Portal infusion of amino acids is more efficient than peripheral infusion in stimulating liver protein synthesis at the same hepatic amino acid load in dogs

Targeting the gut to prevent and counteract metabolic disorders and pathologies during aging

Impairment of gut function is one of the explanatory mechanisms of health status decline in elderly population. These impairments involve a decline in gut digestive physiology, metabolism and immune status, and associated to that, changes in composition and function of the microbiota it harbors. Continuous deteriorations are generally associated with the development of systemic dysregulations and ultimately pathologies that can worsen the initial health status of individuals. All these alterations observed at the gut level can then constitute a wide range of potential targets for development of nutritional strategies that can impact gut tissue or associated microbiota pattern. This can be key, in a preventive manner, to limit gut functionality decline, or in a curative way to help maintaining optimum nutrients bioavailability in a context on increased requirements, as frequently observed in pathological situations. The aim of this review is to give an overview on the alterations that can occur in the gut during aging and lead to the development of altered function in other tissues and organs, ultimately leading to the development of pathologies. Subsequently is discussed how nutritional strategies that target gut tissue and gut microbiota can help to avoid or delay the occurrence of aging-related pathologies.

Prandial hepatic glucose production during hypoglycemia is altered after gastric bypass surgery and sleeve gastrectomy

AIMS/HYPOTHESIS

Roux-en Y gastric bypass surgery (GB) and sleeve gastrectomy (SG) alter prandial glucose metabolism, producing lower nadir glucose values and predisposing susceptible individuals to prandial hypoglycemia. The glycemic phenotype of GB or SG is associated with prandial hyperinsulinemia and hyperglucagonemia along with an increased influx of ingested glucose. Following insulin-induced hypoglycemia, glucagon is the most important stimulus for hepatic glucose production (HGP). It is unclear whether prandial hyperglucagonemia after GB or SG changes HGP under hyperinsulinemic hypoglycemia conditions. This study examined the hypothesis that prandial glucose production is reduced after GB and SG during hypoglycemia.

METHODS

Glucose kinetics and islet-cell and gut hormone secretion during hyperinsulinemic (120 mU.m^-2.min^-1) hypoglycemic clamp (~3.2 mM) were measured before and after mixed meal ingestion in 9 non-diabetic subjects with GB, 7 with SG, and 5 matched non-operated controls (CN).

RESULTS

Systemic appearance of ingested glucose was faster in GB compared to SG, and in SG compared to CN (p < 0.05). Subjects with GB and SG had greater plasma glucagon levels after eating (AUC_Glucagon) compared to CN (p < 0.05). But prandial HGP response during insulin-induced hypoglycemia (AUC_HGP) was smaller and shorter in duration in surgical groups (p < 0.05). In the absence of meal stimuli, however, glucose counterregulatory response to hypoglycemia was comparable among the 3 groups during hyperinsulinemic clamp.

CONCLUSION

After bariatric surgery, prandial glucose counterregulatory response to hypoglycemia is impaired. Considering post-meal hyperglucagonemia after GB or SG the blunted HGP response suggests a lower sensitivity of liver to glucagon that can predispose to hypoglycemia in this population.

Glucagon's effect on liver protein metabolism in vivo

The postprandial state is characterized by a storage of nutrients in the liver, muscle, and adipose tissue for later utilization. In the case of a protein-rich meal, amino acids (AA) stimulate glucagon secretion by the α-cell. The aim of the present study was to determine the impact of the rise in glucagon on AA metabolism, particularly in the liver. We used a conscious catheterized dog model to recreate a postprandial condition using a pancreatic clamp. Portal infusions of glucose, AA, and insulin were used to achieve postprandial levels, while portal glucagon infusion was either maintained at the basal level or increased by three-fold. The high glucagon infusion reduced the increase in arterial AA concentrations compared with the basal glucagon level (-23%, P < 0.05). In the presence of high glucagon, liver AA metabolism shifted toward a more catabolic state with less protein synthesis (-36%) and increased urea production (+52%). Net hepatic glucose uptake was reduced modestly (-35%), and AA were preferentially used in gluconeogenesis, leading to lower glycogen synthesis (-54%). The phosphorylation of AMPK was increased by the high glucagon infusion (+40%), and this could be responsible for increasing the expression of genes related to pathways producing energy and lowering those involved in energy consumption. In conclusion, the rise in glucagon associated with a protein-rich meal promotes a catabolic utilization of AA in the liver, thereby, opposing the storage of AA in proteins.

