Multi-element analysis of the obese subject by in vivo neutron activation analysis.

The Leeds facility for in vivo neutron activation analysis has been modified and calibrated for the simultaneous measurement of nitrogen, potassium, sodium, chlorine, phosphorus and calcium in obese patients weighing up to 210 kg. The effects of body size and shape were incorporated into the calibration by measuring 14 anthropomorphic phantoms of known composition representing individual patients being treated for obesity. The phantoms were constructed from tissue substitutes representing lean, skeletal and adipose tissues, arranged to simulate the distributions of the corresponding tissues within the patients, as visualised by CT scanning. The precision of the method, determined by measuring a single phantom ten times over a period of ten weeks, is between two and three per cent for all elements except calcium, for which it is 11.3%. Accuracy is estimated to be similar to precision. The procedure has been used to study changes in body composition of patients undergoing therapeutic starvation.