Subject who meets inclusion criteria will be enrolled and randomised to one of the 3 groups (outlined above) and invited to attend for baseline investigations (before intervention). Subjects will be asked to fast from midnight (except clear water) prior to attending their first day of investigations: DAY(-4). On this day, baseline investigations will include a DXA scan, fasting specimen (blood glucose, saliva, muscle and fat) collection, vascular ultrasound scans (flow mediated dilation (FMD), leg blood flow (LBF)), and an oral glucose tolerance test (OGTT) with 2-Deoxyglucose (2DOG) tracer infusion to assess glucose uptake and insulin sensitivity. The muscle biopsy will be repeated after the 2DOG infusion, followed by leg ultrasound scans to assess muscle structure. A standard lunch will be provided in the afternoon. A cardiac echocardiogram (ECHO) will be performed to assess cardiac function followed by ultrasound scan (USS) of dominant thigh muscles (for pennation angle, thickness, fascicle length) and cardiopulmonary exercise test (CPET) to determine maximal aerobic capacity and set the load for the HIIT training (Watt Max, where applicable). A loading dose of deuterium oxide (D2O) and D3 creatine tracer solutions will then be given. Before leaving, subjects will be supplied with 3-Methylhistidine (3MH) tracer solution to be taken 2 days later DAY(-2), plus daily 'top-ups' of D2O (all tracer amounts based on body weight). Saliva bottles for daily saliva collection (3 hours after daily D2O top up, to be stored in the fridge) and containers for urine collection for D3 creatine assessment (daily urine sample collection for 3 days in containers begin with 24-hour urine collection, then spot urine at 30h, 48h and 72h) for muscle mass assessment will also be provided. After consuming 3MH on DAY(-2), subjects will then attend our unit the next day (DAY(-1)) in the morning, fasted from midnight (except clear water). Hourly blood samples for 6 hours to measure 3MH levels (the first sample to be taken 20-24 hours after 3MH ingestion) and contrast enhanced ultrasound (CEUS) will be done. Following this, baseline muscle strength/ function will be assessed; 1 repetition maximum (1-RM) assessment on 6 (3 upper body, 3 lower body) exercises and maximal voluntary contraction (MVC) of leg extension. After this visit subjects will begin their intervention period (DAY 1) according to their group. All groups will have a 6-week intervention period of nutritional intervention in the form of LighterLife VLCD meal replacement diets: 4 meals/day providing 600kcal/d with 100% recommended daily allowance (RDA) vitamins and minerals. For RET group, the exercise format will be 2 sets of 12 repetitions at 70% 1-RM for each muscle group with 1-2 minutes rest between sets after a prescribed warm-up 3 sessions/week. The exercises will involve upper and lower body exercises and will be progressive in nature with muscle strength assessed every ~14 days to ensure that the intensity of training remains constant throughout. In the HIIT group, it will be 3 sessions/week, involving 60s high intensity cycling at 95-125% Watt Max, with 90s recovery, for 5 cycles, with 2 minutes warm up and cool down. All subjects will have blood samples (including 3MH tracer) and muscle/ fat biopsies every 3 weeks. DXA scan, ultrasound measurements (vascular and muscle), D3 creatine assessment, 2DOG infusion, OGTT (as explained above), CPET, MVC, muscle strength assessment will be performed before and after the 6-week intervention period. Subjects will be collecting saliva daily into saliva bottles. The collections will be kept in fridge and will be sent to our unit 3 times/week for exercise group (during exercise sessions), and once a week for VLCD only group. Diet diary and physical activity (Actiheart Monitor) also will be monitored (for 4-5 days) before and during interventions, which should include weekend measurement.