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Protein, malnutrition and wasting diseases

Dietary whey protein influences plasma satiety-related hormones and plasma amino acids in normal-weight adult women

Subjects

Abstract

Background/objective:

A distinct suppressive effect of a whey protein (including glycomacropeptide)-enriched preload drink on subsequent food intake in comparison with a maltodextrin carbohydrate-enriched preload was demonstrated in an earlier companion study with the same female subjects; however, the potential mediators underlying the effect are unclear. The objective of this study was to investigate how the ingestion of a whey protein-enriched preload beverage affected postprandial plasma concentrations of several satiety-related gastrointestinal hormones and metabolites in comparison with a maltodextrin carbohydrate-enriched preload.

Subjects/methods:

Eighteen normal-weight women were studied in a single-blind, randomized block design. Blood samples were collected at various time intervals for 120 min after consumption of a test drink (300 ml, ~1300 kJ) enriched (45 g) with either maltodextrin carbohydrate or whey protein containing naturally present glycomacropeptide.

Results:

Plasma-active ghrelin concentrations decreased after both maltodextrin carbohydrate- and whey protein-enriched test drinks (P<0.05). The whey protein-enriched beverage led to increased plasma concentrations of cholecystokinin (CCK) at 60 and 75 min (P<0.05), glucagon-like peptide-1 (GLP-1) at 90 min (P<0.001), peptide tyrosine–tyrosine (PYY) at 90 and 120 min (P<0.01) and pancreatic polypeptide (PP) from 15 to 120 min (P<0.05) compared with maltodextrin carbohydrate. Total amino acid, urea and ammonia plasma concentrations were also higher after whey protein compared with maltodextrin carbohydrate ingestion (P<0.01).

Conclusions:

Increased plasma concentrations of some gastrointestinal hormones related to satiety, particularly PP, and of amino acids and their metabolites, may have acted either singly or together to mediate the observed satiety response to whey protein.

Introduction

The rapid worldwide rise in the prevalence of obesity has led to growing interest in dietary modifications that can enhance satiety and reduce dietary energy intake.1 Several scientific reviews have concluded that protein is the most satiating macronutrient2, 3, 4 and there is evidence that dairy whey protein elicits a strong effect on satiety compared with carbohydrate and other protein sources.5, 6, 7, 8, 9, 10, 11 With respect to the effect on satiety of whey protein relative to maltodextrin carbohydrate specifically, although two studies12, 13 reported no effects, Bertenshaw et al.6 found a greater reduction in subsequent food intake following consumption of a preload enriched with whey protein compared with maltodextrin carbohydrate. However, possible mechanisms underlying these effects were not studied. In a companion study14 to the presently reported work and conducted with the same normal-weight adult female subjects, consumption of a preload drink enriched with whey protein containing naturally present glycomacropeptide was shown to suppress subsequent ad libitum food energy intake compared with a maltodextrin carbohydrate-enriched preload drink, irrespective of the time delay interval between preload drink and test meal (30, 60 or 120 min). Furthermore, for the time delay interval of 120 min, blood samples were collected sequentially for analysis of several plasma hormones and metabolites and these responses are the subject of this report. The hormonal and metabolite mechanisms implicated in the satiating effect of whey protein are not fully understood but in general are considered to involve a postprandial decrease in the circulating peptide ghrelin with a simultaneous increase in circulating amino acids and hormones including cholecystokinin (CCK), glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP) and peptide tyrosine–tyrosine (PYY).15, 16, 17, 18

The objective of this study was to build upon previous work by comparing a wider range of gastrointestinal satiety-related hormones and metabolites. Therefore, changes in plasma glucose, insulin, CCK, GIP, GLP-1, PYY, pancreatic polypeptide (PP), urea, ammonia and amino-acid concentrations, which may account either individually or interactively for the observed14 stronger satiety effect of a preload enriched with whey protein containing glycomacropeptide versus one enriched with maltodextrin carbohydrate, were investigated.

