Test Background Retinol, the dietary form of vitamin A, is a fat-soluble, antioxidant vitamin important in vision and bone growth. Vitamin A synthesis is achieved through hydrolysis of animal-derived retinyl esters to retinol and, similarly, from pro-vitamin A plant carotenoids. Vitamin A is essential for skin growth, bone formation, immune function and vision. One of the earliest signs of vitamin A deficiency is night-blindness, followed by decreased visual acuity. Excess vitamin A can be hepatotoxic and moderately-elevated chronic levels correspond to increased incidence of osteoporotic events.