Plasma Free Amino Acid Responses to Whey Protein and Their Relationships with Gastric Emptying, Blood Glucose- and Appetite-Regulatory Hormones and Energy Intake in Lean Healthy Men

Decoding the nexus: branched-chain amino acids and their connection with sleep, circadian rhythms, and cardiometabolic health

The sleep-wake cycle stands as an integrative process essential for sustaining optimal brain function and, either directly or indirectly, overall body health, encompassing metabolic and cardiovascular well-being. Given the heightened metabolic activity of the brain, there exists a considerable demand for nutrients in comparison to other organs. Among these, the branched-chain amino acids, comprising leucine, isoleucine, and valine, display distinctive significance, from their contribution to protein structure to their involvement in overall metabolism, especially in cerebral processes. Among the first amino acids that are released into circulation post-food intake, branched-chain amino acids assume a pivotal role in the regulation of protein synthesis, modulating insulin secretion and the amino acid sensing pathway of target of rapamycin. Branched-chain amino acids are key players in influencing the brain's uptake of monoamine precursors, competing for a shared transporter. Beyond their involvement in protein synthesis, these amino acids contribute to the metabolic cycles of γ-aminobutyric acid and glutamate, as well as energy metabolism. Notably, they impact GABAergic neurons and the excitation/inhibition balance. The rhythmicity of branched-chain amino acids in plasma concentrations, observed over a 24-hour cycle and conserved in rodent models, is under circadian clock control. The mechanisms underlying those rhythms and the physiological consequences of their disruption are not fully understood. Disturbed sleep, obesity, diabetes, and cardiovascular diseases can elevate branched-chain amino acid concentrations or modify their oscillatory dynamics. The mechanisms driving these effects are currently the focal point of ongoing research efforts, since normalizing branched-chain amino acid levels has the ability to alleviate the severity of these pathologies. In this context, the Drosophila model, though underutilized, holds promise in shedding new light on these mechanisms. Initial findings indicate its potential to introduce novel concepts, particularly in elucidating the intricate connections between the circadian clock, sleep/wake, and metabolism. Consequently, the use and transport of branched-chain amino acids emerge as critical components and orchestrators in the web of interactions across multiple organs throughout the sleep/wake cycle. They could represent one of the so far elusive mechanisms connecting sleep patterns to metabolic and cardiovascular health, paving the way for potential therapeutic interventions.

Impacts of dietary animal and plant protein on weight and glycemic control in health, obesity and type 2 diabetes: friend or foe?

It is well established that high-protein diets (i.e. ~25-30% of energy intake from protein) provide benefits for achieving weight loss, and subsequent weight maintenance, in individuals with obesity, and improve glycemic control in type 2 diabetes (T2D). These effects may be attributable to the superior satiating property of protein, at least in part, through stimulation of both gastrointestinal (GI) mechanisms by protein, involving GI hormone release and slowing of gastric emptying, as well as post-absorptive mechanisms facilitated by circulating amino acids. In contrast, there is evidence that the beneficial effects of greater protein intake on body weight and glycemia may only be sustained for 6-12 months. While both suboptimal dietary compliance and metabolic adaptation, as well as substantial limitations in the design of longer-term studies are all likely to contribute to this contradiction, the source of dietary protein (i.e. animal vs. plant) has received inappropriately little attention. This issue has been highlighted by outcomes of recent epidemiological studies indicating that long-term consumption of animal-based protein may have adverse effects in relation to the development of obesity and T2D, while plant-based protein showed either protective or neutral effects. This review examines information relating to the effects of dietary protein on appetite, energy intake and postprandial glycemia, and the relevant GI functions, as reported in acute, intermediate- and long-term studies in humans. We also evaluate knowledge relating to the relevance of the dietary protein source, specifically animal or plant, to the prevention, and management, of obesity and T2D.

