Introduction

Sarcopenia is defined as the age-related loss of skeletal muscle mass and function and is consistently associated with decreased quality of life, disability, co-morbidity, hospitalization, and mortality.1-7 Sarcopenia is increasingly prevalent with increased age, and data from masters athletes indicate that even in healthy, highly active individuals, muscle function may decline as early as age 30.8 Consequently, sarcopenia is estimated to impact up to 24% of adults below the age of 70 and 43–58% of individuals over the age of 80.9 Because of improvements in living conditions and health care in modern society, mortality is now driven primarily by morbidity and age-associated injuries, which are more likely in those who display low skeletal muscle mass and quality.10-12 Accordingly, the healthcare costs associated with sarcopenia are substantial, as individuals with sarcopenia are two times more likely to be hospitalized than those without, and the annual estimated cost of these hospitalizations is over $40bn in the USA alone.13 Because the global age structure is expected to shift dramatically, such that individuals over the age of 80 years will outnumber those under the age of 5 by two to one by the year 2100,14 there is a dire need to determine effective treatment strategies that are able to combat sarcopenia.

The protein content of skeletal muscle fibres is a primary determinant of skeletal muscle mass. Skeletal muscle protein is predominately regulated by the homeostatic balance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB), which becomes dysregulated in older adults to result in a more negative protein balance and, ultimately, sarcopenia.4 Notably, it does not appear that this derangement is driven by differences in basal muscle amino acid kinetics,15-17 but via a phenomenon termed anabolic resistance (AR) whereby older adults demonstrate reduced MPS responses to anabolic stimuli,18 including amino acid feedings.19 Indeed, older adults have been shown to need nearly twice the relative isolated protein dose to maximally stimulate postprandial MPS.16 Critically, evidence suggests that the potential for amino acid consumption to cause an anabolic response in skeletal muscle is dependent on nutrient delivery, which is a function of both nutritive (blood) flow and concentration. Indeed, multiple studies have demonstrated that a linear relationship exists between (skeletal muscle) blood flow and muscle fractional synthetic rates following amino acid administration.20, 21 Ageing is associated with progressive declines in macrovascular (large conduit arteries) and microvascular (arterioles and capillaries) function22 that are likely to contribute to reduced nutrient delivery and thus impairments in postprandial skeletal muscle anabolism.23-27 In summary, it is likely that AR is a primary driver of sarcopenia in ageing populations and that age-related vascular dysfunction is an important, but overlooked contributory factor to this phenomenon (Figure 1). In this review, we will (i) examine evidence of AR following amino acid administration alone and (ii) following mixed meals, (iii) provide evidence linking insulin resistance and vascular dysfunction to age-related AR, (iv) briefly review potential mechanisms of vascular dysfunction as they relate to age-related AR, and lastly (v) provide a brief overview of important considerations and future research directions. As this review will focus on the role of the vasculature following meal consumption on age-related AR, readers are encouraged to review the contributions of downstream signalling,28, 29 nutrient interactions,30 and the impact of resistance exercise on AR,31 which may be mentioned briefly but are predominantly outside of the scope of this review.

During the ageing process, there are gradual impairments in vascular function, anabolic signalling, arterial compliance, and insulin sensitivity. These impairments may ultimately lead and/or contribute to anabolic resistance, or a reduced ability to mount a muscle protein synthetic response to anabolic stimuli. Over time, anabolic resistance promotes sarcopenia, or the age-related loss in muscle mass and function, resulting in a loss of functional ability and independence in older adults.

