The effects of dietary protein supplementation and energy restriction on whole body protein turnover, substrate oxidation and appetite in lean to obese healthy adults

This study will consist of 4 phases, with each phase lasting 2 weeks; habitual diet 1 (HD1), habitual diet 2 (HD2), high protein energy balance (HPEbal) and high protein energy restriction (HPER). During the habitual diet phases (HD1 and HD2), participants will be instructed to continue to consume their normal diet, which based on being weight stable for greater than or equal to 6 months, should be equal to their current energy requirements. During HPEbal, the participants will be provided with a whey based protein powder supplement (0.5 g/kg/day) which will replace food items in their diet in order to maintain energy balance and a protein intake 0.5 g/kg/day greater than HD1 and HD2. During HPER subjects will continue to be provided with the protein supplement (0.5 g/kg/day) and will have further food excluded from their diet to create an energy restriction of 33% below their HD1 energy intake. The energy restriction will come entirely from carbohydrates and fats in order to maintain protein intake at the level of HPEbal. Energy and macronutrient intake will be recorded using a daily weighed food diary and recording app (myfitnesspal) throughout all study phases. These will be logged into a dietetics database (Foodworks Xyris software) for accuracy. An additional diet recall per week will be undertaken to monitor recording adherence and clarify any items unclear from the weighed records. During HPEbal and HPER, individualised energy and macronutrient goals will be set for all participants based on their energy and protein intake during HD1. Participants will be able to track their own progress in meeting these dietary goals on a daily basis through the app as well as regular feedback from the research team. The research team have over 20 years experience in dietary interventions and the study will be reviewed by a registered clinical dietitian. Participants will attend 5 test days at the laboratory. The initial test session will be prior to starting diet recording to determine baseline values of body composition (Dual Energy X-ray Absorptiometry (DXA) and Air Displacement Plethysmography (BODPOD) and resting metabolic rate. Tests days 2-5 will occur at the end of each 2 week diet block and will include measuring whole body protein turnover (WBPT), resting and postprandial energy expenditure, body composition (bodpod only to monitor changes in body composition), appetite and food hedoncis (liking and wanting). Participants will arrive at the laboratory between 6:00 am and 8:30 am on the morning of their test days after a minimum 10-hour fast. Participants will be required to minimise activity on the test day mornings and to abstain from all exercise for 24 hours prior to test days and abstain from high intensity exercise for greater than 48 hours prior to test days. On test days 2-5, following arrival at the laboratory, participants will undergo measures of their body weight and body composition (Bodpod) followed by a measurement of resting energy expenditure (indirect calorimetry). Participants will then be provided with their first test meal of the day. Postprandial energy expenditure and substrate oxidation will be measured twice, between 20-40 and 70-90 minutes following the breakfast meal. WBPT will be measured using the end products method as described by Picou and Taylor Roberts [1] and modified by Fern et al. [2], with a single dose of 200 mg of 15N Glycine (98% atom 15N) and ammonia as the end product. One hour following a breakfast meal, participants will provide a baseline urine sample for the background abundance of 15N ammonia, followed by ingestion of the 200 mg of 15N labelled glycine (98% atom 15N, Sigma-Aldrich) mixed in water. All urine will be collected for the following 24 hours in three separate urine containers between 0-9 hours and 9-12 hours and 12-24 hours. After 9 hours at the laboratory, participants will be sent home and instructed to continue to collect up to and at 24 hours. Participants will receive 6 isoenergetic and isoproteogenic meals; 1 every 2 hours during the study day to maintain a postprandial state. The total energy and protein intake in the provided meals will be based on each individual’s habitual intake recorded with their food diary during HD1 and modified according to the appropriate study phase (HPEbal or HPER). Appetite and liking and wanting will be assessed fasting and postprandial for 10 hours following the first test meal. 1. Picou, D.F. and T. Taylor-Roberts, The measurement of total protein synthesis and catabolism and nitrogen turnover in infants in different nutritional states and receiving different amounts of dietary protein. Clinical Science, 1969. 36(2): p. 283-296. 2. Fern, E.B., et al., The excretion of isotope in urea and ammonia for estimating protein turnover in man with [15N]glycine. Clinical Science, 1981. 61(2): p. 217-228.