Subjects and methods

Subjects

Nineteen women, aged between 18 and 40 years with a body mass index (BMI) between 19 and 25 kg/m2, were recruited by public advertisement. Individuals completed questionnaires on lifestyle, medical history and dietary habits and the Three Factor Eating Questionnaire19 during an information session. All subjects underwent a blood collection following an overnight fast, for the analysis of glucose and insulin, before the start of the study and participants were excluded if the fasting levels of glucose and insulin were outside of the normal range. Subjects were selected on the basis of being non-smokers, body weight stable (weight gain/loss within 3 kg in the 6 months before the study), consumers of at least three main meals every day, not pregnant or lactating and having regular menstrual cycles. Subjects were excluded if they reported a metabolic disease, or were on medication that may affect appetite or blood clotting. One woman did not tolerate the cannula and was excluded from the study. Eighteen women completed the study and their characteristics are given in Table 1. Written informed consent was obtained from the participants and the study was approved by the Human Ethics Committee of Massey University, Palmerston North, New Zealand. The trial has been registered with the Australian New Zealand Clinical Trials Registry (http://www.anzctr.org.au), registration number ACTRN12614000562673.

Table 1 Subject characteristics at baseline

Study protocol

A single-blind randomized block design was used with the two test beverages randomly allocated to each subject using random numbers. The two test preload drinks consisted of commercially available milk powder (Alfa Foods Distributors, Auckland, New Zealand), sucrose, strawberry flavor, red colorant and either 45 g of maltodextrin (Fieldose 10GV, Penford New Zealand Ltd, Auckland, New Zealand) or 45 g of whey protein isolate with 21% naturally present glycomacropeptide (WPI 894, Fonterra Co-operative Group Ltd, Palmerston North, New Zealand) made up to 300 ml with water. The ‘Maltodextrin’ preload beverage was composed of 71.7 g carbohydrate, 3.1 g protein and 3.2 g fat per 300 ml and was calculated to have a metabolisable energy (ME) content of 1370 kJ per 300 ml. The ‘WheyProtein’ preload drink contained 26.4 g carbohydrate, 45.7 g protein and 3.4 g fat per 300 ml and had a ME content of 1333 kJ per 300 ml. The preload drinks were found to be more acceptable to an independent sensory panel when they were of mixed composition rather than consisting exclusively of whey protein or maltodextrin. The ingredients and macronutrient contents of the two test preload drinks and the preparation protocol were identical to those described earlier.14

Each experimental day was separated by at least 3 days and occurred during the menses and follicular phase of the menstrual cycle. On the day before, subjects were instructed to abstain from alcohol and to consume only water from 2200 hours onwards. On each test day, each participant reported to the research unit at their customary lunchtime (1200–1300 hours) after having consumed a subject-specific breakfast (recorded using food diaries, on average 1462 kJ ME with 59% of ME derived from carbohydrate, 11% protein and 30% fat) at least 3 h before the test session appointment, followed only by water as desired. On arrival, subjects filled in 10 cm visual analogue scales (VAS) for baseline measurements of feelings of appetite (hunger, desire to eat, prospective food consumption and fullness).14 A catheter was inserted into a vein of the subject’s arm for arterialized venous blood sampling and kept patent with saline solution. After a blood sample was collected at baseline, the subject consumed the test drink within 5 min. Following complete ingestion of the test drink (time point t=0 min), a blood sample was drawn and subjects completed appetite VAS ratings. Blood samples and appetite VAS ratings were collected every 15 min for 90 min. A final blood sample and appetite VAS ratings were taken at 120 min, before the cannula was removed. Subjects were then provided with a fried rice test meal (heated) and water to consume ad libitum within 15 min. The ad libitum test meal consisted of a homogeneous combination of white rice, minced chicken meat, egg, peas, carrots, corn, oil and seasoning. The ME and macronutrient contents of the test meal (per 100 g) were determined to be 835 kJ, 28.4 g carbohydrate, 6.6 g protein, 6.4 g fat and 1 g total dietary fiber, respectively. Following the termination of the test meal, subjects completed appetite VAS ratings at 15 and 30 min later, and they were then free to leave the laboratory.

