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 periodsVITAMINS IN MILK AND DAIRY PRODUCTS 271 6.10 cholecalciferol (25(OH)D,; 6.9) which is converted to 1,25-dihydroxy￾cholecalciferol ( 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 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 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 os￾teomalacia occurs most commonly in women who have low calcium intakes and little exposure to sunlight and have had repeated pregnancies or periods