Anabolic resistance following amino acid administration

Age-related alterations in protein kinetics are a complex, multifaceted phenomenon. Initial early work examining protein kinetics appeared to indicate that decrements in basal protein kinetics were driving age-related decrements in muscle mass.32, 33 However, these early studies neglected to also quantify MPB and utilized indirect measurements of MPS.17 More recent studies employing direct MPS measurements indicate that, despite differences in whole-body protein kinetics,34 there is little difference in basal, post-absorptive skeletal MPS rates between younger and older adults.15-17, 34, 35 Instead, divergent responses in older vs. younger adults appear to occur most dramatically during the dynamic changes following amino acid feedings or exercise, where blunted MPS responses are often observed in older adults. For example, Moore et al. investigated the amount of protein needed to maximize postprandial MPS in both younger and older adults and found that nearly twice (0.24 vs. 0.40 g/kg) and over twice (0.25 vs. 0.61 g/kg) the isolated protein dose were needed in older adults to maximize MPS when normalized to body weight and lean body mass, respectively.16 An important caveat to these data, however, is the overlap in the confidence intervals for the protein doses associated with saturated MPS responses in the older vs. younger adults, which appears to be driven primarily by large variability in older adults. Katsanos et al. investigated the effect of age on muscle protein accretion and indicated that, when given a standard dose of 7 g essential amino acids (EAAs), older adults had diminished net phenylalanine uptake (9.9 vs. 25.1 mg/leg) in the 3.5 h period following consumption.35 In a follow-up study by the same group, Katsanos et al. demonstrated that the leucine content of the amino acid mixture is likely a key variable in the subsequent MPS response. In fact, while the older adults displayed an attenuated phenylalanine net balance following consumption of a 7 g EAA dose containing 26% leucine, net balance was not different from the younger adults following consumption of a 7 g EAA dose containing 41% leucine.36 Together, these data suggest that (i) there are no real differences in basal protein kinetics between younger and older adults, (ii) older adults are able to mount similar maximal rates of MPS, but older adults exhibit AR whereby the protein synthetic response to the same submaximal amino acid dose is lower in older than younger adults, and therefore, (iii) larger absolute amino acids doses are needed to achieve similar anabolic responses to younger adults in older individuals. However, although this has been shown in multiple studies when examining amino acid intake in isolation, the results from studies examining age-related differences in the postprandial MPS response to mixed meals have been less clear. Given that most individuals are unlikely to consume amino acids in isolation during the majority of their meals, examination of age-related differences in postprandial protein kinetics following mixed meals is also extremely important.

Anabolic resistance following mixed amino acid and carbohydrate administration

In 2000, Volpi et al. reported that, despite similar basal amino acid turnover in younger and older adults, administration of an amino acid–glucose (40 g crystalline amino acids and 40 g glucose) cocktail increased MPS in younger, but not older adults, despite similar decreases in MPB independent of age.27 Thus, this work by Volpi et al. suggests that AR may actually be more pronounced when amino acids and carbohydrate are co-administered.27 However, in contrast to these findings, Kiskini et al. reported no differences between healthy older and young men in the MPS response to co-ingestion of 20 g casein protein and 40 g carbohydrates, but did observe greater hyperglycaemia and hyperinsulinaemia in the older men.37 In 2014, Gorissen et al. specifically examined whether carbohydrate co-ingestion with protein influenced muscle protein accretion in healthy younger and older men. When MPS rates were examined over a 5 h postprandial period, there were no differences observed among the younger and older men, and carbohydrate co-ingestion did not modulate the MPS response in either group. Importantly, if MPS rates were examined only at 2 h postprandially, MPS was only elevated in the younger and not older men, suggesting that the observation of AR may be, at least in part, a temporal phenomenon.38 Indeed, Volpi et al. only examined the anabolic responses during a 3 h postprandial window.27 Further, whereas the older adults in both the studies conducted by Gorissen et al.38 and Kiskini et al.37 had significantly greater insulin responses to protein and carbohydrate co-ingestion compared with the young participants, the older adults in the study conducted by Volpi et al.27 had similar postprandial insulin responses to the younger adults. Therefore, there were likely differences in the older adult populations used in these studies that may help explain the disparate results, such as in the degree of insulin sensitivity and/or AR. It is also worth noting that these studies rely on isolated nutrient sources and, although outside the scope of this review, additional work is needed to analyse amino acid and glucose kinetics following ingestion of whole food vs. isolated sources to more fully understand the impact of other bioactive compounds present in whole foods on MPS. The reader is referred to Burd et al.30 for a more in depth discussion.

