Vitamin A itself does not occur in plant products, but its precursors, the carotenes, do occur in several forms. These compounds (carotenoids) are commonly called provitamin A because the body can transform them into the active vitamin. The combined potency of a feed, represented by its vitamin A and carotene content, is its vitamin A value. Retinol is the alcohol form of vitamin A. Replacement of the alcohol group (-OH) by an aldehyde group (-CHO) gives retinal, and replacement by an acid group (-COOH) gives retinoic acid. Vitamin A is used in the feed industry as retinol in the esterified forms of retinyl acetate, propionate or palmitate. Vitamin A is a nearly colorless, fat-soluble, long-chain unsaturated alcohol with five double bonds (Illus. 2-1). Since it contains double bonds, vitamin A can exist in various isomeric forms. Only two isomers are of practical importance, namely all-trans-vitamin A, the form with highest biologic activity, and the 13-cisisomer, with a relative biologic activity for chicks of 50% (Ullrey, 1972). Vitamin A and the precursor carotenoids are rapidly destroyed by oxygen, heat, light and acids. Presence of moisture and trace minerals reduces vitamin A activity in feeds (Olson, 1984).
Precursors of vitamin A, the carotenes, occur as orange-yellow pigments mainly in green leaves and to a lesser extent in corn. Carotenoids are normally found in nature in the all-trans form; however, they are easily isomerized to form cis-isomers following exposure to heat and/or light (Lindshield and Erdman, 2006). 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. As an example, both alpha-carotene and cryptoxanthin have about one-half the conversion rate of beta-carotene (Tanumihardjo and Howe, 2005). 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 provided 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 conversion efficiency for vitamin A in rats 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 has been reviewed in more detail elsewhere (Solomons, 2006; Ross and Harrison, 2007). 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 and hog mucosal cleavage enzymes 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). The efficiency of vitamin A 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. Retinol in association with RBP circulates to peripheral tissues complexed to a thyroxine binding protein, transthyretin (Ross and Harrison, 2007). Recently Dever et al., (2011) report that chylomicron-derived retinyl esters, rather than RBP-bound retinol, are likely to be the major source of retinol in milk of vitamin A-deficient lactating sows. When the RBP-transthyretin complex reaches the target cells, the retinol is released at a RBP receptor site on the cell surface. The receptor was found to be located in all tissues known to require retinol for their function, particularly the pigment epithelium of the eye (Wolf, 2007). 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). Retinol first encounters the CRBP class of transport proteins to carry retinol to either esterifying or oxidizing enzymes. These intracellular binding proteins show high specificity and affinity for specific retinoids, and seem to control retinoid metabolism both qualitatively and quantitatively. They protect retinoids from nonspecific interactions, and on the quantitative side, they have been stated to “chaperone” access of metabolic enzymes to retinoids. (Solomons, 2006). 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 increases in vitamin A content in the liver. The liver normally contains about 90% of total body vitamin A and therefore is a good indicator of status for the vitamin. Serum retinol is not always sensitive to vitamin A status. However, recently Surles et al., (2011) suggested that the vitamin A form 3,4-didehydroretinol in lactating swine serum is a good indicator of vitamin A status. The remainder of vitamin A 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. It must be ensured that intakes appearing 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.
Vitamin A is necessary to support growth, health, and life in all higher animals. In the absence of vitamin A, animals will cease to grow and eventually die. Vitamin A and its derivatives, the retinoids, have a profound influence on organ development, cell proliferation, and cell differentiation and their deficiency originates or predisposes a number of disabilities (McDowell, 2000; Esteban-Pretel et al., 2010). Even with a mild vitamin A deficiency in pregnant rats, there was reduction of nephrons in the fetuses resulting in kidney malfunction (Marín et al., 2005; Bhat and Manolescu, 2008). Vitamin A deficiency in rats induced anatomic and metabolic changes comparable to those associated with neurodegenerative disorders. Slowing of cerebral growth was correlated with retinol level (Rahab et al., 2009). The metabolic function of vitamin A, explained in biochemical terms, is still not completely understood. Vitamin A deficiency causes at least four different and probably physiologically distinct lesions: loss of vision due to a failure of rhodopsin formation in the retina; defects in bone growth; defects in reproduction (i.e., failure of spermatogenesis in the male and resorption of the fetus in the female); and defects in growth and differentiation of epithelial tissues, frequently resulting in keratinization. More is known about the role of vitamin A in vision than any of its other functions. Retinol is utilized in the aldehyde form (all-trans-retinol to 11-cis-retinal) in the retina of the eye as the prosthetic group in rhodopsin for dim light vision (rods) and as the prosthetic group in iodopsin for bright light and color vision (cones). It has been found that retinoic acid, which is a form of vitamin A in the body, supports growth and tissue differentiation, but the roles in vision and reproduction are limited. As an example, vision can only be maintained when vitamin A is in the form of retinal (McDowell, 2000). Vitamin A-deficient rats fed retinoic acid were healthy in every respect, with normal estrus and conception, but failed to give birth and resorbed their fetuses. When retinol was given even at a late stage in pregnancy, fetuses were saved. Male rats receiving retinoic acid were healthy but produced no sperm, and without vitamin A both sexes were blind (Anonymous, 1977). Hale (1935) documented the defects resulting from vitamin A deficiency in sows. They included pigs born blind and with incomplete development of eye tissue. Watt and Barlow (1956) presented circumstantial evidence that vitamin A deficiency in sow and gilt diets led to blind piglets with microphthalmia. Chew et al. (1982) and Brief and Chew (1985) suggested that beta-carotene plays a role independent of vitamin A in swine reproduction. Their research suggested that elevation of maternal plasma vitamin A or beta-carotene improves embryonic survival, possibly because more uterine-specific proteins are secreted. Talavera and Chew (1988) found that beta-carotene was more effective in stimulating progesterone secretion by pig corpus luteum in vitro than either retinol or retinoic acid. These earlier studies involved treatments using both injections and enhanced dietary levels. In a study involving over 600 sows, Coffey et al. (1989) found that injecting beta-carotene (0, 50, 100, or 200 mg) at the time of weaning linearly increased by approximately 14% the numbers of live-born pigs at the next farrowing. Coffey et al.(1990) determined that injection of beta-carotene is much more effective than oral supplementation with regard to the influence on plasma and tissue concentrations. Coffey and Britt (1991), reported improvements in the number of pigs born alive when equivalent doses of vitamin A were utilized, confirming the positive effects of vitamin A on reproductive performance. Treatments consisted of injections of 200 mg beta-carotene, 50,000 IU vitamin A (palmitate) or vehicle (control) at weaning, breeding and day seven after breeding. In a subsequent report, Coffey and Britt (1993) provided additional evidence that litter size could be enhanced by the injection of either beta-carotene or vitamin A to sows receiving sufficient quantities of dietary vitamin A. Whaley et al. (1997) presented data leading them to conclude that retinol palmitate treatment may influence the follicular environment and thus would synchronize resumption of meiosis and enhance early embryonic survival in pigs. In this study, retinol palmitate (1 million IU) was injected at 6 days before estrus to gilts fed high-energy diets. Gilts had restored embryo survival if vitamin A was injected, possibly due to decreased variation in embryo size. Whaley et al. (1997) indicated that vitamin A also increased serum concentrations of progesterone in these studies and slightly advanced the development of the embryo. Whaley et al. (1997) concluded that the improvement in litter size is not due to increasing ovulation rate, but to effects on embryonic survival. The lack of an effect on ovulation rate in this report supported earlier findings by Britt and Coffey (1993). Da Silveira et al. (1998) reported similar positive effects of vitamin A (450,000 IU retinol palmitate) injection at weaning or mating on the number of piglets born alive, total number born and litter weight. Darroch et al. (1998) reported on a cooperative regional study combining 417 litters from four universities. Intramuscular injections of 250,000 or 500,000 IU of vitamin A were given at weaning and breeding. The researchers reported that the effect of vitamin A injections increased the number of pigs weaned per litter, but did not influence the number of pigs born alive. In contrast to the numerous studies reporting positive effects associated with vitamin A injections, Pusateri et al. (1996) found no effect of vitamin A injection (1 million IU) on total or live litter size. Injections were given at weaning or on one of the following days: 0, 2, 6, 10, 13, 19, 30, 70, or 110 post breeding. Pusateri et al. (1996) suggested that possibly multiple injections or a sustained release form of vitamin A or beta-carotene was required to elicit a response. Likewise, Washington et al. (1997) reported no effect on the number of embryos or embryo survival in gilts induced to ovulate. The study involved a very limited number of gilts with an injection of 1 million IU of vitamin A prior to breeding. In disagreement with the numerous positive responses that have been reported with beta-carotene injections, Stender et al. (1998) reported no benefits of beta-carotene injection on litter parameters. Stender et al. (1998) did not report the levels or stage when injection was given. Kolb and Seehawer (1997) reviewed the significance of carotenes and of vitamin A for reproduction of cattle, horses and pigs. It was found that retinoic acid is necessary for the function of the germ cell epithelial, Sertoli cells and interstitial cells. Promoting the synthesis of proteins, estrogens and progesterone was included in the actions credited to cis- and all-trans-retinoic acid. During embryogenesis, retinoic acid has been shown to influence processes governing the patterning of neural tissue and cranio-facial, eye and olfactory system development. Retinoic acid continues to influence the development, regeneration, and well-being of neurons (Asson-Batres et. al., 2009). Retinoic acid also regulates the differentiation of epithelial, connective and hematopoietic tissues (Safonova et al., 1994). It has been suggested that retinoic acid is an effective antidiabetic agent that could be considered in the treatment of type 2 diabetes (Wolf, 2009; Manolescu et al., 2010). The nature of the growth and differentiation response elicited by retinoic acid depends upon cell type. Retinoic acid can be an inhibitor of many cell types with a potential to reduce adipose tissues in meat producing animals. (Suryawan and Hu, 1997).
