β-Carotene cleavage and measures of vitamin A activity
THE CONTEXT
Vitamin A content can be expressed using retinol equivalents, retinol activity equivalents, or the older international-unit system.
Cleavage and retinol equivalents
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Symmetrical cleavage of β-carotene by carotene dioxygenase yields retinaldehyde.
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Although one β-carotene molecule appears capable of yielding two retinol molecules, practical nutritional equivalence differs: 6 μg of β-carotene corresponds to 1 μg of preformed retinol.
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Micrograms of retinol equivalents equal micrograms of preformed vitamin A plus one-sixth of the micrograms of β-carotene plus one-twelfth of the micrograms of other provitamin A carotenoids.
Retinol activity equivalents
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Retinol activity equivalents account for incomplete absorption and metabolism of carotenoids.
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One retinol activity equivalent equals 1 μg of all-trans-retinol, 12 μg of β-carotene, or 24 μg of α-carotene or β-cryptoxanthin.
International units
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International units originated from biological assays performed before pure vitamin A was available for chemical analysis. They are obsolete but may still appear on food labels.
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One international unit equals 0.3 μg of retinol; 1 μg of retinol equals 3.33 international units.
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On the retinol activity equivalent basis, one international unit of vitamin A activity equals 3.6 μg of β-carotene or 7.2 μg of other provitamin A carotenoids.