Intestinal Metabolism

Dietary retinyl esters must be hydrolyzed in the lumen of the small intestine before retinol is absorbed, while carotenoids must be absorbed into the intestinal mucosa before being cleaved intracel-lularly. Several enzymes with retinyl ester hydrolase (REH) activity are present in pancreatic juice or on the brush border of duodenal and jujenal enterocytes (Figure 3). Retinol and carotenoids must be solu-bilized in the lumen in mixed micelles composed of bile acids and products of lipid digestion prior to their uptake into enterocytes. These processes require the release of an adequate amount of bile salts and a minimal quantity of dietary fat (approximately 5%), which must be consumed concomi-tantly. The retinol thus liberated diffuses into the enterocyte, is bound by CRBP-II, and is then ester-ified. The newly formed retinyl esters are incorporated into the lipid core of chylomicra, lipoproteins that transport dietary fat into the lymphatic system for absorption. The overall efficiency of retinol absorption is quite high, about 70-90%, and is not significantly downregulated as vitamin A consumption increases.

The efficiency of absorption of ^-carotene is considerably lower (9-22%) and more variable than that of retinol. In fact, in controlled studies some subjects have absorbed little, if any, of a test dose of ^-carotene. In individuals who do absorb dietary carotenoids, the efficiency of absorption tends to fall as intake increases. The type of carote-noid and its physical form in the ingested foodstuff also affect the efficiency of carotene absorption. Pure ^-carotene in an oily solution or supplements is absorbed more efficiently than an equivalent amount of ^-carotene in foods. Much of the carote-noid present in foods is bound within a matrix of

Dietary retinyl esters

Intestinal micelle formation; absorption

Dietary ^-carotene

Intestinal micelle formation; absorption

Plasma transport of holo-RBP + TTR

Figure 3 Absorption of dietary vitamin A (VA) via chylomicrons (CM), vitamin A storage in liver, and the release of retinal to plasma as holo-retinol-binding protein (RBP), which combines with transthyretin (TTR), to deliver retinal to organs that produce retinal (eyes) or retinoic acid (essentially all tissues) for the biological functions attributed to vitamin A.

Plasma transport of holo-RBP + TTR

Target tissue storage, oxidative metabolism, and metabolic responses to bioactive retinoids

Nuclear receptor ^mechanisms/

Figure 3 Absorption of dietary vitamin A (VA) via chylomicrons (CM), vitamin A storage in liver, and the release of retinal to plasma as holo-retinol-binding protein (RBP), which combines with transthyretin (TTR), to deliver retinal to organs that produce retinal (eyes) or retinoic acid (essentially all tissues) for the biological functions attributed to vitamin A.

polysaccharides, fibers, and phenolic compounds that is incompletely digested. Although the absorption of provitamin A carotenoids from fruits is generally better than from fibrous vegetables, it is still low as compared to ^-carotene in oil (see section on Units).

Once in the enterocyte, provitamin A carotenoids are cleaved by one or more carotene monooxy-genases, and the product (initially retinal) is metabolized to form retinol and, subsequently, retinyl esters (see Figure 2). In humans about one-third of ingested ^-carotene escapes cleavage and, instead, is incorporated intact into chylomicrons.

A small fraction of intestinal ^-carotene is oxidized to retinoic acid and absorbed into portal blood. It is speculated that the cleavage of dietary 9-ds-/3-carotene and its subsequent oxidative metabolism may be a significant source of 9-cis-retinoic acid.

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Dieting Dilemma and Skinny Solutions

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