Vascular function, insulin, and muscle protein anabolism

While well known for its direct actions to promote glucose disposal in skeletal muscle and adipose tissue, insulin also plays a critical role in causing the redistribution of blood flow from non-nutritive to nutritive capillary networks to improve nutrient delivery to skeletal muscle in response to nutrient consumption.39 Indeed, insulin stimulates endothelial nitric oxide (NO) production in precapillary muscle arterioles through the phosphoinositide 3-kinase–protein kinase B/AKT–endothelial NO synthase (eNOS) signalling pathway40, 41 and also regulates endothelin-1 (ET-1) synthesis and secretion from the vascular endothelium42 via the mitogen-activated protein kinase-dependent signalling pathway (Figure 2). Thus, insulin plays a central role in increasing the capillary surface area available for nutrient exchange via nutritive capillary recruitment in an endothelium-dependent manner.43 Importantly, insulin-mediated capillary recruitment occurs far prior to44 and perhaps even independently45 of changes in total limb blood flow. Insulin sensitivity decreases substantially across the age span,46 and there is evidence that the vasculature also becomes less responsive to insulin resulting in lower insulin-stimulated microvascular blood flow.39 These reductions in vascular insulin sensitivity thus have critical implications for nutrient delivery and may contribute to subsequent morbidity such as type 2 diabetes and potentially sarcopenia. Accordingly, the decrements in microvascular function seen in type 2 diabetes far precede traditional signs of the disease.47 Specifically, using Sprague Dawley rats, Premilovac et al. demonstrated that microvascular insulin resistance is antecedent and a contributor to impairments in muscle glucose uptake.48 Further, Bradley et al. demonstrated that, when the NO synthase inhibitor l-NG-nitro arginine methyl ester was locally infused during a hyperinsulinaemic–euglycaemic clamp in hooded Wistar rats, the microvascular blood flow response to insulin was completely abolished, providing evidence that NO directly mediated microvascular dilation in response to insulin.49

The impact of meal consumption in an older (left) and younger adult (right). In the older adult, there is an increased tendency for insulin to stimulate endothelin-1 (ET-1) release from the vascular endothelium, rather than nitric oxide (NO) as typically observed in healthy younger adults. Additionally, shear stress subsequent to an increase in blood flow stimulates the glycocalyx to release NO from the vascular endothelium in younger adults, whereas this effect is significantly reduced with ageing. Glucagon-like peptide-1 (GLP-1) also stimulates vasodilation through NO-dependent and NO-independent mechanisms in a postprandial state. Finally, the NO that is produced in older adults is more likely to be scavenged by overproduced and/or unregulated reactive oxygen species (ROS) (i.e. oxidative stress). Consequently, meal consumption in younger adults is ultimately more likely to cause a robust vasodilatory response, thus enhancing the anabolic potential of meal consumption via greater nutrient delivery to skeletal muscle, when compared with older adults.

Adequate nutrient delivery, which is the product of blood flow/tissue perfusion and amino acid concentration, is a key factor in determining the ability to derive a postprandial anabolic response. Because of insulin's effects on skeletal muscle perfusion, it has been well studied as a potential causal factor in AR.21, 50 Early work on the topic mainly examined the role of insulin in stimulating MPS without concomitant amino acid administration. These early studies demonstrated apparently conflicting findings regarding insulin's ability to stimulate MPS, but these are likely due to methodological differences primarily involving local vs. systemic insulin infusion that provide important mechanistic insight regarding insulin's limitations in stimulating MPS. For example, in a 2006 study examining whether local isolated insulin infusion could stimulate MPS in young healthy adults, Fujita et al. reported that both local low-dose (0.05 mU/min per 100 mL) and high-dose (0.30 mU/min per 100 mL) insulin infusion stimulated MPS to a lesser degree than an intermediate dose (0.15 mU/min per 100 mL). Specifically, during the low-dose infusion, insulin did not increase blood flow, thus limiting nutrient delivery; whereas during the high-dose infusion, insulin increased blood flow and also rapidly drove down arterial amino acid concentrations, thereby limiting skeletal muscle amino acid availability.20 Similar to what was observed in the high-dose infusion condition, several studies have shown that when insulin levels increase systemically in a physiological manner, whether by infusion or meal consumption, a marked decrease in amino acid availability occurs if additional amino acids are not simultaneously infused or consumed—an effect that is likely due to insulin signalling that is ubiquitous among the body's tissues.51, 52 Collectively, these studies demonstrate that physiological hyperinsulinaemia stimulates MPS by increasing muscle perfusion and amino acid delivery, provided amino acid availability remains adequate.20 However, when amino acid availability is allowed to decrease, the effect of insulin on MPS becomes self-limiting.51, 53

