THE ROLE OF DIETARY PROTEIN IN THE TREATMENT OF SARCOPENIA

The following are important excerpts from The American Journal of Clinical Nutritionfrom May of 2008, and will formulate a basis for my continued research in the area of preventative health care. 

Sarcopenia is a complex, multifactorial process facilitated by a combination of voluntary and involuntary factors including the adoption of a more sedentary lifestyle and a less than optimal diet (1-2)..After reaching a peak in early adult years, skeletal muscle mass declines by ≈0.5–1.0%·y−1 beginning at about 40 y of age.
— Douglas Paddon Jones et al.
However, a breakpoint can occur when a previously asymptomatic individual experiences an injurious event or is acutely/temporarily disabled (3). In such instances, the loss of skeletal muscle is accelerated and may rapidly facilitate a debilitating loss of functional capacity.
— Douglas Paddon-Jones et al
Chronic muscle loss is estimated to affect 30% of people older than 60 y and may affect >50% of those older than 80 y (4). Sarcopenia is associated with a 3 to 4 fold increased likelihood of disability, which in turn is associated with substantial socioeconomic and health care spending. One analysis estimated that in 2000, sarcopenia was responsible for $18.5 billion in health care costs (5). As the number of people older than 65 y continues to increase, sarcopenia will become an increasingly important public health concern. According the Census Bureau, by the year 2025, the elderly population in the United States is expected to be ≈80% greater than the number in 2000. (6)
— Douglas Paddon-Jones
There are compelling data supporting the efficacy of physical activity and resistance exercise in particular in maintenance of muscle mass and function in aging populations (7 - 10)...however...in many older populations the ability to exercise is compromised by physical disability, frailty, or disease (3). In such instances, targeted control of daily protein consumption and dietary derived amino acids represents one of the few remaining alternatives to slow or prevent muscle protein catabolism
— Douglas Paddon-Jones et al
Moderately increasing dietary protein intake above the Recommended Dietary Allowance may enhance muscle protein anabolism and reduce the progressive loss of muscle mass with age (11, 12). As noted, the beneficial effects of resistance exercise in aging populations are unequivocal. However, the interactive effects of protein supplementation and resistance exercise on muscle mass and function in aging populations are less clear. (13 - 15)
— Douglas Paddon-Jones et al
36 older men and women underwent 12 wk of resistance training in association with a lower protein (0.9 g protein·kg−1·d−1) or higher protein (1.2 g protein·kg−1·d−1) intake. Although all outcome measures improved (eg, increased strength, greater whole-body protein accretion, and reduced fat mass), there were no significant differences between the lower-protein and higher-protein groups...Similarly, (13) Andrews et al suggested that total daily protein intake (1.35 versus 0.72 g protein·kg−1·d−1) does not affect lean mass gains in the context of postexercise protein supplementation. Nevertheless, it has been demonstrated that mixed macronutrient supplementation in frail elderly individuals can result in a compensatory reduction in voluntary food intake and without concomitant resistance exercise may not improve muscle mass or strength. (7)
— Douglas Paddon-Jones et al
Muscle deposition occurs in response to a complex interplay among stimuli such as physical activity and hormonal signaling. However, in all circumstances the prerequisite for muscle protein synthesis is dietary derived amino acids... Insufficient or ineffectual protein intake in elderly individuals may facilitate the loss of muscle by blunting muscle protein synthesis and thus promoting net muscle protein catabolism (16-18). Although there are some reports that basal muscle protein synthesis declines with age (19-20), several other studies disagree (21-23), and current opinion is that in most healthy older people, muscle protein synthesis in the postabsorptive state is similar to that of younger people.
— Douglas Paddon-Jones et al
Increasing evidence points to a blunted anabolic response after a mixed nutrient meal as a likely explanation for muscle loss over time (16, 21, 24). In a recent study examining myofibrillar and sarcoplasmic protein synthesis in young and elderly volunteers after amino acid ingestion, postabsorptive rates of protein synthesis were the same in both age groups, although the elderly demonstrated less anabolic sensitivity to the amino acids.
— Douglas Paddon-Jones et al
In addition to insufficient protein intake, reduced sensitivity to the anabolic action of insulin and amino acids may precede overt changes in skeletal muscle mass (16, 24-26). Furthermore, the reduced vasodilatory response of older muscle to insulin may reduce anabolism by decreasing nutritive blood flow and precursor availability...when carbohydrate is added to an amino acid supplement (24), elderly individuals respond with a diminished anabolic response compared with that of their younger counterparts.
— Douglas Paddon-Jones et al
aging may be associated with reduced anabolic efficiency in response to a normal mixed nutrient meal (16, 24, 25). Ultimately, these data indicate the need to identify nutritional strategies that maximize the stimulation of muscle protein synthesis in elderly individuals. In all likelihood, any such recommendation would take into account the interactive effects of hyperinsulinemia and hyperaminoacidemia and address the amount and quality of protein ingested during each meal rather than the total daily protein intake.
— Douglas Paddon-Jones et al
others have suggested that a moderately higher protein intake of 1.0-1.3 g·kg−1·d−1 may be required to maintain nitrogen balance and offset a potentially lower energy intake, decreased protein synthetic efficiency, and impaired insulin action in elderly individuals (11, 16, 21, 27, 28).
— Douglas Paddon-Jones et al
Very-high-protein diets (>45% energy) have been associated with a host of adverse events, including nausea, diarrhea, increased calcium excretion from diets high in sulfur-containing amino acids, and increased morbidity (29, 30).
— Douglas Paddon-Jones et al
In addition to the quantity of protein ingested, there appears to be subtle inherent differences in the ability of different protein sources to promote muscle protein synthesis. These differences appear to be governed by 2 key determining factors. First, and perhaps most important, is the essential amino acid content of a protein, in particular leucine, that serves as the primary determinant of its anabolic potential (31, 32, 33). Specifically, in terms of muscle protein synthesis, the addition of nonessential amino acids to an essential amino acid supplement does not provide an additional stimulatory effect. Second, differences in digestibility and bioavailability of certain protein-rich foods may influence muscle protein synthesis (33, 34, 35). For example, the combination of fast (whey) and slow (casein) proteins found in milk may provide a greater increase in muscle protein synthesis than an isonitrogenous soy beverage after resistance exercise in young individuals (36, 37).
— Douglas Paddon-Jones et al
There have been a large number of proof of concept studies indicating that free-form essential amino acids and whey protein supplements promote muscle protein synthesis in healthy younger and older people (38). Of greater practical importance, however, is determining whether common protein-rich foods can stimulate muscle protein anabolism in older people. Recent data suggest that a moderate 113 g (4 oz) serving of an intact protein (ie, lean beef) contains sufficient amino acids (30 g total; 10 g essential amino acids) to increase mixed muscle protein synthesis by ≈50% in both young and elderly men and women (21).
— Douglas Paddon-Jones et al
Although targeted amino acid supplementation may indeed be beneficial in cases involving accelerated protein catabolism (eg, advanced sarcopenia, cachexia, and trauma) (39), for the majority of older adults the most practical means of increasing skeletal muscle protein anabolism is to include a moderate serving of protein of high biological value during each meal.
— Douglas Paddon-Jones et al
evidence that a higher protein intake may facilitate a greater decline in renal function in those with modestly impaired renal function (40)...In higher-risk groups, including those with existing renal disease, high-protein diets should be avoided (41).
— Douglas Paddon-Jones et al
Aaron TanasonComment