Nutritional regulation of the anabolic fate of amino acids within the liver in mammals: concepts arising from in vivo studies

At the crossroad between nutrient supply and requirements, the liver plays a central role in partitioning nitrogenous nutrients among tissues. The present review examines the utilisation of amino acids (AA) within the liver in various physiopathological states in mammals and how the fates of AA are regulated. AA uptake by the liver is generally driven by the net portal appearance of AA. This coordination is lost when demands by peripheral tissues is important (rapid growth or lactation), or when certain metabolic pathways within the liver become a priority (synthesis of acute-phase proteins). Data obtained in various species have shown that oxidation of AA and export protein synthesis usually responds to nutrient supply. Gluconeogenesis from AA is less dependent on hepatic delivery and the nature of nutrients supplied, and hormones like insulin are involved in the regulatory processes. Gluconeogenesis is regulated by nutritional factors very differently between mammals (glucose absorbed from the diet is important in single-stomached animals, while in carnivores, glucose from endogenous origin is key). The underlying mechanisms explaining how the liver adapts its AA utilisation to the body requirements are complex. The highly adaptable hepatic metabolism must be capable to deal with the various nutritional/physiological challenges that mammals have to face to maintain homeostasis. Whereas the liver responds generally to nutritional parameters in various physiological states occurring throughout life, other complex signalling pathways at systemic and tissue level (hormones, cytokines, nutrients, etc.) are involved additionally in specific physiological/nutritional states to prioritise certain metabolic pathways (pathological states or when nutritional requirements are uncovered).

Beta Cell Function and the Nutritional State: Dietary Factors that Influence Insulin Secretion

Approximately 366 million people worldwide have been diagnosed with type-2 diabetes (T2D). Chronic insulin resistance, decreased functional β-cell mass, and elevated blood glucose are defining characteristics of T2D. Great advances have been made in understanding the pathogenesis of T2D with respect to the effects of dietary macronutrient composition and energy intake on β-cell physiology and glucose homeostasis. It has been further established that obesity is a leading pathogenic factor for developing insulin resistance. However, insulin resistance may not progress to T2D unless β-cells are unable to secret an adequate amount of insulin to compensate for decreased insulin sensitivity. Therefore, pancreatic β-cell dysfunction plays an important role in the development of overt diabetes. This paper reviews recent research findings on the effects of several micronutrients (zinc, vitamin D, iron, vitamin A), leucine, and the phytochemical, genistein on pancreatic β-cell physiology with emphasis on their effects on insulin secretion, specifically in the context of T2D.

Slight chronic elevation of C-reactive protein is associated with lower aerobic fitness but does not impair meal-induced stimulation of muscle protein metabolism in healthy old men

Ageing impairs the muscle anabolic effect of food intake, which may explain muscle loss and an increased risk of sarcopenia. Ageing is also associated with low grade inflammation (LGI), which has been negatively correlated with muscle mass and strength. In rodents, the muscle anabolic resistance observed during ageing and sarcopenia has been ascribed to the development of the LGI. We aimed to investigate this relationship in humans. We studied protein metabolism and physical fitness in healthy elderly volunteers with slight chronic C-reactive protein. Two groups of healthy elderly volunteers were selected on the presence (or not) of a chronic, slight, elevation of CRP (Control: <1; CRP+: >2 mg l(-1) and <10 mg l(-1) , for 2 months). Body composition, short performance battery test, aerobic fitness and muscle strength were assessed. Whole body and muscle protein metabolism and the splanchnic extraction of amino acids were assessed using [(13) C]leucine and [(2) H]leucine infusion. The anabolic effect of food intake was measured by studying the volunteers both at the post-absorptive and post-prandial states. Slight chronic CRP elevation resulted in neither an alteration of whole body, nor skeletal muscle protein metabolism at both the post-absorptive and the post-prandial states. However, CRP+ presented a reduction of physical fitness, increased abdominal fat mass and post-prandial insulin resistance. Plasma cytokines (interleukin-1, interleukin-6, tumour necrosis factor α) and markers of endothelial inflammation (intercellular adhesion molecule, vascular cell adhesion molecule, selectins) were similar between groups. An isolated elevated CRP in healthy older population does not indicate an impaired skeletal muscle anabolism after food intake, nor an increased risk of skeletal muscle wasting. We propose that a broader picture of LGI (notably with elevated pro-inflammatory cytokines) is required to impact muscle metabolism and mass. However, an isolated chronic CRP elevation could predict a decrease in aerobic fitness and insulin resistance installation in elderly individuals.

Portal glucose delivery stimulates muscle but not liver protein metabolism

Portal vein glucose delivery (the portal glucose signal) stimulates glucose uptake and glycogen storage by the liver, whereas portal amino acid (AA) delivery (the portal AA signal) induces an increase in protein synthesis by the liver. During a meal, both signals coexist and may interact. In this study, we compared the protein synthesis rates in the liver and muscle in response to portal or peripheral glucose infusion during intraportal infusion of a complete AA mixture. Dogs were surgically prepared with hepatic sampling catheters and flow probes. After a 42-h fast, they underwent a 3-h hyperinsulinemic (4× basal) hyperglucagonemic (3× basal) hyperglycemic (≈160 mg/dl) hyperaminoacidemic (hepatic load 1.5× basal; delivered intraportally) clamp (postprandial conditions). Glucose was infused either via a peripheral (PeG; n = 7) or the portal vein (PoG; n = 8). Protein synthesis was assessed with a primed, continuous [(14)C]leucine infusion. Net hepatic glucose uptake was stimulated by portal glucose infusion (+1 mg·kg(-1)·min(-1), P < 0.05) as expected, but hepatic fractional AA extraction and hepatic protein synthesis did not differ between groups. There was a lower arterial AA concentration in the PoG group (-19%, P < 0.05) and a significant stimulation (+30%) of muscle protein synthesis associated with increased expression of LAT1 and ASCT2 AA transporters and p70S6 phosphorylation. Concomitant portal glucose and AA delivery enhances skeletal muscle protein synthesis compared with peripheral glucose and portal AA delivery. These data suggest that enteral nutrition support may have an advantage over parenteral nutrition in stimulating muscle protein synthesis.

