daily routine, muscle in older adults should retain its capacity for a robust anabolic response to dietary protein comparable to that in their younger peers. Therefore, through its ability to “make nutrition better,” physical activity should be viewed as a vital component to maintaining muscle mass and function with age.

FIGURE 1

Schematic of the proposed impact of physical inactivity as a main contributor to the progression of sarcopenia (for further explanation, see text). Inactivity may be voluntary (e.g., sedentary lifestyle) or involuntary (e.g., illness or forced bed rest) but ultimately alters the trajectory away from “healthy” aging through the development of “anabolic resistance” and toward a downward spiral into frailty. Stages of sarcopenia are overlaid with the proposed schematic and correspond to the classifications outlined in the European Working Group on Sarcopenia in Older People (3): “presarcopenic,” loss of muscle mass; “sarcopenic,” loss of muscle mass and muscle strength or performance; “severely sarcopenic,” loss of muscle mass, muscle strength, and muscle performance (e.g., activities of daily living). ADL, activities of daily living.

FIGURE 2

Theoretical representation of the relation between protein intake and maximal postprandial muscle protein synthesis. “Optimal” corresponds to the amount of protein required to maximally stimulate postprandial muscle protein synthesis [i.e., corresponding to the breakpoint in the dose-response curve, as used previously with daily protein requirements (96)]. Factors that can affect the optimal intake by shifting the dose-response curve (outlined in solid lines and denoted as “healthy”) to the left (i.e., increased “sensitivity” to dietary protein) include an active lifestyle (e.g., regular exercise), greater leucine content of the ingested dietary amino acids, and/or rapidly digested proteins. Conversely, the optimal protein intake can be shifted to the right (i.e., decreased “sensitivity” to dietary protein, commonly referred to as “anabolic resistance”) by an inactive lifestyle (e.g., decreased habitual activity or complete immobility) or ingestion of “lower quality” (e.g., lower leucine content) protein (outlined in dashed lines and denoted as “resistant”).