Blood sample collection and analysis

Blood samples (~10 ml) were obtained ~15 min before test drink ingestion (baseline) and 0, 15, 30, 45, 60, 75, 90 and 120 min postprandially. Blood for plasma glucose analysis was collected in chilled fluoride oxalate tubes. Blood for plasma free amino acid, urea and ammonia analysis was collected in chilled tubes containing EDTA. Blood for plasma CCK was collected in chilled EDTA tubes containing aprotinin (500 KIU/ml blood; Phoenix Pharmaceuticals, Burlinghame, CA, USA). Blood for plasma hormones (insulin, ghrelin, GIP, GLP-1, PP and PYY) was collected in chilled EDTA tubes containing aprotinin (500 KIU/ml blood; Phoenix Pharmaceuticals) and dipeptidyl peptidase-IV (DPP-IV) inhibitor (10 μl/ml blood; Millipore, Missouri, MA, USA). Plasma was isolated by centrifugation at 4 °C for 10 min at 3000 g (Heraeus megafuge 1.0R, Hanau, Germany) within 10 min of collection and aliquots were stored at −80 °C for future analysis.

Plasma glucose was determined by a hexokinase method (Roche Diagnostic Kit, Basel, Switzerland) and plasma urea was measured by an enzymatic method (Roche Diagnostic Kit) on a Flexor E analyzer (Vital Scientific NV, The Netherlands). Plasma ammonia was measured with an enzymatic method (Randox Laboratories Inc., Crumlin, UK). Plasma samples for free amino acid analysis were deproteinized by ultrafiltration and γ-amino-butyric acid was used as an internal standard. Amino-acid concentrations were determined with the use of a reverse phase (C18 column) HPLC (Agilent 1200SL, Agilent Technologies, Santa Clara, CA, USA) after precolumn derivatization with o-phthaldialdehyde. The amino acid system used for plasma-free amino-acid analysis gave poor separation for proline; therefore, no data for this amino acid are shown. Ethanol extraction was performed on plasma for CCK analysis according to the manufacturer’s instructions. CCK-33 levels were determined using a commercial ELISA kit (Phoenix Pharmaceuticals) with the intra-assay coefficient of variation ranging between 5 and 10%. Insulin, active ghrelin, GIP, active GLP-1, PP and total PYY were measured using a commercial Human Gut Hormone Milliplex kit (Millipore). Plasma active ghrelin levels were measured using a commercially available multiplex assay kit from Millipore, which uses an antibody raised in guinea pig highly specific for acylated (active) ghrelin. As suggested by the manufacturer, acidification of the plasma samples, to improve stability of acylated ghrelin20 was not appropriate for the multiplex assay. For measurement of active GLP-1, the same multiplex assay kit was used with an antiserum that cross-reacts 100% with GLP-1 (7–36), 72% with GLP-1 (7–37), <3% with GLP-1 (1–36) and <2% with GLP-1 (1–37). The detection limit for insulin was 137 pg/ml, GIP was 2.7 pg/ml and for ghrelin, GLP-1, PP and PYY was all 13.7 pg/ml. The intra-assay coefficient of variation was less than 11% and the inter-assay coefficient of variation was less than 19%.

Statistical analysis

The plasma hormone and metabolite concentrations were first tested for normality of distribution and the presence of outliers.21 Log transformations were conducted on PP and PYY concentrations to adjust for skewed distributions. The response values for glucose, insulin, ghrelin, CCK, GIP, GLP-1, PYY, PP, urea, ammonia and amino acids were analysed using a repeated measures two-way analysis of variance with time and drink and their interaction (time × drink) as factors. Paired (multiple) comparisons were used to examine the statistical significance of the differences between drinks at each time point.21 Net incremental (area under the curve and above the baseline) area under the curve (Net iAUC)22 of the response of each blood parameter after consumption of the test drink (from 0 to 120 min adjusted for baseline) were calculated by the trapezoidal method and tested for a drink effect by using a paired t-test. Pearson’s correlation analysis was used to examine potential relationships within blood variables and measures of satiety. A P-value of less than 0.05 was regarded as being statistically significant. Data are presented as the means and their s.e.'s (mean±s.e.m.). All data were analysed using the Statistical Analysis Systems statistical software package version 9.2 (SAS Institute, Cary, NC, USA).

