Oral arginine does not stimulate an increase in insulin concentration but delays glucose disposal

Metabolic effects of L-citrulline in type 2 diabetes

The prevalence of type 2 diabetes (T2D) is increasing worldwide. Decreased nitric oxide (NO) bioavailability is involved in the pathophysiology of T2D and its complications. L-citrulline (Cit), a precursor of NO production, has been suggested as a novel therapeutic agent for T2D. Available data from human and animal studies indicate that Cit supplementation in T2D increases circulating levels of Cit and L-arginine while decreasing circulating glucose and free fatty acids and improving dyslipidemia. The underlying mechanisms for these beneficial effects of Cit include increased insulin secretion from the pancreatic β cells, increased glucose uptake by the skeletal muscle, as well as increased lipolysis and β-oxidation, and decreased glyceroneogenesis in the adipose tissue. Thus, Cit has antihyperglycemic, antidyslipidemic, and antioxidant effects and has the potential to be used as a new therapeutic agent in the management of T2D. This review summarizes available literature from human and animal studies to explore the effects of Cit on metabolic parameters in T2D. It also discusses the possible mechanisms underlying Cit-induced improved metabolic parameters in T2D.

Effect of metformin on arginine and dimethylarginines in patients with advanced type 2 diabetes: A post hoc analysis of a randomized trial

AIM

To study the effect of metformin on plasma levels of arginine, asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA), indicators of the nitric oxide pathway.

MATERIALS AND METHODS

In this post hoc analysis of the HOME trial, we analysed plasma levels of arginine, ADMA and SDMA during the 4.3-year follow-up (comparing the effects of metformin versus placebo on top of insulin therapy). Statistical analysis was performed with a mixed model approach, in which simultaneously constant treatment effects were estimated, as well as time-dependent treatment effects.

RESULTS

We found that metformin compared with placebo did not affect ADMA or SDMA plasma levels but rapidly decreased arginine plasma levels and hence the arginine to ADMA ratio. The constant treatment effect on ADMA was 0.99 (95% CI 0.97, 1.00) relative to placebo and the time-dependent treatment effect was 1.00 (95% CI 1.00, 1.01). By contrast, the constant treatment effect on arginine was 0.86 (95% CI 0.84, 0.88), with only a minimal time-dependent change of 1.01 (95% CI 1.00, 1.01).

CONCLUSIONS

The potential benefits of metformin on endothelial function cannot be explained by a decrease in ADMA or by improved global arginine availability. The clinical significance of the decreased arginine plasma levels is not clear and can be harmful or beneficial, depending on the mechanism involved. However, a potential effect of metformin on the nitric oxide pathway is not restricted to the studied metabolites.

Pumpkin seed proteins (Cucurbita pepo L.) protect against diet-induced metabolic syndrome by improving insulin resistance and markers of oxidative stress and inflammation in rats

Pumpkin (Cucurbita pepo L.) seeds are enriched in bioactive compounds having functional properties. The aim of this study was to analyze the pumpkin seed proteins (PSP) effects on insulin resistance, oxidative stress damage and inflammation in rats with high fructose-induced metabolic syndrome.

Twenty four male Wistar rats, fed isoenergetic diets supplemented with: (1) 20% casein (C); (2) 20% casein and 1 g/kg/day PSP (P); (3) 20% casein and 64% D-fructose (C-HF); (4) 20% casein, 1 g/kg/day PSP and 64% D-fructose (P-HF). After 8 weeks of treatment, fructose supply impaired white adipose tissue (WAT) weight, deteriorated glucose tolerance and tAUC, plasma glucose, insulin, insulinogenic index, HOMA-IR and HOMA-β, antioxidant status, lipid and protein oxidation, plasma TNF-α and IL-6 as compared to control diets. Interestingly, rats assigned to the PSPs diet with or without fructose displayed lower plasma glucose, insulin and fructose, improved tolerance of glucose, tAUC, HOMA-IR and HOMA-β and increased insulinogenic index as compared to C diets. PSPs consumption lowered thiobarbituric acid reactive substances, hydroperoxides and carbonyls in WAT and carbonyls in muscle. Superoxide dismutase and glutathione peroxidase in WAT were significantly diminished in P-HF but increased in P as compared to C-HF and C. Rats fed P-HF diet had low catalase in WAT and high in muscle than those fed C-HF. Moreover, catalase activity increased in muscle but decreased in WAT in P group than in C group. In conclusion, pumpkin seed proteins exhibit favorable effects on metabolic disorders of fructose-induced metabolic syndrome, suggesting a key role in disease therapy.

Glucokinase-maturity onset diabetes mellitus in the young suggested by factory-calibrated glucose monitoring data: a case report

Glucokinase has an important role in regulating glycolysis as a glucose sensor in liver and pancreatic β cells. Glucokinase-maturity onset diabetes in young (GCK-MODY also known as MODY2) is caused by autosomal dominant gene mutation of the GCK gene; it is characterized by mild fasting hyperglycemia and small 2-h glucose increment during 75 g-oral glucose tolerance test (OGTT) as well as near-normal postprandial glucose variabilities. A 10-year-old girl with family history of diabetes visited her physician after being found positive for urinary glucose by school medical checkup. She received a diagnosis of diabetes based on the laboratory data: 75 g-OGTT (mild fasting hyperglycemia and small 2-h glucose increment) and factory-calibrated glucose monitoring (mild elevation of average glucose level and near-normal glycemic variability), which raised suspicion of GCK-MODY. She was then referred to our institution for genetic examination, which revealed a GCK heterozygous mutation (NM_000162: exon10: c.1324G>T: p.E442X) in the proband as well as in her mother and maternal grandmother, who had been receiving anti-diabetes medications without knowing that they had GCK-MODY specifically. GCK-MODY cases show incidence of microvascular and macrovascular diseases similar to that of normal subjects, and their glucose levels are adequately controlled without anti-diabetes drug use. Thus, early and definitive diagnosis of MODY2 by genetic testing is important to avoid unnecessary medication.

The effects of acute arginine supplementation on neuroendocrine, metabolic, cardiovascular, and mood outcomes in younger men: a double-blind placebo controlled trial

Objectives:

Arginine is an amino-acid supplement and precursor for nitric-oxide synthesis, which affects various biologic processes. The objective of this study was to determine the effects of arginine supplementation on growth hormone (GH) and metabolic parameters.

Methods:

Thirty physically active, healthy men (age 18–39 y; body mass index: 18.5–25 kg/m²) were randomized in a double-blind, placebo-controlled, crossover trial. Arginine (10 g) and placebo (0 g) beverages were consumed after an overnight fast. Blood samples were collected at baseline and 1.5, 3.0, and 24 h after supplementation. The primary outcomes were serum GH and metabolomics. Also, amino acids, glucose, insulin, triacylglycerols, thyroid hormones, testosterone, cortisol, dehydroepiandrosterone, and mood state were assessed. Individuals with detectable increases in GH were analyzed separately (responders: n = 16; < 0.05 ng/mL at 1.5 h). Repeated-measure analyses of variance estimated the treatment effects at each timepoint.

Results:

Arginine levels increased at 1.5 h (146%) and 3.0 h (95%; P ≤ 0.001) and GH (193%) and thyroid-stimulating hormone (TSH; 10%) levels at 24 h (P < 0.05) after arginine versus placebo consumption. Arginine versus placebo increased glucose levels at 1.5 h (5%) and 3.0 h (3%; P ≤ 0.001). Arginine versus placebo did not affect other dependent measures, including mood state (P > 0.05), but changes in the urea, glutamate, and citric-acid pathways were observed. Among responders, arginine versus placebo increased GH at 1.5 h (37%), glucose at 1.5 h (4%) and 3.0 h (4%), and TSH at 24 h (9%; P < 0.05). Responders had higher levels of benzoate metabolites at baseline and 1.5 h, and an unknown compound (X-16124) at baseline, 1.5 h, and 24 h that corresponds to a class of gut microbes (P < 0.05).

Conclusions:

Arginine supplementation modestly increased GH, glucose, and TSH levels in younger men. Responders had higher benzoate metabolites and an unknown analyte attributed to the gut microbiome. Future studies should examine whether the increased prevalence of these gut microorganisms corresponds with GH response after arginine supplementation.

