The high efficiency of the BGO detectors makes them very attractive candidates to replace NaI(T1) detectors, which are widely used in studies of body composition. In this work, the response functions of the BGO and NaI(T1) detectors were determined at 0.662,4.4, and 10.0 MeV using three different Monte Carlo codes: EGS4, MCNP, and PHOTON. These codes differ in their input files and transport calculations, and were used to verify the internal consistency of the setup and of the input data. The energy range of 0.662 to 10 MeV was chosen to cover energies of interest in body composition-studies. The superior efficiency of the BGO-detectors has to be weighed-against their inferior resolution, and their higher price than that of the NaI detectors. Because the price of the BGO detectors strongly depends on the size of the crystal, its optimization is an important component in the design of the entire system.