Retinol Isotope Dilution Is Applied during Restriction of Vitamin A Intake to Predict Individual Subject Total Body Vitamin A Stores at Isotopic Equilibrium.
BACKGROUND
Retinol isotope dilution (RID) equations are used to determine vitamin A status and the efficacy of vitamin A intervention programs. Recent work related to RID methods has focused on modifying the "Olson equation" to improve the accuracy of predictions of vitamin A total body stores (TBS) in individual subjects.


OBJECTIVE
We investigated the hypothesis that short-term restriction of vitamin A intake would result in accurate RID prediction of vitamin A TBS in individuals.


METHODS
We applied model-based compartmental analysis to a 6-component model derived from published retinol kinetic studies on 12 individuals with a wide range of vitamin A stores and determined vitamin A TBS in the steady state. Then we simulated the impact of eliminating or strictly limiting vitamin A intake at the time of isotope administration, while maintaining plasma retinol homeostasis, on retinol specific activity in plasma (SAp; fraction of dose/μmol retinol) and stores, and we calculated TBS using the simplified RID equation TBS = 0.75 × 1/SAp, where the fractional absorption of tracer was set at 0.75 and SApwas simulated 5 d after dosing.


RESULTS
When vitamin A intake was zero or strictly limited (0.25 μmol/d), mean TBS predicted by the equation at 5 d after dose administration divided by TBS determined by using the model was 1.00 (range: 0.959-1.04) or 1.02 (range: 0.983 - 1.06), respectively.


CONCLUSIONS
By eliminating or strictly limiting vitamin A input, isotopic equilibrium was reached by 5 d. At isotopic equilibrium, SApis the same as that in the body's exchangeable vitamin A pools; under these conditions, SApmay be measured at any time from 5 d on and used to calculate TBS.
Department of Nutritional Sciences, College of Health and Human Development, The Pennsylvania State University, University Park, PA mhg@psu.edu.
Michael H
MH
Green
Department of Nutritional Sciences, College of Health and Human Development, The Pennsylvania State University, University Park, PA.
Jennifer Lynn
JL
Ford
Department of Nutritional Sciences, College of Health and Human Development, The Pennsylvania State University, University Park, PA.
Joanne Balmer
JB
Green
D000328:Adult; D006801:Humans; D007201:Indicator Dilution Techniques; D007553:Isotope Labeling; D008954:Models, Biological; D009752:Nutritional Status; D014018:Tissue Distribution; D014801:Vitamin A / Q000008:administration & dosage / Q000378:metabolism* / Q000493:pharmacokinetics*; D014802:Vitamin A Deficiency / Q000175:diagnosis*
D014801:Vitamin A
Olson equation; WinSAAM; humans; isotopic equilibrium; vitamin A status