270 DAIRY CHEMISTRY AND BIOCHEMISTRY 6.7 68 Cholecalciferol. Vitamin D 69 Hydroxycholecalciferol conversion requires about 28 h to convert 50% of previtamin D, to vitamin D,. Thus, production of vitamin D, in the skin can take a number of days. Preformed vitamin D, is obtained from the diet. Vitamin D3 is stored in various fat deposits around the body. Regardless of the source of vitamin Da, it must undergo two hydroxylations to become fully active. Vitamin D3 is transported by a specific binding protein through the circulatory system to the liver where the enzyme, 25-hydroxylase, converts it to 25-hydroxy-
270 DAIRY CHEMISTRY AND BIOCHEMISTRY 6.7 HO 6.8 6.9 conversion requires about 28 h to convert 50% of previtamin D, to vitamin D,. Thus, production of vitamin D, in the skin can take a number of days. Preformed vitamin D, is obtained from the diet. Vitamin D, is stored in various fat deposits around the body. Regardless of the source of vitamin D,, it must undergo two hydroxylations to become fully active. Vitamin D, is transported by a specific binding protein through the circulatory system to the liver where the enzyme, 25-hydroxylase, converts it to 25-hydroxy-
VITAMINS IN MILK AND DAIRY PRODUCTS 271 610 1, 25-Dihydroxycholecalciferol cholecalciferol (25(OH)D3; 6.9) which is converted to 1, 25-dihydroxy cholecalciferol (1, 25(OH)2 D3; 6.10) by the enzyme, 1-hydroxylase, in the 24, 25-dihydroxycholecalciferol(24, 25(OH)2D3). At least 37 on 24 to form kidney. Alternatively, 25(OH)D, can be hydroxylated at posit metabolites of vitamin D, have been identified, but only 3, 25(OH)2D3, 24, 25(OH)2 D3 and 1, 25(OH)2 D, have significant biological activity; 1, 25(OH)2D, is the most biologically active metabolite of vitamin D, Vitamin D,(ergocalciferol) is formed by the photoconversion of ergo- sterol, a sterol present in certin fungi and yeasts, and differs from cholecal ciferol in having an extra methyl group at carbon 24 and an extra double bond between C22 and C23. Ergocalciferol was widely used for many years as a therapeutic agent. The principal physiological role of vitamin D in the body is to maintain plasma calcium by stimulating its absorption from the gastrointestinal tract, its retention by the kidney and by promoting its transfer from bone to the blood. Vitamin d acts in association with other vitamins hormones and nutrients in the bone mineralization process. In addition, vitamin d has a wider physiological role in other tissues in the body, including the brain and nervous system, muscles and cartilage, pancreas, skin, reproductive organs and immune cells. The RDA for vitamin D is 10 and 5 ug day-I for persons aged 19- 24 years or over 25 years, respectively. RNI values for vitamin D are 10 ug day-I for persons over 65 years and for pregnant or lactating women With the exception of these and other at-risk groups, the RNI value for dietary vitamin D is O ug day. The classical syndrome of vitamin D deficency is rickets, in which bone is inadequately mineralized, resulting in growth retardation and skeletal abnormalities. Adult rickets or os- omalacia occurs most commonly in women who have low calcium intakes and little exposure to sunlight and have had repeated pregnancies or periods
VITAMINS IN MILK AND DAIRY PRODUCTS 271 6.10 cholecalciferol (25(OH)D,; 6.9) which is converted to 1,25-dihydroxycholecalciferol ( 1,25(OH),D,; 6.10) by the enzyme, 1-hydroxylase, in the kidney. Alternatively, 25(OH)D, can be hydroxylated at position 24 to form 24,25-dihydroxycholecalciferol (24,25(OH),D3). At least 37 metabolites of vitamin D, have been identified, but only 3,25(OH),D,, 24,25(OH),D, and 1,25(OH),D, have significant biological activity; 1,25(OH),D, is the most biologically active metabolite of vitamin D,. Vitamin D, (ergocalciferol) is formed by the photoconversion of ergosterol, a sterol present in certin fungi and yeasts, and differs from cholecalciferol in having an extra methyl group at carbon 24 and an extra double bond between C,, and C23. Ergocalciferol was widely used for many years as a therapeutic agent. The principal physiological role of