Retinol pool size

To look at the determinants of vitamin A disposal rate, we used multiple regression analysis to relate our data on disposal rate for 62 rats to vitamin A intake, liver vitamin A levels, and plasma retinol pool size (Fig. 10) (Kelley et al., 1994). These three variables predict 91% of the variation in disposal rate 68% of the reduction in sum of squares comes from plasma retinol, 18% from liver vitamin A, and 14% from vitamin A intake. Our data indicate that disposal rate does not fall until liver vitamin A levels are essentially depleted. That is, as long as plasma retinol levels are normal, degradation rate is high, implying a nonfunctional utilization of the vitamin. We call this degradative preservation. We conclude that plasma retinol is the major determinant of vitamin A disposal rate and hypothesize that... 

FIG. 10. Relationship between vitamin A disposal rate (DR nmol/day), plasma retinol pool size (ROH nmol), vitamin A intake (IN nmol/day), and liver vitamin A (LIVA nmol). 

Requirements for nongrowing adults are based on a limited number of depletion-repletion studies carried out on small numbers of subjects. These studies have been reviewed by Rodriguez and Irwin (1972). An abnormal dark adaptation was the criterion used to determine an inadequate vitamin A status in the early studies. This test provided no measure of the body pool size of vitamin A when abnormal dark adaptation occurred. Dark adaptation in adults is known to be affected by factors other than vitamin A (Fisher et al., 1970 Ripps, 1982). Thus it was difficult, even in the classic Sheffield depletion study of the British Medical Research Council (Hume and Krebs, 1949), to attribute aberrant dark adaptation specifically to the relative vitamin A status of subjects. This was particularly true because there was a poor correlation between blood levels of vitamin A and evidence of impaired dark adaptation until blood levels fell critically low and clinical signs became obvious. It is therefore most unfortunate that a carefully controlled study of adult human requirements that used labeled retinol has not been fully reported (Hodges and Kolder, 1971). From the data available (Sauberlich et al., 1974), individual variation in blood and clinical responses to... 

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