Intro: A typical High Intensity Interval Training (HIIT) programme involves only around 4 min exercise per session, yet improves cardiorespiratory function and increases fatty acid oxidation. It remains unclear whether such short duration, high intensity exercise will lead to significant loss of fat mass over time. Methods: Men and women (n=42, 57% male, mean (SD) age: 33 (±5) yrs;) from the general Manchester population gave written informed consent to participate. Measurements of body composition by Dual energy X-ray absorptiometry (DXA), maximal oxygen uptake (VO2max) and maximal rates of fat oxidation (FATmax) were taken in the fasted state before and after 12 weeks HIIT. HIIT consisted of 4 x 20 sec sprints on a cycle ergometer at a workload of approximately two-times higher than VO2max; 3 times per week, totalling 4 mins exercise per week. Results: VO2max increased by 7%(pre-training: 2.9 ml/min-1(±0.9) vs post-training:3.1ml/min-1 (±0.88) p=0.0003). The maximal rate of fat oxidation during steady-state exercise increased by 19%, from 7.8 mg/kg/min-1 (±3.2) to 9.3 mg/kg/min-1 (±3.9) (p=0.02). There were no significant changes to total body lean mass, lean mass of the legs or of the arms. Total body fat mass decreased by 5.42% (pre-training: 18.25 kg (±6.10) vs post-training 17.24 kg (±5.96) P=0.0002). Trunk fat decreased by 3% (pre-training: 9.1 kg (±3.9) vs post-training 8.8 kg (±3.8) p=0.007) and leg fat decreased by 6% (pre-training: 6.8 kg (±1.1) vs post-training: 6.4 kg (±1.1) p=<0.0005). Conclusion: A total of 4 min exercise per week over 12 weeks significantly increased VO2max and fat oxidation. It also decreased total body, trunk and leg fat mass. These adaptations might improve health status and reduce risk of metabolic and cardiovascular disease.