While the study by Fujita et al. was a vital step in furthering our understanding of insulin on protein kinetics, it did not include older adults. In the same year, Rasmussen et al. examined muscle protein metabolism during a hyperinsulinaemic (0.15 mU/min per 100 mL)–euglycaemic clamp via local insulin infusion and observed that, whereas younger adults experience significant increases in muscle blood flow and MPS in response to hyperinsulinaemia, older adults experience no changes in either. Importantly, the authors reported that the observed changes in MPS were highly related to changes in blood flow (r = 0.90) and leg amino acid delivery (r = 0.89).21 While the participants in the study by Rasmussen et al. were otherwise healthy, the older adults displayed a resistance to the vasodilatory effects of insulin following infusion, thus ultimately limiting nutrient delivery. Finally, Fujita et al. examined whether age-related decrements in insulin-induced muscle protein anabolism could be overcome by supraphysiological hyperinsulinaemia.54 The authors reported that in otherwise healthy older adults, a local physiological postprandial insulin dose (i.e. 0.15 mU/min per 100 mL) was unable to stimulate amino acid delivery into the muscle nor increase MPS, while a high dose (0.30 mU/min per 100 mL) was able to improve leg blood flow and induce an MPS response. Consequentially, the older adults needed roughly twice the insulin dose to stimulate vasodilation and MPS compared with a body mass index-matched, younger cohort, providing further evidence of tissue-specific, vascular insulin resistance in otherwise glucose-tolerant older adults. Thus, the contributory role of insulin to AR seems to be mainly in its ability, or inability in the presence of age-related AR, to stimulate vasodilation and thus improve nutrient (i.e. amino acid) delivery to the muscle.54

Although the previous studies provide excellent insight into the influence of isolated insulin on protein kinetics, meals are often composed of both protein and carbohydrate, thus examining the interaction between amino acid and insulin concentrations in a postprandial state is critical for a full, ecologically valid understanding in any attempt to characterize AR. Cuthbertson et al. examined the muscle anabolic response to 0–20 and 0–40 g of EAA in younger and older men, respectively, while maintaining basal insulin and glucose concentrations. The authors reported that MPS was maximized in younger men in response to just 10 g of EAA, whereas MPS increased, but remained lower in the older men even at a 40 g dose of EAA. Further, anabolic signalling responses were similarly greater in the younger than older adults in response to EAA administration.15 The postprandial plasma leucine concentration was markedly greater in the older men, perhaps signifying lower perfusion and delivery of amino acids into the muscle in agreement with the aforementioned 2006 study by Rasmussen et al.21 Similarly, Condino et al. observed that older adults had significantly greater plasma EAA concentrations 60–180 min after consuming 8 g of EAA compared with the younger adults.55 In another study examining the physiological interaction between amino acids and insulin on muscle protein metabolism in older adults, Volpi et al. observed blunted MPS responses in older adults following co-administration of an amino acid and glucose cocktail, despite observing similar postprandial hyperinsulinaemia, similar rates of muscle glucose uptake, and similar decreases in MPB.27 Consequently, the increase in net protein balance caused by amino acid and glucose administration was also blunted in the older compared with younger adults. Interestingly, leg blood flow actually decreased postprandially in the older adults, but increased in the younger adults. It is plausible that insulin may preferentially increase mitogen-activated protein kinase and ET-1 signalling and/or altered sympathetic nervous system activity in older adults, blunting the vasodilatory response to hyperinsulinaemia, although this possibility needs to be explored further. Regardless, while hyperinsulinaemia caused similar glucose uptake across age groups, there was clearly an alteration in the response of MPS and peripheral blood flow to the endogenous hyperinsulinaemia induced by glucose and amino acid co-administration,27 suggestive of impaired vasodilatory responses and an unresponsiveness of MPS to endogenous hyperinsulinaemia in otherwise healthy, glucose-tolerant older adults. Collectively, these studies indicate that older adults exhibit diminished vasodilatory responses to insulin and have similar or greater postprandial plasma EAAs compared with younger adults, perhaps suggesting age-related impairments in amino acid delivery that obstruct the anabolic effect of protein-rich feedings.

