VITAMIN-A HOMEOSTASIS AND DIABETES-MELLITUS

Recent studies have shown that plasma concentrations of vitamin A (ret inol) and its carrier proteins, retinol-binding protein (REP), and tra nsthyretin (TTR), are decreased in human subjects with insulin-depende nt (IDDM) but not with noninsulin dependent diabetes mellitus (NIDDM). Rats made diabetic with streptozotocin (STZ) have also been shown to have reduced levels of plasma vitamin A while its hepatic concentratio ns elevate. The circulatory vitamin A levels remained low while its he patic concentrations were further elevated following supplementation o f the vitamin. The reduced circulatory status of vitamin A in diabetic animals was not caused by its impaired intestinal absorption. Further experimental studies have pointed to the fact that IDDM is associated with a deficiency of vitamin A, which is secondary to an impaired tra nsport mechanism of this vitamin from its hepatic storage to the targe t site, such as retina of the eyes. The diabetes-associated changes in vitamin A metabolism were reversed to normal by insulin treatment. Th e underlying cause for decreased metabolic availability in uncontrolle d diabetes, is not clearly understood. It appears that the increased h epatic store of vitamin A is attributed to a decreased availability of its carrier proteins. Subnormal vitamin A status in poorly controlled diabetic subjects may not respond to vitamin A supplementation, rathe r it may increase its load in the liver lending to hepatoxicity. These results clearly suggest that there is a need for further research ide ntifying the importance of vitamin A in diabetes mellitus. (C) Elsevie r Science Inc. 1997.