Nutrient Combinations Sensed by L-Cell Receptors Potentiate GLP-1 Secretion

Obesity is a risk factor for cardiometabolic diseases. Nutrients stimulate GLP-1 release; however, GLP-1 has a short half-life (<2 min), and only <10-15% reaches the systemic circulation. Human L-cells are localized in the distal ileum and colon, while most nutrients are absorbed in the proximal intestine. We hypothesized that combinations of amino acids and fatty acids potentiate GLP-1 release via different L-cell receptors. GLP-1 secretion was studied in the mouse enteroendocrine STC-1 cells. Cells were pre-incubated with buffer for 1 h and treated with nutrients: alpha-linolenic acid (αLA), phenylalanine (Phe), tryptophan (Trp), and their combinations αLA+Phe and αLA+Trp with dipeptidyl peptidase-4 (DPP4) inhibitor. After 1 h GLP-1 in supernatants was measured and cell lysates taken for qPCR. αLA (12.5 µM) significantly stimulated GLP-1 secretion compared with the control. Phe (6.25-25 mM) and Trp (2.5-10 mM) showed a clear dose response for GLP-1 secretion. The combination of αLA (6.25 µM) and either Phe (12.5 mM) or Trp (5 mM) significantly increased GLP-1 secretion compared with αLA, Phe, or Trp individually. The combination of αLA and Trp upregulated GPR120 expression and potentiated GLP-1 secretion. These nutrient combinations could be used in sustained-delivery formulations to the colon to prolong GLP-1 release for diminishing appetite and preventing obesity.

Oral hydrolyzed whey protein in the perioperative period of cesarean section increases postoperative prolactin in mothers: a randomized controlled trial

ABSTRACT Objective This study aimed to investigate the effect of oral administration of hydrolyzed whey protein during the perioperative period of cesarean section on postoperative prolactin levels and breast milk production in mothers. Methods triple-blind randomized trial was conducted on women who gave birth in a Chinese tertiary care hospital. Participants were randomly assigned to either the hydrolyzed whey protein or placebo group and were administered the respective substances at specific times during the perioperative period. The primary outcomes were prolactin levels and additional fluid milk intake during specific postoperative periods. Secondary observations included mothers’ subjective preoperative feelings and postoperative exhaust time. Results The study found that mothers in the hydrolyzed whey protein group had reduced visual hunger (p=0.002) and anxiety (p<0.001) while having higher neonatal blood glucose levels (p=0.041) compared to the placebo group. This group also had higher prolactin levels (p=0.009) and a reduced the number of milk additions in the first 48 hours of a newborn’s life (No-BreakpNo-Break=0.035). Conclusion Hydrolyzed whey protein intake during the cesarean section perioperative period reduces maternal hunger and anxiety, increases serum prolactin levels 3-4 hours after surgery, increases the infant’s blood glucose level 30 minutes after birth, and reduces milk supplementation in the first 48 hours.

Absorption of egg white hydrolysate in the intestine: Clathrin-dependent endocytosis as the main transport route

This paper aimed to investigate the in vivo absorption of egg white hydrolysate (EWH) in rats and the transport route across the intestinal epithelium. Results showed that the level of plasma peptide-bound amino acid (PAA) of the EWH-supplemented rats (EWH-R) was determined to be 2012.18 ± 300.98 μmol/L, 10.72% higher than that of the control group, and was significantly positively correlated to that of EWH. Thirty-three egg white-derived peptides were successfully identified from the plasma of EWH-R, and 20 of them were found in both EWH-R plasma and EWH, indicating that these peptides tend to be absorbed through the intestinal epithelium in intact forms into the blood circulation. In addition, 637 up-regulated and 577 down-regulated genes in Caco-2 cells incubated with EWH were detected by RNA-sequencing and the clathrin-dependent endocytosis was the most enriched pathway in KEGG analysis. EWH significantly increased the mRNA levels of the key genes involved in the clathrin-dependent endocytosis but these changes would be inhibited by the clathrin-dependent endocytosis inhibitor of chlorpromazine. Moreover, the transepithelial transport of EWH across Caco-2 cell monolayers was significantly reduced by chlorpromazine. This study provided molecular-level evidence for the first time that clathrin-dependent endocytosis might be the main transport route of EWH in the intestinal epithelium.

