Effects of oral phenylalanine load on plasma glucagon, insulin, amino acid and glucose concentrations in man

Amino acid-stimulated insulin secretion: a path forward in type 2 diabetes

Qualitative and quantitatively appropriate insulin secretion is essential for optimal control of blood glucose. Beta-cells of the pancreas produce and secrete insulin in response to glucose and non-glucose stimuli including amino acids. In this manuscript, we review the literature on amino acid-stimulated insulin secretion in oral and intravenous in vivo studies, in addition to the in vitro literature, and describe areas of consensus and gaps in understanding. We find promising evidence that the synergism of amino acid-stimulated insulin secretion could be exploited to develop novel therapeutics, but that a systematic approach to investigating these lines of evidence is lacking. We highlight evidence that supports the relative preservation of amino acid-stimulated insulin secretion compared to glucose-stimulated insulin secretion in type 2 diabetes, and make the case for the therapeutic potential of amino acids. Finally, we make recommendations for research and describe the potential clinical utility of nutrient-based treatments for type 2 diabetes including remission services.

The Metabolic Switch of Physical Activity in Non-Obese Insulin Resistant Individuals

Healthy non-obese insulin resistant (IR) individuals are at higher risk of metabolic syndrome. The metabolic signature of the increased risk was previously determined. Physical activity can lower the risk of insulin resistance, but the underlying metabolic pathways remain to be determined. In this study, the common and unique metabolic signatures of insulin sensitive (IS) and IR individuals in active and sedentary individuals were determined. Data from 305 young, aged 20-30, non-obese participants from Qatar biobank, were analyzed. The homeostatic model assessment of insulin resistance (HOMA-IR) and physical activity questionnaires were utilized to classify participants into four groups: Active Insulin Sensitive (ISA, n = 30), Active Insulin Resistant (IRA, n = 20), Sedentary Insulin Sensitive (ISS, n = 21) and Sedentary Insulin Resistant (SIR, n = 23). Differences in the levels of 1000 metabolites between insulin sensitive and insulin resistant individuals in both active and sedentary groups were compared using orthogonal partial least square discriminate analysis (OPLS-DA) and linear models. The study indicated significant differences in fatty acids between individuals with insulin sensitivity and insulin resistance who engaged in physical activity, including monohydroxy, dicarboxylate, medium and long chain, mono and polyunsaturated fatty acids. On the other hand, the sedentary group showed changes in carbohydrates, specifically glucose and pyruvate. Both groups exhibited alterations in 1-carboxyethylphenylalanine. The study revealed different metabolic signature in insulin resistant individuals depending on their physical activity status. Specifically, the active group showed changes in lipid metabolism, while the sedentary group showed alterations in glucose metabolism. These metabolic discrepancies demonstrate the beneficial impact of moderate physical activity on high risk insulin resistant healthy non-obese individuals by flipping their metabolic pathways from glucose based to fat based, ultimately leading to improved health outcomes. The results of this study carry significant implications for the prevention and treatment of metabolic syndrome in non-obese individuals.

Metabolomics of Lean/Overweight Insulin-Resistant Females Reveals Alterations in Steroids and Fatty Acids

BACKGROUND

The global diabetes epidemic is largely attributed to obesity-triggered metabolic syndrome. However, the impact of insulin resistance (IR) prior to obesity on the high prevalence of diabetes and the molecular mediators remain largely unknown. This study aims to compare the metabolic profiling of apparently healthy lean/overweight participants with IR and insulin sensitivity (IS), and identify the metabolic pathways underlying IR.

METHODS

In this cross-sectional study, clinical and metabolic data for 200 seemingly healthy young female participants (100 IR and 100 IS) was collected from Qatar Biobank. Orthogonal partial least square analysis was performed to assess the extent of separation between individuals from the 2 groups based on measured metabolites. Classical linear models were used to identify the metabolic signature of IR, followed by elastic-net-regularized generalized linear model (GLMNET) and receiver operating characteristic (ROC) analysis to determine top metabolites associated with IR.

