A method of in-vivo fat measurement of humans or animals by scanning the ear of the subject using a fibre optic probe delivering a light beam of Near infrared wavelengths provided by a NIR source. Passing the beam through an interferometer to encode data from the whole spectral range simultaneously. Detecting reflected light by a detector and applying Fourier Transform techniques to determine the intensity of the light in at least one narrow wave band selected for its correspondence to a form of fat. Recording the NIR response and determining the fat content of the body by either comparison to known reference samples or by use of an empirical formula.