With evidence mounting that AR may be partially attributable to an inability of insulin to promote adequate perfusion of skeletal muscle, Timmerman et al. further investigated the interaction between perfusion, or endothelial function, and protein kinetics by directly controlling for blood flow in two 2010 studies.25, 26 In the first study, Timmerman et al. investigated the role of eNOS in protein kinetics by infusing either isolated insulin, or insulin combined with the eNOS inhibitor NG-monomethyl-l-arginine in 14 healthy, young adults. During the combined infusion of insulin and NG-monomethyl-l-arginine, blood flow and capillary recruitment were significantly reduced. In addition, ET-1 levels remained unchanged, whereas they significantly decreased in the isolated insulin group. Ultimately, fractional synthetic rate, MPS, and mTORC1 signalling significantly increased in response to isolated insulin but did not change during combined L-NNMA and insulin infusion. However, the combined infusion did cause a decrease in proteolysis, which remained unchanged following isolated insulin, resulting in no differences in net protein balance between the two groups.25

The second study by Timmerman et al. investigated the impact of blood flow on protein kinetics by again infusing either insulin alone or in combination with sodium nitroprusside (SNP) in older adults (71 years). SNP is an NO donor that promotes vasodilation independently of endothelial NO production.56 Strikingly, the addition of SNP to insulin elicited a two-fold increase in blood flow, a nine-fold increase in microvascular perfusion, and a significantly greater increase in Akt phosphorylation, MPS (43–129 vs. 41–53 nmol/min per 100 mL), and net phenylalanine balance (−16 to 26 vs. −17 to −2 nmol/min per 100 mL) compared with isolated insulin.26

In another study, Timmerman et al. examined whether an acute bout of aerobic exercise performed the night prior to consumption of 20 g of EAA and 35 g of sucrose could improve the postprandial anabolic response, skeletal muscle perfusion, or insulin signalling in older adults.24 The authors illustrated that EAA and sucrose consumption alone failed to stimulate an increase in MPS, conduit artery blood flow, or skeletal muscle perfusion in older adults. However, the prior bout of aerobic exercise restored nutrient-mediated increases in blood flow, muscle microvascular perfusion, and amino acid delivery and subsequently enhanced the MPS response to EAA and sucrose consumption. Interestingly, these differences were realized despite the observation of equivalent systemic insulin levels in the control and aerobic exercise conditions. Thus, the increase in skeletal muscle protein anabolism was due to an increase in microvascular perfusion and amino acid delivery subsequent to the prior bout of aerobic exercise and did not appear to be driven by an insulin-related effect, although an exercise-induced improvement in vascular insulin sensitivity cannot be ruled out. While largely outside the scope of this review, this study also describes a novel exercise and nutrient interaction and illustrates an underappreciated mechanism by which exercise may be expected to improve anabolism to nutrient consumption. When taking the aforementioned studies by the Timmerman group using either aerobic exercise24 or SNP to increase perfusion26 together, it is apparent that nutritive flow is likely a key determinant for deriving an anabolic response to amino acid consumption, providing further evidence that the vasodilatory role of insulin is potentially vital when examining age-related AR. As noted by Timmerman et al., these data support a link between muscle protein anabolism and endothelial function that is a critical component of the complex response to nutrient consumption in older adults.24 Collectively, these studies indicate that impairments in blood flow, specifically in the microvasculature, are contributing to AR and provide strong evidence for a link between muscle protein anabolism and endothelial microvascular function in older adults.