RESULTS

Compared to lean/overweight participants with IS, those with IR showed increased androgenic steroids, including androsterone glucuronide, in addition to various microbiota byproducts, such as the phenylalanine derivative carboxyethylphenylalanine. On the other hand, participants with IS had elevated levels of long-chain fatty acids. A ROC analysis suggested better discriminatory performance using 20 metabolites selected by GLMNET in comparison to the classical clinical traits (area under curve: 0.93 vs 0.73, respectively).

CONCLUSION

Our data confirm the multifactorial mechanism of IR with a diverse spectrum of emerging potential biomarkers, including steroids, long-chain fatty acids, and microbiota metabolites. Further studies are warranted to validate these markers for diagnostic and therapeutic applications.

Differential effects of L- and D-phenylalanine on pancreatic and gastrointestinal hormone release in humans: A randomized crossover study

AIM

To investigate the effects of L-phenylalanine on gastroenteropancreatic hormone release, glucose levels, subjective appetite and energy intake in humans, and to determine whether these effects were stereoisomer-specific by comparing them with D-phenylalanine.

MATERIALS AND METHODS

A dose-finding, non-randomized, unblinded, crossover study was conducted during October-December 2017 at the NIHR Imperial Clinical Research Facility in five participants, in which the tolerability of escalating doses of oral L-phenylalanine was assessed (0, 3, 6 and 10 g). Also, an acute, randomized, double-blind, placebo-controlled crossover study was conducted during January-May 2018 at the NIHR Imperial Clinical Research Facility in 11 participants, in which the effects of oral 10 g L-phenylalanine relative to D-phenylalanine and placebo on gastroenteropancreatic hormone (insulin, glucagon, glucose-dependent insulinotropic polypeptide [GIP], peptide tyrosine tyrosine [PYY], glucagon-like peptide-1) and glucose concentrations, visual analogue scales for subjective appetite and energy intake at an ad libitum meal served 70 minutes postingestion, were investigated.

RESULTS

L-phenylalanine was well-tolerated and increased insulin and glucagon concentrations prior to meal ingestion at several time points relative to placebo and D-phenylalanine (P < .05). L-phenylalanine also increased GIP concentrations relative to D-phenylalanine (P = .0420) and placebo (P = .0249) 70 minutes following ingestion. L-phenylalanine reduced postprandial glucose area under the curve (AUC)_70-150mins relative to placebo (P = .0317) but did not affect subjective appetite or energy intake (P > .05). D-phenylalanine increased postprandial PYY AUC_70-150mins concentrations relative to placebo (P = .0002).

CONCLUSIONS

Ingestion of L-phenylalanine, but not D-phenylalanine, increases insulin, glucagon and GIP concentrations without appearing to have a marked effect on appetite.

Short-term lactation and mammary metabolism responses in lactating goats to graded removal of methionine from an intravenously infused complete amino acid mixture

To investigate the possible pathways of Met deficiency to depress milk protein synthesis, 4 lactating goats fitted with jugular vein, mammary vein, and carotid artery catheters and transonic blood flow detectors on the external pudic artery were used in a 4 × 4 Latin square experiment. Goats were fasted for 24 h followed by a 9-h intravenous infusion of an AA mixture plus glucose. Milk yield was recorded and samples were taken in h 2 to 8 of the infusion period, and mammary biopsy was performed in the last hour. Treatments were graded removal of Met from the infused AA mixture to achieve Met content in the infusate of 100 (complete), 60, 30, or 0% of that in casein. Graded Met removal decreased yield of milk, milk protein, and lactose linearly and tended to decrease yield of milk fat linearly. Milk protein yield decreased to 82, 78, and 69% that of complete mixture infusion, respectively, when the 60, 30, and 0% Met infusate was infused. Circulating Met decreased linearly with graded Met removal. Arterial and venous Met decreased to 36 and 23% that of complete mixture infusion, respectively, when all Met was removed out of the mixture. Concomitant with the decreased circulating concentration was a similar increase in mammary Met affinity as reflected by the linearly increased mammary Met clearance rate. The increased affinity plus the linearly increased mammary blood flow totally offset the negative effect of decreased circulating Met concentration on mammary Met uptake. The overall result was similar mammary Met uptakes across treatments ranging from 285.9 to 339.5 μmol/h. Mammary uptakes of the other AA measured were generally not affected by treatments except for a linearly decreased Thr uptake and a trend of linearly increased Glu uptake. Consistent with the behavior of an AA mainly catabolized in the liver and mainly used for protein synthesis in peripheral tissues, mammary uptake to milk output ratios of Met measured in the present study ranged from 1.25 to 1.49 and was not affected by treatments. For the other AA measured, the ratio of Thr was linearly decreased and that of Glu was linearly increased by graded Met removal. Graded Met removal linearly elevated circulating urea N and glucose concentrations, indicating enhanced whole-body catabolism of AA and hepatic gluconeogenesis. Treatments had no significant effects on circulating insulin, growth hormone, and the other hormones and metabolites measured. Phosphorylation status of eIF4E binding protein 1 tended to decrease linearly and that of p70S6k was linearly decreased by graded Met removal, indicating depressed signal in the intracellular mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway. In conclusion, results of the present study indicated that the mTORC1 pathway and whole-body AA catabolism rather than mammary uptake appeared the drivers for changes in milk protein synthesis in response to varying Met supply.