Impact of a Whey Protein Hydrolysate Treated by Electrodialysis with Ultrafiltration Membrane on the Development of Metabolic Syndrome and the Modulation of Gut Microbiota in Mice

The development of Metabolic Syndrome (MetS) affects a large number of people around the world and represents a major issue in the field of health. Thus, it is important to implement new strategies to reduce its prevalence, and various approaches are currently under development. Recently, an eco-friendly technology named electrodialysis with ultrafiltration membrane (EDUF) was used successfully for the first time at a semi-industrial scale to produce three fractions concentrated in bioactive peptides (BPs) from an enzymatically hydrolyzed whey protein concentrate (WPC): the initial (F1), the final (F2) and the recovery fraction (F3), and it was demonstrated in vitro that F3 exhibited interesting DPP-IV inhibitory effects. Therefore, the present study aimed to evaluate the effect of each fraction on in vivo models of obesity. A daily dose of 312.5 mg/kg was administered to High Fat/High Sucrose diet (HFHS) induced C57BL6/J mice for eight weeks. The physiological parameters of each group and alterations of their gut microbiota by the fractions were assessed. Little effect of the different fractions was demonstrated on the physiological state of the mice, probably due to the digestion process of the BP content. However, there were changes in the gut microbiota composition and functions of mice treated with F3.

The Break-Fast study protocol: a single arm pre-post study to measure the effect of a protein-rich breakfast on autophagic flux in fasting healthy individuals

BACKGROUND

Autophagy is a cellular process that cleanses cells and is particularly important during ageing. Autophagy has been extensively studied in vitro and in animal models and is known to be sensitive to nutrition. However, human data are limited because autophagic flux (autophagic degradative activity) has been challenging to measure in humans. This protocol paper describes the Break-Fast study, in which autophagic flux will be measured using a recently developed blood test, before and after ingestion of whey protein. This aims to determine whether an acute nutritional intervention can change autophagy in humans.

METHODS

A minimum of forty healthy participants (both male and female) aged 20-50 years, BMI 18.5-29.9 kg/m² will be recruited into this single arm pre-post study. Participants will visit the clinic after an overnight fast for a first blood collection after which they will consume a whey protein-rich drink. A second blood collection will be performed 60 minutes after consumption of the drink. The primary outcome is the change in autophagic flux at 60 minutes post drink. Secondary outcomes include changes in blood glucose, autophagy-related proteins and mRNA, plasma hormones (e.g. insulin, C-peptide, adiponectin, GLP-1, GIP, ghrelin), cytokines, amino acids and lipids, protein synthesis, and correlation between molecular cell damage and autophagic flux.

DISCUSSION

This study will provide information about whether autophagy responds to nutrients in humans, and if nutritional strategies could be used to treat or prevent autophagy-related diseases such as Alzheimer's disease or cancer.

TRIAL REGISTRATION

Australian New Zealand Clinical Trials Registry (ANZCTR), anzctr.org.au ACTRN12621001029886. Registered on 5 August 2021.

Circulatory amino acid responses to milk consumption in dairy and lactose intolerant individuals

Background/objectives

Self-reported digestive intolerance to dairy foods is common. As dairy can be an important source of dietary protein, this study aimed to identify whether milk protein digestion is compromised in individuals with digestive intolerance.

Subjects/methods

Adult women (n = 40) were enroled in this double-blinded, randomised cross-over trial, with digestive symptoms characterised using a lactose challenge and self-reported digestive symptom questionnaire. Participants were classified as either lactose intolerant (LI, n = 10), non-lactose dairy intolerant (NLDI, n = 20) or dairy tolerant (DT, n = 10). In a randomised sequence, participants consumed three different kinds of milk (750 ml); conventional milk (CON), a2 Milk™ (A2M), and lactose-free conventional milk (LF-CON). Circulatory plasma amino acid (AA) concentrations were measured at baseline and every 30 min until 3 h post-ingestion.

