40 The nutrition handbook for food processors 12: 1 ratio is referred to as the retinol activity equivalence(RAE)(Northrop Clewes, 2001a) In the calculation, the IOM used the mean ratio from only one ' oil study here were in fact 5'oil studies(Booher et al, 1939: Wagner, 1940: Hume and Krebs, 1949; Sauberlich et al, 1974; Tang et al, 2000)and if the data from all the studies had been used a bioconversion value of 3.5 ug B-carotene to l ug retinol would have been calculated giving an RAE of 21 instead of 12 as quoted. The lOM are at present reconsidering the published data(van Lieshout, 2001). How we interpret the bioconversion factors for B-carotene proposed by IOM or by others still needs much more thought before any of them can be put to practical use. However, even if the RaE is only 12: 1, there are probably not sufficient vegetables in developing countries to meet that required, therefore, there must be other, as yet undiscovered factors, that influence bioefficacy. Despite this, intake of fruit and vegetable-sources of provitamin A should be encouraged, for although the bioefficacy of B-carotene is apparently so poor, it has to be remembered that he majority of the worlds children are not VAD 3.5 Function Vitamin A, its analogues and metabolites function in vision, cell differentiation embryogenesis, the immune response, reproduction and growth 3.5.1 Vision n vision, vitamin A is required in two forms for two processes 1. As 1l-cis-retinal in rhodopsin which on exposure to light in the retina iso- merises to a transoid intermediate, triggering a series of conformational changes in membrane potential which is transmitted to the brain 2. As retinoic acid to maintain normal differentiation of the cells of the con junctival membranes, cornea and other ocular structures hence preventin xerophthalmia(Ross, 1999) 3.5.2 Cell differentiation The role of vitamin a in cell differentiation has been clarified since the identifi- cation of two sets of nuclear receptors, RXR and RAR, which are activated by retinoic acid isomers(Olson et al, 2000). Each of the receptors has three distinct forms a, B and y and six domains involved in transcription of genes. RAR binds either all-trans or 9-cis retinoic acid, while rXR binds only 9-cis retinoic acid Both receptors are dimers, however, the rXR receptors form homodimers with themselves as well as heterodimers with RAR, the vitamin D receptor, the tri- iodothyronine receptor and several other nuclear transcription receptors(see also sections 3. 12.3, 3.20.2 and 3.21.3). These interactions usually activate gene expression but RAR can also be inhibitory12 :1 ratio is referred to as the retinol activity equivalence (RAE) (Northrop￾Clewes, 2001a). In the calculation, the IOM used the mean ratio from only one ‘oil study’. There were in fact 5 ‘oil’ studies (Booher et al, 1939; Wagner, 1940; Hume and Krebs, 1949; Sauberlich et al, 1974; Tang et al, 2000) and if the data from all the studies had been used a bioconversion value of 3.5mg b-carotene to 1mg retinol would have been calculated giving an RAE of 21 instead of 12 as quoted. The IOM are at present reconsidering the published data (van Lieshout, 2001). How we interpret the bioconversion factors for b-carotene proposed by IOM or by others still needs much more thought before any of them can be put to practical use. However, even if the RAE is only 12 :1, there are probably not sufficient vegetables in developing countries to meet that required, therefore, there must be other, as yet undiscovered factors, that influence bioefficacy. Despite this, intake of fruit and vegetable-sources of provitamin A should be encouraged, for although the bioefficacy of b-carotene is apparently so poor, it has to be remembered that the majority of the world’s children are not VAD. 3.5 Function Vitamin A, its analogues and metabolites function in vision, cell differentiation, embryogenesis, the immune response, reproduction and growth. 3.5.1 Vision In vision, vitamin A is required in two forms for two processes: 1. As 11-cis-retinal in rhodopsin which on exposure to light in the retina iso￾merises to a transoid intermediate, triggering a series of conformational changes in membrane potential which is transmitted to the brain. 2. As retinoic acid to maintain normal differentiation of the cells of the con￾junctival membranes, cornea and other ocular structures hence preventing xerophthalmia (Ross, 1999). 3.5.2 Cell differentiation The role of vitamin A in cell differentiation has been clarified since the identifi- cation of two sets of nuclear receptors, RXR and RAR, which are activated by retinoic acid isomers (Olson et al, 2000). Each of the receptors has three distinct forms a, b and g and six domains involved in transcription of genes. RAR binds either all-trans or 9-cis retinoic acid, while RXR binds only 9-cis retinoic acid. Both receptors are dimers, however, the RXR receptors form homodimers with themselves as well as heterodimers with RAR, the vitamin D receptor, the tri￾iodothyronine receptor and several other nuclear transcription receptors (see also sections 3.12.3, 3.20.2 and 3.21.3). These interactions usually activate gene expression but RAR can also be inhibitory. 40 The nutrition handbook for food processors