Oral L-Arginine (5 g/day) for 14 Days Improves Microcirculatory Function in Healthy Young Women and Healthy and Type 2 Diabetes Mellitus Elderly Women

OBJECTIVE

The aim of this study was to investigate the effect of oral supplementation with L-arginine on serum biochemical profile, blood pressure, microcirculation, and vasoreactivity/endothelial function in young controls, and elderly women with and without type 2 diabetes mellitus (T2DM).

METHODS

Healthy young (n = 25), healthy elderly (n = 25), and elderly women with type 2 diabetes mellitus (T2DME, n = 23, glycated Hb ≥6.4% and mean of 7.7 years for duration of the disease), aged 18-30 and older than 65 years, respectively, were included in the study. All patients underwent biochemical analysis (fasting glycemia and lipidogram), arterial blood pressure, nailfold videocapillaroscopy (capillary diameters, functional capillary density [FCD], peak red blood cell velocity [RBCVmax] after 1 min ischemia, time to reach peak RBCV [TRBCVmax]), and venous occlusion plethysmography (vasoreactivity), before and after 14 days of oral supplementation with L-arginine (5 g/day).

RESULTS

L-Arginine did not change fasting glycemia and lipidogram, but it decreased systolic, diastolic, and mean arterial pressure in elderly women, increased RBCVmax in all groups, and did not decrease TRBCVmax in T2DME. Capillary diameters and FCD remained unchanged in all groups. L-Arginine improved vasoreactivity during reactive hyperemia and after sublingual nitroglycerin (0.4 mg) in all groups.

CONCLUSION

L-Arginine supplementation (5g/day during 14 days) was able to improve vascular/microvascular health in the elderly women with or without T2DM.

Effects of L-arginine supplementation on biomarkers of glycemic control: a systematic review and meta-analysis of randomised clinical trials

The present meta-analysis aimed to determine the effectiveness of L-arginine supplementation in improving biomarkers of glycemic control in adults. Electronic databases including PubMed, ISI Web of Science, Scopus, and the Cochrane Collaboration Library were searched up to January 2020. The meta-analysis of twelve randomised clinical trials indicated that L-arginine had no significant effect on serum fasting blood sugar (FBS) (weighted mean difference [WMD]: -3.38 mg/dl, 95% CI: -6.79 to 0.04, p = .53), serum insulin (WMD: -0.12 Hedges' g 95% CI: -0.33 to 0.09, p = .27), glycated haemoglobin A1c (HbA1c; WMD: -0.04%, 95% CI: -0.25 to 0.17, p = .71), and homeostasis model assessment for insulin resistance (WMD: -0.48, 95% CI: -1.15 to 0.19, p = .15). Although several animal studies have proposed that L-arginine supplementation might improve blood glucose control, the present study could not confirm this benefit in humans.

The Impact of Amino Acids on Postprandial Glucose and Insulin Kinetics in Humans: A Quantitative Overview

Different amino acids (AAs) may exert distinct effects on postprandial glucose and insulin concentrations. A quantitative comparison of the effects of AAs on glucose and insulin kinetics in humans is currently lacking. PubMed was queried to identify intervention studies reporting glucose and insulin concentrations after acute ingestion and/or intravenous infusion of AAs in healthy adults and those living with obesity and/or type 2 diabetes (T2DM). The systematic literature search identified 55 studies that examined the effects of l-leucine, l-isoleucine, l-alanine, l-glutamine, l-arginine, l-lysine, glycine, l-proline, l-phenylalanine, l-glutamate, branched-chain AAs (i.e., l-leucine, l-isoleucine, and l-valine), and multiple individual l-AAs on glucose and insulin concentrations. Oral ingestion of most individual AAs induced an insulin response, but did not alter glucose concentrations in healthy participants. Specific AAs (i.e., leucine and isoleucine) co-ingested with glucose exerted a synergistic effect on the postprandial insulin response and attenuated the glucose response compared to glucose intake alone in healthy participants. Oral AA ingestion as well as intravenous AA infusion was able to stimulate an insulin response and decrease glucose concentrations in T2DM and obese individuals. The extracted information is publicly available and can serve multiple purposes such as computational modeling.

A novel nutritional supplement containing amino acids and chromium decreases postprandial glucose response in a randomized, double-blind, placebo-controlled study

High postprandial blood glucose levels are associated with increased mortality, cardiovascular events and development of diabetes in the general population. Interventions targeting postprandial glucose have been shown to prevent both cardiovascular events and diabetes. This study evaluates the efficacy and safety of a novel nutritional supplement targeting postprandial glucose excursions in non-diabetic adults. Sixty overweight healthy male and female participants were recruited at two centers and randomized in a double-blind, placebo-controlled, crossover design. The supplement, a water-based drink containing 2.6g of amino acids (L-Leucine, L-Threonine, L-Lysine Monohydrochloride, L-Isoleucine, L-Valine) and 250 mcg of chromium picolinate, was consumed with a standardized carbohydrate-rich meal. The primary endpoint was the incremental area under the curve (iAUC) for venous blood glucose from 0 to 120 minutes. Secondary endpoints included glucose iAUC _{0–180 minutes} and the maximum glucose concentration (C_max), for both venous and capillary blood glucose. In the intention-to-treat-analysis (n = 60) the supplement resulted in a decreased venous blood glucose iAUC_0-120min compared to placebo, mean (SE) of 68.7 (6.6) versus 52.2 (6.8) respectively, a difference of -16.5 mmol/L•min (95% CI -3.1 to -30.0, p = 0.017). The C_max for venous blood glucose for the supplement and placebo were 6.45 (0.12) versus 6.10 (<0.12), respectively, a difference of -0.35 mmol/L (95% CI -0.17 to -0.53, p<0.001). In the per protocol-analysis (n = 48), the supplement resulted in a decreased C_max compared to placebo from 6.42 (0.14) to 6.12 (0.14), a difference of -0.29 mmol/L (95% CI -0.12 to -0.47, p = 0.002). No significant differences in capillary blood glucose were found, as measured by regular bed-side glucometers. The nutritional supplement drink containing amino acids and chromium improves the postprandial glucose homeostasis in overweight adults without diabetes. Future studies should clarify, whether regular consumption of the supplement improves markers of disease or could play a role in a diet aiming at preventing the development of diabetes.

The Association between Maternal Dietary Protein Intake and Risk of Gestational Diabetes Mellitus

BACKGROUND

The amount and type of dietary protein affect glucose metabolism. However, the association between dietary protein intake and gestational diabetes mellitus (GDM) risk is vague. We examined this association. Methods: We included 152 GDM and 168 non-GDM participants (total 320), age 18-45 years from Arash Women's Hospital, Tehran, Iran. Protein intake was ascertained from 168-item Food Frequency Questionnaire at 24-40 weeks' gestation. GDM was defined as fasting blood sugar >95 mg/dL and/or oral glucose tolerance test >155 mg/dL. Dietary data were assessed using N4 software and statistical analysis was performed using SPSS 21. We tested the association between the amount of protein consumed from red and processed meat, poultry, dairy, egg, seafood, and vegetable plus sociodemographic and lifestyle covariates and GDM risk using multivariate logistic regression analysis.

RESULTS

There was a significant association between the physical activity (P < 0.035), socioeconomic status (P < 0.013), body mass index, age, and each trimester's weight (P < 0.001), and risk of GDM. No significant association was observed between the intake of protein from major protein sources and risk of GDM. The only significant association was observed for egg consumption which was lower in GDM participants (P = 0.004), yet this association turned nonsignificant after adjustment for confounders, except for the fourth quartile (odds ratio: 0.43, 95% confidence interval: 0.208, 0.893).

CONCLUSIONS

According to our findings, dietary intake of total and major protein sources could not affect the GDM risk. Differences between Iranian and Western population and the reverse causality might be the main reasons for this nonsignificant association.

