1. A method for analyzing bioimpedance data for a body segment of a subject, said body segment having an external skin surface, said method comprising: (a) applying alternating current at a plurality of frequencies to at least two points on the external skin surface so as to cause current to pass through the segment; (b) for each frequency, recording the voltage difference between at least two other points on the external skin surface, said recorded voltage differences comprising both magnitude and phase information; and (c) using the recorded voltage differences at the plurality of frequencies to determine at least one numerical value indicative of the muscle, fat, and/or extracellular fluid content of the segment, said numerical value being determined using an impedance model for the segment which at least comprises three parallel paths, one of which consists of a capacitor C M and a resistor R I in series which represent primarily the muscle component of the segment, one of which consists of a capacitor C F and a resistor R F in series which represent primarily the fat component of the segment, and one of which consists of a resistor; wherein the three parallel path impedance model is used at least one frequency less than or equal to 10 kilohertz and at least one frequency greater than 10 kilohertz.