It would appear that the role of retinoic acid in cell differentiation regulates fat cells in growing animals (Brandebourg and Hu, 2005). These researchers found that retinoic acid effectively inhibited the differentiation of swine preadipocytes. The RAR receptor was activated while other receptors were inhibited; thus controlling fat tissue development.
Except for its role in vision, the function of vitamin A at the cellular level is not yet clear. It is thought that retinyl phosphate may act to regulate cell differentiation. Research findings have indicated that vitamin A increased RNA synthesis by polymerase II in rat testes and that the induced change in transcription was due in part to altered chromatin structure (Porter et al., 1986).
Vitamin A is required for normal disease resistance, which is related to maintenance of the mucous membranes and normal functioning of the adrenal gland for production of corticosteroids needed to combat disease. An animal’s ability to resist disease depends on a responsive immune system. A vitamin A deficiency causes a reduced immune response.
In general, vitamin A is one of the central nutrients influencing immune function. It also is central in the development and differentiation of neutrophils, monocytes, lymphocytes, and many other immunologic cells. Activation of T cells and B cells also appears to require vitamin A. Recent evidence indicates that retinoic acid, a metabolite of vitamin A, regulates factors required for Ig isotype switching and development of certain B cells into IgG-expressing cells. (Chen and Ross, 2005).
In many experiments with laboratory and domestic animals, the effects of both clinical and subclinical deficiencies of vitamin A on the production of antibodies and on the resistance of the different tissues to microbial infection or parasitic infestation have frequently been demonstrated (Kelley and Easter, 1987). Vitamin A-deficient chicks showed rapid loss of lymphocytes, and deficient rats showed atrophy of the thymus and spleen and reduced response to diphtheria and tetanus toxoids (Krishnan et al., 1974). A protective effect of dietary vitamin A supplementation against experimental Staphylococcus aureus mastitis in mice has been reported (Chew et al., 1984). Harman et al. (1963) studied the effect of a vitamin A deficiency on antibody production in baby pigs and found a high correlation coefficient between serum vitamin A and antibody titer. Baby pigs infected with Trichuris suis responded to supplemental vitamin A with an enhanced immunologic response compared with that of controls (Bebravicius et al., 1987).
Animal studies indicate that certain carotenoids with antioxidant capacities, but without vitamin A activity, can enhance many aspects of immune functions; can act directly as antimutagens and anticarcinogens; can protect against radiation damage; and can block the damaging effects of photosensitizers. Also, carotenoids can directly affect gene expression and this mechanism may enable carotenoids to modulate the interaction between B cells and T cells, thus regulating humoral and cell-mediated immunity (Koutsos, 2003).
Beta-carotene can modulate T-helper cell functions for antibody production (Jyonouchi et al., 1995; Zomborszky-Kovaes et al., 1998). Hoskinson et al. (1989) reported the effects of beta-carotene and vitamin A on mitogen-induced lymphocyte proliferation. Both beta-carotene and vitamin A were found to stimulate lymphocyte proliferation in pigs, and beta-carotene was reported to play a specific role in modulating lymphocyte response. Chew et al. (1991) determined that beta-carotene is taken up by circulating lymphocytes but not by neutrophils or erythrocytes. A historical overview of immunity with particular regard to vitamin A has been published by Beisel (1995).
