268 DAIRY CHEMISTRY AND BIOCHEMISTRY may be cleavaged at its centre by the enzyme, B-carotene- 15, 15'-oxygenase (present in the intestinal mucosa), to yield 2 mol retinol per mol. However, cleavage of other bonds results in the formation of only I molecule of retinol per molecule of B-carotene. In practice, 6 ug B-carotene will yield only 1 ug of retinol. Likewise, 12 ug other carotenes which are vitamin A precursors (i.e. which contain one B-ionone ring) are required to yield 1 ug of retinol Thus, I retinol equivalent(RE)is defined as 1 ug retinol, 6 ug B-carotene or 2 ug of other precursor carotenes Retinol can be oxidized to retinal (6. 2)and further to retinoic acid (6.3) Cis-trans isomerization can also occur, e.g. the conversion of all trans- retinal to 11-cis-retinal(6.4), which is important for vision Vitamin a has a number of roles in the body: it is involved in the vision process, in cell differentiation, in growth and bone remodelling and in the immune system. US RDAs for vitamin A are 1000 ug RE day"for men and 800 ug RE day- for women. UK RNI values for vitamin a are 700 and 600 ug Re day for adult men and women, respectively. The body will tolerate a wide range of vitamin A intakes(500-15 000 ug Re day" )but sufficient or excessive intakes result in illness. Vitamin A deficiency (<500 ug RE day- )results in night blindness, xerophthalmia(progressive blindness caused by drying of the cornea of the eye), keratinization(accu mulation of keratin in digestive, respiratory and urogenital tract tissues)and finally exhaustion and death. At excessive intake levels(>15 000 ug RE day ) vitamin A is toxic. Symptoms of hypervitaminosis A include skin rashes, hair loss, haemorrhages, bone abnormalities and fractures, and in extreme cases. liver failure and death he major dietary sources of retinol are dairy products, eggs and liver, while important sources of B-carotene are spinach and other dark-green leafy vegetables, deep orange fruits (apricots, cantaloupe) and vegetables (squash, carrots, sweet potatoes, pumpkin). The richest natural sources of vitamin A are fish liver oils, particularly halibut and shark Vitamin A activity is present in milk as retinol, retinyl esters and as carotenes. Whole cows'milk contains an average of 52 ug retinol and 21 ug carotene per 100g. The concentration of retinol in raw sheeps and pas- teurized goats'milks is 83 and 44 ug per 100 g, respectively, although milks of these species are reported(Holland et al., 1991) to contain only trace amounts of carotenes. Human milk and colostrum contain an average of 58 and 155 ug retinol per 100 g, respectively. In addition to their role as provitamin A, the carotenoids in milk are reponsible for the colour of milk fat( Chapter 11) The concentration of vitamin a and carotenoids in milk is strongly influenced by the carotenoid content of the feed. Milk from animals fed on pasture contains higher levels of carotenes than that from animals fed on concentrate feeds. There is also a large seasonal variation in vitamin A concentration;summer milk contains an average of 62 ug retinol and 31 ug carotene per 100g while the values for winter milk are 41 and 11 ug per268 DAIRY CHEMISTRY AND BIOCHEMISTRY may be cleavaged at its centre by the enzyme, p-carotene-1 5,15'-oxygenase (present in the intestinal mucosa), to yield 2 mol retinol per mol. However, cleavage of other bonds results in the formation of only 1 molecule of retinol per molecule of p-carotene. In practice, 6 pg 8-carotene will yield only 1 pg of retinol. Likewise, 12 pg other carotenes which are vitamin A precursors (i.e. which contain one p-ionone ring) are required to yield 1 pg of retinol. Thus, 1 retinol equivalent (RE) is defined as 1 pg retinol, 6 pg p-carotene or 12 pg of other precursor carotenes. Retinol can be oxidized to retinal (6.2) and further to retinoic acid (6.3). Cis-trans isomerization can also occur, e.g. the conversion of all tvans￾retinal to 11-cis-retinal (6.4), which is important for vision. Vitamin A has a number of roles in the body: it is involved in the vision process, in cell differentiation, in growth and bone remodelling and in the immune system. US RDAs for vitamin A are 1000 pg RE day- for men and 800 pg RE day-' for women. UK RNI values for vitamin A are 700 and 600 pg RE day- ' for adult men and women, respectively. The body will tolerate a wide range of vitamin A intakes (500-15OOOpg REday-') but insufficient or excessive intakes result in illness. Vitamin A deficiency ( < 500 pg RE day- ') results in night blindness, xerophthalmia (progressive blindness caused by drying of the cornea of the eye), keratinization (accu￾mulation of keratin in digestive, respiratory and urogenital tract tissues) and finally exhaustion and death. At excessive intake levels (> 15 000 pg REday-'), vitamin A is toxic. Symptoms of hypervitaminosis A include skin rashes, hair loss, haemorrhages, bone abnormalities and fractures, and in extreme cases, liver failure and death. The major dietary sources of retinol are dairy products, eggs and liver, while important sources of p-carotene are spinach and other dark-green leafy vegetables, deep orange fruits (apricots, cantaloupe) and vegetables (squash, carrots, sweet potatoes, pumpkin). The richest natural sources of vitamin A are fish liver oils, particularly halibut and shark. Vitamin A activity is present in milk as retinol, retinyl esters and as carotenes. Whole cows' milk contains an average of 52 pg retinol and 21 pg carotene per 1OOg. The concentration of retinol in raw sheep's and pas￾teurized goats' milks is 83 and 44 pg per 100 g, respectively, although milks of these species are reported (Holland et al., 1991) to contain only trace amounts of carotenes. Human milk and colostrum contain an average of 58 and 155pg retinol per lOOg, respectively. In addition to their role as provitamin A, the carotenoids in milk are reponsible for the colour of milk fat (Chapter 11). The concentration of vitamin A and carotenoids in milk is strongly influenced by the carotenoid content of the feed. Milk from animals fed on pasture contains higher levels of carotenes than that from animals fed on concentrate feeds. There is also a large seasonal variation in vitamin A concentration; summer milk contains an average of 62 pg retinol and 31 pg carotene per 100 g while the values for winter milk are 41 and 11 pg per