Effects of maternal nutrition and rumen-protected arginine supplementation on ewe performance and postnatal lamb growth and internal organ mass

The hypothesis of this study was that arginine supplementation would overcome negative effects of restricted maternal feed intake during the last two-thirds of gestation on ewe performance and positively affect postnatal lamb growth and development. Multiparous, Rambouillet ewes (n = 32) were allocated to 3 treatments in a completely random design at 54 ± 3.9 d of gestation. Dietary treatments were 100% of nutrient requirements (control, CON), 60% of control (restricted, RES), or RES plus a rumen-protected arginine supplement dosed at 180 mg/kg BW once daily (RES-ARG). Ewes were penned individually in a temperature-controlled facility. At parturition, lambs were immediately removed from dams and reared independently. At day 54 ± 3 of age, lambs were stunned using captive bolt, exsanguinated, and organs were collected and weighed. Ewe BW from day 68 of gestation through parturition was greater (P ≤ 0.03) in CON compared with RES or RES-ARG. Similarly, ewe BCS from day 68 of gestation through parturition was greater (P ≤ 0.03) in CON than either RES or RES-ARG. Total ewe colostrum mass (g) at 3 h after parturition was greater (P ≤ 0.001) in CON than RES or RES-ARG. Lamb birth weight was greater (P = 0.04) in CON than RES ewes and tended (P = 0.10) to be greater in CON vs. RES-ARG. Lambs born to CON ewes had greater (P ≤ 0.03) BW than lambs from RES ewes at 7, 14, and 33 d postpartum. On day 19, lambs from CON and RES-ARG ewes both had greater (P ≤ 0.04) BW than lambs from RES ewes (12.0 and 11.5 vs. 10.3 ± 0.41 kg, respectively). Lambs born to CON and RES-ARG ewes had greater (P ≤ 0.04) ADG than lambs from RES ewes on day 19 (355.0 and 354.0 vs. 306.4 ± 15.77 g, respectively). Lambs from CON and RES-ARG ewes also had greater (P ≤ 0.02) girth circumference than lambs from RES ewes on day 19 (55.4 and 54.6 vs. 51.3 ± 0.97 cm, respectively). On day 54, lambs from RES-ARG ewes had greater (P = 0.003) curved crown rump length than lambs from RES ewes (99.8 vs. 93.9 ± 1.28 cm, respectively). Adrenal glands in lambs from CON dams had greater (P = 0.01) mass than adrenal glands in lambs from RES dams. Livers from lambs born to RES-ARG ewes weighed more (P = 0.05) than livers from lambs born to RES ewes. These results confirm our hypothesis that arginine supplementation during the last two-thirds of gestation can mitigate offspring, but not maternal negative consequences associated with restricted maternal nutrition.

Intracutaneous delivery of gelatins induces lipolysis and suppresses lipogenesis of adipocytes

Due to growing interest in cosmetics and medical applications, therapeutic medications that reduce the amount of local subcutaneous adipose tissue have potential for obesity treatment. However, conventional methods such as surgical operation are restricted due to risk of complications. Here, we report a simple and effective method for local reduction of subcutaneous adipose tissue (AT) by using microneedle-assisted transdermal delivery of natural polymers. After in vitro screening tests, gelatin was selected as a therapeutic polymer to reduce accumulation of AT. An in vitro study showed that the level of released glycerol as an indicator of lipolysis was elevated in isolated adipocytes after gelatin treatment. In addition, gelatins suppressed expression levels of lipogenesis-associated genes. Following application of gelatin microneedle (GMN) patches to high-fat diet (HD)-induced obese rats, the amount of subcutaneous AT at the site of GMN application was significantly reduced, which was also confirmed by histological analysis and micro-computed tomography scanning. In addition, lipogenesis-associated genes were down-regulated in GMN-treated subcutaneous AT. These findings suggest that GMN patches induce lipolysis and simultaneously inhibit lipogenesis, thereby reducing deposition of subcutaneous AT. This platform using GMNs may provide a new strategy to treat excess subcutaneous AT with minimal complications. STATEMENT OF SIGNIFICANCE: (1) Significance This work reports a new approach for the local reduction of subcutaneous adipose tissue using a dissolving microneedle patch prepared using gelatin to enable suppression of lipogenesis and acceleration of lipolysis in adipocytes. The gelatin microneedle patch exhibited a significant reduction of local subcutaneous fat up to 60% compared to control groups without any change in total weight. (2) Scientific impact This is the first report demonstrating the direct anti-obesity effects of gelatin administrated in a transdermal route and the feasibility of natural polymer therapeutics for regional reduction of subcutaneous fat. We believe that our work will excite interdisciplinary readers of Acta Biomaterialia, those who are interested in the natural polymers, drug delivery, and obesity.

Influence of Amino Acids in Dairy Products on Glucose Homeostasis: The Clinical Evidence

Dairy products have been hypothesized to protect against type 2 diabetes because of their high content of whey proteins, rich in branched-chain amino acids (BCAAs) - leucine, isoleucine and valine - and lysine, which may decrease postprandial glucose responses and stimulate insulin secretion. Paradoxically, epidemiologic studies also show that higher levels of plasma BCAAs have been linked to insulin resistance and type 2 diabetes. Therefore, the objective was to review the recent clinical evidence concerning the intake of amino acids found in dairy proteins so as to determine their impact on glucose homeostasis in healthy persons and in those with prediabetes and type 2 diabetes. Clinical studies have reported that the major dairy amino acids, namely, leucine, isoleucine, glutamine, phenylalanine, proline and lysine, have beneficial effects on glucose homeostasis. Yet the reported doses of amino acids investigated are too elevated to be reached through adequate dairy product intake. The minor dairy amino acids, arginine and glycine, may improve glucose homeostasis by improving other risk factors for type 2 diabetes. Further, the combination of amino acids may also improve glucose-related outcomes, suggesting additive or synergistic effects. Nevertheless, additional long-term studies in individuals with prediabetes and type 2 diabetes are needed to ascertain the benefits for glucose homeostasis of amino acids found in dairy foods.

L-arginine supplementation and risk factors of cardiovascular diseases in healthy men: a double-blind randomized clinical trial

Context: The effect of L-arginine on risk factors of cardiovascular diseases (CVD) has mostly focused on western countries. Since cardiovascular diseases is the second cause of death in Iran and, as far as we are aware, there have been no studies about the effect of L-arginine on CVD risk factors, the aim of this trial was to assess the effects of L-arginine supplementation on CVD risk factors in healthy men.

Objective: The purpose of this study was to evaluate the effect of low-dose L-arginine supplementation on CVD risk factors (lipid profile, blood sugar and blood pressure) in Iranian healthy men.

Design, setting, participants: We conducted a double-blind randomized controlled trial in 56 patients selected from sport clubs at the Isfahan University of Medical Science between November 2013 and December 2013.

Interventions: Healthy men received L-arginine supplementation (2000 mg daily) in the intervention group or placebo (2000 mg maltodextrin daily) in the control group for 45 days.

Main outcome measure: The primary outcome measures were we measured the levels of fasting blood sugar, blood pressure and lipid profile including triglyceride (TG), cholesterol, LDL and HDL in healthy subjects. It was hypothesized that these measures would be significantly improved in those receiving L–arginine supplementation. at the beginning and end of the study.

Results: In this trial, we had complete data for 52 healthy participants with mean age of 20.85±4.29 years. At the end of study, fasting blood sugar (P=0.001) and lipid profile (triglycerideTG (P<0.001), cholesterol (P<0.001), LDL (P=0.04), HDL (P=0.015)) decreased in the L-arginine group but we found no significant change in the placebo group. In addition, the reduction of fasting blood sugar and lipid profile in L-arginine was significant compared with placebo group. No significant changes were found about systolic (P=0.81) and diastolic blood pressure either in L-arginine or placebo group. (P=0.532).

Conclusion: The use of L-arginine significantly improved outcomes compared to placebo.