Regulation of hepatic glucose uptake and storage in vivo

In the postprandial state, the liver takes up and stores glucose to minimize the fluctuation of glycemia. Elevated insulin concentrations, an increase in the load of glucose reaching the liver, and the oral/enteral/portal vein route of glucose delivery (compared with the peripheral intravenous route) are factors that increase the rate of net hepatic glucose uptake (NHGU). The entry of glucose into the portal vein stimulates a portal glucose signal that not only enhances NHGU but concomitantly reduces muscle glucose uptake to ensure appropriate partitioning of a glucose load. This coordinated regulation of glucose uptake is likely neurally mediated, at least in part, because it is not observed after total hepatic denervation. Moreover, there is evidence that both the sympathetic and the nitrergic innervation of the liver exert a tonic repression of NHGU that is relieved under feeding conditions. Further, the energy sensor 5'AMP-activated protein kinase appears to be involved in regulation of NHGU and glycogen storage. Consumption of a high-fat and high-fructose diet impairs NHGU and glycogen storage in association with a reduction in glucokinase protein and activity. An understanding of the impact of nutrients themselves and the route of nutrient delivery on liver carbohydrate metabolism is fundamental to the development of therapies for impaired postprandial glucoregulation.

A high-fat, high-fructose diet accelerates nutrient absorption and impairs net hepatic glucose uptake in response to a mixed meal in partially pancreatectomized dogs

The aim of this study was to elucidate the impact of a high-fat, high-fructose diet (HFFD; fat, 52%; fructose, 17%), in the presence of a partial (~65%) pancreatectomy (PPx), on the response of the liver and extrahepatic tissues to an orally administered, liquid mixed meal. Adult male dogs were fed either a nonpurified, canine control diet (CTR; fat, 26%; no fructose; n = 5) or a HFFD (n = 5) for 8 wk. Diets were provided in a quantity to maintain neutral or positive energy balance in CTR or HFFD, respectively. Dogs underwent a sham operation or PPx at wk 0, portal and hepatic vein catheterization at wk 6, and a mixed meal test at wk 8. Postprandial glucose concentrations were significantly greater in the HFFD group (14.5 ± 2.0 mmol/L) than in the CTR group (9.2 ± 0.5 mmol/L). Impaired glucose tolerance in HFFD was due in part to accelerated gastric emptying and glucose absorption, as indicated by a more rapid rise in arterial plasma acetaminophen and the rate of glucose output by the gut, respectively, in HFFD than in CTR. It was also attributable to lower net hepatic glucose uptake (NHGU) in the HFFD group (5.5 ± 3.9 μmol · kg(-1) · min(-1)) compared to the CTR group (26.6 ± 7.0 μmol · kg(-1) · min(-1)), resulting in lower hepatic glycogen synthesis (GSYN) in the HFFD group (10.8 ± 5.4 μmol · kg(-1) · min(-1)) than in the CTR group (30.4 ± 7.0 μmol · kg(-1) · min(-1)). HFFD also displayed aberrant suppression of lipolysis by insulin. In conclusion, HFFD feeding accelerates gastric emptying and diminishes NHGU and GSYN, thereby impairing glucose tolerance following a mixed meal challenge. These data reveal a constellation of deleterious metabolic consequences associated with consumption of a HFFD for 8 wk.

Leucine metabolism in regulation of insulin secretion from pancreatic beta cells

Leucine, a branched-chain amino acid that must be supplied in the daily diet, plays an important role in controlling protein synthesis and regulating cell metabolism in various cell types. In pancreatic beta cells, leucine acutely stimulates insulin secretion by serving as both metabolic fuel and allosteric activator of glutamate dehydrogenase to enhance glutaminolysis. Leucine has also been shown to regulate gene transcription and protein synthesis in pancreatic islet beta cells via both mTOR-dependent and -independent pathways at physiological concentrations. Long-term treatment with leucine has been shown to improve insulin secretory dysfunction of human diabetic islets via upregulation of certain key metabolic genes. In vivo, leucine administration improves glycemic control in humans and rodents with type 2 diabetes. This review summarizes and discusses the recent findings regarding the effects of leucine metabolism on pancreatic beta-cell function.