The recent requirement of vitamin A listed in the NRC (1998) is 1,300 to 2,200 IU per kg of diet (591 to 1,000 IU per lb) for weanling to finishing pigs. Breeding animals require 4,000 IU per kg (1,818 IU per lb), while the estimated requirement for lactating gilts and sows is 2,000 IU per kg (909 IU per lb). The reason that lactating animals require less vitamin A per unit of feed than for breeding is related to allowed feed consumption. Lactating sows would consume an average of 5.3 kg of feed (11.7 lbs), while breeding animals are restricted to about 1.9 kg (4.2 lbs) of feed to prevent excessive weight gains. These requirements are deemed sufficient to provide optimal growth, satisfactory reproduction and prevention of deficiency signs. Suggested requirements should be designed to be adequate for these purposes under practical conditions of feeding and management, as well as to allow for a certain amount of storage. The decision as to the minimum vitamin A requirement of young swine depends on whether the criterion to determine the requirement is based on growth and feed utilization or also considers liver vitamin A storage. Sheffy et al. (1954) reported that 18 mg of oral vitamin A per kg body weight was required to supply the minimum requirement to promote a trace of liver storage of vitamin A. Storage of vitamin A in the liver certainly appears to be desirable, since under conditions of little or no liver storage, stresses and diseases may precipitate vitamin A deficiency. The vitamin A reserves of the sow make it difficult to establish requirements. Braude et al. (1941) reported that mature sows fed diets without supplemental vitamin A completed three pregnancies normally; only in the fourth pregnancy did deficiency signs appear. Gilts receiving adequate vitamin A levels until nine months of age completed two reproductive cycles without signs of vitamin A deficiency (Selke et al., 1967). Heaney et al. (1963) indicated that liver stores at birth were less important to newborn pigs than the vitamin A in the colostrum and milk. Thomas et al. (1947) reported that the extent of placental and mammary transfer of vitamin A in swine and dairy goats could be elevated by adding large amounts of vitamin A to the diet of pregnant sows or does. Thomas et al. (1947) indicated that feeding massive doses of vitamin A during late pregnancy increased the vitamin A content in the colostrum by 2.5 to 3 times that of the control group. Rearing sows on pasture versus dry lot also increased the concentration of vitamin A in milk and colostrum (Bowland et al., 1949a; 1949b). In establishing a satisfactory vitamin A level for practical diets, it is necessary to consider a number of factors that may alter the vitamin A requirement. Practical factors influencing vitamin A requirements are listed in Table 2-1. The conversion of carotenoids to vitamin A is one of the most important considerations in determining swine vitamin A requirements (see Section I).
Table 2-1: Factors Influencing Vitamin A Requirements
Adapted from McDowell (1999).
Stress conditions, such as extremely hot weather, viral infections, and altered thyroid function, have been suggested as causes of reduced conversion of carotene to vitamin A. Swick et al. (1952) investigated the effect of thyroid activity on carotenoid metabolism in swine, while Frape et al. (1959b) evaluated the influence of vitamin A on thyroid function in the young pig exposed to two environmental temperatures. Vitamin A requirements are higher under stressful conditions, such as abnormal temperatures and exposure to disease. 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. This decreases absorption of vitamin A and, at the same time, causes the chickens to stop eating for several days (Scott et al., 1982). In humans, a range of vitamin A stores were positively associated with several measures of innate immune activity, suggesting that vitamin A enhances protection against diverse pathogens even at concentrations above those needed to maintain normal vision (Ahmad et al, 2009). Likewise, other factors may possibly affect the metabolism and increase requirements of vitamin A. These include free nitrates in feeds, inadequate protein, a zinc deficiency, and low dietary phosphorus (Harris, 1975). Wood et al. (1967) reported that when nitrites or nitrates were supplied to pigs through drinking water, a significant reduction of vitamin A liver stores resulted. Hutagalung et al. (1968) investigated the effects of nitrates and nitrites in the feed on utilization of carotene in swine. Although there was a definite trend toward a decrease in liver vitamin A stores when nitrite level in the diet increased, none of the nitrate or nitrite treatments significantly decreased liver vitamin A stores of the pigs. Considerable work and controversy has been reported on the relationship between nitrates and vitamin A nutrition. In a review of this subject, Rumsey (1975) concluded that although nitrates can be shown to have an adverse effect on vitamin A in vitro, this does not appear to translate into a significant effect under most feeding conditions. Whether carotene has a rachitogenic effect that may increase the vitamin D requirement of the pig has been investigated. Hendricks et al. (1967) found that under the conditions of their experiment, beta-carotene did not increase the need for ergocalciferol above that normally required by baby pigs.
Concentrations of vitamin A in feedstuffs are highly variable. The richest natural sources of vitamin A are fish oils and liver. Among the common foods of animal origin, milk fat, egg yolk and liver are rich sources, but this is not the case if the animal from which they came had been receiving a vitamin A-deficient diet for an extended period.
Rovitamin A carotenoids, mainly beta-carotene in green feeds, are principal sources of vitamin A for livestock. All green parts of growing plants are rich in carotene and therefore have a high vitamin A value. In fact, the degree of green color in roughage is a good index of its carotene content.
Although the yellow color of carotenoids is masked by chlorophyll, all green parts of growing plants are rich in carotene and thus have a high vitamin A value. Good pasture always provides a liberal supply, and type of pasture plant—whether grass or legume—appears to be of minor importance. At maturity, however, leaves contain much more than stems. Thus, legume hay is richer in vitamin A value than grass hay (Maynard et al., 1979). With all hays and other forage, vitamin A value decreases after the bloom stage. Plants at maturity can have 50% or less of the maximum carotenoid value of immature plants.