L-arginine supplementation and risk factors of cardiovascular diseases in healthy men: a double-blind randomized clinical trial

Context: The effect of L-arginine on risk factors of cardiovascular diseases (CVD) has mostly focused on western countries. Since cardiovascular diseases is the second cause of death in Iran and, as far as we are aware, there have been no studies about the effect of L-arginine on CVD risk factors, the aim of this trial was to assess the effects of L-arginine supplementation on CVD risk factors in healthy men. Objective: The purpose of this study was to evaluate the effect of low-dose L-arginine supplementation on CVD risk factors (lipid profile, blood sugar and blood pressure) in Iranian healthy men. Design, setting, participants: We conducted a double-blind randomized controlled trial in 56 patients selected from sport clubs at the Isfahan University of Medical Science between November 2013 and December 2013. Interventions: Healthy men received L-arginine supplementation (2000 mg daily) in the intervention group or placebo (2000 mg maltodextrin daily) in the control group for 45 days. Main outcome measure: The primary outcome measures were we measured the levels of fasting blood sugar, blood pressure and lipid profile including triglyceride (TG), cholesterol, LDL and HDL in healthy subjects. It was hypothesized that these measures would be significantly improved in those receiving L-arginine supplementation. at the beginning and end of the study. Results: In this trial, we had complete data for 52 healthy participants with mean age of 20.85±4.29 years. At the end of study, fasting blood sugar (P=0.001) and lipid profile (triglycerideTG (P<0.001), cholesterol (P<0.001), LDL (P=0.04), HDL (P=0.015)) decreased in the L-arginine group but we found no significant change in the placebo group. In addition, the reduction of fasting blood sugar and lipid profile in L-arginine was significant compared with placebo group. No significant changes were found about systolic (P=0.81) and diastolic blood pressure either in L-arginine or placebo group. (P=0.532). Conclusion: The use of L-arginine significantly improved outcomes compared to placebo.

Acute lysine supplementation does not improve hepatic or peripheral insulin sensitivity in older, overweight individuals

Context

Lysine supplementation may have a positive influence on the regulation of glucose metabolism but it has not been tested in the geriatric population. Objective: We evaluated the impact of acute lysine supplementation using three randomized experimental scenarios: 1) oral glucose alone (control), 2) oral glucose and low-dose lysine (2 grams), and oral glucose and high dose lysine (5 grams) lysine in 7 older (66 ± 1 years/age), overweight/obese (BMI = 28 ± 2 kg/m²) individuals.

Methods

We utilized a dual tracer technique (i.e., [6,6-²H₂] glucose primed constant infusion and 1-[¹³C] glucose oral ingestion) during an oral glucose tolerance test (OGTT) to examine differences in hepatic and peripheral insulin sensitivity under all three scenarios.

Results

Post-absorptive plasma glucose and insulin concentrations were not different between the three trials. Similarly, the response of glucose and insulin concentrations during the oral glucose tolerance tests (OGTT) was similar in the three trials. The results of the Matsuda index (ISI/M) were also not different between the three trials. As an index of hepatic insulin sensitivity, there were no significant differences in the endogenous glucose rate of appearance (glucose R_a) for control, 2 g lysine and 5 g lysine (1.2 ± 0.1, 1.1 ± 0.1, 1.3 ± 0.1 mg•kg^-1•min^-1), respectively. With respect to peripheral insulin sensitivity, there were no significant differences in the glucose rate of disappearance (glucose R_d) for control, 2 g lysine and 5 g lysine (4.2 ± 0.1, 4.3 ± 0.2, and 4.5 ± 0.4 mg•kg^-1•min^-1), respectively.

Conclusions

Previous studies in younger participants have suggested that lysine may have a beneficial effect on glucose metabolism. However, acute lysine supplementation in the older population does not facilitate beneficial changes in glucose R_a or glucose R_d.

Electronic supplementary material

The online version of this article (doi:10.1186/1743-7075-11-49) contains supplementary material, which is available to authorized users.

Pre-exercise arginine supplementation increases time to exhaustion in elite male wrestlers

Dietary supplements containing arginine are among the most popular ergogenics intended to enhance strength, power and muscle recovery associated with both anaerobic and aerobic exercise. The aim of the present study was to evaluate the possible effect of pre-exercise acute intake of arginine on performance and exercise metabolism during incremental exhaustive exercise in elite male wrestlers. Nine volunteer elite male wrestlers (24.7±3.8 years) participated in this study. The test-retest protocol was used on the same subjects. The study was conducted using a cross-over design. A single dose of arginine (1.5 g · 10 kg^-1 body weight) or placebo was given to the subjects after 12 hours fasting (during the night) for both test and retest. Subjects were allowed to drink water but not allowed to eat anything between arginine or placebo ingestion and the exercise protocol. An incremental exercise protocol was applied and oxygen consumption was measured during the exercise. Heart rate and plasma lactate levels were measured during the exercise and recovery. Results showed that in the same working loads there was no significant difference for the mean lactate levels and no difference in maximum oxygen consumption (arginine 52.47±4.01 mL · kg^-1 · min^-1, placebo 52.07±5.21 mL · kg^-1 · min^-1) or in maximum heart rates (arginine 181.09±13.57 bpm, placebo 185.89±7.38 bpm) between arginine and placebo trials. Time to exhaustion was longer with arginine supplementation (1386.8±69.8 s) compared to placebo (1313±90.8 s) (p < 0.05). These results suggest that L-arginine supplementation can have beneficial effects on exercise performance in elite male wrestlers but cannot explain the metabolic pathways which are responsible from these effects.

Acute effects of pistachio consumption on glucose and insulin, satiety hormones and endothelial function in the metabolic syndrome

BACKGROUND/OBJECTIVE

Nut consumption has been found to decrease risk of coronary heart disease and diabetes and to promote healthy body weights possibly related to their favorable macronutrient profile. We therefore assessed the effect of pistachios on postprandial glucose and insulin levels, gut hormones related to satiety and endothelial function.

SUBJECTS/METHODS

In this randomized crossover study, 20 subjects with metabolic syndrome consumed five study meals over 5-10 weeks. The meals differed in fat type and quantity, but were matched according to available carbohydrates (CHOs). Three meals had 50 g available CHO: white bread (WB50g), white bread, butter and cheese (WB+B+Ch) and white bread and pistachios (WB+P). Two meals had 12 g available CHO: white bread (WB12g) and pistachios (P).

RESULTS

Within each group of available CHO meals, postprandial glucose levels were the highest following the white bread-only meals, and glucose response was significantly attenuated when butter and cheese or pistachios were consumed (P<0.05). Postprandial insulin levels were highest after the WB+B+Ch meal (P<0.05), but did not differ between the white bread-only and pistachio meals. Both endothelial function (reactive hyperemia index) and arterial stiffness (augmentation index) significantly increased after the white bread-only meals compared with the WB+B+Ch meal (all P<0.05). Insulin secretagogue levels were higher when butter and cheese or pistachios were consumed than when white bread only was consumed (P<0.05).

CONCLUSIONS

Compared with white bread, pistachio consumption reduced postprandial glycemia, increased glucagon-like-peptide levels and may have insulin-sparing properties. These effects could be beneficial for individuals with diabetes and metabolic syndrome.

Stimulation of insulin secretion by large-dose oral arginine administration in healthy adults

The effects of large-dose oral arginine administration on the secretion of insulin by islet β-cells in healthy adults were determined. Eight non-obese healthy volunteers with normal glucose tolerance participated randomly in tests with four stages (with an interval of at least 3 days): the 300 ml purified water stage (PWS), the 75 g glucose stage (GSS), the 30 g arginine stage (ARS) and the 75 g glucose with 30 g arginine stage (GAS). Venous blood samples were collected to detect the concentrations of glucose and insulin at baseline (0) and at 15, 30, 45, 60 and 120 min after drug administration. The glucose and insulin levels were steady in the PWS. The remaining three stages had similarly shaped insulin concentration-time curves, which differed from that of the PWS. The peak concentration of blood insulin and the net incremental area under the curve of blood insulin in the GSS, ARS and GAS were significantly higher compared with those in the PWS (P<0.05). In the ARS, the glucose levels remained stable; however, the net incremental area under the curve for blood insulin in the ARS was much lower compared with that in the GSS or GAS (P<0.05). Large-dose oral arginine administration may slightly stimulate insulin secretion by islet β-cells in healthy adults with normal glucose tolerance in a manner that is independent of glucose concentration.

Insulin signalling to the kidney in health and disease

Ninety-one years ago insulin was discovered, which was one of the most important medical discoveries in the past century, transforming the lives of millions of diabetic patients. Initially insulin was considered only important for rapid control of blood glucose by its action on a restricted number of tissues; however, it has now become clear that this hormone controls an array of cellular processes in many different tissues. The present review will focus on the role of insulin in the kidney in health and disease.

