Precursors of vitamin A, the carotenes, occur as orange-yellow pigments, mainly in green leaves and to a lesser extent in corn. Four of these carotenoids, alpha-carotene, beta-carotene, gamma-carotene, and cryptoxanthin (the main carotenoid of corn), are of particular importance because of the provitamin A activity. Vitamin A activity of beta-carotene is substantially greater than that of other carotenoids. However, biological tests have consistently shown that pure vitamin A has twice the potency of beta-carotene on a weight-to-weight basis. Only one molecule of vitamin A is formed from one molecule of beta-carotene. Lycopene is an important carotenoid for its antioxidant function but does not possess the beta-ionone ring structure (required for vitamin A activity) and, therefore, is not a precursor of vitamin A. In humans, beta-carotene and lycopene are the predominant carotenoids in tissue (Ribaya-Mercado et al., 1995).
A number of factors influence digestibility of carotene and vitamin A. Working with lambs, Donoghue et al. (1983) reported that dietary levels of vitamin A ranging from mildly deficient to toxic levels affect digestion and uptake. Percentage transfer from the digestive tract from supplemental dietary levels of 0, 100 and 12,000 µg retinol per kg were 91%, 58% and 14%, respectively. Wing (1969) reported that the apparent digestibility of carotene in various forages fed to dairy cattle averaged about 78%. Variables that influenced carotene digestibility included month of forage harvest, type of forage (hay, silage, greenchop or pasture), species of plant and plant dry matter. In general, carotene digestibility was higher during warmer months than during winter.
In most mammals the product ultimately absorbed from the intestinal tract as a result of feeding carotenoids is mainly vitamin A itself. There is considerable species specificity regarding the ability to absorb dietary carotenoids. In some species, such as the rat, pig, goat, sheep, rabbit, buffalo and dog, almost all the carotene is cleaved in the intestine. In humans, some breeds of cattle, horses and carp, significant amounts of carotene can be absorbed. Absorbed carotene can be stored in the liver and fatty tissues. Hence, these latter animals have yellow body and milk fat, whereas animals that do not absorb carotene have white fat. In the case of cattle, there is a strong breed difference in absorption of carotene. The Holstein is an efficient converter, having white adipose tissue and milk fat. The Guernsey and Jersey breeds, however, readily absorb carotene, resulting in yellow adipose fat and also butterfat.
Beta-carotene present in feed is converted in the intestinal mucosa by an enzyme to retinal, which is then reduced to retinol (vitamin A). However, extensive evidence exists also for random (excentric) cleavage, resulting in retinoic acid and retinal, with a preponderance of apocarotenals formed as intermediates (Wolf, 1995). The cleavage enzyme has been found in many vertebrates but is not present in the cat or mink. Therefore, these species cannot utilize carotene as a source of vitamin A. The main site of vitamin A and carotenoid absorption is the mucosa of the proximal jejunum. Although carotenoids are normally converted to retinol in the intestinal mucosa, they may also be converted in the liver and other organs, especially in yellow fat species such as Guernsey and Jersey breeds of cattle (McGinnis, 1988). Either dietary retinol or retinol resulting from conversion of carotenoids is then esterified with a long-chain fatty acid, usually palmitate.
A number of factors affect absorption of carotenoids. Cis-trans isomerism of the carotenoids is important in determining their absorbability, with the trans forms being more efficiently absorbed (Stahl et al., 1995). Among carotenoids, there is a differential uptake and clearance of specific carotenoids (Bierer et al., 1995; Johnson et al., 1997). Dietary fat is important (Roels et al., 1958; Fichter and Mitchell, 1977). Dietary antioxidants (e.g., vitamin E) also appear to have an important effect on the utilization and perhaps absorption of carotenoids. It is uncertain whether the antioxidants contribute directly to the efficient absorption or whether they protect both carotene and vitamin A from oxidative breakdown. Protein deficiency reduces absorption of carotene from the intestine.