Carbohydrate status in patients with phenylketonuria

BACKGROUND

In patients with phenylketonuria (PKU), a low-phenylalanine (Phe) diet supplemented with low-protein foods and a Phe-free amino acid mixture favors a dietary intake rich in carbohydrates, but little is known about how these molecules are metabolized in this setting. The objective of the present study was to analyze carbohydrate metabolism in patients with hyperphenylalaninemia.

METHODS

We conducted a multicenter cross-sectional study to investigate biochemical markers of basal and postprandial carbohydrate metabolism in PKU patients according to age, Phe tolerance, waist circumference and body mass index (BMI), diet, tetrahydrobiopterin (BH4) supplementation, and adherence to treatment. Basal biomarkers and anthropometric parameters were also evaluated in patients with mild hyperphenylalaninemia (MHPA) and in healthy controls.

RESULTS

A total of 83 patients aged 4-52 years were studied; 68.7% had PKU and 31.3% had MHPA. 68 healthy controls of similar sex and age were also evaluated Metabolic control was adequate in 71.9% of PKU patients. Fasting glucose levels (mean 80.77 ± 8.06 mg/dL) were high in just one patient, but fasting insulin levels, with a mean of 12.74 ± 8.4 mIU/L, were altered in 15 PKU patients (26.3%) and markedly higher than in patients with MPHA (p = 0.035). Fasting insulin levels and Homeostasis Model Assessment Insulin Resistance (HOMA-IR) were significantly higher than in healthy controls and correlated with body mass index, waist circumference, age, and also showed statistically significant differences according to diagnosis and Phe tolerance (p < 0.05). Patients under BH4 therapy had lower insulin levels and HOMA-IR. A higher mean carbohydrate intake from AA mixtures was observed in classic PKU patients. The caloric intake in the form of carbohydrates was also higher in PKU than MHPA patients (p = 0.038) and it was correlated with basal insulin (rho = 0.468, p = 0.006), HOMA-IR (rho = 0.423, p = 0.02), BMI (rho 0.533, p = 0.002), and waist circumference (rho 0.584, p = 0.0007).

CONCLUSIONS

This study shows that PKU patients are at risk of carbohydrate intolerance and insulin resistance, more evident in adults and overweight patients, probably related to their higher caloric intake in form carbohydrate content. A higher dependency of AA mixtures was demonstrated in PKU patients.

Plasma and Pancreas Islet Hormone Concentrations in Lactating Rats Are Associated with Dietary Protein Amounts

Background

Circulating amino acid (AA) and nitric oxide (NO) concentrations and hepatic gluconeogenesis are affected by previous protein intake. However, information about their relations and islet hormone responses is limited.

Objective

This study investigated the associations between islet hormone concentrations with circulating AA and NO concentrations as well as with hepatic gluconeogenesis in lactating rats.

Methods

At delivery, 18 Wistar rats aged 14 wk were assigned either to low-protein (LP; 9% protein), standard-protein (SP; 21% protein), or high-protein (HP; 35% protein) diets for 15 d in groups of 6 pups/dam. Circulating AA and NO concentrations, circulating and pancreas islet hormone concentrations, and the activities and gene expressions of hepatic phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) were measured at the end of treatment.