Effects of milk and milk constituents on postprandial lipid and glucose metabolism in overweight and obese men

Studies have suggested that two major milk constituents, casein and Ca, favourably affect postprandial responses. However, effects of milk on postprandial metabolism are unknown. We therefore investigated effects of using milk with a fat-containing meal on lipid and glucose responses in overweight men. To identify the constituent responsible for possible effects, we also studied responses to Ca and protein. A total of sixteen men (BMI >27 kg/m2) participated in four postprandial tests. They consumed a breakfast (44 g of fat) plus a drink: a control drink, low-fat milk or a protein and Ca drink (500 ml). Blood samples were taken before the meals and at regular time points during 6 h thereafter. Compared with control, the incremental AUC (iAUC) for serum TAG was increased by 44 % after the protein meal (P= 0·015). Although the iAUC were not different (P= 0·051), peak glucose concentrations were reduced by 24 % after protein intake, as compared with control (P= 0·021). The decrease of 18 % after milk intake did not reach statistical significance. Compared with the milk meal, the iAUC for insulin was 52 % lower after the control meal (P= 0·035) and 51 % after the protein meal (P= 0·005). The present results indicate that the intake of milk with a fat-containing meal enhances postprandial TAG and insulin responses and may blunt glucose increases. The protein fraction of milk seems to be the main determinant for the effects on TAG and glucose. Ca did not change any of the postprandial responses.

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.

Arginine-rich coconut kernel diet influences nitric oxide synthase activity in alloxandiabetic rats

BACKGROUND

Coconut kernel protein (CKP) has been reported to contain significant amounts of L-arginine. Its potential effect on glucose homeostasis, possibly through the nitric oxide synthase (NO) pathway, was therefore investigated in alloxan-induced diabetic rats. Diabetes was induced by a single intraperitoneal dose of alloxan (150 mg kg⁻¹ body weight). Experimental rats were grouped as follows: Group I, normal control; Group II, diabetic control; Group III, diabetic + CKP; Group IV, diabetic + L-arginine; Group V, diabetic + L-arginine + L-N(G)-Nitroarginine methyl ester (L-NAME). Purified CKP isolated from dried coconut kernel and L-arginine was administered to rats along with a semi-synthetic diet for 45 days. L-NAME (0.5 mg kg⁻¹ body weight) was given to Group V animals. After the experimental period, serum glucose, insulin, activities of liver nitric oxide synthase and arginase, liver glycogen levels and histopathology of the pancreas were evaluated.

RESULTS

Serum glucose, insulin and antioxidant enzyme activities and liver glycogen levels were found to be restored to basal levels in CKP-fed rats. Decreased arginase and increased nitric oxide synthase (NOS) activities were found in CKP- and arginine-fed rats. L-NAME treatment showed a partial effect on these parameters. Histopathology revealed that CKP and L-arginine feeding reduced the diabetes-related pancreatic damage in treated rats compared to the diabetic control.

CONCLUSION

The results observed in this study indicate that the potential antidiabetic activity of CKP may be through an arginine-NO pathway leading to pancreatic beta cell regeneration.

Effects of a whey protein supplementation on intrahepatocellular lipids in obese female patients

BACKGROUND & AIMS

High protein diets have been shown to improve hepatic steatosis in rodent models and in high-fat fed humans. We therefore evaluated the effects of a protein supplementation on intrahepatocellular lipids (IHCL), and fasting plasma triglycerides in obese non diabetic women.

METHODS

Eleven obese women received a 60 g/day whey protein supplement (WPS) for 4-weeks, while otherwise nourished on a spontaneous diet, IHCL concentrations, visceral body fat, total liver volume (MR), fasting total-triglyceride and cholesterol concentrations, glucose tolerance (standard 75 g OGTT), insulin sensitivity (HOMA IS index), creatinine clearance, blood pressure and body composition (bio-impedance analysis) were assessed before and after 4-week WPS.

RESULTS

IHCL were positively correlated with visceral fat and total liver volume at inclusion. WPS decreased significantly IHCL by 20.8 ± 7.7%, fasting total TG by 15 ± 6.9%, and total cholesterol by 7.3 ± 2.7%. WPS slightly increased fat free mass from 54.8 ± 2.2 kg to 56.7 ± 2.5 kg, p = 0.005). Visceral fat, total liver volume, glucose tolerance, creatinine clearance and insulin sensitivity were not changed.

CONCLUSIONS

WPS improves hepatic steatosis and plasma lipid profiles in obese non diabetic patients, without adverse effects on glucose tolerance or creatinine clearance.

TRIAL NUMBER

NCT00870077, ClinicalTrials.gov.

Dietary intake of total, animal, and vegetable protein and risk of type 2 diabetes in the European Prospective Investigation into Cancer and Nutrition (EPIC)-NL study

OBJECTIVE

Dietary recommendations are focused mainly on relative dietary fat and carbohydrate content in relation to diabetes risk. Meanwhile, high-protein diets may contribute to disturbance of glucose metabolism, but evidence from prospective studies is scarce. We examined the association among dietary total, vegetable, and animal protein intake and diabetes incidence and whether consuming 5 energy % from protein at the expense of 5 energy % from either carbohydrates or fat was associated with diabetes risk.

RESEARCH DESIGN AND METHODS

A prospective cohort study was conducted among 38,094 participants of the European Prospective Investigation into Cancer and Nutrition (EPIC)-NL study. Dietary protein intake was measured with a validated food frequency questionnaire. Incident diabetes was verified against medical records.

RESULTS

During 10 years of follow-up, 918 incident cases of diabetes were documented. Diabetes risk increased with higher total protein (hazard ratio 2.15 [95% CI 1.77-2.60] highest vs. lowest quartile) and animal protein (2.18 [1.80-2.63]) intake. Adjustment for confounders did not materially change these results. Further adjustment for adiposity measures attenuated the associations. Vegetable protein was not related to diabetes. Consuming 5 energy % from total or animal protein at the expense of 5 energy % from carbohydrates or fat increased diabetes risk.

CONCLUSIONS

Diets high in animal protein are associated with an increased diabetes risk. Our findings also suggest a similar association for total protein itself instead of only animal sources. Consumption of energy from protein at the expense of energy from either carbohydrates or fat may similarly increase diabetes risk. This finding indicates that accounting for protein content in dietary recommendations for diabetes prevention may be useful.

Lysine ingestion markedly attenuates the glucose response to ingested glucose without a change in insulin response

BACKGROUND

Ingested proteins are known to stimulate a rise in insulin and glucagon concentrations. In our effort to explain this effect, we have begun to measure the effect of individual amino acids.

OBJECTIVES

The objectives were to determine the effect of lysine ingestion on insulin and glucagon concentrations and whether the effect is moderated by glucose ingestion.

DESIGN

Thirteen healthy subjects were studied on 4 occasions. Water, 25 g glucose, 1 mmol lysine/kg lean body mass, or lysine plus glucose was given on separate occasions at 0800 after a 12-h fast. Serum lysine, glucose, insulin, and glucagon were measured during a 2.5-h period. The amount of lysine provided was equivalent to that present in a 672-g (24-oz) steak.

RESULTS

Lysine ingestion resulted in an approximately 3-fold increase in lysine concentration and in a small decrease in glucose concentration. When lysine was ingested with glucose, the 2.5-h glucose area response decreased by 44% (P < 0.02). Lysine alone increased the insulin area response modestly; the insulin increase when lysine was ingested with glucose was similar to that when only glucose was ingested. Lysine stimulated an increase in glucagon (P < 0.02), whereas glucose decreased glucagon.

CONCLUSIONS

Lysine ingestion results in a small decrease in serum glucose and an increase in glucagon and insulin concentrations. Lysine ingested with glucose dramatically attenuated the glucose-stimulated glucose response, but there was no change in insulin response. Whether similar effects will be observed with more physiologic doses of lysine remains to be determined.

