A × B × C × D × E × F
A = Percent of body vitamin A stores lost per day when ingesting a vitamin A-free diet
B = Minimum acceptable liver vitamin A reserve
C = The liver weight:body weight ratio
D = Reference weight for a specific age group and gender
E = Ratio of total body:liver vitamin A reserves
F = Efficiency of storage of ingested vitamin A.
By using this approach, a daily vitamin A intake can be determined that will assure vitamin A reserves to cover increased needs during periods of stress and low vitamin A intake. That value can be used for estimating the average requirement for vitamin A.
The portion of body vitamin A stores lost per day has been estimated to be 0.5 percent based on the rate of excretion of radio-activity from radiolabeled vitamin A and by the calculation of the half-life of vitamin A. The minimal acceptable liver reserve is estimated to be 20 μg/g and is based on the concentration at which (1) no clinical signs of a deficiency are observed, (2) adequate plasma retinol concentrations are maintained (Loerch et al., 1979), (3) induced biliary excretion of vitamin A is observed (Hicks et al., 1984), and (4) there is a protection against a vitamin A deficiency for approximately 4 months while the person consumes a vitamin A-deficient diet. The liver weight:body weight ratio is 1:33 (0.03) and is an average of ratios for infants and adults. The reference weights for adult women and men are 61 and 76 kg, respectively (see Chapter 1). The ratio of total body:liver vitamin A reserves is 10:9 (1.1) and is based on individuals with adequate vitamin A status. Finally, the efficiency of storage can be determined by isotope dilution methods following the administration of either radioactive or stable-isotopically labeled vitamin A to subjects adequate in vitamin A (Bausch and Reitz, 1977; Haskell et al., 1997). Recent studies by Haskell and coworkers (1997) suggest that the efficiency of storage is approximately 40 percent, rather than the 50 percent that was previously reported (Olson, 1987). Based on these current estimations, the EAR of preformed vitamin A required to assure an adequate body reserve in an adult man is 0.005 × 20 μg/g × 0.03 × 76 kg × 1.1 × 2.5, or 627 μg RAE/day. With a reference weight of 61 kg for women, the EAR would be 503 μg RAE/day.
Based on the study of Sauberlich and coworkers (1974), Olson (1987) estimated that the liver vitamin A concentration was less than 10 μg/g at the time the first clinical signs of vitamin A defi-