Glucagon-like peptide-1 (GLP-1) is a gut-derived, incretin hormone with insulin-independent effects that is a key regulator of postprandial glucose metabolism following a feeding.57 Additionally, in both humans and animal models, GLP-1 has been shown to augment eNOS expression58 and microvascular blood flow59 and enhance insulin secretion60 and muscle glucose uptake by enhancing skeletal muscle microvascular recruitment and blood flow independently of insulin.61 Recent evidence has emerged to suggest that GLP-1 may also augment postprandial MPS by enhancing microvascular blood flow through both NO-dependent62 and NO-independent mechanisms63 (Figure 2). For example, Abdulla et al. examined the acute effects of GLP-1 infusion on muscle protein anabolism in response to amino acid infusion during a postprandial, euglycaemic–insulinaemic clamp. Importantly, both leg blood flow and skeletal muscle microvascular perfusion were also assessed. Consistent with previous studies, the authors reported that MPS was resistant to the effects of intravenous amino acid administration under postprandial insulin conditions in older adults. However, adjuvant GLP-1 infusion rescued the MPS response to the amino acid infusion, which was observed alongside a significant improvement in skeletal muscle perfusion.23 Thus, when considering experimental designs, GLP-1 should be accounted for when executing and comparing studies examining postprandial anabolism and blood flow, as meal composition can greatly alter its response.64 Further, these data provide additional evidence to reinforce the hypothesis that (micro)vascular dysfunction plays a key role in skeletal muscle AR.

Although the results of the aforementioned studies are extremely insightful, they are all acute in nature and ultimately do not definitively demonstrate that (micro)vascular dysfunction contributes to a chronic inability to derive an anabolic response to anabolic stimuli. If skeletal muscle blood flow is a limiting factor in the ability to promote skeletal muscle anabolism in AR, individuals with a lower capacity for nutritive flow would be expected to be resistance to, and thus display impaired adaptation to, the anabolic effect of known anabolic stimuli (i.e. resistance exercise training). Moro et al. indirectly investigated this concept in 2019 by placing 19 male and female older adults, stratified by their pretraining capillary density, in a 12 week resistance training programme.65 The authors found that the individuals with low pretraining capillary density were almost completely resistant to the anabolic effect of the resistance training programme, while the individuals with high pretraining capillary density saw significant improvements in appendicular skeletal muscle mass, skeletal muscle index, leg lean mass, and type II fibre cross-sectional area. Additionally, the individuals with high pretraining capillary density showed a significant improvement in basal fractional synthetic rate, with no changes observed in the low pretraining capillary density group.65 Thus, these initial findings suggest that older individuals with a lower capacity for skeletal muscle perfusion, and thus nutrient delivery, derive less benefit from chronic exposure to a known anabolic stimulus and lend support for the role of vascular function in AR. It is clear that additional longitudinal trials are needed in the context of AR to more fully understand this relationship.