Wing (1969) reported carotene digestibility in plants was greater during the warmer months. Variations were found in the digestibility of carotenes in plants according to year, species of plant, dry matter content, and form of forage. Carotene digestibility was somewhat lower in silages than in pastures or hay. Average published values of carotene content can serve only as an approximate guide in feeding practice because of the many factors affecting actual potency of individual as-fed samples (NRC, 1982).
Cooking processes commonly used in human food preparation do not cause much destruction to vitamin A potency. The blanching and freezing process generally causes little loss of carotenoid content in vegetables and fruits. Heat, however, does isomerize the all-trans-carotenoids to cis-forms. In a report from Indonesia, isomerication during traditional cooking caused a loss of up to 9% of vitamin A potency (Van der Pol et al., 1988). Hydrogenation of fats lessens their vitamin A value, while saponification does not destroy the vitamin if oxidation is avoided.
Both carotene and vitamin A are destroyed by oxidation. This is the most common cause of any depreciation that may occur in the potency. The process is accelerated at high temperatures, but heat without oxygen has a minor effect. Butter exposed in thin layers in air at 50°C loses all its vitamin A potency in 6 hours, but in the absence of air there is little destruction at 120°C over the same period. Cod liver oil in a tightly corked bottle has shown activity after 31 years, but it may lose all its potency in a few weeks when incorporated in a feed mixture stored under usual conditions (Maynard et al., 1979). All-trans beta-carotene is the predominant isomer in feeds, thermal processing can substantially increase the proportions of cis isomers (9-c and 13-c) (Deming et al., 2002).
Of the grains, only yellow corn has significant quantities of carotene. The potency of yellow corn is only about one-eighth that of good green dehydrated forage. Roots and tubers as a class, supply practically no vitamin A value, but carrots are a very rich source, as are sweet potatoes, as might be expected from their yellow color. Pumpkins and squash also supply considerable amounts, and green leafy vegetable used in human nutrition are rich in carotene (Maynard et al., 1979). Tankage, meat scraps, and similar animal by-products have little if any vitamin A potency. Certain fish meals are fair sources, but variation in the raw material and in methods of processing may entirely destroy any potency originally present.
Sources of supplemental vitamin A were derived primarily from fish liver oils, in which the vitamin A occurs largely in esterified form, and chemical synthesis. Before the era of chemical production of vitamin A, the principal source of vitamin A concentrates was the liver and (or) body oils of marine fish. Since chemical synthesis was developed in 1949, the synthesized form has become the major source of the vitamin.
The major source of vitamin A used in swine feed is all-trans-retinyl acetate (McGinnis, 1988). The acetate, like the propionate and palmitate esters, is chemically synthesized by basic manufacturers, often in beadlet form. It is designed to be used in mash and pelleted feeds. The beadlet generally contains carbohydrates, gelatin and antioxidants, which stabilize the vitamin A by providing physical and chemical protection against adverse factors normally found in feed. Schoenbeck et al. (1994) compared the effects of commercially available injectable vitamin compounds, including vitamin A, beta-carotene, and (or) vitamin E supplements, on reproductive performance in weaned sows. Only the vitamin A and E injections increased the total number of pigs born alive per litter compared with the control group. The most popular vitamin A acetate product used in swine feed contains 1,000,000 IU or United States Pharmacopeia (USP) units per gram of product. Note that IU and USP units are equal in value, and one unit equals the activity of 0.3 µg of all-trans-retinol or 0.344 µg of all-trans-retinyl acetate.
Rahman et al. (1996) recorded signs associated with vitamin A deficiency on a commercial hog farm and found agreement with observations reported by earlier workers (Hughes et al., 1928). Goodwin and Jennings (1958) and Palludan (1961) provided a comprehensive description of the congenital malformations associated with vitamin A deficiency in pregnant sows. In pigs, the absence of vitamin A results principally in nervous signs, such as unsteady gait, incoordination, trembling of the legs, spasms and paralysis (Hentges et al., 1952a) (Illus. 2-2). Eye lesions are less common. Reduced growth is a sign of vitamin A deficiency in all species. Insufficient vitamin A for rats reduces efficiency of the urea synthesis pathway, thus accounting for the increased amino nitrogen excretion seen with the deficiency (John and Sivakumar, 1989). The effect of vitamin A deficiency in swine on appetite or rate of gain often does not occur until eventual paralysis and weakness prohibit movement to the feeder (Cunha, 1977). Increased cerobrospinal fluid (CSF) pressure is often associated with vitamin A deficiency. Nelson et al. (1962) reported that 8 to 16 mg of vitamin A per lb of body weight per day can produce lower CSF pressure. Nelson et al. (1964) reported CSF fluid pressure to be a very sensitive criterion for establishing vitamin A status of pigs fed vitamin A or vitamin A acid.
Pig exhibits partial partial paralysis and seborrhea.
Pig is in the initial stages of spasm.
Courtesy of J.F. Hentges, R.H. Grummer and University of Wisconsin.
