Microbiome in highly trained athletes and response to dietary change

21 d physical training intervention where participants will receive one of three separate dietary interventions, involving: 1) High carbohydrate availability ~ 65% carbohydrate, 15% protein, 20% fat; 2) High-fat low carbohydrate diet (LCHF) : 70-80% fat, 15-25% protein, <50g/d CHO; or 3) Periodised diet, involving the same macronutrient composition as the HCHO diet (~ 65% carbohydrate, 15% protein, 20% fat) that will involve a careful integration of sessions with high CHO availability (high muscle glycogen, CHO feeding during session), low CHO availability (low pre-exercise glycogen or overnight fasted or delayed refuelling) and well-timed protein intake (post exercise and around meal times). The physical training involves a 21 d training program, including endurance sessions, interval training, sustained high-intensity sessions, resistance training and a weekly performance monitoring session. Training sessions will be undertaken as a group and will be monitored by the research team as well as recorded by the participants in a daily online log. The template for the weekly training programme has been developed in collaboration with a world-class race walker and several coaches to blend the typical intensified training practices of competitive race walkers with opportunities to implement the desired dietary intervention. The weekly training program will involve six mandatory sessions (key sessions include: Monday - high intensity interval session, 10 x 1 km repititions on 6 mins; Wednesday and Saturday - long walk, =<40 km; Friday - Hill session, 16 km) conducted under standardised conditions with external monitoring, while the remaining sessions will be undertaken according to the preference of individuals and noted in the participants training log. As such, sessions will be completed under instruction of the head coach, to achieve the intended training outcome and confirmed by physiological monitoring by the research team. Adherence to the dietary intervention - All foods and fluids consumed will be provided by the research team and recorded. Menu construction and the preparation of meals/snacks will be undertaken by professional chefs, food service dietitians and sports dietitians. Meal plans will be individually developed for each athlete to integrate personal food preferences and nutrition requirements within the allocated dietary treatment. Meals will be eaten in a separate dining area in a group setting with individual meals being served for each athlete and according to their meal plans. During each meal service, the weight of each food item will be recorded using calibrated scales (accurate to 2 g). Individualised snacks will be provided for intake between meals and before or during training sessions, with the requirement for their consumption to be cross-checked at the next meal. A range of 'free foods and drinks' (foods with low energy) will be provided in the participants living area with a check list to allow each participant to report on their daily intake at the first meal of the following day. Nutrition support during longer training sessions and after key training sessions will be provided at the training site by members of the research team and intake will be recorded. Compliance to the dietary prescription and reporting requirements will be checked on a daily basis. Substitutions of foods will be considered if individuals cannot consume the volume of foods prescribed with alternatives to achieve macronutrient targets.