While these studies provide strong evidence that AR and vascular dysfunction occur in tandem and that improvements in tissue perfusion occur alongside improvements in muscle anabolism in older adults, not all studies have supported the relationship between blood flow and MPS. Specifically, in 2014, Phillips et al.66 indicated that muscle microvascular blood volume was not closely coupled to MPS nor did pharmacological enhancement of tissue perfusion enhance muscle anabolism. Using a between-leg, bilateral comparison model, the authors infused an amino acid and glucose mixture and then infused the vasodilator methacholine into the femoral artery of one leg. The authors reported that microvascular blood flow increased 25% following administration of the amino acid and glucose alone and 79% in the leg that also received methacholine infusion. However, the increase in blood flow was not associated with additional increases in MPS or changes in net protein balance compared with amino acid and glucose administration alone.66 On the surface, the results of this study appear to indicate that augmenting blood flow does not increase anabolism following feeding and thus that blood flow may not play a causal role in AR. However, several considerations are warranted before drawing such conclusions. For example, the study population consisted of young, healthy men, and therefore, feeding of amino acids and glucose alone enhanced blood flow and microvascular perfusion and caused a robust increase in muscle anabolism. Thus, it is highly likely that the MPS response to the feeding was saturated, even without concomitant methacholine administration. Such findings should not be readily extended to older adults, where these vascular and anabolic responses would be diminished, and dismissal of the role of vascular function (and skeletal muscle perfusion) in AR based on these data alone is unwarranted. Indeed, Mitchell et al. indicated that in response to EAA supplementation, and unlike their younger counterparts, older adults had completely absent macrovascular and microvascular responses following meal consumption, even with an equal insulin response between age groups.67

However, in a study examining the effect of supplementation with the NO precursor sodium nitrate on the anabolic response to a protein feeding in older, type 2 diabetic participants, Kouw et al. reported no augmentation of MPS despite significant elevations in both plasma nitrate and nitrite. The authors concluded that microvascular function is likely not a key contributor to protein kinetics following protein consumption.68 However, two major primary limitations to this study include the lack of a negative control group to confirm the presence of AR in their subject population and, perhaps most importantly, the lack of any blood flow or perfusion measurements. In fact, in direct opposition to a core underlying assumption made by Kouw et al., it has very recently been reported that despite significantly increasing plasma nitrate and nitrite, nitrate supplementation does not enhance skeletal muscle blood flow in the lower limbs of older adults.69 Thus, it is possible that there was no or little change in microvascular perfusion in the subjects examined in the study by Kouw et al. Lastly, Mitchell et al.70 showed that older adults consuming a combination of 3 g of arginine with 15 g EAAs elicited significant increases in microvascular blood flow without improvements in MPS when compared with EAAs alone. However, when examining the time course of both plasma EAA concentrations and changes in microvascular blood flow, it is apparent that EAA concentrations had peaked ~75 min prior to maximal improvements in microvascular blood flow. Thus, the improvement in microvascular blood flow occurred when EAA concentrations were returning near basal levels, thus limiting nutrient delivery-related impacts on the MPS response.70 Still, this study provides insight into the importance of early skeletal muscle capillary recruitment coincident with optimal amino acid availability following a meal to optimize the MPS response. This is especially apparent when juxtaposed by the previously described study of Abdulla et al., where GLP-1 was infused alongside amino acids to immediately increase both microvascular perfusion and amino acid concentrations, which robustly enhanced the postprandial MPS response in older adults.23 In sum, the studies that have examined the association of microvascular perfusion with postprandial skeletal MPS in populations who are characterized by impaired perfusion and diminished anabolism have seemingly demonstrated that augmenting microvascular perfusion does enhance postprandial MPS provided that the increase in microvascular perfusion occurs while postprandial amino acid availability is also elevated. Additional well-controlled studies are needed to provide further confirmation of this association, but this evidence supports (micro)vascular function as an important factor in the ability to derive an anabolic response to meal consumption, a factor that is often compromised in the context of ageing.

Connecting age-related anabolic resistance and vascular function

The endothelium was originally thought only to serve as a barrier between the wall of the vessel and the lumen, but is now known to be an active organ that lines the entire vascular system and plays a major role in maintaining vascular tone.71 Ageing is associated with gradual impairments in vascular function, and dysfunction at the level of the endothelium is a primary contributing factor. Endothelial dysfunction is predominantly driven by altered bioavailability and signalling of endothelial-derived signalling molecules such as NO and ET-172 (Figure 2).

