The retention of deuterium fuel during MAST discharges has been studied using the method of global gas balance. The results show that with inter-shot 4 He- GDC, the total number of particles retained for a discharge increases with total number of injected particles, and the total retention fraction stays very high, mostly around 80%, and can be almost 100%. However, it is observed that disruptions can reduce the wall inventory. With inter-shot 4 He-GDC to recondition surfaces on MAST, the retention for all shots analyzed has not been observed to saturate, while if there is no 4 He-GDC before a shot, the following plasma density can become uncontrollable owing to lower wall pumping capacity. This indicates a majority of injected particles is retained in the walls/divertors during each discharge and most of them can be recovered with 4 He-GDC, suggesting that on the short time-scale of MAST pulses, the particle balance is dominated by dynamic retention, which is mostly attributed to direct implantation of ions and neutral particles in a shallow surface layer.