Should We Restrict Vitamin A Intake, a Minor Contributor to Plasma Retinol Turnover, When Using Retinol Isotope Dilution Equations to Estimate an Individual's Vitamin A Status, or Should Vitamin A Balance Be Maintained?

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We discuss whether dietary vitamin A intake should be restricted or maintained at balance when retinol isotope dilution equations are applied to estimate an individual's vitamin A total body stores (TBS) after oral administration of a labeled dose of vitamin A. Although, at first glance, restriction makes sense as a way to prevent dilution of tracer in plasma, further investigation of the assumptions underlying the widely used isotope dilution equation presented by Olson's laboratory in 1989, as well as the compartmental modeling results presented in this article, indicate that, in fact, restriction leads to an incorrect prediction of TBS if steady state retinol isotope dilution equations are applied at the traditional time (21 d). Our results show that newly ingested vitamin A is a minor contributor to total plasma retinol turnover and that restriction of vitamin A intake leads to a higher plasma retinol specific activity than the value obtained when vitamin A input equals output (balance). When that higher specific activity is used in the traditional retinol isotope dilution equation, it results in a small but notable underestimation of vitamin A TBS. We conclude that, especially if blood is sampled at the traditional time, the most accurate results will be obtained when vitamin A balance is maintained. If sampling is done soon after dosing (e.g., 4 d), dietary intake has less effect on plasma retinol specific activity and thus on predictions of vitamin A status. Vitamin A status can also be estimated if intake is completely restricted and a different (non–steady state) equation is applied at an appropriate time after isotopic equilibrium has been reached.