Endothelial dysfunction

Nitric oxide is a potent vasodilator and likely the most well-known paracrine factor secreted from the endothelium. NO plays an important role in the maintenance of basal vascular tone.73 When stimulated by shear stress, the glycocalyx lining the wall of the endothelium initiates a cell signalling cascade resulting in NO production.74 Oxidative stress results when production of reactive oxygen species, which are otherwise vital for normal physiological function, overwhelms antioxidant capacity.75 The main sources of reactive oxygen species are nicotinamide adenine dinucleotide phosphate oxidases and the mitochondria.76 Superoxide is particularly damaging because it directly reduces NO bioavailability by scavenging NO to produce peroxynitrite. Peroxynitrite oxidizes tetrahydrobiopterin (BH4), the essential cofactor involved in NO formation, to dihydrobiopterin (BH2). Because eNOS has roughly equal affinity for BH4 and BH2, overproduction of BH2 promotes eNOS uncoupling, causing superoxide production (rather than NO) and perpetuating the process.77 Because of the role of NO in maintenance of vascular tone and inhibition of platelet aggregation, secretion, and adhesion, decreased bioavailability ultimately leads to accumulation of lipids within the artery and formation of fibrous plaque, all of which eventually lead to wall thickening and promote adverse cardiovascular events.78 Thus, endothelial function (and NO bioavailability) is an extremely important marker of overall health, as its impairment is a major contributor to the initiation and progression atherosclerosis and antecedent to traditional early signs of atherosclerosis and possibly also hypertension.79, 80 Endothelial function has also been shown to decline with age independent of traditional cardiovascular disease risk factors.81 Ageing and endothelial dysfunction have been associated with impaired angiogenic responses to vascular endothelial growth factor,82 likely due to NO's role in mediating vascular endothelial growth factor stimulated angiogenesis.83 Therefore, endothelial dysfunction results in both impaired vasodilatory function via decreased NO bioavailability and impairments in angiogenesis, both of which are major contributors to microvascular dysfunction in ageing.76 These aberrations may be particularly detrimental to skeletal muscle anabolism, due to the important, previously discussed link between anabolism and blood flow,20, 25 or, more specifically, microvascular perfusion and nutrient delivery.23, 26

Endothelin-1 is a potent vasoconstrictor that acts as a paracrine molecule at the endothelium, playing a key role in vascular tone through binding at the ETA and ETB receptors on the endothelium and vascular smooth muscle cells.84-87 Ageing is associated with the up-regulation of ET-121, 88 and augmented ET-1 mediated vasoconstriction,87, 89 and ET-1 has been shown to impair glucose uptake into skeletal muscle86 and interfere with insulin signalling in arterial smooth muscle cells.90 At low concentrations, ET-1 is also capable of potentiating the effects of other vasoconstrictive molecules, such as norepinephrine.91 Accordingly, acute antagonism of ETA/B receptors results in greater leg blood flow in older vs. younger adults (29% vs. 10%) and improved endothelial function in obese and diabetic populations when compared with insulin-sensitive individuals.87, 92, 93 ET-1 has been shown to inhibit both insulin receptor substrate 2 (IRS-2) associated insulin-stimulated phosphatidylinositol 3-kinase and p85 phosphatidylinositol 3-kinase subunit activity.90 Shemyakin et al. indicated that blocking the ETA/B receptors in vivo resulted in a 63% increase in forearm blood glucose uptake, which was further doubled upon co-infusion with insulin. As expected, ETA/B receptor blockade also resulted in 30% greater forearm blood flow. Insulin-stimulated and basal glucose uptake was also decreased upon direct incubation of skeletal muscle cells with ET-1.86 In the aforementioned 2006 study by Rasmussen et al., greater blood flow and lower ET-1 concentrations were reported at rest and during a hyperinsulinaemic–euglycaemic clamp in younger compared with older adults. Ultimately, after infusing insulin to levels commonly seen in the postprandial period, the authors reported that insulin was able to significantly elevate MPS in the younger, but not older subjects, with no changes in MPB.21 It is possible in the previous study that either the elevated ET-1 concentrations in the older adults