Results

Measures of satiety

Food intake at the ad libitum test meal (120 min preload time delay) was lower following ingestion of the WheyProtein preload drink compared with the Maltodextrin drink (2442±290 versus 2920±230 kJ, respectively, P=0.0345). VAS-rated subjective feelings of appetite (hunger, desire to eat, prospective food consumption and fullness) over time and expressed as net iAUC 0–120 min did not differ between the maltodextrin carbohydrate and whey protein beverages (data reported in Chungchunlam et al.14).

Gastrointestinal peptide hormones and glucose

There was a significant drink × time interaction effect for plasma concentrations of glucose (P<0.0001), insulin (P=0.0002), active GLP-1 (P<0.0001), PP (P<0.0001) and PYY (P=0.0042). Following ingestion of the Maltodextrin drink, plasma glucose concentrations from 15 to 120 min (P<0.01), plasma insulin concentrations at 45 (P<0.0001), 60 (P<0.0001) and 90 (P=0.011) min, plasma active GLP-1 concentrations at 15 (P=0.0054) and 30 (P=0.0011) min and plasma PYY concentrations at 30 min (P=0.0439) were significantly higher than after that of the WheyProtein drink (Figure 1). The WheyProtein drink elicited greater plasma active GLP-1 concentrations at 90 min (P=0.0005), plasma PYY concentrations at 90 (P=0.003) and 120 (P=0.0006) min, and plasma PP concentrations from 15 to 120 min (P<0.05) compared with the Maltodextrin drink (Figure 1).

Figure 1
figure1figure1

Plasma concentrations of (a) glucose, (b) insulin, (c) ghrelin, (d) CCK-33, (e) GIP, (f) GLP-1, (g) PP and (h) PYY in 18 normal-weight women in response to ingestion of maltodextrin carbohydrate-enriched () and whey protein-enriched () beverages. P-values were derived by repeated-measures analysis of variance (ANOVA) and by pairwise comparisons if the interaction between time and drink was significant: ****P<0.0001; ***P<0.001; **P<0.01; *P<0.05; NS, nonsignificant. Significant probability levels shown on the curve are for differences between the two drinks at each time point. No significant interaction between time and drink was observed for GIP (P>0.05), but there was a significant main effect of drink (P<0.05), with overall plasma GIP levels being higher following consumption of the maltodextrin carbohydrate-enriched beverage compared with whey protein. The net incremental area under the curve (AUC) is presented for each response.

When all data were included in the analysis, there was no significant interaction between drink and time for the plasma CCK-33 response (P>0.05) and there were no significant differences because of time (P>0.05) or drink (P>0.05). However, there were seven statistical outliers from 322 total observations for the CCK-33 data. When these outliers were removed, a significant interaction effect between drink and time (P=0.0492) was observed (data shown in Figure 1). The WheyProtein drink induced greater CCK-33 responses 60 (P=0.0406) and 75 (P=0.0035) min postprandially than the Maltodextrin drink.

There was no significant drink × time interaction effect (P>0.05) for the active ghrelin response (Figure 1) and there was no significant effect of drink (P>0.05); however, plasma active ghrelin concentrations were significantly influenced by time (P<0.0001). Postprandially, active ghrelin concentrations decreased rapidly after both test drinks and remained below the baseline level throughout the duration of the study. There was no significant drink × time interaction effect (P>0.05); however, time (P<0.0001) and drink (P=0.0222) both had a significant effect on plasma GIP concentrations (Figure 1). When the main effect of drink was examined, GIP concentrations were found to be more elevated after ingestion of the Maltodextrin drink than after ingestion of the WheyProtein drink from 0 to 120 min (163.9±7.1 versus 122.6±4.1 pg/ml, respectively).