Leucine, when ingested with glucose, synergistically stimulates insulin secretion and lowers blood glucose

Our laboratory is interested in the metabolic effects of ingested proteins. As part of this research, we currently are investigating the metabolic effects of ingested individual amino acids. The objective of the current study was to determine whether leucine stimulates insulin and/or glucagon secretion and whether, when it is ingested with glucose, it modifies the glucose, insulin, or glucagon response. Thirteen healthy subjects (6 men and 7 women) were studied on 4 different occasions. Subjects were admitted to the special diagnostic and treatment unit after a 12-hour fast. They received test meals at 8:00 am. On the first occasion, they received water only. Thereafter, they received 25 g glucose or 1 mmol/kg lean body mass leucine or 1 mmol/kg lean body mass leucine plus 25 g glucose in random order. Serum leucine, glucose, insulin, glucagon, and alpha-amino nitrogen concentrations were measured at various times during a 2.5-hour period after ingestion of the test meal. The amount of leucine provided was equivalent to that present in a high-protein meal, that is, that approximately present in a 350-g steak. After leucine ingestion, the leucine concentration increased 7-fold; and the alpha-amino nitrogen concentration increased by 16%. Ingested leucine did not affect the serum glucose concentration. When leucine was ingested with glucose, it reduced the 2.5-hour glucose area response by 50%. Leucine, when ingested alone, increased the serum insulin area response modestly. However, it increased the insulin area response to glucose by an additional 66%; that is, it almost doubled the response. Ingested leucine stimulated an increase in glucagon. Ingested glucose decreased it. When ingested together, the net effect was essentially no change in glucagon area. In summary, leucine at a dose equivalent to that present in a high-protein meal, had little effect on serum glucose or insulin concentrations but did increase the glucagon concentration. When leucine was ingested with glucose, it attenuated the serum glucose response and strongly stimulated additional insulin secretion. Leucine also attenuated the decrease in glucagon expected when glucose alone is ingested. The data suggest that a rise in glucose concentration is necessary for leucine to stimulate significant insulin secretion. This in turn reduces the glucose response to ingested glucose.

A specific blend of intact protein rich in aspartate has strong postprandial glucose attenuating properties in rats

Three studies were carried out to help define an optimal protein blend for use in a nutritional product for diabetic patients. To this end, we tested the effects of coinfusions of combinations of different types of carbohydrates and proteins on the postprandial glycemic plasma response in healthy rats. Expt. 1 compared the effects of administering different forms of soy protein (intact protein, its hydrolysate, or an equivalent amount of the same amino acids), all in combination with a fixed amount of glucose (Glu), on postprandial Glu and insulin plasma concentrations. Intact soy protein (SI) had stronger insulinogenic properties compared with its hydrolysate but was equally potent in reducing the postprandial Glu response. In Expt. 2, we compared the effect of replacing 50% of the SI with the whey-derived protein alpha-lactalbumin when coingested with maltodextrin as the carbohydrate source. Only the specific aspartate-rich blend of SI and alpha-lactalbumin significantly improved the postprandial Glu response. In Expt. 3, we studied the effect of using the blend of SI and alpha-lactalbumin combined with a slowly digestible carbohydrate. The protein blend was still capable of significantly decreasing the postprandial Glu response even when a slow-release carbohydrate source was included. Combining this aspartate-rich protein blend with a slow-release carbohydrate might therefore lead to a low-glycemic nutritional product beneficial for dietary management in diabetic patients.

Reduced carbohydrate intake in citrin-deficient subjects

Citrin is the liver-type aspartate-glutamate carrier that resides within the inner mitochondrial membrane. Citrin deficiency (due to homozygous or compound heterozygous mutations in the gene SLC25A13) causes both adult-onset type II citrullinaemia (CTLN2) and neonatal intrahepatic cholestasis (NICCD). Clinically, CTLN2 is characterized by hyperammonaemia and citrullinaemia, whereas NICCD has a much more varied and transient presentation that can include multiple aminoacidaemias, hypoproteinaemia, galactosaemia, hypoglycaemia, and jaundice. Personal histories from CTLN2 patients have repeatedly described an aversion to carbohydrate-rich foods, and clinical observations of dietary and therapeutic outcomes have suggested that their unusual food preferences may be directly related to their pathophysiology. In the present study, we monitored the food intake of 18 Japanese citrin-deficient subjects whose ages ranged from 1 to 33 years, comparing them against published values for the general Japanese population. Our survey confirmed a marked decrease in carbohydrate intake, which accounts for a smaller proportion of carbohydrates contributing to the total energy intake (PFC ratio) as well as a shift towards a lower centile distribution for carbohydrate intake relative to age- and sex-matched controls. These results strongly support an avoidance of carbohydrate-rich foods by citrin-deficient patients that may lead to worsening of symptoms.

Meal amino acids with varied levels of arginine do not affect postprandial vascular endothelial function in healthy young men

Postprandial endothelial dysfunction is a key event mediating the effects of diet on early atherogenesis. The potential effects of protein intake have been overlooked in the past, although amino acids are precursors for homocysteine and nitric oxide (NO). Our objective was to study the effect of amino acids on postprandial vascular function, in relation to the utilization of meal arginine for NO production. In a crossover design, 9 men ingested 50 g of a complete amino acid mixture, trace-labeled with (13)C-glycine and (15)N(2)-arginine, without (meal A) or with (meal B) 3 g extra arginine. The postprandial utilization of meal arginine for NO production was determined from urinary (15)NO(3). We monitored endothelial function of the brachial artery, the stiffness of the common carotid artery, aortic pulse wave velocity and soluble markers related to endothelial function for 8 h. Meal A did not significantly increase plasma homocysteine and did not alter endothelial function markers. The amount of NO synthesized from meal arginine doubled after meal B (107.1 +/- 16.5% increase vs. meal A, P < 0.01) but was very low (271 +/- 84 ppm vs. 332 +/- 73 ppm, P < 0.05, respectively). After meal B, flow-mediated and nitroglycerine-induced dilation decreased but common carotid artery compliance, pulse wave velocity, plasma soluble intercellular adhesion molecule-1, and von Willebrand factor, and urinary cGMP did not differ when compared with meal A. Together, the data indicate that, in healthy men, meal amino acids do not adversely affect endothelial function, and meal arginine only slightly enters the NO pathway. Unexpectedly, arginine in physiological amounts may acutely lessen smooth muscle cell reactivity to a high dynamic NO release.

Adverse gastrointestinal effects of arginine and related amino acids

Oral supplements of arginine and citrulline increase local nitric oxide (NO) production in the small intestine and this may be harmful under certain circumstances. Gastrointestinal toxicity was therefore reviewed with respect to the intestinal physiology of arginine, citrulline, ornithine, and cystine (which shares the same transporter) and the many clinical trials of supplements of the dibasic amino acids or N-acetylcysteine (NAC). The human intestinal dibasic amino acid transport system has high affinity and low capacity. L-arginine (but not lysine, ornithine, or D-arginine) induces water and electrolyte secretion that is mediated by NO, which acts as an absorbagogue at low levels and as a secretagogue at high levels. The action of many laxatives is NO mediated and there are reports of diarrhea following oral administration of arginine or ornithine. The clinical data cover a wide span of arginine intakes from 3 g/d to>100 g/d, but the standard of reporting adverse effects (e.g. nausea, vomiting, and diarrhea) was variable. Single doses of 3-6 g rarely provoked side effects and healthy athletes appeared to be more susceptible than diabetic patients to gastrointestinal symptoms at individual doses>9 g. This may relate to an effect of disease on gastrointestinal motility and pharmacokinetics. Most side effects of arginine and NAC occurred at single doses of >9 g in adults (>140 mg/kg) often when part of a daily regime of approximately>30 g/d (>174 mmol/d). In the case of arginine, this compares with the laxative threshold of the nonabsorbed disaccharide alcohol, lactitol (74 g or 194 mmol). Adverse effects seemed dependent on the dosage regime and disappeared if divided doses were ingested (unlike lactitol). Large single doses of poorly absorbed amino acids seem to provoke diarrhea. More research is needed to refine dosage strategies that reduce this phenomenon. It is suggested that dipeptide forms of arginine may meet this criterion.

Metabolic effects of amino acid mixtures and whey protein in healthy subjects: studies using glucose-equivalent drinks

BACKGROUND

Milk protein, in particular the whey fraction, has been shown to display insulinotrophic properties in healthy persons and persons with type 2 diabetes. In parallel to the hyperinsulinemia, a pronounced postprandial rise of certain amino acids and of glucose-dependent insulinotrophic polypeptide (GIP) was observed in plasma.

OBJECTIVE

The objective of the study was to determine to what extent the insulinotrophic properties of whey could be simulated by specific amino acid mixtures.

DESIGN

Twelve healthy volunteers were served drinks consisting of pure glucose (reference drink) or glucose supplemented with free amino acids or whey proteins (test drinks).