Hjarde et al. (1961) reported that the influence of vitamin A deficiency depends on the age of the animal at the time of depletion. During reproduction and lactation, vitamin A deficiency in the sow produces the following clinical signs: failure of estrus; resorption of young; wobbly gait; weaving and crossing of the hind legs while walking; dropping of the ears; curving with head down to one side; spasms; loss of control of hindquarters and forequarters, hence inability to stand up; and impaired vision (Cunha, 1977). Depending on degree of severity of vitamin A deficiency, fetuses were resorbed, born dead or carried to term. Fetuses carried to term showed a variety of defects, including various stages of arrested formation of the eyes (sometimes complete lack of eyeballs), harelips, cleft plate, misplaced kidneys, accessory ear-like growths, some with one eye, some with one large and one small eye and bilateral cryptorchidism (Guilbert et al., 1937; Cunha, 1977) (Illus. 2-3). Hjarde et al. (1961) reported that vitamin A deficiency at breeding age led to impaired spermiogenesis; metaplasia of oviduct, cervix and vagina; and compression of the brain.
Malformations in newborn piglet due to deficiency of vitamin A in the diet of the sow.
Vitamin A appears to improve reproductive performance of gilts by decreasing embryonic mortality, resulting in more pigs per litter (Brief and Chew, 1985; Lindemann et al., 2008). Weekly injections to supply 12,800 IU vitamin A and 32.6 mg beta-carotene per gilt daily resulted in elevated levels of plasma vitamin A and beta-carotene, reduced embryonic mortality and produced more and heavier pigs alive at birth and weaning (Brief and Chew, 1985). A regional study in the U.S. involving 182 sows, was conducted in five cooperating experiment stations to determine the effects of I.M. injection of high levels of vitamin A (control, 250,000 and 500,000 IU) at weaning and breeding on subsequent litter size of sows (Lindemann et al., 2008). The results of this project demonstrated that injection of high doses of vitamin A in young sows at weaning and breeding improves the subsequent number of pigs born and weaned per litter. This indicates that vitamin A requirements for maximal performance may vary with age. Only sows in parity 1 and 2 were affected, compared to parity sows 3 to 6. Using the same high-energy-fed gilt model, Whaley et al. (2000) subsequently reported that treatment with vitamin A (100,000 IU) stimulated an earlier resumption of meiosis and altered development of oocytes before ovulation, resulting in more uniform and advanced oocytes and early embryos. Previous reports have also shown improvements in number of pigs born alive with injectable vitamin A (Coffey and Britt, 1991; 1993; Whaley et al., 2000). Coffey et al. (1989) indicated that injecting multiparous sows with beta-carotene at weaning increased litter size at the subsequent farrowing.
The vitamin A activity in ingredients of typical swine rations is unpredictable. Therefore, the total requirement is usually added to the diet as a commercially synthesized, stabilized vitamin A product. Available means of supplementing vitamin A to swine are as part of a concentrate or liquid supplement and in drinking water preparations. The most convenient and often most effective means to provide vitamin A to swine is inclusion with concentrate mixtures that provide uniform consumption of the vitamin. Because of the lack of stability of vitamin A, particularly regarding exposure to oxygen, trace minerals, pelleting, feed storage, and other factors, the feed industry has readily accepted the dry stabilized forms of the vitamin. Stabilized and protectively coated (or beadlet) forms of vitamin A slow destruction of the vitamin, but for highest potency, fresh supplies of the mixture should be available on a regular basis. Practical considerations that affect vitamin A stability are listed in Table 2-2.Moisture has a negative effect on both feeds and premixes (NRC, 1998). Water causes vitamin A beadlets to soften and become more permeable to oxygen. Both high humidity and presence of free choline chloride (which is very hygroscopic) enhance vitamin A destruction. Trace minerals also exacerbate vitamin A losses in premixes exposed to moisture. The destruction of vitamins mixed with trace minerals is insignificant in relatively dry premixes. For maximum retention of vitamin A activity, premixes should be as moisture-free as possible and have a pH above five. Low pH causes isomerization of all-trans vitamin A to less potent cis forms and also results in de-esterification of vitamin A esters to more labile retinol (De Ritter, 1976).
