Use of the modified relative dose response (MRDR) assay in rats and its application to humans for the measurement of vitamin A status.

After an appropriate dose of 3,4-didehydroretinol (vitamin A2, DR) is given orally in corn oil to retinol (R)-depleted rats, the ratio of plasma DR/R values at 3.5 h is inversely related to the liver concentration of vitamin A (Tanumihardjo & Olson, 1988). In the present study, a similar procedure, termed the modified relative dose response (MRDR) assay, was employed with rats containing a much broader range of liver reserves; ie, less than 2 to 107 nmol of vitamin A per g wet weight of liver (less than 1 to 30 micrograms/g). The DR/R ratio for 15 of 16 rats with liver reserves less than 17 nmol/g (less than 5 micrograms/g) was greater than 0.15, whereas the ratio was less than 0.15 for 7 of 8 rats with liver reserves greater than 17 nmol/g. This distribution provides sensitivity, specificity and positive predictive values of 94, 88 and 94 per cent, respectively. The DR/R ratios reached a maximal plateau in two other groups of rats between 3.5 and 8 h. At all times up to 24 h, mean DR/R ratios for vitamin A-depleted rats (45 +/- 6 nmoles of vitamin A/g liver) were approximately twice those for vitamin A-sufficient rats (230 +/- 40 nmoles of vitamin A/g liver). In three well-nourished adults, presumably with liver reserves of greater than 300 nmol/g wet weight liver (greater than 80 micrograms retinol equivalents/g), and in two young (1- and 3-years old) well-nourished children, maximal DR/R ratios were less than 0.023. In these cases, peak DR/R ratios were observed, with one possible exception, between 8 and 12 h.(ABSTRACT TRUNCATED AT 250 WORDS)