Results

Circulating insulin and glucagon concentrations were greater in the HP than in the LP (25% and 17%, respectively) and SP (37% and 31%) diet groups, whereas compared with the SP group, pancreatic concentrations were lower in the LP (32% and 49%) and HP (34% and 46%) groups (P < 0.01). Hepatic PEPCK and G6Pase activities in the HP group were greater than those in the SP (15% and 15%) and LP (8% and 19%) groups (P < 0.05). In all groups, plasma NO concentrations were correlated negatively to circulating insulin (r = -0.77, P = 0.0003) and positively to pancreas insulin and glucagon concentrations and the insulin-to-glucagon ratio (r = 0.50-0.63; P < 0.05). Some circulating AAs correlated positively to circulating insulin and pancreas insulin and glucagon (r = 0.50-0.82, P < 0.05) but negatively to circulating glucagon (r = -0.53-0.68, P < 0.05).

Conclusion

Variations in circulating AA and NO concentrations and hepatic gluconeogenic enzyme activities are likely intermediary responses involved in the effects of dietary protein amounts on the synthesis and secretion of islet hormones in lactating rats.

Role of proteins in insulin secretion and glycemic control

Dietary proteins are essential for the life of all animals and humans at all stages of the life cycle. They serve many structural and biochemical functions and have significant effects on health and wellbeing. Dietary protein consumption has shown an upward trend in developed countries in the past two decades primarily due to greater supply and affordability. Consumption is also on the rise in developing countries as affluence is increasing. Research shows that proteins have a notable impact on glucose homeostasis mechanisms, predominantly through their effects on insulin, incretins, gluconeogenesis, and gastric emptying. Since higher protein consumption and impaired glucose tolerance can be commonly seen in the same population demographics, a thorough understanding of the former's role in glucose homeostasis is crucial both toward the prevention and management of the latter. This chapter reviews the current state of the art on proteins, amino acids, and their effects on blood glucose and insulin secretion.

Effects of a nutritional supplement containing Salacia oblonga extract and insulinogenic amino acids on postprandial glycemia, insulinemia, and breath hydrogen responses in healthy adults

OBJECTIVE

This study evaluated the postprandial glycemic, insulinemic, and breath hydrogen responses to a liquid nutritional product containing Salacia oblonga extract, an herbal alpha-glucosidase inhibitor, and two insulinogenic amino acids.

METHODS

In a randomized, double-masked, crossover design, 43 healthy subjects were fed the following meals on separate days after overnight fasting: control (C; 480 mL of a study beverage containing 82 g of carbohydrate, 20 g of protein, and 14 g of fat), control plus 3.5 g each of phenylalanine and leucine (AA), control plus 1000 mg of S. oblonga extract (S), and control plus S and AA (SAA). Postprandially, fingerstick capillary plasma glucose and venous serum insulin levels were measured for 180 min, and breath hydrogen excretion was measured for 480 min.

RESULTS

The baseline-adjusted peak glucose response was not different across meals. However, changes in plasma glucose areas under the curve (0 to 120 min and 0 to 180 min, respectively) compared with C were -9% and -11% for AA (P>0.05 each), -27% and -24% for S (P=0.035 and 0.137), and -27% and -29% for SAA (P<0.05 each). Changes in insulin areas under the curve were +5% and +5% for AA (P>0.05 each), -35% and -36% for S (P<0.001 each), and -6% and -7% for SAA (P>0.05 each). Breath hydrogen excretion was 60% greater (P<0.001) in the S-containing meals than in the C- and AA-containing meals and was associated with mild flatulence.

CONCLUSIONS

Salacia oblonga extract is a promising nutraceutical ingredient that decreased glycemia in this study. Supplementation with amino acids had no significant additional effect on glycemia.