Table 2-2: Practical Factors that Affect Vitamin A Stability
Adapted from Hoffmann-La Roche (1989)
The stability of vitamin A in feeds and premixes has been improved tremendously in recent years. This has been accomplished through chemical stabilization as an ester and by physical protection using antioxidants, emulsifying agents, and stabilized materials in spray-dried, beadlet or prilled products (Bauernfeind and DeRitter, 1972; Hoffmann-La Roche, 1988). These vitamin A product forms result not only in enhanced chemical and physical stability, but also in excellent biologic availability. Nevertheless, vitamin A supplements should not be stored for long periods prior to use and feeding. Several factors can influence the loss of vitamin A from feedstuffs during storage. The trace minerals in feeds and supplements, particularly copper, are detrimental to vitamin A stability. Dash and Mitchell (1976) reported the vitamin A content of 1,293 commercial feeds over three years. The loss of vitamin A was over 50% in one year’s time. Vitamin A loss in commercial feeds was evident even if the commercial feeds contained stabilized vitamin A supplements. There is evidence that yellow corn may lose carotene rapidly during storage. For instance, a hybrid corn high in carotene lost about half of its carotene in eight months storage at 25°C and approximately 75% in three years. Less carotene was lost during storage at 7°C (Quackenbush, 1963). Because of vitamin A variability in feeds and losses during processing and storage, most animal nutritionists tend to ignore vitamin A activity in feedstuffs and rely exclusively on dietary fortification to arrive at vitamin allowances for swine. Vitamin A and carotene destruction also occurs during processing of feeds with steam and pressure. Pelleting effects on vitamin A in feed are caused by die thickness and hole size, which produce frictional heat and a shearing effect that can expose the vitamin to destructive processes. In addition, steam application exposes feed to heat and moisture. Running fines back through the pellet mill exposes vitamin A to the same factors a second time. Between 30% and 40% of vitamin A present at mixing may be destroyed during pelleting (Shields et al., 1982). Diseases and mycotoxins increase the need for supplemental vitamin A. Furthermore, diseases can interfere with the absorption and utilization of vitamin A (i.e., enteric diseases such as malabsorption syndrome, mycotoxins and diarrhea). The effect of mycotoxins in the feed should also be considered, since aflatoxin is known to interfere with protein synthesis (Doerr, 1987). Thus, it can be seen how mycotoxins may interfere with RBP synthesis and the transport of vitamin A. Mycotoxins in feed can substantially decrease antioxidant nutrient assimilation from the feed and increase the vitamin A requirement to prevent damaging effects of free radicals and the toxic products of their metabolism. It is now increasingly recognized that at least 25% of the world’s grains are contaminated with mycotoxins (Surai, 2002). Mycotoxins are known to cause digestive disturbances, such as vomiting and diarrhea, as well as internal bleeding and interfere with absorption of dietary vitamin A, as well as vitamins D3, E and K (McDowell and Ward, 2008). Short-term administration of vitamin A in drinking water or by injection is recommended to support any specific measures used in treatment of diseased and convalescent animals. This is true in swine in which vitamin A stores may have been depleted by fever and in animals suffering from intestinal disorders, where vitamin A absorption is seriously impaired.
The decision for vitamin A supplementation should be based mainly on whether a deficiency could be a practical problem. As with most nutrients, a borderline deficiency is much more likely than a severe deficiency. Likewise, a marginal deficiency adversely affecting performance by a few percentage points is not easily detected. Based on the positive results that may be derived, and taking into account that vitamin A supplementation is inexpensive and no toxicity problems have been reported when given at recommended levels, it seems beneficial to supplement vitamin A at all times and to increase the supplementation substantially for pigs that are diseased or under stress.
The amount of vitamin A added to swine diets is usually in excess of the NRC requirements because no safety factors are built into these values (Olson, 1984). Additional vitamin A is added to allow for loss of activity due to oxidative destruction of the vitamin A ester during feed processing and storage, variability of carotenes in feedstuffs, changes in feed consumption, genetic differences in animals and stress due to disease and other environmental factors. Stahly et al. (1997) reported evidence that requirements for one or more antioxidant vitamins, i.e., vitamin A, E and C, are greater than NRC (1988) estimates when pigs are exposed to a high level of antigens. Optimum vitamin nutrition (OVN), including vitamin A, is essential for optimizing the productive potential for swine.
