Effect of Protein Quantity and Quality on Plasma Response to an Oral Dose of Vitamin A as an Indicator of Hepatic Vitamin A Reserves in Rats

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Abstract
Rats were fed diets deficient (-A) or sufficient (+A) in vitamin A (3 mg retinol equivalents/kg), containing 7% rice protein (RP) or 7% casein (CP). Rats fed vitamin A-free casein diets were pair-fed with the comparable rice protein-fed group. Growth of rice protein-fed rats was less than that of casein-fed rats, regardless of vitamin A status. Plasma vitamin A levels after 34 days of feeding were similar, and in normal range (>40 µg/dl), for the vitamin-A supplemented rice protein group and the unsupplemented casein groups, in spite of widely differing liver reserves (>130 versus <20 µg/g, respectively). Unsupplemented groups had similar liver reserves (10–20 µg/g), regardless of protein source, but widely differing plasma levels. The relative dose response (RDR) over a 5-hour period to a small (20 µg) oral dose of vitamin A was under 25% for all groups on day 34. On day 49, plasma vitamin A levels had dropped and were similar for both vitamin A-deprived groups (about 25–30 µg/dl), as were liver levels (50%). In contrast, the vitamin A-supplemented, rice protein-fed group had an RDR of <50%. Thus, the RDR approach effectively identified rats with reduced liver stores (<10 µg/g) and plasma levels in the 20–30 µg/dl range, even when diets contained protein suboptimal in quantity and quality.