Metabolic impact of certain dietary proteins and/or amino acids - Glycaemic and hormonal responses to carbohydrate meals in healthy subject

Abstract: Re-occurring hyperglycaemic episodes promote subclinical low-grade inflammation and CVD in type 2 diabetes, emphasising the therapeutic role of tight blood glucose regulation. A tight blood glucose regulation is probably beneficial also in healthy subjects and mild elevations in postprandial glycaemia and triglycerides are associated with impaired flow-mediated dilation and increased markers of oxidative stress in young healthy subjects. Certain dietary proteins and amino acids (AA) have insulinogenic properties and might facilitate glycaemic regulation following a carbohydrate challenge. However, there is a lack of knowledge regarding the impact of different food proteins, and to what extent their effects can be influenced by supplementation with AA. Also, limited information exists with respect to influence of proteins/AA on metabolic response to carbohydrates in a composite meal. The objective of the present thesis was to investigate the impact of whey and soy protein on postprandial blood glucose, plasma AA (p-AA) and hormonal responses when administered to healthy subjects in a glucose drink, as part of milk meals, or in combination with a composite carbohydrate meal. The effect of exchanging half of the protein for specific AA mixtures (5AA: isoleucine, leucine, lysine, threonine and valine) or (6AA: 5AA+arginine) was also examined. Additionally appetite rating in the postprandial phase was performed using VAS scales. Whey protein (4.5-18g) reduced postprandial glycaemia, and increased insulinaemia and p-AA in a dose dependent way to a glucose challenge, and the p-5AA (iAUC 0-60 min) correlated to the insulin response (iPeak; P < 0.009). Lactose-equivalent amounts of bovine and human milk resulted in similar postprandial glycaemia and insulinaemia. A rapid response in GIP, GLP-1 and p-AA correlated to an early insulinogenic effect that was associated to reduction of glycaemia (iAUC 0-90 min; P < 0.001). Hydrolysed and intact whey had similar effects on glycaemic responses when co-ingested with glucose, although hydrolysed way tended to be more insulinogenic, possible due to its higher early insulin and faster p-AA response compared with intact whey. Exchanging half of the intact or hydrolysed whey protein for 5AA magnified the insulinogenic effect and reduced postprandial glycaemia (iAUC 0-120min; P < 0.05). Intake of whey or soy protein with or without addition of 5AA or 6AA, as a pre-meal protein drink (PMPD) prior a composite meal, considerably attenuated postprandial blood glucose incremental peak value (iPeak; P < 0.05). Also, all whey PMPDs with or without added AA reduced glycaemia (iAUC 0-120min; P < 0.05) and increased the Glycaemic Profile (GP; P < 0.05). Arginine had no additional effect on glycaemic responses when added to the 5AA mixture. Early GLP-1 and p-AA responses (iAUC 0-15 min) were associated with early insulin response (iAUC 0-15min). Early increment in insulin possibly explain the attenuation of over-all course of post-prandial glycaemia to the composite carbohydrate meal post the PMPDs. Interestingly, the lowering of glycaemic excursions was observed in the absence of elevated insulinaemic peak. Intake of a PMPD prior a composite meal had no effects on appetite rating (VAS) or plasma ghrelin.