The chemical structure of biotin includes a sulfur atom in its ring (like thiamin) and a transverse bond across the ring (Illus. 11-1). It is a monocarboxylic acid with the sulfur as a thioether linkage. Biotin with its rather unique structure contains three asymmetric carbon atoms; therefore, eight different isomers are possible. Of these isomers, only d-biotin has vitamin activity.
Biotin crystallizes from water as long, white needles. Its melting point is 232° to 233°C. Free biotin is soluble in dilute alkaline solutions and hot water and practically insoluble in fats and organic solvents. Biotin is quite stable under ordinary conditions. It is destroyed by nitric acid, other strong acids, strong bases and formaldehyde, and also is inactivated by oxidative rancidity reactions (Scott et al., 1982). It is gradually destroyed by ultraviolet (UV) radiation. Structurally related analogs of biotin can vary from no activity, to partial replacement of biotin activity, to anti-biotin activity. Mild oxidation converts biotin to the sulfoxide, and strong oxidation converts it to sulfone. Strong agents result in sulfur replacement by oxygen, resulting in oxybiotin and desthiobiotin. Oxybiotin has some biotin activity for chicks (one-third) but less for rats (one-tenth). A prominent nutrient-drug interaction is that biotin-dependent enzymes are reduced with the epilepsy drug carbamazepine (Rathman et al., 2002). Biotin is present in feedstuffs in both bound and free forms, and much of the bound biotin is apparently unavailable to animal species. For poultry (and presumably other species), often less than one-half of the microbiologically determined biotin in a feedstuff is biologically available (Scott, 1981; Frigg, 1984, 1987). Naturally occurring biotin is found partly in the free state (fruit, milk, vegetables) and partly in the form bound to protein in animal tissues, plant seeds and yeast. Naturally occurring biotin is often bound to the amino acid lysine. Biotinidase, present in pancreatic juice and intestinal mucosa, releases biotin from biocytin (bound form of biotin) during the luminal phase of proteolysis. In most species that have been investigated, physiological concentrations of biotin are absorbed from the intestinal tract by a sodium-dependent active transport process, which is inhibited by dethiobiotin and biocytin (Said and Derweesh, 1991). Absorption of biotin by a Na+-dependent process was noted to be higher in the duodenum than the jejunum, which was in turn higher than that in the ileum, and it was concluded that the proximal part of the human small intestine was the site of maximum transport of biotin (Said et al., 1988). Biotin is absorbed intact in the first third to half of the small intestine (Bonjour, 1991). Also, biotin is absorbed from the hind gut of the pig with 50% to 61% of infused biotin disappearing between the cecum and feces. This is accompanied by more than a four-fold increase of plasma biotin concentration and more than a six-fold increase of urinary biotin excretion (Barth et al., 1986). Scholtissek et al. (1990) suggested that under basal conditions 1.7% to 17% of the swine requirement for biotin is provided by colonic bacteria. Biotin appears to circulate in the bloodstream both free and bound to a serum glycoprotein, which also has biotinidase activity, catalyzing the hydrolysis of biocytin. In humans, 81% of biotin in plasma was free and the remainder bound (Mock and Malik, 1992). Information is very limited on biotin transport, tissue deposition, and storage in animals and humans. Mock (1990) reported that biotin is transported as a free water-soluble component of plasma, is taken up by cells via active transport and is attached to its apoenzymes. Said et al. (1992) reported that biotin is transported into the human liver via a specialized, carrier-mediated transport system. This system is Na+ gradient-dependent and transports biotin via an electroneutral process.
All cells contain some biotin, with larger quantities in the liver and kidney. Intracellular distribution of biotin corresponds to known locations of biotin-dependent enzymes (carboxylases). Investigations of biotin metabolism in animals are difficult to interpret, as biotin-producing microorganisms are present in the intestinal tract distal to the cecum. Often the amount of biotin excreted in urine and feces together exceeds the total dietary intake, whereas urinary biotin excretion is usually less than intake. Efficient conservation of biotin, together with the recycling of biocytin released from the catabolism of biotin-containing enzymes, may be as important as intestinal bacterial synthesis of the vitamin in meeting biotin requirements (Bender, 1992). 14C-labeled biotin showed the major portion of intraperitoneally injected radioactivity to be excreted in the urine and none in the feces or as expired CO2 (Lee et al., 1973). In rats and pigs, biliary excretion of biotin and metabolites is negligible (Zempleni et al., 1997).
