DAIRY CHEMISTRY AND BIOCHEMISTRY (RND) is the quantity of a nutrient sufficient to meet the needs of 97% of the population. Nutrient intakes equal to the rNi thus pose only a very small risk of deficiency. United Kingdom RNI values(Department of Health, 1991)are also quoted below. 6.2 Fat-soluble vitamins 6.2.1 Retinol ( vitamin A Vitamin A(retinol, 6.1)is the parent of a range of compounds known as retinoids, which possess the biological activity of vitamin A. In general, animal foods provide preformed vitamin A as retinyl esters( e.g. 6.5, which are easily hydrolysed in the gastrointestinal tract) while plant foods provide precursors of vitamin A, i. e carotenoids. Only carotenoids with a B-ionone ring (e. g. B-carotene)can serve as vitamin A precursors. B-Carotene(6.6) 6.1 6.2 Retinal 6.3 Retinoic acid 64 11-cis-retinal266 DAIRY CHEMISTRY AND BIOCHEMISTRY (RNI) is the quantity of a nutrient sufficient to meet the needs of 97% of the population. Nutrient intakes equal to the RNI thus pose only a very small risk of deficiency. United Kingdom RNI values (Department of Health, 1991) are also quoted below. 6.2 Fat-soluble vitamins 6.2.1 Retinol (vitamin A) Vitamin A (retinol, 6.1) is the parent of a range of compounds known as retinoids, which possess the biological activity of vitamin A. In general, animal foods provide preformed vitamin A as retinyl esters (e.g. 6.5, which are easily hydrolysed in the gastrointestinal tract) while plant foods provide precursors of vitamin A, i.e. carotenoids. Only carotenoids with a /3-ionone ring (e.g. p-carotene) can serve as vitamin A precursors. p-Carotene (6.6) 6.1 3 6.2 6.3 6.4