Results

In all participants across all milk types, plasma AA concentrations (AUC_0-180) increased after milk ingestion with no significant differences in responses observed between milk types or participants (P > 0.05), with the exception of the suppressed lysine response in the DT group following A2M ingestion, relative to the other two groups and milk types (P < 0.05).

Conclusion

Milk protein digestion, as determined by circulatory AAs, is largely unaffected by dairy- and lactose- intolerances.

Comparative Effects of the Branched-Chain Amino Acids, Leucine, Isoleucine and Valine, on Gastric Emptying, Plasma Glucose, C-Peptide and Glucagon in Healthy Men

(1) Background: Whey protein lowers postprandial blood glucose in health and type 2 diabetes, by stimulating insulin and incretin hormone secretion and slowing gastric emptying. The branched-chain amino acids, leucine, isoleucine and valine, abundant in whey, may mediate the glucoregulatory effects of whey. We investigated the comparative effects of intragastric administration of leucine, isoleucine and valine on the plasma glucose, C-peptide and glucagon responses to and gastric emptying of a mixed-nutrient drink in healthy men. (2) Methods: 15 healthy men (27 ± 3 y) received, on four separate occasions, in double-blind, randomised fashion, either 10 g of leucine, 10 g of isoleucine, 10 g of valine or control, intragastrically, 30 min before a mixed-nutrient drink. Plasma glucose, C-peptide and glucagon concentrations were measured before, and for 2 h following, the drink. Gastric emptying of the drink was quantified using 13C-acetate breath-testing. (3) Results: Amino acids alone did not affect plasma glucose or C-peptide, while isoleucine and valine, but not leucine, stimulated glucagon (p < 0.05), compared with control. After the drink, isoleucine and leucine reduced peak plasma glucose compared with both control and valine (all p < 0.05). Neither amino acid affected early (t = 0-30 min) postprandial C-peptide or glucagon. While there was no effect on overall gastric emptying, plasma glucose at t = 30 min correlated with early gastric emptying (p < 0.05). (4) Conclusion: In healthy individuals, leucine and isoleucine lower postprandial blood glucose, at least in part by slowing gastric emptying, while valine does not appear to have an effect, possibly due to glucagon stimulation.

Effects of intragastric administration of L-tryptophan on the glycaemic response to a nutrient drink in men with type 2 diabetes - impacts on gastric emptying, glucoregulatory hormones and glucose absorption

BACKGROUND

The rate of gastric emptying and glucoregulatory hormones are key determinants of postprandial glycaemia. Intragastric administration of L-tryptophan slows gastric emptying and reduces the glycaemic response to a nutrient drink in lean individuals and those with obesity. We investigated whether tryptophan decreases postprandial glycaemia and slows gastric emptying in type 2 diabetes (T2D).

METHODS

Twelve men with T2D (age: 63 ± 2 years, HbA1c: 49.7 ± 2.5 mmol/mol, BMI: 30 ± 1 kg/m2) received, on three separate occasions, 3 g ('Trp-3') or 1.5 g ('Trp-1.5') tryptophan, or control (0.9% saline), intragastrically, in randomised, double-blind fashion, 30 min before a mixed-nutrient drink (500 kcal, 74 g carbohydrates), containing 3 g 3-O-methyl-D-glucose (3-OMG) to assess glucose absorption. Venous blood samples were obtained at baseline, after tryptophan, and for 2 h post-drink for measurements of plasma glucose, C-peptide, glucagon and 3-OMG. Gastric emptying of the drink was quantified using two-dimensional ultrasound.

RESULTS

Tryptophan alone stimulated C-peptide (P = 0.002) and glucagon (P = 0.04), but did not affect fasting glucose. In response to the drink, Trp-3 lowered plasma glucose from t = 15-30 min and from t = 30-45 min compared with control and Trp-1.5, respectively (both P < 0.05), with no differences in peak glucose between treatments. Gastric emptying tended to be slower after Trp-3, but not Trp-1.5, than control (P = 0.06). Plasma C-peptide, glucagon and 3-OMG increased on all days, with no major differences between treatments.