Molecular basis for nonphenylketonuria hyperphenylalaninemia

Nonphenylketonuria hyperphenylalaninemia (non-PKU HPA) is defined as phenylalanine hydroxylase (PAH) deficiency with blood phenylalanine levels below 600 mumol/liter (i.e., within the therapeutic range) on a normal dietary intake. Haplotype analysis at the PAH locus was performed in 17 Danish families with non-PKU HPA, revealing compound heterozygosity in all individuals. By allele-specific oligonucleotide (ASO) probing for common PKU mutations we found 12 of 17 non-PKU HPA children with a PKU allele on one chromosome. To identify molecular lesions in the second allele, individual exons were amplified by polymerase chain reaction and screened for mutations by single-strand conformation polymorphism. Two new missense mutations were identified. Three children had inherited a G-to-A transition at codon 415 in exon 12 of the PAH gene, resulting in the substitution of asparagine for aspartate, whereas one child possessed an A-to-G transition at codon 306 in exon 9, causing the replacement of an isoleucine by a valine in the enzyme. It is further demonstrated that the identified mutations have less impact on the heterozygote's ability to hydroxylate phenylalanine to tyrosine compared to the parents carrying a PKU mutation. The combined effect on PAH activity explains the non-PKU HPA phenotype of the child. The present observations that PKU mutations in combination with other mutations result in the non-PKU HPA phenotype and that particular mutation-restriction fragment length polymorphism haplotype combinations are associated with this phenotype offer the possibility of distinguishing PKU patients from non-PKU individuals by means of molecular analysis of the hyperphenylalaninemic neonate and, consequently, of determining whether a newborn child requires dietary treatment.

Glucose tolerance, blood lipid, insulin and glucagon concentration after single or continuous administration of aspartame in diabetics

A nutritive sweetener, aspartame (L-aspartyl-L-phenylalanine methylester) was administered orally to normal controls and diabetic patients in order to evaluate effects on blood glucose, lipids and pancreatic hormone secretion. An oral glucose tolerance test was also performed in the same subjects as a control study of aspartame administration. In 7 normal controls and 22 untreated diabetics, a single dose of 500 mg aspartame, equivalent to 100 g glucose in sweetness, induced no increase in blood glucose concentration. Rather, a small but significant decrease in blood glucose was noticed 2 or 3 h after administration. The decrease in blood glucose was found to be smallest in the control and became greater as the diabetes increased in severity. No significant change in blood insulin or glucagon concentration during a 3-h period was observed in either the controls or the diabetics. The second study was designed to determine the effects of 2 weeks' continuous administration of 125 mg aspartame, equal in sweetness to the mean daily consumption of sugar (20-30 g) in Japan, to 9 hospitalized diabetics with steady-state glycemic control. The glucose tolerance showed no significant change after 2 weeks' administration. Fasting, 1 h and 2 h postprandial blood glucose, blood cholesterol, triglyceride and HDL-cholesterol were also unaffected. From these and other published results, aspartame would seem to be a useful alternative nutrient sweetener for patients with diabetes mellitus.

Regulation of the activity of hepatic phenylalanine hydroxylase

Rat liver phenylalanine hydroxylase catalyzes the tetrahydropterin-dependent oxidation of phenylalanine to tyrosine, according to equation 1. In addition to the naturally-occurring coenzyme, tetrahydrobiopterin (BH4), certain synthetic analogs of BH4 such as 6-methyltetrahydropterin (6MPH4) have high cofactor activity. (formula; see text) The hydroxylase can be activated by a variety of reversible and irreversible modifications, including those caused by partial proteolysis, by interaction with phospholipids such as lysolecithin, by alkylation of a single sulfhydryl group, by phosphorylation catalyzed by cAMP-dependent protein kinase, and by preincubation with its substrate, phenylalanine. All of these modes of activation greatly increase the hydroxylase activity in the presence of BH4, whereas the activity in the presence of 6MPH4 is increased only slightly. The ratio of hydroxylase activity in the presence of BH4 compared to the activity in the presence of 6MPH4, therefore, is a useful index of the state of activation of the enzyme. Of the various activation mechanisms listed above, only phosphorylation of the enzyme and phenylalanine-activation appear to operate in vivo. The evidence indicates that these two regulatory mechanisms act synergistically. Thus, phosphorylation of the enzyme by cAMP-dependent protein kinase is stimulated by phenylalanine, especially in the presence of BH4, (which by itself inhibits), whereas phosphorylation sensitizes the enzyme to activation by phenylalanine. One of the consequences of these interlocking control mechanisms is to enhance the responsiveness of the activity of the hydroxylase to alterations in tissue levels of phenylalanine. As a result, elevated concentrations of phenylalanine can be rapidly metabolized, thereby protecting the fetal and neonatal brain from possible damage by excess phenylalanine.