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Many authors have argued that muscle loss in older persons partly depends upon inadequate nutritional intake and/or an impaired response of skeletal muscle to nutrients, e.g. amino acids [13, 15]. By using femoral arterio‐venous balance and biopsies of vastus lateralis, the rates of muscle protein synthesis and breakdown have been measured in response to oral or intravenous infusions of amino acid mixture in young and older persons [16, 17]. When amino acids are infused intravenously, a significant increase in amino acid delivery to the leg, amino acid transport, and muscle protein synthesis is reported irrespective of age [16]. So, although protein breakdown was not modified during amino acid infusion, a positive net balance of amino acids across the muscle was achieved. These data suggest that despite an age‐related decline in muscle mass, muscle protein anabolism can be stimulated by high amino acid availability in older persons [16]. Muscle protein turnover and amino acid transport in healthy young and older people were also determined during an oral administration of an amino acid mixture. As already shown by Boirie et al. for leucine [18], amino acid first‐pass splanchnic extraction of phenylalanine was significantly higher in older persons during ingestion of amino acids, but the delivery to the leg increased to the same extent in both groups. In addition, amino acid transport into muscle cells, muscle protein synthesis, and net balance increased similarly in both the young and older persons [17]. Thus, despite an increased splanchnic extraction, muscle protein anabolism can be stimulated by oral amino acids in older as well as in young subjects as far as the dietary protein intakes or the amount of amino acid mixture administered is high enough to induce a sufficient amino acid availability for skeletal muscles. Interestingly, muscle protein synthesis increased to the same extent after an oral intake of either balanced amino acids or essential amino acids (EAAs) only in healthy older persons [19], suggesting that nonessential amino acids may not be required to stimulate muscle protein anabolism in older adults, although specific studies on which and how much of these EAAs has to be performed. From these studies it seems that the amount of amino acids/protein intake is crucial for triggering protein synthesis. Indeed, it was demonstrated that a smaller amount of protein during meal intake was not able to stimulate muscle protein synthesis as efficiently as higher doses [20]. This concept of a limited response to feeding was already demonstrated in animal studies involving old rodents [21, 22]. When amino acids and glucose are administered either orally or intravenously during an euglycemic hyperinsulinemic clamp [8, 23] an increased amino acid delivery and transport into the muscle and a decreased muscle protein breakdown occurs in both groups. However, the stimulation of muscle protein synthesis in the young subjects was not depicted in older persons despite the elevation in plasma amino acids and an even higher insulinemia [8, 23]. More recently, an impaired capacity to increase muscle protein synthesis rates in response to protein intake was confirmed in large cohorts of healthy young and older men [9]. Interestingly, the response of whole body protein breakdown was lower in older persons in response to insulin or to the meal [24–26]. The adaptation of muscle protein anabolism to hyperaminoacidemia and hyperinsulinemia seems to be due to an impaired response to insulin as demonstrated by direct infusion into the arterial side of the muscle area [27]. When specific muscle protein fractions within the skeletal muscle were separated, it was shown that the mitochondrial proteins were particularly affected by the age‐related insulin resistance [8, 28]. Investigations from Elena Volpi's group have indicated that the blunted response of protein synthesis in healthy older persons was the result of a decreased adaptation of leg blood flow to insulin [29]. Indeed blood flow is a crucial determinant of amino acid delivery and of the protein synthesis process through amino acids transport [30]. When blood flow was restored by using nitroprussiate in older persons, muscle protein synthesis was also normalized [31]. These studies lead us to open the question of muscle sensitivity to hormones like insulin or to substrates like amino acids, suggesting that a threshold needs to be reached to induce muscle protein synthesis and that this threshold may be higher in the case of inflammation (Figure 5.2). Previous studies have shown that reducing inflammation by using anti‐inflammatory agents is able to restore muscle protein synthesis in old rats [32]. It also demonstrates that substrates delivery has to be controlled by acting on the plasma availability or the blood flow. In addition, it is known that lipid intakes modulate insulin sensitivity of glucose metabolism and inflammation. In an animal study, an oleate‐enriched diet decreased inflammation markers in the plasma and in the adipose tissue of old rats [33]. However, the major effect of feeding old rats a high‐oleate diet was to restore muscle protein synthesis in response to amino acid and insulin.

Therefore, the lack of anabolic response to a complete meal in skeletal muscle is likely to contribute to the development, over the long term, of sarcopenia in older persons. The consecutive issue is whether nutrition, i.e. an adapted protein intake, could reverse the phenomenon.

Apart from dietary protein, other nutrients can play a key role in protein anabolism in older people. Mechanistically, these nutrients could help the anabolic effectors of the meal to stimulate protein anabolism. For example, vitamin D is able to increase the effects of insulin and amino acids on the rate of muscle protein synthesis. Vitamin D deficiency is very common in older population living in institutions or in the community, reaching up to 90% of older adults. The clinical relevance is that hypovitaminosis D is accompanied by adverse health events. For instance, hypovitaminosis D is associated with poorer physical performance in older adults [34]. The explanation most commonly offered is based on the possible involvement of vitamin D in muscle health and function [35]. In animals, i.e. old rats, a long‐term vitamin D deficiency resulted in decreased skeletal muscle mass and increased adiposity with a subsequent intramyocellular lipid depot, suggesting that vitamin D deficiency could exacerbate the effects of aging on muscle, i.e. sarcopenia [36]. The related lipotoxicity was shown to be detrimental for type II muscles at least, with a significant decrease in fasted muscle protein synthesis. Interestingly, vitamin D supplementation in old vitamin D‐deficient rats prevented these changes. In addition, it was recently reported that a six‐month treatment with vitamin D showed beneficial effects on appendicular muscle mass in pre‐sarcopenic older men and women [37]. With regard to the cellular mechanism, as revealed in murine C2C12 skeletal myotubes, vitamin D sensitizes the Akt/mTOR‐dependent pathway to the stimulating effect of leucine and insulin, resulting in a further activation of protein synthesis [38].

Figure 5.2 Activation of insulin signaling pathway by amino acids.