Retinol Isotope Dilution Is Applied during Restriction of Vitamin A Intake to Predict Individual Subject Total Body Vitamin A Stores at Isotopic Equilibrium

• Published in: The Journal of nutrition Vol. 146; no. 11; pp. 2407 - 2411
• Main Authors: Green, Michael H, Ford, Jennifer Lynn, Green, Joanne Balmer
• Format: Journal Article
• Language: English
• Published: United States American Institute of Nutrition 01.11.2016
• Subjects: Adult; Homeostasis; Humans; Indicator Dilution Techniques; Intervention; Isotope Labeling; Models, Biological; Nutrition; Nutritional Status; Tissue Distribution; Vitamin A; Vitamin A - administration & dosage; Vitamin A - metabolism; Vitamin A - pharmacokinetics; Vitamin A Deficiency - diagnosis; Olson equation; vitamin A status; WinSAAM; humans; isotopic equilibrium
• ISSN: 0022-3166; 1541-6100; 1541-6100
• DOI: 10.3945/jn.116.238899

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. We investigated the hypothesis that short-term restriction of vitamin A intake would result in accurate RID prediction of vitamin A TBS in individuals. 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 (SA ; fraction of dose/μmol retinol) and stores, and we calculated TBS using the simplified RID equation TBS = 0.75 × 1/SA , where the fractional absorption of tracer was set at 0.75 and SA was simulated 5 d after dosing. 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. By eliminating or strictly limiting vitamin A input, isotopic equilibrium was reached by 5 d. At isotopic equilibrium, SA is the same as that in the body's exchangeable vitamin A pools; under these conditions, SA may be measured at any time from 5 d on and used to calculate TBS.