RESULTS

A test drink with the branched-chain amino acids isoleucine, leucine, and valine resulted in significantly higher insulin responses than did the glucose reference. A drink containing glucose and leucine, isoleucine, valine, lysine, and threonine mimicked the glycemic and insulinemic responses seen after whey ingestion. With consumption of this drink, the glucose area under the curve (AUC) was 44% smaller (P < 0.05) and the insulin AUC was 31% larger (NS) than with consumption of the reference drink. With consumption of the whey drink, the AUCs were 56% smaller (glucose; P < 0.05) and 60% larger (insulin; P < 0.05), respectively, than with the reference drink. The whey drink was accompanied by an 80% greater GIP response (P < 0.05), whereas the drinks containing free amino acids did not significantly affect GIP secretion.

CONCLUSION

A mixture of leucine, isoleucine, valine, lysine, and threonine resulted in glycemic and insulinemic responses closely mimicking those seen after whey ingestion in the absence of an additional effect of GIP and glucagon-like peptide 1.

Effects of L-arginine supplementation on exercise metabolism

PURPOSE OF REVIEW

To describe the influence of acute and chronic administration of L-arginine on metabolism at rest and during exercise.

RECENT FINDINGS

There has been substantial examination of the effect of infusion and ingestion of L-arginine at rest. It has been clearly demonstrated that L-arginine administration improves endothelial function in various disease states. In addition, L-arginine infusion at rest increases plasma insulin, growth hormone, glucagon, catecholamines and prolactin. Such hormonal changes affect metabolism. There has, however, been very little examination of the effect of increases in L-arginine availability during exercise. This is important to study as there is preliminary evidence that L-arginine infusion, probably via increases in nitric oxide (NO), alters skeletal-muscle metabolism during exercise. There is a need for further research, especially to understand the mechanisms of how L-arginine affects exercise metabolism and also to determine whether the hormonal responses that occur in response to L-arginine at rest are also present to some extent during exercise.

SUMMARY

This line of research may have important therapeutic implications as there are indications that L-arginine augments the effects of exercise training on insulin sensitivity and capillary growth in muscles.

The nutritional control of ghrelin secretion in humans: the effects of enteral vs. parenteral nutrition

BACKGROUND

The nutritional control of ghrelin has not been fully clarified yet. Particularly, the influence of aminoacids and lipids is controversial and, moreover, whether the intraluminal gastric contact with nutrients is required or if the modulatory action of nutrients on ghrelin secretion is mediated by insulin is still matter of debate.

AIM OF THE STUDY

To clarify the role of nutrients in the control of ghrelin secretion evaluating the effects of intravenous and oral lipids and aminoacids compared with glucose and fructose load in healthy subjects.

METHODS

A total of 6 healthy overnight-fasted volunteers underwent the following testing sessions: (a) iv arginine (ARG, 0.5 g/kg); (b) oral protein load (PRO, 50 g); (c) iv lipid-heparin infusion (Li He, Intralipid 10% 250 ml); (d) oral fat load (OIL, soy oil 40 g); (e) oral glucose load (OGL, 100 g); (f) oral fructose load (OFL, 100 g); (g) iv saline (SAL, 3 ml); (h) oral water load (WL, 200 ml). Total ghrelin, insulin, and glucose were assayed every 15 min from 0 up to +180 min.

RESULTS

WL and SAL did not modify insulin, glucose and ghrelin. ARG induced a prompt but transient increase (P < 0.05) of insulin and glucose (P < 0.01), without modifying ghrelin secretion. PRO induced a mild but sustained increase of insulin secretion (P < 0.05) without affecting glucose and ghrelin. Li-He progressively increased circulating glucose (P < 0.01) without modifying insulin and ghrelin secretion. No significant variations in circulating glucose, insulin, and ghrelin occurred after OIL. OGL significantly (P < 0.01) increased insulin and glucose levels and progressively decreased (P < 0.05) ghrelin levels. OFL induced a mild (P < 0.05) increase of insulin without modifying glucose levels. Similarly, OFL was followed by a milder decrease (P < 0.05) of ghrelin levels.

CONCLUSIONS

Differently from carbohydrates and independently from their modulatory effect on insulin secretion and glucose levels, both lipids and aminoacids play a negligible role in the acute control of ghrelin secretion either after acute enteral and parenteral administration.

The reduced energy intake of rats fed a high-protein low-carbohydrate diet explains the lower fat deposition, but macronutrient substitution accounts for the improved glycemic control

The metabolic effect of high-protein low-carbohydrate (HP) diets on body composition and glucose homeostasis remains incompletely understood. This study assesses the respective roles of the increased protein:carbohydrate ratio (P:C) and the resulting moderate decrease in energy intake in the metabolic effects of HP diets. Rats had free access to normal (NP; 14%) or high (HP; 53%) total milk protein isoenergetic diets, or were fed the NP diet but restricted to the energy intake of HP rats (NPr), which was 89.1 +/- 9.3% that of NP rats. After 8 wk, body weight was lower in HP and NPr rats than in NP rats. In HP rats, the lower body weight was associated with a lower adipose tissue mass and a reduced proportion of large adipocytes. HP rats also had an improved oral glucose tolerance and insulin sensitivity, as assessed by the homeostatic model assessment index, compared with NPr and NP rats, and these effects were related solely to the increased P:C. These data suggest that the reduced energy intake of rats fed a high-protein, low-carbohydrate diet explains the lower fat deposition but an increased P:C per se improves glucose homeostasis.

Chronic oral administration of arginine induces GH gene expression and insulin resistance

Arginine (Arg) presents a potent growth hormone (GH) releasing activity. In vivo and in vitro studies carried out in our laboratory have demonstrated that acute treatment with Arg also increases GH gene expression. Taking into account the recognizable diabetogenic role of GH and that Arg increases insulin release, this study aimed at evaluating the effects of oral chronic administration of Arg on GH gene expression, by Northern blotting analysis, and on the insulin sensitivity, by means of the Insulin Tolerance Test (ITT), blood glucose decay rate (kitt) and insulin plasma concentration. We demonstrated that rats that consumed Arg ( approximately 35 mg/day) in drinking water, during 4 weeks, presented an increase in GH mRNA content (p < 0.01), a decreased peripheral response to insulin, as shown by the reduced blood glucose decay rate (p < 0.05), and a higher insulin plasma concentration (p < 0.01) than control group. Arg treatment did not modify the animals' food and water intake, while it decreased the heart rate and the arterial blood pressure compared to control group (p < 0.05). According to the results presented herein we conclude that chronic oral administration of arginine increases GH gene expression and induces insulin resistance. The arterial blood pressure decrease has already been pointed out in the literature, and seems to occur in response to the dilating effect of nitric oxide generated from Arg, as well as from NO generation in central and peripheral neuronal populations that express NOS and are involved in the autonomic regulation of the cardiac function.

Glucose and insulin responses in healthy women after intake of composite meals containing cod-, milk-, and soy protein

OBJECTIVE

To evaluate the metabolic effect of three different kinds of dietary proteins as part of composite meals with similar macronutrient composition in healthy subjects.

DESIGN

A randomised meal study.

SETTING

Metabolic ward.

SUBJECTS AND METHODS

In total, 17 healthy women, 30-65 years old, consumed three meals in randomised order. The meals consisted of foodstuffs with similar nutrient composition but different types of protein (cod, cottage cheese, or soy protein isolate). The distribution of energy from protein, fat and carbohydrates was 33, 26, and 41 energy percent, respectively. Total amount of energy was 2300 kJ. Blood samples were drawn for assay of B-glucose, S-insulin, S-free fatty acids, S-triglycerides, and C-peptide in the fasting state and at seven times (20, 40, 60, 90, 120, 180, and 240 min) after starting to eat the test meal.

RESULTS

The blood glucose response after the cod protein meal differed from that of the soy protein meal, with a larger area under the curve (AUC) calculated up to 120 min. The serum insulin response after the milk protein meal differed from that of the cod protein meal with a larger AUC calculated up to 240 min. The insulin/C-peptide and the insulin/glucose ratios differed between the meals; the insulin/C-peptide ratio was higher after the milk protein meal compared to the cod, and soy protein meal at 120 min. The insulin/glucose ratio was lower after the cod protein meal compared to the milk, and soy protein meals at 120 min. The results showed that the metabolic responses differed after meals with similar macronutrient composition containing cod-, milk-, or soy protein.