Biotin is an essential coenzyme in carbohydrate, fat and protein metabolism. It is involved in conversion of carbohydrate to protein and vice versa, as well as conversion of protein and carbohydrate to fat. Biotin also plays an important role in maintaining normal blood glucose levels from metabolism of protein and fat when the dietary intake of carbohydrate is low. As a component of several carboxylating enzymes, it has the capacity to transport carboxyl units and to fix carbon dioxide (as bicarbonate) in tissue (Camporeale and Zemplen, 2006). In carbohydrate metabolism, biotin functions in both carbon dioxide fixation and decarboxylation, with the energy-producing citric acid cycle dependent upon the presence of this vitamin. Specific biotin-dependent reactions in carbohydrate metabolism are:
Estimated biotin requirements for various poultry species vary from 0.1 to 0.3 mg per kg (0.05 to 0.14 mg per lb) of ration (NRC, 1994). Biotin requirements are difficult to establish because of variation in feed content and bioavailability of biotin. Likewise, it is difficult to obtain a quantitative requirement for biotin, as the vitamin is synthesized by many different microorganisms and certain fungi. These microorganisms are found in the lower part of the intestinal tract, a region in which absorption of nutrients is generally reduced. However, it is believed that intestinal microflora make a significant contribution to the body pool of available biotin. In general, combined urinary and fecal excretion of biotin exceeds the dietary intake. What is not known for the various species is the extent of microbial synthesis or the biotin availability to the host. It is concluded that microorganisms contribute to meet animal and human biotin requirements, as the use of some sulfa drugs, such as sulfathalidine, can induce deficiency under some circumstances. Microorganisms that provide significant quantities of biotin to most species apparently supply a variable, undependable amount of biotin for poultry (Frigg, 1987). Biotin deficiency was less severe in cecetomized than in normal chickens, indicating that cecal microorganisms do not supply chickens with significant amounts of biotin, but instead compete with the host animal for dietary biotin, thus increasing the requirement (Sunde et al., 1950). Rate and extent of biotin synthesis may depend on the level of other dietary components. It has been shown that in poultry polyunsaturated fats, ascorbic acid and other B vitamins may influence the demand for biotin. Addition of polyunsaturated fatty acids (PUFA) to fat-free, biotin-deficient diets increased severity of dermal lesions (Roland and Edwards, 1971). Biotin is rapidly destroyed as feeds become rancid. Pure biotin was inactivated to an extent of 96% in 12 hours when linoleic acid of a high peroxide number was added to the diet (Pavcek and Shull, 1942). In the presence of alpha-tocopherol, this destruction amounted to only 40% after 48 hours. Biotin requirements can be influenced by a number of factors, one of which is the genetic selection for performance (growth and feed efficiency). Whitehead (1988) suggested that the biotin requirement for broilers in 1976 was 170 µg per kg (77 µg per lb) of diet, whereas Whitehead (1998) estimated the requirement to be 180 µg per kg (82 µg per lb) of feed for the fastest growing strains of broilers. Li et al.(1994) reported biotin deficiency lesions (foot pads) in broilers and suggested that 300 µg per kg (136.4 µg per lb) was the requirement for birds on a wheat-based diet. Whitehead (2000) reported a biotin requirement of 200-300 µg per kg (90.9 to 136.4 µg per lb) of feed, depending on whether linear or curvilinear statistical analysis was performed on the data. In work with turkey breeders, both Atkinsonet al. (1976) and Robel (1991) observed improvements in hatchability of fertile eggs when the diets of hens were fortified with 550 µg per kg (250 µg per lb) or 520 µg per kg (236.4 µg per lb) of d-biotin, respectively. In either experiment, the majority of hatchability improvement was noted after hens were in lay for more than eight weeks. Also, Chen et al. (1994) reported that higher levels of supplemental biotin were beneficial in supporting later reproductive performance in one of two experiments. Furthermore, Atkinson et al. (1976) noted heavier 10-week body weights when the diet of poults was supplemented with 250 µg per kg (113.6 µg per lb) of d-biotin regardless of the biotin status of hens. Robel (2002) found that commercial turkey breeder diets did not contain sufficient biotin to meet hatchability demands as age progressed in turkey hens.