vitamin D in the body is to maintain plasma calcium by stimulating its absorption from the gastrointestinal tract, its retention by the kidney and by promoting its transfer from bone to the blood. Vitamin D acts in association with other vitamins, hormones and nutrients in the bone mineralization process. In addition, vitamin D has a wider physiological role in other tissues in the body, including the brain and nervous system, muscles and cartilage, pancreas, skin, reproductive organs and immune cells. The RDA for vitamin D is 10 and 5pgday-' for persons aged 19- 24years or over 25 years, respectively. RNI values for vitamin D are 10 pg day- ' for persons over 65 years and for pregnant or lactating women. With the exception of these and other at-risk groups, the RNI value for dietary vitamin D is Opgday-'. The classical syndrome of vitamin D deficency is rickets, in which bone is inadequately mineralized, resulting in growth retardation and skeletal abnormalities. Adult rickets or osteomalacia occurs most commonly in women who have low calcium intakes and little exposure to sunlight and have had repeated pregnancies or periods
272 DAIRY CHEMISTRY AND BIOCHEMISTRY of lactation. Hypervitaminosis D(excess intake of vitamin D)is character ized by enhanced absorption of calcium and transfer of calcium from bone to the blood. These cause excessively high concentrations of serum calcium which can precipitate at various locations in the body, causing kidney stones or calcification of the arteries. Vitamin d can exert these toxic effects if consumed continuously at only relatively small amounts in excess of the RDA Relatively few foods contain significant amounts of vitamin D. In addition to conversion in situ by the body, the principal sources of vitamin D are foods derived from animal sources, including egg yolk, fatty fish and liver. Unfortified cows milk is not an important source of vitamin D The major form of vitamin D in both cows and human milk 25(OH)D3. This compound is reported to be responsible for most of the vitamin D in the blood serum of exclusively breast-fed infants. Whole cows milk contains only about 0.03 ug vitamin d per 100 g and 1 litre of milk per day will supply only 10-20% of the RDA. Therefore, milk is often fortified (at the level of c. 1-10 ug I")with vitamin D. Fortified milk, dairy products or margarine are important dietary sources of vitamin D. The concentration of vitamin D in unfortified dairy products is usually quite low. Vitamin D levels in milk vary with exposure to sunlight As with other fat-soluble vitamins the concentration of vitamin d in dairy products is increased pro rata by concentration of the fat (e.g. in the production of butter or cheese). Vitamin D is relatively stable during storage nd to most dairy processing operations. Studies on the degradation of vitamin D in fortified milk have shown that the vitamin may be degraded by exposure to light. However, the conditions necessary to cause significant losses are unlikely to be encountered in practice. Extended exposure to light and oxygen are needed to cause significant losses of vitamin d 6.2.3 Tocopherols and related compounds(vitamin E, Eight compounds have vitamin E activity, four of which are derivatives of tocopherol (6.11)and four of tocotrienol(6. 12);all are derivatives of 6-chromanol. Tocotrienols differ from tocopherols in having three carbon carbon double bonds in their hydrocarbon side chain. a, B-, y-or 8- tocopherols and tocotrienols differ with respect to number and position of methyl groups on the chromanol ring. The biological activity of the diferent forms of the tocopherols and tocotrienols varies with their structure. D-and L-enantiomers of vitamin E also occur; the biological activity of the D-form is higher than that of the L-isomer. Vitamin e activity can be expressed as tocopherol equivalents(TE), where 1 TE is equivalent to the vitamin E activity of I mg a-D-tocopherol. The biological activity of B.and7- tocopherols and a-tocotrienol is 50, 10 and 33% of the activity of a-D