The net iAUC for plasma concentrations of glucose and insulin from 0 to 120 min was greater after consumption of the Maltodextrin drink than after the WheyProtein drink (P<0.05; Figure 1), but the net iAUC for the ghrelin, CCK-33, GIP, GLP-1, PP and PYY responses across the 120-min period were not significantly different between test beverages (P>0.05; Figure 1).

Protein metabolites

There were differences in plasma concentrations of urea, ammonia, total amino acids (TAA) and branched-chain amino acids (BCAA: isoleucine, leucine and valine; Figure 2) between drinks over time, characterized by a significant drink × time interaction effect (P<0.0001). The WheyProtein drink elicited a higher urea response from 45 min to the end of the study period (P<0.05) and led to significant increases in plasma ammonia, TAA and BCAA concentrations from 15 to 120 min (P<0.01) compared with Maltodextrin. The net iAUC for urea, ammonia, TAA and BCAA was higher after ingestion of the WheyProtein beverage than after the Maltodextrin beverage (Figure 2). Pearson correlation analysis revealed a significant positive relationship between overall plasma concentrations of TAA and overall plasma concentrations of urea (r=0.17, P=0.0334) and ammonia (r=0.27, P=0.0009) following consumption of the WheyProtein preload drink. Postprandial plasma BCAA concentrations were significantly positively correlated with plasma GLP-1 concentrations overall (r=0.26, P=0.0013), particularly at 75 min (r=0.48, P=0.0405) and 120 min (r=0.50, P=0.0309) after ingestion of the WheyProtein beverage. A positive correlation between plasma BCAA and PYY concentrations overall (r=0.15, P=0.0635) and at 45 min (r=0.48, P=0.0505) following consumption of the WheyProtein preload drink approached significance. The net iAUC for plasma concentrations of glucose was inversely correlated with the net iAUC for plasma levels of TAA (r=−0.54, P=0.0207) and BCAA (r=−0.71, P=0.0008).

Figure 2
figure2

Postprandial plasma responses of (a) urea, (b) ammonia, (c) TAAs and (d) BCAAs in 18 normal-weight women following consumption of maltodextrin carbohydrate-enriched () and whey protein-enriched () beverages. By repeated-measures ANOVA, there was a significant time × drink effect for urea, ammonia, TAA and BCAA (all P<0.0001). Significance indicators shown on the curve are for differences between the two drinks at each time point: ****P0.0001; ***P<0.001; **P<0.01, *P<0.05. The net incremental AUC is presented for each variable.

The difference in individual plasma amino-acid concentrations between the two test drinks was determined by net iAUC (Table 2). The net iAUC responses for glutamine, glycine, histidine, taurine and tryptophan did not differ significantly after ingestion of the two test beverages (P>0.05). However, for α-amino-butyric acid, alanine, arginine, asparagine, aspartic acid, citrulline, cysteine, glutamic acid, isoleucine, leucine, lysine, methionine, ornithine, phenylalanine, serine, threonine, tyrosine and valine, the net iAUC responses were significantly higher after ingestion of the WheyProtein beverage compared with the Maltodextrin beverage (P<0.05).

Table 2 Net iAUC for plasma concentrations of individual amino acid following consumption of the maltodextrin carbohydrate-enriched and whey protein-enriched test drinks

Correlations between blood variables and measures of satiety

When the hormonal and metabolite responses from 0 to 120 min were tested by Pearson’s correlation analysis with food intake at the ad libitum test meal, significant inverse relationships between food energy intake and plasma concentrations of GIP (r=−0.24, P=0.0032), PP (r=−0.23, P=0.0055) and PYY (r=−0.23, P=00056) were found following consumption of the Maltodextrin preload drink. Pearson correlations showed a significant negative relationship between food energy intake and plasma levels of PP (r=−0.17, P=0.0359) and urea (r=−0.21, P=0.0083) and there was a tendency towards a negative correlation with plasma PYY (r=−0.16, P=0.0574) concentrations across all time points following ingestion of the WheyProtein preload drink. No other significant (P<0.05) correlations were observed (data not shown).