Biotin is present in many feedstuffs; however, corn, wheat, other cereals, meat and fish are relatively poor sources (Table 11-1). There is considerable variation in biotin content within individual sources. For example, 59 samples of corn analyzed for biotin varied between 56 and 115 µg per kg (26 and 52 µg per lb) and 62 samples of meat meal ranged from 17 to 323 µg (8 to 146 µg per lb) (Frigg, 1987). In comparison to cereal grains, oilseed meals are better sources of total biotin. Soybean meal, for instance, contains a mean biotin content of 270 µg per kg (122 µg per lb) with a range of 200 to 387 µg per kg (91 to 176 µg per lb) (Frigg and Volker, 1994).
Less than one-half of the biotin in various feedstuffs, as determined by microbiological assay, is biologically available (Frigg, 1976; 1984; 1987). Thus, it is important to know the form of biotin (e.g., bound or unbound) as well as its overall content in the feed. For alfalfa meal, corn, cottonseed meal and soybean meal, bioavailability of biotin is estimated at 100%.
However, biotin availability is variable for other feedstuffs, for example, 20% to 50% in barley, 62% in corn gluten meal, 30% in fish meal, 20% to 60% in sorghum, 32% in oats and 0% to 62% in wheat (McDowell, 2004).
Frigg (1976) reported that the biotin content of wheat and barley may be totally unavailable. Buenrostro and Kratzer (1984) found biotin in sorghum to have an availability of 10% to 20%, while the bioavailability of corn ranged from 75 to 100%. Sorghum has an availability of 10% to 20%, whereas the bioavailability of corn ranged from 75% to 100% (Buenrostro and Kratzer, 1984).
Information is lacking on the availability of feedstuff sources of biotin to ruminants. It seems likely, however, that much of the dietary biotin would also be unavailable to rumen microorganisms. The primary source of biotin to ruminants is that synthesized by microorganisms is the rumen, and biotin is synthesized to some extent regardless of dietary composition (Hayes et al., 1966). Milk produced by ruminants is an excellent source of biotin; cow colostrum contained from 1.0 to 2.7 µg biotin per 100 ml (Foley and Otterby, 1978). Biotin in feedstuffs may be destroyed by heat curing, solvent extraction, and improper storage conditions, while pelleting has little effect on biotin content of feed (McGinnis, 1986). Milling of wheat or corn and canning of corn, carrots, spinach or tomatoes reduced biotin concentrations (Bonjour, 1991). Biotin is unstable to oxidizing conditions and, therefore, is destroyed by heat, especially under conditions that support simultaneous lipid peroxidation.
Biotin is commercially available as a 100% crystalline product or as various dilutions, premixed and low potency, spray-dried preparations. The d form of biotin is the biologically active form. It is the form that occurs in nature, and is also the commercially available form. A 2% spray-dried biotin product is also commercially available for use either in feed or drinking water.
Biotin is important for normal function of the thyroid and adrenal glands, the reproductive tract and the nervous system. Its effect on the cutaneous system is most dramatic, since severe dermatitis is the major obvious clinical sign of biotin deficiency in livestock and poultry. Biotin requirements in the turkey are higher than that of the chick, so more field problems with biotin deficiency have arisen in turkeys. Biotin deficiency in chicks and poults results in a wide range of clinical signs (Illus. 11-2, 11-3, 11-4 and 11-5) with considerable variation in time of appearance of individual signs (NRC, 1994). Principal effects in both species are reduced growth rate and feed efficiency, disturbed and broken feathering, dermatitis and leg and beak deformities. First signs of a deficiency are usually growth depression and loose feathering; signs of dermatitis appear next; and finally, disorders of the leg (perosis) and beak become apparent. However, Li et al. (1994) noted that the first signs of biotin deficiency of broiler chicks were lesions on the foot pads in the second week, while growth reduction did not occur until the third week.