272 DAIRY CHEMISTRY AND BIOCHEMISTRY of lactation. Hypervitaminosis D (excess intake of vitamin D) is characterized by enhanced absorption of calcium and transfer of calcium from bone to the blood. These cause excessively high concentrations of serum calcium which can precipitate at various locations in the body, causing kidney stones or calcification of the arteries. Vitamin D can exert these toxic effects if consumed continuously at only relatively small amounts in excess of the RDA. Relatively few foods contain significant amounts of vitamin D. In addition to conversion in situ by the body, the principal sources of vitamin D are foods derived from animal sources, including egg yolk, fatty fish and liver. Unfortified cows’ milk is not an important source of vitamin D. The major form of vitamin D in both cows’ and human milk is 25(OH)D,. This compound is reported to be responsible for most of the vitamin D in the blood serum of exclusively breast-fed infants. Whole cows’ milk contains only about 0.03 pg vitamin D per 100 g and 1 litre of milk per day will supply only 10-20% of the RDA. Therefore, milk is often fortified (at the level of c. 1-10 pg 1-’) with vitamin D. Fortified milk, dairy products or margarine are important dietary sources of vitamin D. The concentration of vitamin D in unfortified dairy products is usually quite low. Vitamin D levels in milk vary with exposure to sunlight. As with other fat-soluble vitamins, the concentration of vitamin D in dairy products is increased pro rata by concentration of the fat (e.g. in the production of butter or cheese). Vitamin D is relatively stable during storage and to most dairy processing operations. Studies on the degradation of vitamin D in fortified milk have shown that the vitamin may be degraded by exposure to light. However, the conditions necessary to cause significant losses are unlikely to be encountered in practice. Extended exposure to light and oxygen are needed to cause significant losses of vitamin D. 6.2.3 Eight compounds have vitamin E activity, four of which are derivatives of tocopherol (6.11) and four of tocotrienol (6.12); all are derivatives of 6-chromanol. Tocotrienols differ from tocopherols in having three carboncarbon double bonds in their hydrocarbon side chain. a-, p-, y- or 6- tocopherols and tocotrienols differ with respect to number and position of methyl groups on the chromanol ring. The biological activity of the different forms of the tocopherols and tocotrienols varies with their structure. D- and L-enantiomers of vitamin E also occur; the biological activity of the D-form is higher than that of the L-isomer. Vitamin E activity can be expressed as tocopherol equivalents (TE), where 1 TE is equivalent to the vitamin E activity of 1 mg u-D-tocopherol. The biological activity of p- and ytocopherols and u-tocotrienol is 50, 10 and 33% of the activity of a-Dtocopherol, respectively. Tocopherols and related compounds (vitamin E)
VITAMINS IN MILK AND DAIRY PRODUCTS 6.11 6.12 Tocotrienols R=量=RE Vitamin E is a very effective antioxidant. It can easily donate a hydrogen from the phenolic -oh group on the chromanol ring to free radicals. The sulting vitamin E radical is quite unreactive as it is stabilized by delocal ization of its unpaired electron into the aromatic ring. Vitamin E thus protects the lipids(particularly polyunsaturated fatty acids ) and membranes in the body against damage caused by free radicals. the role of vitamin E is of particular importance in the lungs where exposure of cells to oxygen is greatest. Vitamin E also exerts a protective effect on red and white blood cells. It has been suggested that the body has a system to regenerate active vitamin E(perhaps involving vitamin C)once it has acted as an antioxidant. Vitamin e deficiency is normally associated with diseases of fat mal- absorption and is rare in humans. Deficiency is characterized by erythrocyte haemolysis and prolonged deficiency can cause neuromuscular dysfunction Hypervitaminosis E is not common, despite an increased intake of vitamin E supplements. Extremely high doses of the vitamin may interfere with the blood clotting process The RDAs for vitamin e are 10 mg and 8 mg a-tE day for men and women, respectively. UK RNI values have not been established for vitamin E since its requirement is largely dependent on the content of polyunsatu rated lipids in the diet. However, the Department of Health(1991)suggested that 4 and 3 mg a-TE day-l are adequate for men and women, respectively The major food sources of vitamin E are polyunsaturated vegetable oils and products derived therefrom(e.g. maragrine, salad dressings), green and leafy