Regarding VAS-rated feelings of appetite, the 120-min net iAUC for plasma glucose was correlated with the 120-min net iAUC for hunger (r=−0.48, P=0.0027), desire to eat (r=−0.47, P=0.0034), prospective food consumption (r=−0.51, P=0.0013) and fullness (r=0.32, P=0.0506) following consumption of both preload drinks. There was an overall correlation between the net iAUC for plasma ghrelin and the net iAUC for hunger (r=0.45, P=0.0563) and prospective food consumption (r=0.35, P=0.0339). Following consumption of the Maltodextrin drink, the net iAUC for GIP was correlated with the net iAUC for hunger (r=−0.54, P=0.0202), desire to eat (r=−0.61, P=0.007), prospective food consumption (r=−0.48, P=0.0415) and fullness (r=0.64, P=0.0042), whereas a significant positive correlation was shown between the net iAUC for plasma insulin and feelings of fullness (r=0.57, P=0.0127) following ingestion of the WheyProtein drink. No other significant (P<0.05) correlations between blood variables and subjective feelings of appetite were found (data not shown).

Discussion

The presently reported investigation of the response of several plasma hormones and metabolites extends the finding of our previously published work.14 For the preload time delay of 120 min, food intake at an ad libitum test meal was lower following consumption of a whey protein (containing naturally present glycomacropeptide)-enriched preload compared with a maltodextrin carbohydrate-enriched preload and the suppressive effect on subsequent food intake of whey protein may be related to an increase in plasma gastrointestinal hormones and metabolites.

Ingestion of the two preload drinks enriched with maltodextrin carbohydrate or whey protein isolate (containing naturally present glycomacropeptide) influenced several of the blood parameters differently, but plasma active ghrelin concentrations were not affected by the beverage type. Plasma active ghrelin was found to decrease immediately after consumption of both test drinks and the decrease was not statistically significantly different between the two test drinks. This is in line with several studies showing that high protein and high carbohydrate meals do not have a differential effect on plasma active ghrelin levels.23, 24, 25 Nonetheless, plasma active ghrelin levels were overall positively correlated with VAS-rated feelings of prospective food consumption and there was a tendency towards an association with feelings of hunger. Thus, postprandial suppression of plasma active ghrelin levels may be related to the energy consumed in the liquid test preload but not to the macronutrient composition of the test beverage. In contrast with other studies,26, 27, 28, 29 an increase in postprandial plasma insulin and GIP following consumption of the whey protein-enriched test drink compared with the maltodextrin carbohydrate-enriched beverage was not observed. This was unexpected but it may be that the postprandial plasma insulin and incretin responses were blunted by the presence of available carbohydrate (26 and 72 g in 300 ml of the whey protein and maltodextrin carbohydrate test drinks, respectively) in the whey protein-enriched test preload.27, 30 It would appear that an increased plasma insulin concentration and GIP concentration with whey protein are not prerequisites for a satiety response to a whey protein-enriched preload.

In the present study, ingestion of the whey protein-enriched drink led to increased mean plasma concentrations of CCK-33 at 60 and 75 min, active GLP-1 at 90 min and PYY from 90 to 120 min compared with the maltodextrin carbohydrate-enriched drink. An effect of dietary whey protein in comparison with carbohydrate on responses of plasma CCK, GLP-1 and PYY has been observed in previous studies.16, 17, 18, 24, 27, 31, 32, 33, 34 It should be noted that our approach differs from some other studies in that the whey protein was administered to the subjects along with some carbohydrate. All or some of these hormones are potential candidates as mediators of the increased satiety response to whey protein. Of particular interest was the significant elevation in the mean plasma PP concentrations from 15 to 120 min after whey protein ingestion compared with maltodextrin carbohydrate. Although infusions of PP have been demonstrated to suppress food intake,35, 36, 37 little is known about the postprandial release of PP following the ingestion of a protein-rich meal.38, 39, 40, 41 Of all of the gastrointestinal peptide hormones investigated in the present study, only for hormone PP was a significant inverse relationship between food energy intake at the ad libitum test meal and the plasma concentration of the hormone found following consumption of the whey protein-enriched drink. Therefore, not only were the mean plasma PP concentrations significantly higher at most times examined post-drink ingestion for the whey protein versus maltodextrin carbohydrate, but the effect was also seen among individual subjects, with subjects having higher plasma PP levels also showing lower food energy intake at the subsequent test meal. The hormone PP may have a role in mediating the satiating effect of whey protein, and this needs to be investigated further using PP administration and techniques to block the effect of released PP. The possibility that PP acts in concert with some of the other gastrointestinal hormones, the mean plasma concentrations of which were also elevated with whey protein (namely CCK, GLP-1 and PYY), is also worthy of investigation.

