Postprandial Metabolic and Appetite Responses Following Whey Protein Supplementation at Breakfast

The prevailing lifestyle pattern in western society results in humans spending the majority of non-sleeping hours in the postprandial state. Given that impaired postprandial metabolism may have adverse consequences for health, interventions that positively influence the postprandial milieu are pertinent considering the global rise in prevalence of chronic metabolic disorders. The aim of this thesis was to investigate whether consumption of whey protein, which has reported beneficial properties relevant to metabolic health, could impact favourably upon acute and second-meal postprandial metabolic and appetite responses. More specifically, a series of studies was designed to establish the conditions under which a rationale for whey protein supplementation may be strongest, through investigation of co-ingestion with other macronutrients, timing of supplementation and interactions with low-moderate intensity exercise. Study one revealed that consuming whey (20 g) alongside a high-carbohydrate or high-fat breakfast significantly augmented the plasma insulin response to that meal in physically active, normal-weight males, without significantly affecting postprandial glycaemia, lipaemia or subjective appetite. Whey protein consumption at breakfast did not affect second meal responses or 24-hour glycaemia when co-ingested with either macronutrient. Study two compared the effects of the same dose of protein administered before, during or after a mixed-macronutrient breakfast in centrally-obese males. Consuming whey as a preload 15 minutes prior to breakfast significantly reduced postprandial glycaemia compared to consuming it afterwards or not at all, with evidence indicating that insulin-independent mechanisms may be responsible. Again, prior whey consumption, irrespective of timing, did not influence glycaemic, insulinaemic or appetite profiles following a standard lunch meal. The final study investigated the effects of a post-exercise whey protein preload on postprandial metabolism and appetite in physically inactive, centrally-obese males. Postprandial glycaemia was moderately impaired following brisk walking exercise without supplemental protein, however this effect was negated when exercise was followed by a whey preload, with a reduced peak glycaemic excursion observed compared to other conditions. Ad libitum energy intake, assessed at lunch, did not differ between conditions. This thesis has shown that whey protein supplementation at breakfast may acutely influence postprandial glycaemia in centrally-obese individuals, however this beneficial effect is not carried over to subsequent meal occasions. Timing of protein intake appears to be important, and this effect is not diminished by prior walking exercise in previously sedentary individuals. Further studies to investigate the effects of whey protein supplementation at multiple meals are required to determine whether this may be a worthwhile strategy to prevent day-long elevated glycaemic exposure