System using on- or in-body communication technologies such as body-sound communication (BSC) or body-coupled communication (BCC) to obtain information on the body composition of a patient (e.g. water content/hydration level on bone density/joint status). The system, in a preferred embodiment, comprises a transmitter, receiver and processor connected to the receiver. The transmitter is arranged to transmit a signal through a medium (the body of the patient), the signal comprising a plurality of different frequency components and transmission technologies. The receiver is arranged to receive the signal following propagation through the said medium. The processor is arranged to generate, at a first time, one or more transfer functions from the received signal, each transfer function defining values for a predetermined signal parameter at different frequencies. The processor then generates, at a second time, one or more further transfer functions from the received signal and compares a transfer function with the further transfer function for the same signal parameter. An output is generated if the difference between the transfer function and the further transfer function exceeds a preset threshold.
