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al., 1996), and pregnant and postpartum women (West et al., 1999). Meta-analyses of the results from these and other community-based trials are consistent with a 23 to 30 percent reduction in mortality of young children beyond 6 months of age after vitamin A supplementation (Beaton et al., 1993; Fawzi et al., 1993, Glasziou and Mackerras, 1993). WHO recommends broad-based prophylaxis in vitamin A-deficient populations. It also recommends treating children who suffer from xerophthalmia, measles, prolonged diarrhea, wasting malnutrition, and other acute infections with vitamin A (WHO, 1997). Furthermore, the American Academy of Pediatrics (AAP, 1993) recommends vitamin A supplementation for children in the United States who are hospitalized with measles.

SELECTION OF INDICATORS FOR ESTIMATING THE REQUIREMENT FOR VITAMIN A

Dark Adaptation

The ability of the retina to adapt to dim light depends upon an adequate supply of vitamin A, because 11-cis retinal is an integral part of the rhodopsin molecule of the rods. Without adequate levels of vitamin A in the retina, the function of the rods in dim light situations becomes compromised, resulting in abnormal dark adaptation (night blindness). Before clinically apparent night blindness occurs, abnormal rod function may be detected by dark adaptation testing. In addition to vitamin A deficiency, zinc deficiency and severe protein deficiency also may affect dark adaptation responses (Bankson et al., 1989; Morrison et al., 1978).

Dark Adaptation Test

To perform a dark adaptation test, the eye is first dilated and the subject fixates on a point located approximately 15 degrees above the center of the test light. The test stimulus consists of light flashes of approximately 1-second duration separated by 1-second intervals of darkness. A tracking method is used with the luminance of the test light being increased or decreased depending upon the response of the subject. The ascending threshold is the intensity at which the subject first sees the test light as its luminance is increased. The descending threshold is the intensity at which the subject ceases to see the test light as its luminance is lowered. Each threshold intensity is plotted versus time and the values are read from the graph at the end of a test session. Testing is continued



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