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.

Leucine reduces the duration of insulin-induced PI 3-kinase activity in rat skeletal muscle

Leucine (Leu) is known to stimulate translation initiation of protein synthesis at mammalian target of rapamycin (mTOR) in the insulin signaling pathway. However, potential feedback from mTOR to upstream aspects of the insulin signaling pathway remains controversial. This study evaluates the impact of a physiological oral dose of Leu and/or carbohydrate (CHO) on upstream elements of the insulin signaling pathway using phosphatidylinositol 3-kinase (PI 3-kinase) activity and glucose uptake as markers for insulin sensitivity and glucose homeostasis. Rats (approximately 200 g) were fasted 12 h and administered oral doses of CHO (1.31 g glucose, 1.31 g sucrose), Leu (270 mg), or CHO plus Leu. Animals were killed at 15, 30, 60, and 90 min after treatment. Plasma and gastrocnemius muscles were collected for analyses. Treatments were designed to produce elevated blood glucose and insulin with basal levels of Leu (CHO); elevated Leu with basal levels of glucose and insulin (Leu); or a combined increase of glucose, insulin, and Leu (CHO + Leu). The CHO treatment stimulated PI 3-kinase activity and glucose uptake with no effect on the downstream translation initiation factor eIF4E. Leu alone stimulated the release of the translation initiation factor eIF4E from 4E-BP1 with no effects on PI 3-kinase activity or glucose uptake. The CHO + Leu treatment reduced the magnitude and duration of the PI 3-kinase response but maintained glucose uptake similar to the CHO treatment and eIF4E levels similar to the Leu treatment. These findings demonstrate that Leu reduces insulin-stimulated PI 3-kinase activity while increasing downstream translation initiation and with no effect on net glucose transport in skeletal muscle.

Sepsis: an arginine deficiency state?

OBJECTIVE

Sepsis is a major health problem considering its significant morbidity and mortality rate. The amino acid L-arginine has recently received substantial attention in relation to human sepsis. However, knowledge of arginine metabolism during sepsis is limited. Therefore, we reviewed the current knowledge about arginine metabolism in sepsis.

DATA SOURCE

This review summarizes the literature on arginine metabolism both in general and in relation to sepsis. Moreover, arginine-related therapies are reviewed and discussed, which includes therapies of both nitric oxide (NO) and arginine administration and therapies directed toward inhibition of NO.

DATA

In sepsis, protein breakdown is increased, which is a key process to maintain arginine delivery, because both endogenous de novo production from citrulline and food intake are reduced. Arginine catabolism, on the other hand, is markedly increased by enhanced use of arginine in the arginase and NO pathways. As a result, lowered plasma arginine levels are usually found. Clinical symptoms of sepsis that are related to changes in arginine metabolism are mainly related to hemodynamic alterations and diminished microcirculation. NO administration and arginine supplementation as a monotherapy demonstrated beneficial effects, whereas nonselective NO synthase inhibition seemed not to be beneficial, and selective NO synthase 2 inhibition was not beneficial overall.

CONCLUSIONS

Because sepsis has all the characteristics of an arginine-deficiency state, we hypothesise that arginine supplementation is a logical option in the treatment of sepsis. This is supported by substantial experimental and clinical data on NO donors and NO inhibitors. However, further evidence is required to prove our hypothesis.

Postprandial lipid and carbohydrate responses after the ingestion of a casein-enriched mixed meal

BACKGROUND

Postprandial lipemia is markedly modulated when carbohydrates are added to a fatty meal. The effect of added protein is less known, however, and the data are controversial.

OBJECTIVE

We investigated the effects of casein added to various fat-rich meals in the absence and presence of oligosaccharides.

DESIGN

Four different test meals were given to 24 healthy volunteers: 1) fat alone, consisting of 3 g cream/kg body wt; 2) fat plus 75 g oligosaccharides; 3) fat plus 50 g sodium caseinate; and 4) a combination of all 3 components. Blood samples were taken before the meals and 1, 2, 3, 4, 5, 6, 7, and 8 h thereafter. The variables measured were serum free fatty acids, arginine, glucose, insulin, and C-peptide as well as triacylglycerol in serum, in chylomicrons, and in VLDL. Gastric emptying was monitored with the use of a (13)C breath test.

RESULTS

Addition of oligosaccharides resulted in the known delay and reduction in postprandial lipemia. Casein caused additional effects: chylomicrons were further reduced and delayed, independently of gastric emptying. C-peptide and insulin, as expressed by their areas under the curves, were raised not only during the early response to the glucose load but also in the postabsorptive state. Concentrations of free fatty acids, which were markedly suppressed by 24% after oligosaccharides alone, were lowered a further 20% after the addition of casein.

CONCLUSIONS

Casein added to a fatty meal lowers free fatty acids markedly in the postprandial and postabsorption phases, probably via its insulinotropic activity. Postprandial lipemia is also moderately reduced.

The metabolic response to ingestion of proline with and without glucose

Ingested protein results in an increase in circulating insulin and glucagon concentrations and no change, or a slight decrease, in circulating glucose. In subjects with type 2 diabetes, when protein is ingested with glucose, insulin is further increased and the glucose rise is less than when glucose is ingested alone. Presumably these effects are due to the amino acids present in the proteins. The effects of individual amino acids, ingested in physiologic amounts, with or without glucose, have not been determined. Therefore, we have begun a systematic study of the response to ingested amino acids. Eight young, non-obese, subjects (4 men, 4 women) ingested 1 mmol proline/kg lean body mass, 25 g glucose, 25 g glucose + 1 mmol proline/kg lean body mass or water only on 4 separate occasions at 8 am. Blood was obtained before and after ingestion of the test meal over the following 150 minutes. Proline ingestion resulted in a 13-fold increase in the plasma proline concentration. This was decreased by 50% when glucose was ingested with proline. Proline alone had little effect on glucose, insulin, or glucagon concentrations. However, ingestion of proline with glucose resulted in a 23% attenuation of the glucose area response and no change in insulin response compared with the response to that of glucose alone. A glucose-stimulated decrease in glucagon was further facilitated by proline. Ingested proline is readily absorbed. It reduces the glucose-induced increase in glucose concentration in the presence of an unchanged insulin and a decreased glucagon response.

A decrease in glucose production is associated with an increase in plasma citrulline response to oral arginine in normal volunteers

Acute arginine administration (30 g) increases insulin secretion and reduces glucose production (GP). A slower administration of L-arginine may have direct effect on the liver without increasing C-peptide or insulin secretion. We tested the direct effect of oral L-arginine on fasting GP in 15 normal-weight volunteers and compared these to a group of L-alanine-treated controls (placebo). Volunteers were admitted to the General Clinical Research Center for a 3-day stay. Three grams of freebase arginine or alanine was ingested hourly between 4 am and 2 pm. Neither arginine nor alanine had an effect on C-peptide or insulin concentration. Oral arginine, but not alanine, increased plasma arginine, citrulline, and ornithine concentrations. Arginine-treated volunteers had a greater fall in GP as compared to the alanine-treated group (16.2% +/- 1.9% v 9.7% +/- 3.6%, respectively; P<.05). Five volunteers treated with arginine had less than a 30% increase in citrulline concentration (26 +/- 2 to 32 +/- 2 micromol/L, mean +/- SEM) and 10 volunteers had equal to or greater than a 30% increase in plasma citrulline concentration (29 +/- 2 to 49 +/- 4 micromol/L, P<.05). Since citrulline is generated in the conversion of arginine to nitric oxide (NO), the failure of oral arginine to increase citrulline concentration suggests that NO generation may be varied in different individuals. The increased plasma citrulline group reduced GP by 18.2% +/- 1.9% over the final 4 hours of arginine administration (2.00 +/- 0.08 to 1.64 +/- 0.07 mg/kg/min; P<.01). In contrast, GP only decreased by 12.4% +/- 3.9% (1.97 +/- 0.13 to 1.73 +/- 0.13 mg/kg/min; not significant [NS]) in those who had little to no increase in plasma citrulline concentration. The 12% decrease in GP in the hyporesponders was similar to the 10% decrease seen in the alanine-treated normal volunteers (9.7% +/- 3.6%). Individuals may have a variable NO response from an oral arginine administration. GP is suppressed in those who have a greater increase in plasma citrulline concentration.