Plasma Free Amino Acid Responses to Intraduodenal Whey Protein, and Relationships with Insulin, Glucagon-Like Peptide-1 and Energy Intake in Lean Healthy Men.

Natalie Luscombe-Marsh,Michael Horowitz,Amy Hutchison,Peter Clifton,Stijn Soenen,Christine Feinle-Bisset,Robert Steinert

doi:10.3390/nu8010004

Abstract

This study determined the effects of increasing loads of intraduodenal (ID) dairy protein on plasma amino acid (AA) concentrations, and their relationships with serum insulin, plasma glucagon-like peptide-1 (GLP-1) and energy intake. Sixteen healthy men had concentrations of AAs, GLP-1 and insulin measured in response to 60-min ID infusions of hydrolysed whey protein administered, in double-blinded and randomised order, at 2.1 (P2.1), 6.3 (P6.3) or 12.5 (P12.5) kJ/min (encompassing the range of nutrient emptying from the stomach), or saline control (C). Energy intake was quantified immediately afterwards. Compared with C, the concentrations of 19/20 AAs, the exception being cysteine, were increased, and this was dependent on the protein load. The relationship between AA concentrations in the infusions and the area under the curve from 0 to 60 min (AUC0–60 min) of each AA profile was strong for essential AAs (R2 range, 0.61–0.67), but more variable for non-essential (0.02–0.54) and conditional (0.006–0.64) AAs. The AUC0–60 min for each AA was correlated directly with the AUC0–60 min of insulin (R2 range 0.3–0.6), GLP-1 (0.2–0.6) and energy intake (0.09–0.3) (p < 0.05, for all), with the strongest correlations being for branched-chain AAs, lysine, methionine and tyrosine. These findings indicate that ID whey protein infused at loads encompassing the normal range of gastric emptying increases plasma concentrations of 19/20 AAs in a load-dependent manner, and provide novel information on the close relationships between the essential AAs, leucine, valine, isoleucine, lysine, methionine, and the conditionally-essential AA, tyrosine, with energy intake, insulin and GLP-1.

Highlights

Calorie-controlled diets with moderately increased protein content are effective in the management of obesity due to their capacity to (i) suppress appetite [1] and, under ad-libitum feeding conditions, reduce energy intake [2]; (ii) promote loss of fat while preserving muscle mass [3,4];(iii) increase insulin release and reduce postprandial glycemic excursions [1,3]; and (iv) improve lipidNutrients 2016, 8, 4; doi:10.3390/nu8010004 www.mdpi.com/journal/nutrientsNutrients 2016, 8, 4 metabolism and blood pressure [1,3]
Co-ingested with carbohydrate, improves glycaemia by slowing gastric emptying and stimulating the release of glucagon-like peptide-1 (GLP-1) and/or glucose-dependent insulinotropic polypeptide (GIP) as well as insulin [10], our recent observations indicate that the effects of lipid–protein combinations on these gut hormones are related directly to the amount of lipid, and the effects on insulin and glucagon are related to the amount of protein, whereas the effects on energy intake appear dependent on a threshold load of ~12.5 kJ/min of either nutrient being delivered to the duodenum [11]
“P12.5”; or (iv) a saline control “C”, at a rate of 4 mL/min for 60 min, on antropyloroduodenal motility (APD), and gut hormone, insulin, glucose, appetite and energy intake responses [7], we have evaluated the effects on plasma free amino acid (AA) to determine their relationships with GLP-1, insulin, and energy intake [7]

Summary

Introduction

Calorie-controlled diets with moderately increased protein content are effective in the management of obesity due to their capacity to (i) suppress appetite [1] and, under ad-libitum feeding conditions, reduce energy intake [2]; (ii) promote loss of fat while preserving muscle mass [3,4];(iii) increase insulin release and reduce postprandial glycemic excursions [1,3]; and (iv) improve lipidNutrients 2016, 8, 4; doi:10.3390/nu8010004 www.mdpi.com/journal/nutrientsNutrients 2016, 8, 4 metabolism and blood pressure [1,3]. Oral and intraduodenal (ID) administration of dietary protein, including whey protein isolate (in both un-hydrolysed and hydrolysed form), like lipid and carbohydrate, modulates gastrointestinal (GI) motor activity and hormone release in a nutrient load-dependent manner [5,6,7]. These GI factors influence gastric emptying [8], postprandial glycaemia [9] and energy intake [5,6,7]. This study demonstrated that the effects of ID infused hydrolysed whey protein isolate (which did not contain fat or carbohydrate and was delivered at 12.5 kJ/min) on the GI factors were weaker than those of isocaloric lipid [11] suggesting the suppression of energy intake by protein may be dependent on additional mechanisms

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