CONCLUSIONS

In people with T2D, intragastric administration of 3 g tryptophan modestly slows gastric emptying, associated with a delayed rise, but not an overall lowering of, postprandial glucose.

Intragastric administration of leucine and isoleucine does not reduce the glycaemic response to, or slow gastric emptying of, a carbohydrate-containing drink in type 2 diabetes

AIMS

In healthy individuals, intragastric administration of the branched-chain amino acids, leucine and isoleucine, diminishes the glycaemic response to a mixed-nutrient drink, apparently by stimulating insulin and slowing gastric emptying, respectively. This study aimed to evaluate the effects of leucine and isoleucine on postprandial glycaemia and gastric emptying in type-2 diabetes mellitus (T2D).

METHODS

14 males with T2D received, on 3 separate occasions, in double-blind, randomised fashion, either 10 g leucine, 10 g isoleucine or control, intragastrically 30 min before a mixed-nutrient drink (500 kcal; 74 g carbohydrates, 18 g protein, 15 g fat). Plasma glucose, insulin and glucagon were measured from 30 min pre- until 120 min post-drink. Gastric emptying of the drink was also measured.

RESULTS

Leucine and isoleucine stimulated insulin, both before and after the drink (all P < 0.05; peak (mU/L): control: 70 ± 15; leucine: 88 ± 17; isoleucine: 74 ± 15). Isoleucine stimulated (P < 0.05), and leucine tended to stimulate (P = 0.078), glucagon before the drink, and isoleucine stimulated glucagon post-drink (P = 0.031; peak (pg/mL): control: 62 ± 5; leucine: 70 ± 9; isoleucine: 69 ± 6). Neither amino acid affected gastric emptying or plasma glucose (peak (mmol/L): control: 12.0 ± 0.5; leucine: 12.5 ± 0.7; isoleucine: 12.0 ± 0.6).

CONCLUSIONS

In contrast to health, in T2D, leucine and isoleucine, administered intragastrically in a dose of 10 g, do not lower the glycaemic response to a mixed-nutrient drink. This finding argues against a role for 'preloads' of either leucine or isoleucine in the management of T2D.

Role of Amino Acids in Blood Glucose Changes in Young Adults Consuming Cereal with Milks Varying in Casein and Whey Concentrations and Their Ratio

BACKGROUND

Increasing the total protein content and reducing the casein to whey ratio in milks consumed with breakfast cereal reduce postprandial blood glucose (BG).

OBJECTIVES

We aimed to explore associations between plasma amino acids (AAs), BG, and glucoregulatory hormones.

METHODS

In this repeated-measures design, 12 healthy adults consumed cereal (58 g) and milks (250 mL) with 3.1 wt% or high 9.3 wt% protein concentrations and with casein to whey ratios of either 80:20 or 40:60. Blood was collected at 0, 30, 60, 120, 140, 170, and 200 min for measurement of the primary outcome, BG, and for the exploratory outcomes such as plasma AA, gastric emptying, insulin (INS), and glucoregulatory hormones. Measures were made prior to and after an ad libitum lunch at 120 min. Exploratory correlations were conducted to determine associations between outcomes.

RESULTS

Pre-lunch plasma AA groups [total (TAA), essential (EAA), BCAA, and nonessential (NEAA)] were higher after 9.3 wt% than 3.1 wt% milks by 12.7%, 21.4%, 20.9%, and 7.6%, respectively (P ≤ 0.05), while post-lunch AA groups were higher by 10.9%, 19.8%, 18.8%, and 6.0%, respectively (P ≤ 0.05). Except for NEAA, pre-lunch AAs were higher after 40:60 than 80:20 ratio milks by 4.5%, 8.3%, and 9.3% (P ≤ 0.05). When pooled by all treatments, pre-lunch AA groups associated negatively with BG (r/ρ ≥ -0.45, P ≤ 0.05), but post-lunch only TAA and NEAA correlated (r ≥ -0.37, P < 0.05). Pre-lunch BG was inversely associated with Leu, Ile, Lys, Met, Thr, Cys-Cys, Asn, and Gln (r/ρ ≥ -0.46, P ≤ 0.05), but post-lunch, only with Thr, Ala, and Gly (r ≥ -0.50, P ≤ 0.05). Pre-lunch associations between AA groups and INS were not found.

