High- versus low-energy density preloads in overweight/obese men: acute effects on gastric emptying, gut hormone release and energy expenditure

Experimental design - The study will utilise a single-blind, randomized cross-over design and measure gastric emptying, gut hormone release, diet-induced thermogenesis, appetite perceptions, and subsequent energy intake in response to three oral preloads: 1. High energy density (HED) 2. Low energy density, high fat (LedHF); and, 3. Low energy density, high protein (LedHP). Details of the preloads are described below. Preloads - The energy densities and macronutrient compositions of the 3 semi-solid test preloads are: 1. High energy density (HED; 1.48 kcal/g; 999 kcal; g fat/protein is 62/30); 2. Low energy density, high fat (LedHF; 1.15 kcal/g; 777 kcal; g fat/protein is 37/29); and, 3. Low energy density, high protein (LedHP; 1.15 kcal/g; 777 kcal; g fat/protein is 22/63). All preloads will be matched for grams of carbohydrate, volume (673 g), palatability, smell, texture and look. The total energy of all preloads will represent between ~30% of estimated daily energy requirement for overweight men. All preloads will comprise of varying quantities of milk, natural yoghurt, frozen raspberries, cream, water, sugar, cornflour and gelatine. Each will also contain 5 ml of egg yolk that is labeled with 75 kBq of Octanoic acid sodium salt [1 – 14C] for the assessment of gastric emptying rate. In addition, whey protein isolate and cream will be used to manipulate the protein and fat contents of the preloads. Experimental protocol - On 3 occasions, each separated by 3-7 days, subjects will have their fasting and postprandial energy expenditure, appetite profile, gastric emptying, gut hormone release and subsequent energy intake measured. Fasting energy expenditure will be determined by indirect calorimetry using a clear plastic ventilated hood and the TrueOne metabolic monitor while subjects lay in the supine position at rest (no fidgeting or talking allowed) for 40 minutes from 0800h. The amount of oxygen consumed and carbondioxide (CO2) produced in the breath is used to measure the amount of energy expended. Afterwards, a cannula will be inserted into a forearm vein to collect blood. In addition, a baseline sample of their breath will be collected for the assessment of gastric emptying rate; for this, subjects will be asked to blow through a straw which is connected to a glass scintillation vial that contains 0.5 mmol of a proprietary CO2 trapping agent. The 14C-Octanoic acid breath test is a validated, indirect marker for the rate of liquid and semisolid gastric emptying. The quantity of breath 14C02 (disintegrations per min, DPM) will be measured using Liquid Scintillation Counting. The activity of 14CO2 in the breath and expressed as a percentage of the original dose ingested in the preload, at the respective time points, will be plotted against time and non-linear regression modelling of the data will be used to determine the lag time (defined as the time before any radioactivity is excreted via the breath), 50% emptying time, the amount of meal retained at 180 mins and gastric emptying coefficient. Subjects will then consume, in random order, one of the three semi-solid experimental preloads. 10 minutes will be allowed to finish the preload. Subjects will then return to the bed and rest quietly under the ventilated hood for a further 3 hrs to measure postprandial energy expenditure for the determination of diet-induced thermogenesis. Blood will be drawn for the samples for the determination of plasma concentrations of glucose, insulin, gut hormones (ghrelin, glucagon like peptide-1 (GLP-1), peptide YY (PPY), cholecystokinin (CCK). At the same time points appetite ratings (fullness, hunger, desire to eat and amount of food that could be eaten) will be recorded using validated 100-mm visual analogue scales. Breath samples to assess gastric emptying will also be collected. At the end of the 3 hours, a cold buffet-style meal will be offered to subjects and they will be asked to eat until ‘comfortably full’ over a 30 min period.