To effectively recommend appropriate levels of vitamin A and estimate vitamin A status using retinol isotope dilution, it is imperative to understand vitamin A absorption efficiency under various conditions.  Work is continuing to develop and optimize better methodologies to accurately assess vitamin A absorption.  Green and Green adapted an existing method to determine cholesterol absorption and used it to measure vitamin A absorption in rats.  Their results are published in the July 2020 issue of The Journal of Nutrition.

Rats were given an oral dose of [3H]retinyl acetate and an intravenous dose of [14C]vitamin A-labeled lymph derived from donor rats.  Plasma was analyzed for 3H and 14C in samples collected on days 1, 2, 3, 6, 9, and 12, and vitamin A absorption was calculated (fraction of oral dose/fraction of intravenous dose).  Feces and urine collected on days 3, 6, 9, and 12 were pooled for analysis of radioactivity, which was also measured in the liver and the remaining carcass on day 12.

Using the plasma isotope ratio led to a calculated vitamin A absorption of over 100% for day 1 and decreased to 74% by day 12.  Only 6.2% of the oral dose was recovered in feces, and the isotope ratio in urine from days 9-12 was lower than that in plasma.  These observations led the authors to conclude that this approach needs further work in order to optimize the approach, but that it holds promise as a way of estimating vitamin A absorption.

References Green MH, Green JB.  Vitamin A absorption determined in rats using a plasma isotope ratio method.  The Journal of Nutrition, Volume 150, Issue 7, July 2020, Pages 1977–1981, https://doi.org/10.1093/jn/nxaa092.

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Nancy Turner, PhD

Nancy Turner is a Professor of Nutrition at Michigan State University.  Her research focus is on the role of diet on colon cancer and inflammatory bowel disease.  She is the author of many papers on this topic, as well as the co-editor of a book, and co-author of a textbook.