Illustration 11-2: Biotin Deficiency, Perosis
Adapted from Cunha (1984) - Courtesy of Hoffman-La Roche, Inc.
Note difference in growth between normal (left) and biotin-deficeint Broad Breasted Bronze male turkeys at three weeks of age.
Courtesy of D.C. Dobson, Utah State University.
Courtesy of L.S. Jensen, Washington State University.
Courtesy of Richard Miles, University of Florida
With dermal lesions, bottoms of feet become rough and callused and contain deep fissures that show some hemorrhaging. Foot problems are usually exacerbated by bacterial invasion of lesions. Also, toes may become necrotic and slough off. Tops of feet and legs usually show only a dry scaliness. Lesions appear in the corner of the mouth and slowly spread to the whole area around the beak. Eyelids eventually swell and stick together. Dermal lesions have a characteristic order of appearance, although speed of onset depends on severity of deficiency. For chicks fed severely biotin-deficient diets, dryness and flakiness of the feet first become noticeable at about 14 days of age, and slight encrustations and superficial fissures develop on the undersurfaces of the feet at about 18 days (Whitehead, 1978). These increase in severity by about 25 days, when the fissures are hemorrhagic. Between three and four weeks, dermatitis may also appear on the eyelids, and as this develops, the bird becomes unable to keep the lids apart, and they eventually stick together. Dermal lesions are similar to those of pantothenic acid deficiency. However, with biotin deficiency, lesions occur first on feet and later around the beak and eyes, whereas in pantothenic acid deficiency, signs occur first in corners of mouth and eyes and only in prolonged cases appear on the feet. Because of the difficulty of making a differential diagnosis between the two vitamins, it is often necessary to examine the diet composition and decide which is more likely to be deficient. In commercial poultry production both vitamins should be supplemented in a corn-soybean meal ration. Biotin deficiency is a cause of hock disorders in both poults and chicks. The major deficiency sign affecting market turkeys is severe leg weakness. Lesions caused by biotin deficiency are brought about by chondrodystrophy, a condition in which bone mineralization is normal but linear growth of long bones is impaired.
Chondrodystrophy caused by biotin deficiency can result in shortening of metatarsal bones and perosis. Perosis occurs when irregular bone development results in enlargement and deformity of the hock joint (Illus. 11-6). Crippling in turkeys can occur as early as three to four weeks of age. Often it seems to disappear at six to seven weeks. Then it reappears with great severity between 13 to 16 weeks (Scott, 1981). At this stage the birds are unable to walk and thus can be trampled or cannibalized by other turkeys. Perosis can occur at any stage. In general, young chickens are less susceptible to leg disorders than poults, although biotin deficiency does cause problems of the same type in chicks as in poults (Whitehead, 1978). Once the deformities of perosis occur, biotin administration is not effective.
Bain and Newbrey (1988) report that “twisted leg” is the most common limb disorder in broiler chickens and that biotin deficiency adversely affected tibiotarsal bone growth. Tibiotarsal bones are frequently longitudinally distorted in biotin-deficient poultry. Presumably, reduced biotin prevents ready formation of prostaglandins from essential fatty acids, and bone growth fails to respond to stresses during development (Watkins et al., 1989). Low dietary fat and the necessity for fatty acid synthesis lead to an abnormal array of fatty acids that predisposes poultry to a fatty liver and kidney syndrome (FLKS) (Whitehead, 1988). This condition, which has caused heavy economic losses in commercial broiler flocks, was found to be due to suboptimal dietary biotin coupled with certain nutritional and environmental stress factors. Situations increasing the metabolic rate of biotin-dependent enzymes, such as low fat or protein levels, aggravate the condition. Although the signs of FLKS are not those of classic biotin deficiency, they can be virtually eliminated by supplementation of chick starter or breeder diets with biotin. For turkeys, dry and brittle feathers usually accompany the other signs of clinical biotin deficiency. Bronze poults can exhibit white barring of the feathers, usually affecting just tom turkeys. Likewise, deficient chicks have rough and broken feathering, with head and breast feathers often having a spiky, matted appearance. Poor egg production and hatchability result from clinical biotin deficiency (Robel, 1991; NRC, 1994). For breeder chickens, biotin deficiency reduces hatchability but is less likely to affect egg production. Clinical signs and conditions associated with biotin deficiency in chick embryos and (or) newly hatched chicks include bone deformities (perosis); impaired muscular coordination (ataxia); skeletal deformities (e.g., crooked legs); extensive foot webbing; abnormal cartilage development (chondrodystrophy); embryonic mortality; twisted, malformed beak (“parrot beak”); and reduced size. Ferguson et al. (1961) reported that biotin deficiency in turkeys resulted in a marked decrease in hatchability and a high rate of embryonic mortality during the first week of incubation. At the end of the second week, hatchability decreased from 83% to 14%. At the end of the third week hatchability was zero. Embryonic mortality because of inadequate biotin occurs largely during the last three days of incubation. Feeding the biotin-deficient diet resulted in an abrupt decrease in egg production after 13 weeks. A marginal biotin deficiency is found to be common in normal human pregnancy and is highly teratogenic (fetal malformations) in mice (Mock, 2009).