VITAMINS IN MILK AND DAIRY PRODUCTS 273 6.11 6.12 R, I Tocotrienols I R3 Vitamin E is a very effective antioxidant. It can easily donate a hydrogen from the phenolic -OH group on the chromanol ring to free radicals. The resulting vitamin E radical is quite unreactive as it is stabilized by delocalization of its unpaired electron into the aromatic ring. Vitamin E thus protects the lipids (particularly polyunsaturated fatty acids) and membranes in the body against damage caused by free radicals. The role of vitamin E is of particular importance in the lungs where exposure of cells to oxygen is greatest. Vitamin E also exerts a protective effect on red and white blood cells. It has been suggested that the body has a system to regenerate active vitamin E (perhaps involving vitamin C) once it has acted as an antioxidant. Vitamin E deficiency is normally associated with diseases of fat malabsorption and is rare in humans. Deficiency is characterized by erythrocyte haemolysis and prolonged deficiency can cause neuromuscular dysfunction. Hypervitaminosis E is not common, despite an increased intake of vitamin E supplements. Extremely high doses of the vitamin may interfere with the blood clotting process. The RDAs for vitamin E are 10 mg and 8 mg c(-TE day- for men and women, respectively. UK RNI values have not been established for vitamin E since its requirement is largely dependent on the content of polyunsaturated lipids in the diet. However, the Department of Health (1991) suggested that 4 and 3 mg a-TE day- are adequate for men and women, respectively. The major food sources of vitamin E are polyunsaturated vegetable oils and products derived therefrom (e.g. maragrine, salad dressings), green and leafy
274 DAIRY CHEMISTRY AND BIOCHEMISTRY vegetables, wheat germ, whole-grain cereal products, liver, egg yolk, nuts The concentration of vitamin E in cows'milk is quite low (0.09 mg per 100 g) and is higher in summer than in winter milks. Human milk and colostrum contain somewhat higher concentrations(0.3 and 1.3 mg pe 100 g, respectively). Most dairy products contain low levels of vitamin E (Appendix 6A)and thus are not important sources of this nutrient. How ever, levels are higher in dairy products supplemented with vegetable fat(e. g some ice-creams, imitation creams, fat-filled dried skim milk). Like other fat-soluble vitamins, the concentration of vitamin E in dairy products increased ta with fat content. Vitamin E is relatively stable below .00C but is destroyed at higher temperatures(e. g. deep-fat frying). The vitamin may also be lost through oxidation during processing. Oxidative losses are increased by exposure to light, heat or alkaline ph, and are promoted by the presence of pro-oxidants, lipoxygenase or catalytic trace elements(e.g. Fe,cut). Pro-oxidants increase the production of free radicals and thus accelerate the oxidation of vitamin E. Exogenous vitamin E in milk powders supplemented with this nutrient appears to be stable for long storage periods if the powders are held at or below room temperaure The potential of feed supplemented with vitamin e to increase the oxidative stability of milk has been investigated, as has the potential use of exogenous tocopherols added directly to the milk fat 6.2.4 Phylloquinone and related compounds( vitamin K) The structure of vitamin K is characterized by methylnaphthoquinone rings with a side chain at position 3. It exists naturally in two forms: phyllo- quinone(vitamin Ki; 6.13)occurs only in plants, while menaquinones (vitamin K2; 6.14)are a family of compounds with a side chain consisting of between 1 and 14 isoprene