Duration of retinol isotope dilution studies with compartmental modeling affects model complexity, kinetic parameters, and calculated Vitamin A stores in US women

Background: Retinol isotope dilution (RID) indirectly estimates vitamin A (VA) status. Multicompartment modeling of RID data is used to refine study designs and equations to calculate VA stores. Previous studies suggest that VA in slowly turning over pools is not traced if follow-up is not long enough; however, shorter RID studies are being investigated. Few long-term models have been published. Objective: We determined the effect of time on mathematical models of VA kinetics, model parameters, and outcomes. Methods: In this longitudinal study, women (mean ± SD age: 22 ± 3 y; n = 7) were given 2.0 µmol [14,15]-¹³C₂-retinyl acetate. Blood samples were staggered from 4 h to 152 d; the fraction of dose in serum was modeled with compartmental models. Four model-time categories were created: full models that used all data (median: 137 d; range 97-152 d) and truncated shorter studies of 14, 27, and 52 d (range: 42-62 d). Outcomes included number of compartments to adequately model serum data, kinetic parameters, total traced VA mass, and time-to-dose equilibration. To gain insight into longer follow-up, an additional participant was given 17.5 µmol¹³C₄-VA, and data were modeled as long as enrichment was above baseline (5 y). Results: Longer follow-up times affected kinetic parameters and outcomes. Compared with the 14-d models, long-term full models required an additional compartment for adequate fit (14.3% compared with 100%; P= 0.0056) and had longer [median (quartile 1, quartile 3)] whole-body half-life [15.0 d (10.5, 72.6 d) compared with 135 d (115, 199 d); P = 0.0006], time-to-dose equilibration [3.40 d (3.14, 6.75 d) compared with 18.9 d (11.2, 25.7 d); P < 0.0001], and total traced mass [166 µmol VA (162, 252 µmol VA) compared with 476 µmol VA (290, 752 µmol VA); P = 0.0031]. Conclusions: Extended RID sampling alters numerous mathematically modeled, time-dependent outcomes in women. Length of study should be considered when using mathematical models for calculating total-body VA stores or kinetic parameters related to VA turnover.