The economics of biotin supplementation in poultry rations is often given much attention in formulation of vitamin premixes. The reason for this is that biotin is thought to be the most expensive vitamin included in poultry diets. However, based on the requirements and recommended industry levels for biotin supplementation, biotin costs less than other vitamins, such as choline and vitamin E, in poultry rations. To this point the nutritionist must consider the extreme variation of biotin content and bioavailability of feedstuffs used in poultry diets, which makes them unreliable as a consistent source of biotin in the diet. Therefore, under practical commercial production conditions, criteria such as growth rate, feed conversion, reproductive efficiency and other parameters associated with biotin-responsive conditions should be considered when formulating poultry diets. Although clinical signs were observed in poultry and swine under experimental conditions in the 1940s, for many years it was believed that supplemental biotin was not needed in poultry and swine diets because of the wide distribution of biotin in feedstuffs and the synthesis by the animal’s intestinal microflora. In the late 1960s and early 1970s, interest in poultry and swine was rekindled when more field cases of biotin deficiency occurred than in the past. Also, since then, well-controlled research has more clearly defined the occurrence of biotin deficiency and the value of supplementation in the practical poultry diet. Interferences with the biosynthesis of biotin by intestinal bacteria can individually or collectively lead to a biotin deficiency.
These interferences can be in the form of therapeutic administration of antibacterial agents and of modern housing systems limiting animals’ access to feces. Additionally, biotin deficiencies are now more prevalent because of restricted use of biotin-rich feedstuffs and limited bioavailability of biotin in some grains (e.g., wheat, barley, sorghum) and in some animal protein sources (e.g., meat meal, poultry by-product meal). Biotin antagonists, including molds, rancid fat and improved genetic characteristics for greater crop production, have also contributed to limiting biotin utilization by poultry. Cunha (1984) has summarized the possible reasons for more frequent occurrence of biotin deficiencies in swine. These reasons are likely applicable to the poultry industry.
It has been presumed that biotin deficiency does not result when corn and soybean meal are the main diet ingredients. However, conditions suggesting biotin deficiency have been observed in broilers in the field. In various university studies, a marginal biotin deficiency associated with a high incidence of foot pad lesions and breast blisters, which can cause downgrades at meat processing, was seen in broilers fed practical corn-soy rations (Harms and Simpson, 1975; Harms et al., 1977). Likewise, Oloyo and Ogunmodede (1989) indicated that a dietary biotin level of 120 µg per kg (54.5 µg per lb) was needed to prevent signs of FLKS. These studies, conducted in Nigeria, attempted to relate the level of biotin necessary to prevent FLKS in broilers fed palm kernel oil. Biotin supplementation has an additional benefit in that it favorably affects vitamin C metabolism (Lechowski and Nagorna-Stasiak, 1993). Results indicate that biotin supplementation accelerates ascorbic acid synthesis in some tissues of chickens and that increasing ascorbic acid levels indirectly affects all processes involving ascorbic acid. In battery experiments, biotin deficiency signs were observed in turkey poults fed a commercial type ration containing 227 mg of biotin per ton (0.25 mg per kg, or 0.11 mg per lb) (Marusich et al., 1970). Concurrent feeding of increasing levels of biotin reduced or prevented the deficiency, as measured by foot pad, beak, shank and toe dermatitis and perosis (leg abnormalities) in three-week-old poults. About 409 mg of biotin per ton (0.45 mg per kg, or 0.2 mg per lb) of feed was required to prevent deficiency signs. Feeding this quantity of biotin for three additional weeks to poults previously fed the commercial ration without supplemental biotin for three weeks, reduced the incidence of deficiency signs. Various factors influence footpad dermatitis in