Retinol metabolism in rats with low vitamin A status: a compartmental model

A compartmental model was developed to describe the metabolism of vitamin A in rats with low vitamin A status maintained by a low dietary intake of vitamin A (approximately 2 micrograms retinol equivalents/day). After the IV bolus injection of [3H]retinol in its physiological transport complex, tracer and trace data were obtained from plasma, organs (liver, kidneys, small intestine, eyes, adrenals, testes, lungs, carcass), and tracer data were obtained from urine and feces. The dietary protocol developed for this study resulted in animals having plasma vitamin A levels less than 10 micrograms retinol/dl and total liver vitamin A levels of approximately 1 microgram retinol equivalent. Four compartments were used to model the plasma: one to describe retinol, one to describe the nonphysiological portion of the dose, and two to simulate polar metabolites derived from retinol. The liver required two compartments and a delay, the carcass (small intestine, eyes, adrenals, testes, and lungs, plus remaining carcass) required three compartments, and the kidneys required two. The model predicted a vitamin A utilization rate of 1.65 micrograms retinol equivalents/day with the urine and feces accounting for most of the output. The plasma retinol turnover rate was approximately 20 micrograms retinol equivalents/day; this was 12 times greater than the utilization rate. This indicated that, of the large amount of retinol moving through the plasma each day, less than 10% of this was actually being irreversibly utilized. Similarly, as compared to the whole-body utilization rate, there was a relatively high turnover rate of retinol in the kidneys, carcass, and liver (9.0, 8.2, and 5.8 micrograms retinol equivalents/day, respectively), coupled with a high degree of recycling of vitamin A through these tissues. Of the total vitamin A that entered the liver from all sources including the diet, approximately 86% was mobilized into the plasma. Similarly, of the vitamin A that entered the carcass, approximately 76% was returned to the plasma. All of the retinol that entered the kidneys was modeled as recycling to the plasma. The present studies provide quantitative and descriptive evidence of an efficient metabolism of vitamin A from absorption through turnover and utilization in rats with very low vitamin A status. Furthermore, although their body stores of vitamin A were extremely low, these rats maintained a high level of recycling of vitamin A throughout the body.