A modified isotope dilution equation predicts vitamin A total body stores in individuals

Isotope dilution is used to successfully predict vitamin A (VA) total body stores (TBS) in populations but does not typically work for individuals. Using plasma kinetic data from 33 healthy young adults dosed with [13C10]retinyl acetate, we determined TBS using model‐based compartmental analysis (Simulation, Analysis and Modelling software [SAAM]) vs the equation TBS = F x S x (1/SA), where F= fraction of the orally administered stable isotope dose in stores, S= ratio of specific activity of retinol in plasma to that in stores, and SA = VA specific activity in plasma at 3 d post dosing. A high correlation (R=0.92) was found for the population. We found that the fractional catabolic rate (FCR) was a proxy for F and developed a mono‐exponential equation using 7 and 14 d data to predict F in individual subjects (R=0.999). Since S is ~1 at 3 d (when VA SA in stores crosses over plasma), the equation for d 3 simplifies to TBS = F x (1/SA). That is, by collecting blood samples at 3, 7 and 14 d and then calculating FCR based on 7 and 14 d data, we can accurately estimate an individual's TBS as individual F x (1/SA) at 3 d (R=0.94). This study was supported by BBSRC.