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ARS Home » Pacific West Area » Davis, California » Western Human Nutrition Research Center » Immunity and Disease Prevention Research » Research » Publications at this Location » Publication #235896

Title: A Minute Dose of 14C-b-Carotene is Absorbed and Converted to Retinoids in Humans

item Burri, Betty

Submitted to: Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/13/2009
Publication Date: 6/17/2009
Citation: Ho, C.C., De Moura, F.F., Kim, S., Burri, B.J., Clifford, A.J. 2009. A Minute Dose of 14C-b-Carotene is absorbed and Converted to Retinoids in Humans. Journal of Nutrition, 139:1480-1486, 2009.

Interpretive Summary: Vitamin A is essential for normal growth and development. “Vitamin A” is actually an umbrella term for a small group of retinoids: retinal, retinol, and retinyl esters. Beta-carotene rich foods are the primary source of vitamin A for most of the world’s people, yet few studies have analyzed beta-carotene metabolism and its conversion to vitamin A in healthy individuals. We measured the absorption and metabolism of isotopically-labeled beta-carotene with accelerator mass spectrometry, a very sensitive method capable of measuring trace amounts of beta-carotene and retinoids in blood, urine and feces. This allowed us to measure the time course of beta-carotene absorption and conversion to vitamin A, and to calculate mass balance changes for beta-carotene and its metabolites. We report that beta-carotene absorption is highly variable—as expected—but discovered that most of the reason for this variability is urinary excretion, a pathway that was previously considered to have minor importance. Furthermore, we discovered that retinyl esters appear simultaneously with beta-carotene in the blood, while retinol appears much later. This is a very important finding. In vitro studies show that beta-carotene is cleaved to retinal, converted to retinol, and then esterified to form retinyl esters. Naturally, scientists assumed that when beta-carotene was cleaved to form vitamin A in the human body retinol would appear first, before retinyl esters. Thus, scientists and public health workers assumed that the conversion of beta-carotene to vitamin A could be accurately estimated by measuring the appearance of retinol in the blood. Indeed, vitamin A concentrations are evaluated in terms of retinol equivalents or retinol activity equivalents. However, our results clearly show that retinyl esters appear in blood before retinol, which means that one must measure retinyl ester formation to determine the conversion ratio of beta-carotene to vitamin A.

Technical Abstract: We dosed 8 adults with 14C-all-trans [10,10',11,11'-14C]-B-carotene (1.01 nmol) to quantify its absorption and metabolism. We used accelerator mass spectrometry (AMS) to measure 14C eliminated in feces over 14 days, in urine over 30 days, and that was retained in plasma over 166 days since dose. We then isolated plasma B-carotene, retinol (ROH), and total retinyl esters (RE) and measured their 14C to profile intact 14C-B-carotene and its metabolites by time since dose. The apparent digestibility of the 14C-dose was 50%. The metabolic fecal elimination and AUC were 0.05 +/- 0.02/d and 41 +/- 15 %, respectively. The portion of the 14C-dose that was eliminated in urine varied (8 +/- 6%). All subjects (but one) had a distinct plasma-14C-peak at 0.25 d post-dose, preceded by a “shoulder” at ~ 0.1 d and followed by a “broad” 14C-peak that returned to baseline within ~ 40 d. Plasma 14C-B-carotene and 14C-RE accounted for most of the absorbed 14C-B-carotene. Plasma 14C-B-carotene peaked just after 0.1, 0.2, and 1.0 d post-dose, presumably coinciding with remnant-chylomicra, -VLDL, and LDL + HDL. Plasma 14C-RE also peaked just after 0.1 and 0.2 d.. However, plasma 14C-ROH peaked at 0.3 and 1.0 d, presumably coinciding with retinol binding protein and LDL + HDL. Our data suggests that variable elimination of 14C-B-carotene via urine may result in the variable plasma responses reported for B-carotene. Furthermore, they suggest that it is necessary to measure RE concentrations to derive accurate estimates of the retinol equivalency of B-carotene.