Different species of animals convert beta-carotene and vitamin A with varying degrees of efficiency. Some factors that influence the rate at which carotenoids are converted to vitamin A are type of carotenoid, class and production level of animal, individual genetic differences in animals and level of carotene intake (NRC, 1996). Efficiency of vitamin A conversion from beta-carotene is decreased with higher levels of intake (Van Vliet et al., 1996). The conversion rate of the rat has been used as the standard value, with 1 mg of beta-carotene equal to 1,667 IU of vitamin A. Based on this standard, the comparative efficiencies of various species are shown in Table 1. Of the species studied, only poultry are equal to the rat in vitamin conversion, with cattle being only 24% as efficient. For the chicken, as beta-carotene level is increased, conversion efficiency drops from a ratio of 2:1 to 5:1 and from 8:1 to 16:1 for the calf (Bauernfeind, 1972).
Stress conditions, such as extremely hot weather, viral infections and altered thyroid function, have also been suggested as causes for reduced carotene to vitamin A conversion. Vitamin A requirements are higher under stressful conditions such as abnormal temperatures or exposure to disease conditions. As an example with poultry, coccidiosis not only causes destruction of vitamin A in the gut but also injures the microvilli of the intestinal wall.
The efficiency of beta-carotene in meeting the vitamin A requirements of trout and salmon apparently is dependent on water temperature. Cold-water fish utilize precursors of vitamin A at 12.4 to 14°C, but do not at 9°C (Poston et al., 1977). Activity of beta-carotene-15,15'-dioxygenase, which oxidizes beta-carotene to retinal in the intestinal mucosa, may be restricted at cold temperatures.
Dietary retinyl esters are hydrolyzed to retinol in the intestine; they are absorbed as the free alcohol and then re-esterified in the mucosa. The retinyl esters are transported mainly in association with lymph chylomicrons to the liver where they are hydrolyzed to retinol and re-esterified for storage. Hydrolysis of the ester storage form mobilizes vitamin A from the liver as free retinol.
Retinol is released from the hepatocyte as a complex with retinol-binding protein (RBP). Retinol in association with RBP circulates to peripheral tissues complexed to a thyroxine-binding protein, transthyretin (Blomhoff et al., 1991; Ross, 1993). The retinol-transthyretin complex is transported to target tissues, where the complex binds to a cell-surface receptor and the retinol is transported into cells of target tissue.
Once the retinoids are transferred into the cell they are quickly bound by specific binding proteins in the cell cytosol. The intracellular retinoid-binding proteins bind retinol, retinal and retinoic acid for purposes of protection against decomposition, solubilize them in aqueous medium, render them nontoxic and transport them within cells to their site of action. These binding proteins also function by presenting the retinoids to the appropriate enzymes for metabolism (Wolf, 1991). Some of the principal forms of intracellular (cytoplasmic) retinoid-binding proteins are cellular retinol-binding proteins (CRBP, I and II), cellular retinoic acid-binding proteins (CRABP, I and II), cellular retinaldehyde binding protein (CRALBP) and six nuclear retinoic acid receptors (RAR and RXR, with alpha, beta and gamma forms). There are two classes of nuclear receptors, with all-trans retinoic acid the ligand for RAR, and 9-cis-retinoic acid the ligand for RXR (Kasner et al., 1994; Kliewer et al., 1994).
Retinol is readily transferred to the egg in birds, but the transfer of retinol across the placenta is marginal and mammals are born with very low liver stores of vitamin A. In the pig, uterine RBP has been identified in the uterus, with the function of delivering retinol to the fetus (Clawitter et al., 1990). The main excretory pathway for vitamin A is by elimination as glucuronide conjugates in the bile prior to fecal excretion.
The liver normally contains about 90% of the total body vitamin A. The remainder is stored in the kidneys, lungs, adrenals and blood, with small amounts also found in other organs and tissues. Several studies have shown that the liver can store enough vitamin A to protect the animal from long periods of dietary scarcity. This large storage capacity must be considered in studies of vitamin A requirements to ensure that intakes that appear adequate for a given function are not being supplemented by reserves stored prior to the period of observation. Measurement of the liver store of vitamin A at slaughter or a biopsy is a useful technique in studies of vitamin A status and requirements.
During periods of low dietary carotene, stored vitamin A can be mobilized and utilized without signs of vitamin A deficiency. At birth, the mammal usually does not have sufficient vitamin A reserves to provide for its needs for any substantial time. Accordingly, it is important that young animals receive colostrum, which generally is high in vitamin A, within a few hours after birth. If the dam has received a diet low in vitamin A activity, the newborn animal is likely to be susceptible to a vitamin A deficiency because body reserves are low and colostrum will have a subnormal content (Miller et al., 1969).