units. Menaquinones are synthesized only by bacteria(which inhabit the human gastrointestinal tract and thus provide some of the vitamin K required by the body) Menadione(vitamin K3; 6.15) is a synthetic compound with vitamin K activity. Unlike K, and K menadione is water soluble and is not active until it is alkylated in vivo The physiological role of vitamin K is in blood clotting and is essential for the synthesis of at least four of the proteins(including prothrombin) involved in this process. Vitamin K also plays a role in the synthesis of a rotein(osteocalcin) in bone. Vitamin k deficiency is rare but can result from impaired absorption of fat. Vitamin K levels in the body are also reduced if the intestinal flora is killed(e.g. by antibiotics). Vitamin K toxicity is rare but can be caused by excessive intake of vitamin K supplements Symptoms include erythrocyte haemolysis, jaundice, brain damage and reduced effectiveness of anticoagulants The RDAs for vitamin K for people aged 19-24 years are 70 ug and 60 ug day-I for men and women, respectively. Corresponding values for
274 DAIRY CHEMISTRY AND BIOCHEMISTRY vegetables, wheat germ, whole-grain cereal products, liver, egg yolk, nuts and seeds. The concentration of vitamin E in cows' milk is quite low (0.09mg per lOOg) and is higher in summer than in winter milks. Human milk and colostrum contain somewhat higher concentrations (-0.3 and - 1.3 mg per 100 g, respectively). Most dairy products contain low levels of vitamin E (Appendix 6A) and thus are not important sources of this nutrient. However, levels are higher in dairy products supplemented with vegetable fat (e.g. some ice-creams, imitation creams, fat-filled dried skim milk). Like other fat-soluble vitamins, the concentration of vitamin E in dairy products is increased pro rafa with fat: content. Vitamin E is relatively stable below 100°C but is destroyed at higher temperatures (e.g. deep-fat frying). The vitamin may also be lost through oxidation during processing. Oxidative losses are increased by exposure to light, heat or alkaline pH, and are promoted by the presence of pro-oxidants, lipoxygenase or catalytic trace elements (e.g. Fe3+, Cu2+). Pro-oxidants increase the production of free radicals and thus accelerate the oxidation of vitamin E. Exogenous vitamin E in milk powders supplemented with this nutrient appears to be stable for long storage periods if the powders are held at or below room temperaure. The potential of feed supplemented with vitamin E to increase the oxidative stability of milk has been investigated, as has the potential use of exogenous tocopherols added directly to the milk fat. 6.2.4 Phylloquinone and related compounds (vitamin K) The structure of vitamin K is characterized by methylnaphthoquinone rings with a side chain at position 3. It exists naturally in two forms: phylloquinone (vitamin K,; 6.13) occurs only in plants, while menaquinones (vitamin K,; 6.14) are a family of compounds with a side chain consisting of between 1 and 14 isoprene units. Menaquinones are synthesized only by bacteria (which inhabit the human gastrointestinal tract and thus provide some of the vitamin K required by the body). Menadione (vitamin K,; 6.15) is a synthetic compound with vitamin K activity. Unlike K, and K,, menadione is water soluble and is not active until it is alkylated in uiuo. The physiological role of vitamin K is in blood clotting and is essential for the synthesis of at least four of the proteins (including prothrombin) involved in this process. Vitamin K also plays a role in the synthesis of a protein (osteocalcin) in bone. Vitamin K deficiency is rare but can result from impaired absorption of fat. Vitamin K levels in the body are also reduced if the intestinal flora is killed (e.g. by antibiotics). Vitamin K toxicity is rare but can be caused by excessive intake of vitamin K supplements. Symptoms include erythrocyte haemolysis, jaundice, brain damage and reduced effectiveness of anticoagulants. The RDAs for vitamin K for people aged 19-24 years are 70pg and 60 pg day- for men and women, respectively. Corresponding values for