CONCLUSIONS

Protein concentration and the ratio of casein to whey in milks consumed at breakfast with cereal affect plasma AA concentrations and their associations with decreased BG. The decrease in BG could be explained by INS-independent mechanisms. This trial was registered at www.clinicaltrials.gov as NCT02471092.

Role of Bioactive Peptide Sequences in the Potential Impact of Dairy Protein Intake on Metabolic Health

For years, there has been an increasing move towards elucidating the complexities of how food can interplay with the signalling networks underlying energy homeostasis and glycaemic control. Dairy foods can be regarded as the greatest source of proteins and peptides with various health benefits and are a well-recognized source of bioactive compounds. A number of dairy protein-derived peptide sequences with the ability to modulate functions related to the control of food intake, body weight gain and glucose homeostasis have been isolated and characterized. Their being active in vivo may be questionable mainly due to expected low bioavailability after ingestion, and hence their real contribution to the metabolic impact of dairy protein intake needs to be discussed. Some reports suggest that the differential effects of dairy proteins-in particular whey proteins-on mechanisms underlying energy balance and glucose-homeostasis may be attributed to their unique amino acid composition and hence the release of free amino acid mixtures enriched in essential amino acids (i.e., branched-chain-amino acids) upon digestion. Actually, the research reports reviewed in this article suggest that, among a number of dairy protein-derived peptides isolated and characterized as bioactive compounds in vitro, some peptides can be active in vivo post-oral administration through a local action in the gut, or, alternatively, a systemic action on specific molecular targets after entering the systemic circulation. Moreover, these studies highlight the importance of the enteroendocrine system in the cross talk between food proteins and the neuroendocrine network regulating energy balance.

Effects of intraduodenal coadministration of lauric acid and leucine on gut motility, plasma cholecystokinin, and energy intake in healthy men

The fatty acid, lauric acid (C12), and the amino acid, leucine (Leu) stimulate gut hormones, including CCK, associated with suppression of energy intake. In our recent study, intraduodenal infusion of a combination of C12 and l-tryptophan, at loads that individually did not affect energy intake, reduced energy intake substantially, associated with much greater stimulation of CCK. We have now investigated whether combined administration of C12 and Leu would enhance the intake-suppressant effects of each nutrient, when given at loads that each suppress energy intake individually. Sixteen healthy, lean males (age: 23 ± 2 yr) received, in randomized, double-blind fashion, 90-min intraduodenal infusions of control (saline), C12 (0.4 kcal/min), Leu (0.45 kcal/min), or C12+Leu (0.85 kcal/min). Antropyloroduodenal pressures were measured continuously and plasma CCK at 15-min intervals, and energy intake from a standardized buffet-meal, consumed immediately postinfusion, was quantified. All nutrient infusions stimulated plasma CCK compared with control (P < 0.05). Moreover, C12 and C12+Leu stimulated CCK compared with Leu (P < 0.05) (mean concentration, pmol/L; control: 2.3 ± 0.3, C12: 3.8 ± 0.3, Leu: 2.7 ± 0.3, and C12+Leu: 4.0 ± 0.4). C12+Leu, but not C12 or Leu, stimulated pyloric pressures (P < 0.05). C12+Leu and C12 reduced energy intake (P < 0.05), and there was a trend for Leu to reduce (P = 0.06) energy intake compared with control, with no differences between the three nutrient treatments (kcal; control: 1398 ± 84, C12: 1226 ± 80, Leu: 1260 ± 92, and C12+Leu: 1208 ± 83). In conclusion, combination of C12 and Leu, at the loads given, did not reduce energy intake beyond their individual effects, possibly because maximal effects had been evoked.