turkeys. While wet litter is a leading cause, and will increase both the severity and incidence, biotin insufficiency can be a primary contributing factor. For turkeys, high dietary levels of biotin or zinc to lowered the severity of foot pad lesions, but only on dry litter and not on wet litter (Youssef et al., 2011). Biotin is active in skin formation and maintenance, and a deficiency causes abnormal keratinization and cornification of the epidermis, leading to low tensile strength and more skin lacerations. Perhaps more importantly, biotin deficiency can slow the process of wound healing. The biotin requirement is highest for turkeys, the specie that suffers the greatest from footpad dermatitis. Although several factors influence the actual biotin requirement, a conclusion can be drawn: biotin is most effective when fed at high levels early in the life of the poult. Evaluating the natural phenomenon in turkey breeder hens indicates that as the hen ages, the hatchability of eggs decreases. Robel (1983) suggested that biotin in eggs from certain hens may limit embryonic survival. In field studies, eggs from Large White turkey hens were injected with 87 µg of d-biotin, and subsequent hatchability values of fertile eggs were 4% to 5% higher for exogenous d-biotin-injected eggs than for noninjected controls (Robel and Christensen, 1987). Later, Robel (1991) reported that in turkey breeders a minimum of 38 µg of biotin per egg was needed to maintain adequate hatchability of fertile eggs. This translated into dietary levels of 500 to 800 µg per kg (227 to 364 µg per lb) of feed depending on the feed intake and age of the birds. Chen et al. (1994) reported that biotin supplementation of turkey breeder feeds is particularly important for older birds. Similar to Robel, these workers found that 750 µg per kg (341 µg per lb) of supplemental biotin was needed to support adequate hatchability of fertile eggs. The level of biotin in the yolk of turkey eggs was fairly constant regardless of supplemental biotin, while albumen biotin increased proportionally to dietary biotin levels. White et al. (1987) found that avidin tightly binds to biotin as it is deposited in the albumen through the oviduct. In a follow-up study, White et al. (1992) determined that the biotin content of yolk appeared to nurture the embryo, while the biotin content of the albumen appeared available only to the hatchling. Therefore, the biotin content of both yolk and albumen serve critical needs during the growth and maturation of both prehatch and posthatch chicks and poults.
Krueger and Brown (1987) reported a 3% to 4% improvement in hatch of total eggs in broiler breeder eggs when biotin was supplemented in the breeder hen diet at 1.8 to 2.3 times the NRC (1994) requirement. This improvement in hatch was mainly attributed to reduction in the percentage of eggs that did not hatch.
Losses of biotin during storage can occur. Biotin is readily destroyed by rancid fat (Pavcek and Shull, 1942). Preparing fresh feeds, storing them for only short periods and keeping them dry and in a well-ventilated storage area will minimize rancidity problems. Hamilton and Veum (1984) have suggested that overdrying, poor storage conditions and presence of mold may reduce the availability of biotin in corn. Also, the diet should be low in pro-oxidants and (or) properly protected by an effective antioxidant to avoid destruction of biotin, vitamin E, selenium and other nutrients.
Biotin is relatively stable in multivitamin premixes unless combined with choline or trace minerals. Biotin is relatively stable in natural sources in feeds and is fairly stable during processing. Supplemental d-biotin should be added to poultry rations at levels that provide an appropriate margin of safety to offset the factors influencing the biotin needs of poultry.
Studies with poultry indicate that these species can safely tolerate dietary biotin levels four to 10 times their nutritional requirements (NRC, 1987). In some studies with poultry, detrimental effects with toxic levels of biotin resulted in lowered egg production and fertility. Birds are very tolerant of high levels of biotin, and because the vitamin is excreted intact, toxicity is very rare (Leeson and Summer, 2001).
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