In general, the possibility of vitamin A toxicity for swine is remote. However, of all vitamins, vitamin A is most likely to be provided in concentrations toxic to swine. Excessive vitamin A has been demonstrated to have toxic effects in most species studied. Presumed upper safe levels are four to 10 times the nutritional requirements for nonruminant animals, including swine (NRC, 1998). Most of the harmful effects have been obtained by feeding more than 100 times the daily requirements over time. Thus, small excesses of vitamin A for short periods should not exert any harmful effects. Recommended upper safe levels of vitamin A for swine are 20,000 IU per kg (9,091 IU per lb) of diet for growing pigs and 40,000 IU per kg (18,182 IU per lb) of diet for breeding animals. The most characteristic signs of hypervitaminosis A are skeletal malformations, spontaneous fractures, and internal hemorrhage (Anderson et al., 1966; NRC, 1998). Gross toxicity signs also include a roughened hair coat, scaly skin, hyperirritability and sensitivity to touch, bleeding from cracks which appear in the skin about the hooves, blood in urine and feces, loss of control of legs accompanied by inability to rise, and periodic tremors, (NRC, 1998). Other signs include loss of appetite, slow growth, loss of weight, skin thickening, suppressed keratinization, increased blood-clotting time, reduced erythrocyte count, enteritis, congenital abnormalities and conjunctivitis. Degenerative atrophy, fatty infiltration and reduced function of liver and kidney are also typical. A variety of effects of chronic hypervitaminosis A in weanling pigs were reported by Hurt et al. (1966). Dobson (1969) documented the effects of massive induced hypervitaminosis A on newborn pigs. In addition to reduced growth and high plasma, liver and kidney vitamin A levels as well as a characteristic gait, Pryor et al. (1969) reported pathologic changes in the bones of pigs subjected to levels of vitamin A sufficient to produce hypervitaminosis A.Bone abnormalities may include extensive bone resorption and narrowing of the bone shaft, bone fragility and short bones because of retarded growth. Wolke et al. (1968) reported the effects of excessive vitamin A on endochondral and intramembranous bone formation in the pig. Abnormalities in bone modeling are the essential causes of fractures, and the cartilage matrix of bone may be destroyed.For humans, signs of vitamin A toxicity include nausea and vomiting, headache, dizziness, blurred vision, lack of muscular coordination, and pain in weight-bearing bones and joints (Ross and Harrison, 2007). Since the advent of high-dose treatment with systemic retinoic acid for leukemia and other malignant conditions, a condition known as “retinoic acid syndrome” has been recognized (Solomons, 2006). It is characterized by weight gain, hypotension, kidney failure and respiratory distress. High doses of retinoid analogs can also increase intracranial pressure (Friedman, 2005). The effects of supplementing the diets of weanling pigs with up to 100 times the NRC (1988) estimated requirements for vitamin A have been investigated (Blair et al., 1989, 1992). Blair et al. (1989) indicated that a level of only 10 times the requirement, even after just over four weeks, may induce osteochondrosis. The allowable range of vitamin A set out in the Canadian Feeds Regulations (1983) was found to be appropriate for practical pig production (Blair et al., 1992). Hidiroglou (1996) reported that an intramuscular dose of 2.5 x 106 IU vitamin A produced livers with levels of vitamin A high enough to be considered hazardous to health.
Another area of attention has focused on whether or not excessive vitamin A can affect vitamin E status. Vitamin A has been reported to have a significant depressing effect on vitamin E status (Ching et al., 2002). Blair et al. (1996) indicated that a tolerable dietary range of vitamin A for young pigs in the range of 10 to 30 kg is up to 10 times the requirement. Ching et al. (2002) fed weanling pigs 6 times the vitamin A requirement (13,200 IU per kg or 6,000IU per lb). As dietary vitamin A level increased, serum and liver alpha-tocopherol concentrations declined, suggesting a reduced absorption and retention of alpha-tocopherol when weaned pigs were fed high dietary vitamin A concentrations. Fuhrmann et al. (1997) reported that 20,000 IU per kg (9,072 IU/lb) of diet vitamin A reduced plasma and tissue vitamin E levels, which in turn led to an increase of lipid peroxidation as indicated by a higher production of hydrocarbons. Blair et al. (1996) considered that this raised concern about further increases of vitamin A supplementation in early-weaned pigs. However, Anderson et al. (1997) concluded that during early gestation, the vitamin E status of gilts was not detrimentally influenced by three 350,000 IU injections of vitamin A shortly before, at and shortly after breeding. Anderson et al. (1995) reported that feeding a high level of retinyl acetate (20,000 IU/kg) did not influence animal performance or blood serum and tissue alpha-tocopherol concentrations in growing-finishing pigs. Weaver et al. (1989) reported a significant interaction for vitamin A and vitamin E for plasma tocopherol. Higher levels of vitamin A when supplied with a high level of vitamin E, lowered plasma tocopherol concentrations. Hoppe et al. (1992) indicated that dietary retinol at up to 10,000 IU per kg (4,536 IU per lb) of diet does not affect alpha-tocopherol concentrations in plasma or in tissues selected with the exception of cardiac muscle. Alpha-tocopherol levels in heart and liver displayed an inverse relationship with levels (5,000, 10,000, 20,000 or 40,000 IU per kg) of dietary retinol, while dietary retinol had no effect on levels of alpha-tocopherol in Longissimus muscle or backfat. Zomborszky-Kovacs et al. (1998) reported a numeric decrease in plasma vitamin E concentration in unsupplemented versus carotene-supplemented weaned pigs. Beginning one week prior to weaning, the supplemented group received 855 mg beta-carotene per kg (388 mg/lb) of diet (10 times the NRC  recommendation). The carotene-supplemented pigs and control pigs had vitamin E concentrations in plasma of 2.12 and 5.58 mg/dl, respectively.
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