Use of Compartmental Modeling to Study Long-Term Adaptation to Increases or Decreases in Daily Vitamin A Intake in...

Whole-body vitamin A (VA) stores, an index of VA status, result from the cumulative difference between VA input (dietary intake times fractional absorption) and output (disposal).

Our objective was to apply compartmental modeling to determine how long-term changes in VA intake influence VA storage and disposal in theoretical subjects.

We generated 7 theoretical women with VA total body stores (TBS) of 225-1140 μmol and VA intakes [1.92-2.95 μmol/d (549-844 μg RAE/d)] that maintained a VA steady state [steady state (SS); i.e., VA balance]. Using Simulation, Analysis and Modeling software, we either adjusted VA intakes to 0.25, 0.5, 2, and 3 times initial SS intakes or added a daily supplement (3.5 μmol/d); then, we simulated TBS over 3 decades, with plasma VA homeostatically controlled, to determine whether and if so, when, a new VA SS was attained and by what amount stores changed.

After >5 y, the specified decreases or increases in VA intake led to a new SS TBS plateau, with mean fractional changes reaching 0.14 (at 0.25× initial SS intake), 0.43 (0.5×), 2.1 (2×), and 3.2 (3×). Similarly, adding 3.5 μmol/d above SS intake led to a new SS TBS after >5 y, with TBS increasing 426-1666 μmol; the greatest gain was in subjects with higher initial stores.

For these subjects, the VA intake required to maintain VA balance increased with TBS. Results have implications for dietary VA recommendations: assuming that 1) only a small part of total VA disposal is related to essential VA functions and 2) functional needs do not change with TBS, one wonders whether one's VA needs should be viewed as the intake needed to support functional requirements, with higher recommendations providing a buffer that includes nonfunctional VA disposal and a moderate store.