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low plasma or serum vitamin A concentrations (Coutsoudis et al., 1992; Semba et al., 1992, 1996). However, there are no human studies using controlled diets that have evaluated immune function tests as a means to assess the adequacy of different levels of dietary vitamin A. In addition to a lack of relevant dietary studies, there are some inherent limitations to using immune functions as indicators to establish dietary recommendations. Most changes in immune functions that have been associated with a nutrient deficiency are not specific to the nutrient under study (e.g., low T cell-mediated immunity may be caused by a lack of vitamin A, but also by a deficiency of protein or energy, zinc, or other specific nutrient deficiencies or imbalances). Thus, human dietary studies would have to be highly controlled with respect to the contents of potentially confounding nutrients. Another limitation of many immune function tests is related to difficulties encountered in standardizing tests of immunity (e.g., proliferative responses to antigen or mitogen challenge which are often used within studies to assess T and B cell responses). These tests are affected by many factors, such as the type and quality of mitogen used, cell culture conditions, and how subjects’ cells have been collected, that cannot be readily controlled among laboratories or over time. Thus, for these reasons, immune function tests could not be used as an indicator for establishing the EAR for vitamin A.

FACTORS AFFECTING THE VITAMIN A REQUIREMENT

Intestinal Absorption

Dietary Fat

Dietary vitamin A is digested in mixed micelles and absorbed with fat. In some studies, increasing the level of fat in a low fat diet has been shown to improve retinol and carotene absorption (Reddy and Srikantia, 1966) and vitamin A nutriture (Jalal et al., 1998; Roels et al., 1963). Other studies, however, have not demonstrated a beneficial effect of fat on vitamin A absorption (Borel et al., 1997; Figueira et al., 1969).

For optimal carotenoid absorption, a number of research groups have demonstrated that dietary fat must be consumed along with carotenoids. Roels and coworkers (1958) reported that the addition of 18 g/day of olive oil improved carotene absorption from 5 to 25 percent. Jayarajan and coworkers (1980) reported that the addition 5 g of fat to the diet significantly improved serum vitamin A concen-



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