The amino-acid composition of dairy whey protein may have a role in the stronger satiating effect of whey protein in relation to maltodextrin and potentially other protein types. A considerably elevated concentration of plasma amino acids within 15 min of consumption of the whey protein infers that plasma amino acids are possible candidate metabolites associated with the mechanism by which whey protein induces satiety. A sharp rise in the mean circulating amino-acid concentrations has been shown previously to relate to a reduction in appetite.42 Dairy whey protein has a relatively high concentration of BCAAs (isolecucine, leucine and valine)43, 44 and this was reflected in the increased plasma BCAA concentrations for whey protein compared with carbohydrate. In the present study, although there was no statistically significant correlation between plasma TAA and BCAA concentrations with subsequent food intake and VAS-rated feelings of appetite, a rise in plasma BCAA concentrations was found to be related to an increase in plasma concentrations of GLP-1 and PYY, suggesting that plasma amino acids may be implicated in the release of satiety-related gastrointestinal peptide hormones.45 The products of postprandial protein metabolism, urea and ammonia also increased after ingestion of the whey protein compared with carbohydrate, which has been reported by others.46 A secondary effect of plasma urea and ammonia concentrations to an effect of plasma amino acids on satiety should also not be excluded.

In conclusion, consumption of a preload drink enriched with whey protein (including glycomacropeptide) led to a decrease in subsequent food intake and increased plasma concentrations of satiety-related gastrointestinal hormones and higher plasma levels of amino acids and products of amino-acid catabolism. The present results suggest that the greater satiating effect of whey protein compared with carbohydrate may be mediated, at least in part, by elevated plasma concentrations of CCK, GLP-1, PYY and amino acids, and extend similar results of other studies by revealing that the peptide hormone PP may have a significant role in the satiating potential of whey protein. Further studies are required to examine how proteins from other sources compared with dairy whey protein influence satiety-related gastrointestinal hormones, particularly PP, and metabolites of amino-acid catabolism postprandially. There is a need now for controlled intervention studies where potential regulatory hormones are administered or techniques for blocking the hormonal effects are applied. Further work is needed to identify the importance of these gastrointestinal hormone signaling responses, with underlying possible interactions, in relation to satiety to gain a better understanding of the control mechanisms of food intake and appetite.

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Acknowledgements

The study was funded by the Riddet Institute, a New Zealand government supported Centre of Research Excellence. We wish to gratefully thank the volunteers who participated in this study. We acknowledge Miss Ying Jin, Miss Christina Streicher and Miss Jacinta Lee for their technical assistance, Dr Mark Morris from the Massey University Medical Centre, Mrs Maria-Tine Biersteker and Mrs Chris Booth for blood sample collection. We would also like to thank Miss Shirley Ling from Abacus-Als, Massey University Nutrition Laboratory and Mr Sofian Tijono from the University of Auckland for their assistance with the blood assays.

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Correspondence to S M S Chungchunlam.

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Chungchunlam, S., Henare, S., Ganesh, S. et al. Dietary whey protein influences plasma satiety-related hormones and plasma amino acids in normal-weight adult women. Eur J Clin Nutr 69, 179–186 (2015). https://doi.org/10.1038/ejcn.2014.266

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