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 their provitamin A activity. Vitamin A activity of beta-carotene is substantially greater than that of other carotenoids. However, biologic tests have consistently shown that pure vitamin A has twice the potency of beta-carotene on a weight-to-weight basis (Hendricks et al., 1967). Thus, only one molecule of vitamin A is formed from one molecule of beta-carotene. Early researchers (Parrish et al., 1951) recognized that unit for unit carotene is less effective than preformed vitamin A as a supplement for swine during early gestation and early lactation. In addition, Myers et al. (1959) reported that per unit of intake, vitamin A gave a greater rate of change than a corresponding increase in carotene intake. In poultry, one IU of vitamin A is equivalent to 0.6 µg of beta-carotene (NRC, 1984). This seems to indicate that under normal conditions poultry obtain the equivalent activity of only one molecule of vitamin A for each molecule of beta-carotene. This, however, is a better efficiency of utilization than is found in swine and most other animals. Pigs are less efficient than poultry in converting beta-carotene to vitamin A (Ullrey, 1972). Poultry are able to convert 1 mg of beta-carotene to 1667 IU of vitamin A. Based on liver storage, the biopotency of 1 mg of carotene in corn fed to weanling pigs is 261 IU of vitamin A (Wellenreiter et al., 1969). This is less than 16% of the efficiency for vitamin A conversion by the rat or poultry. The activity of beta-carotene decreases further with increasing intake. At higher levels of all-trans-carotene intake from corn gluten meal, 1 mg of beta-carotene had a vitamin A potency of only 123 to 174 IU. Ullrey et al. (1965) reported that 1 mg of beta-carotene from a fermentation process was equal to 192 IU of all-trans-vitamin A palmitate.
The biosynthesis, absorption and transport of vitamin A have been reviewed in more detail elsewhere (Goodman, 1979, 1980). Beta-carotene in feed is cleaved in the intestinal mucosa by dioxygenase, an enzyme, to retinal, which is then reduced to retinol (vitamin A) in agreement with results reported by Swick et al. (1952). Early researchers, including Fidge et al. (1969), compared properties of the rat mucosal cleavage enzyme with that of the hog mucosal cleavage enzyme and found similar mechanisms of catalyzing beta-carotene to retinal. The absorption of vitamin A in the intestine is believed to be 80% to 90%, while that of beta-carotene is about 50% to 60% (Olson, 1984). Vitamin A efficiency of absorption decreases somewhat with very high doses.
The main site of vitamin A and carotenoid absorption is the mucosa of the proximal jejunum. Carotenoids are normally converted to retinol in the intestinal mucosa but may also be converted in the liver and other organs, especially in yellow-fat species, such as poultry (McGinnis, 1988). Poor et al. (1987) determined that species vary in their ability to absorb a variety of carotenoids intact. The authors indicated that pigs do not appear to absorb or store significant quantities of intact beta-carotene. For the pig, almost all of the carotene is converted to vitamin A in the intestine. Either dietary retinol or retinol resulting from conversion of carotenoids is then esterified with a long-chain fatty acid, usually palmitate. Dietary retinyl esters are hydrolyzed to retinol in the intestine; they are absorbed as the free alcohol and then reesterified in the mucosa. In swine, the retinyl esters are transported mainly in association with lymph chylomicrons to the liver, where they are hydrolyzed to retinol and reesterified 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); it is transported in this form to the tissues. When the RBP-retinol complex reaches the target cells, the retinol is released at a RBP receptor site on the cell surface. Once the retinol passes through the cell membrane, it combines with a cellular retinol-binding protein (CRBP). The CRBP is believed to carry the retinol to its intracellular site of action (Olson, 1984). The main excretory pathway for vitamin A is by elimination as glucuronide conjugates in the bile prior to fecal excretion.
Grummer et al. (1948) profiled the concentrations of vitamin A in plasma from birth through weaning in comparison with concentrations in mature pregnant sows. Vitamin A blood values for mature pregnant sows were similar to those of weanling pigs but quite different from the blood values of the newborn. The researchers could not detect any measurable amount of carotene in swine blood. Christensen et al. (1958) reported early on that administration of vitamin A caused rapid storage and sharp increase in vitamin A content in the liver. Liver normally contains about 90% of 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 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. For example, Wemheuer et al. (1996) reported that serum retinol of the lowest vitamin A-treated boars began to decrease only after 3 months because of the high retinol reserve capacity of the liver. Likewise, Hentges et al. (1952b) recognized the need to incorporate a lengthy depletion phase before evaluating the requirement for vitamin A in young pigs. Measurement of the liver store of vitamin A at slaughter is a useful technique in studies of vitamin A status and requirements.
