The effects of reducing of maintaining protein intake during energy restriction on body composition, metabolism and appetite in lean to obese healthy adults.

The study will involve 3 phases, 1) energy balance with no food diary (3 x diet recalls) (1 week), 2) Energy balance with weighed food diary (1 week) 3) Energy restriction with weighed food diary (2 weeks). During energy restriction, participants will be randomly assigned to either the main intervention group (Protein maintenance) or the control group (Protein reduction). During energy balance, 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 energy restriction, participants will be required to reduce their energy intake by 33% . This will be undertaken by either reducing all macronutrients equally from their baseline intake (protein reduction), or by maintaining their habitual protein intake and reducing fat and carbohydrates equally (protein maintenance). Female participants will have a 2 week break between week 2 and 3 to monitor for timing of the menstrual cycle. Participants will continue to consume their own diet throughout the study. Energy and macronutrient intake will be recorded using 3 diet recalls during week 1 then using a weighed food diary and recording app (myfitnesspal) during week 2-4. These will be logged into a dietetics database (Foodworks Xyris software) for accuracy. During energy restriction, participants will be given advice from the leading researcher (5+ years’ experience in dietary interventions), on how to change their energy and macronutrient goals in the app and appropriate changes to their individual diets to meet these goals. This advice will indicate for participants to reduce all portion sizes by 1/3rd or to specifically target and eliminate food with high carbohydrate and fat content from their diets. Participants will be able to track their own progress in meeting these macronutrient and energy goals on a daily basis through the app. Participants will attend the laboratory for testing on 9 occasions (10 for females due to a 2-week break), equating to, 2 times per week for the duration of the study. During these sessions participants will continue to receive face-to-face feedback and advice on meeting their dietary goals. During test session 1 (~1.5hrs), measurements of body composition and resting energy expenditure will be undertaken. During all midweek test sessions and at the end of week 3 (sessions 2,4,6,7,8) (~20 -30minutes) body weight and body composition will be measured. At the end of week 1,2 and 4 (session 3,5,and 9) (~2.5 hours) participants will undertake measures of the body composition, resting energy expenditure, appetite and food hedoncis (liking and wanting). Appetite and liking and wanting will be assessed fasting and for 1 hour following an individualised breakfast meal comprising, muesli, milk, yogurt, fruit salad, juice and whey protein powder as required. The breakfast meal will provide 25% of each individual's habitual energy balance energy and protein intake as determined during week one from the average of 3 diet recalls. During energy restriction, the breakfast meal will be reduced by 33% according to the level of dietary energy restriction and protein intake will be either maintained or all macronutrients evenly reduced depending on the participant's allocation to the protein maintenance or protein reduction group. The energy and protein content of the participants’ meals will be individualised and adapted to the dietary interventions by changing between high and low fat dairy products, adjusting the quantities of individual food items and the inclusion of protein supplements with meals. All meals will be weighed pre and post meal consumption to determine exact energy and macronutrient intakes. Palatability will be assessed after the breakfast meal to determine any changes in the palatability of the test meal and impact on appetite and food hedonics. The study was supervised by 2 professors with greater than 20 years experience in dietary interventions and was reviewed by an accredited practising dietician.