Concentrations of 520 to 590 μg/L of vitamin A in milk from Holstein cows have been reported (Tomlinson et al., 1976), which is significantly less than the levels observed in human milk (Table 4-5). The majority of vitamin A and carotenes are located in the fat globule and fat globule membrane in cow milk (Patton et al., 1980; Zahar et al., 1995). The concentrations of retinol and β-carotene in cow milk averaged 18 to 27 μg/g of milk fat in one study (Jensen and Nielsen, 1996). Retinol in cow milk is bound to β-lactoglobulin, which has a structure very similar to retinol binding protein (Papiz et al., 1986). There is minimal isomerization of trans-retinol to cis-retinol in unheated cow milk (Panfili et al., 1998), the latter being less well absorbed. Cow milk submitted to pasteurization resulted in 3 to 6 percent isomerization to cis-retinol. Greater isomerization was observed with severe heat treatments (16 percent in ultra high temperature milk and 34 percent in sterilized milk).
No data are available to estimate an average requirement for children and adolescents. A computational method is used that includes an allowance for adequate liver vitamin A stores to set the Estimated Average Requirement (EAR) (see “Adults Ages 19 Years and Older”). The EAR for children and adolescents is extrapolated from adults by using metabolic body weight and the method described in Chapter 2. If total body weight is used, the RDA for children 1 through 3 years would be 200 μg RAE/day. If metabolic weight (kg0.75) is used, the RDA would be 300 μg RAE/day. Studies conducted in developing countries indicate that xerophthalmia and serum retinol concentrations of less than 20 μg/dL exist among preschool children with daily intakes of up to 200 μg of vitamin A, whereas 300 μg/day of vitamin A is associated with serum retinol concentrations greater than 30 μg/dL (Reddy, 1985). Although similar data are lacking in developed countries, to ensure that the RDA will meet the requirement of almost all North American preschool children, metabolic weight was used to extrapolate from adults.