Calculation of short-term changes in body fat from measurement of respiratory gas exchange

A method is presented where the gain in body fat of patients receiving intravenous nutrition (IVN) may be computed using indirect calorimetry. When patients are administered their nutritional requirements soley in the form of glucose and amino acids, metabolism of these will result in changes in the exchange of carbon dioxide and oxygen. Since net fat synthesis results in the production of excess carbon dioxide whereas degradation results in a net reduction in the production of carbon dioxide, changes in RQ will reflect underlying changes in net fat balance. A correction may be applied to these measurements to allow for net protein balance. The basic equations and stoichiometry for the synthesis of tripalmitin from glucose are shown together with the derivation of the formulae that enable the calculation of fat changes to be made. A group of 13 patients was used to assess the technique and given energy at 1.5 times resting metabolic expenditure together with adequate nitrogen as amino acids to maintain protein balance. The patients were monitored daily in the resting state and fed at a constant rate so that errors due to discontinuous intake or exercise were avoided. The group was found to gain 874 +/- 243 g fat during the 14 day study. Although individual changes of fat in patients ranged from -169 to +2487 g, net synthesis of fat was found in ten of them. It is suggested that indirect calorimetry is useful not only in the assessment of actual expenditure in patients but can also be used to quantify the fate of administered fuels during intravenous nutrition.