Functions of Vitamin D

Abstract

From the early work of Nicolaysen it was firmly established that vitamin D improves intestinal calcium absorption (Nicolaysen, 1937a, b, c; Nicolaysen et al., 1953). Nicolaysen and his associates further demonstrated conclusively that vitamin D is essential for the ability of the intestine to adapt its intestinal calcium absorption to the needs for calcium (Nicolaysen et al., 1953). These two fundamental biological observations still remain today to be elucidated in molecular detail. Following the work of NICOLAYSEN, several investigators using a variety of techniques confirmed the idea that vitamin D increases intestinal calcium absorption (Wasser-Man, 1963). However, little progress was made until Schachter and Rosen introduced the technique of everted intestinal sacs in vitro as a technique to measure calcium transport (Schachter and Rosen, 1959). Using this technique, these pioneering investigations revealed that vitamin D increases the transport of calcium against a concentration gradient in a process which required metabolic energy and thus appeared to be active transport (DowDle et al., 1960; Schachter, 1963). SchachTer and colleagues continued their investigations and demonstrated that the ability of the small intestine to transport calcium was not only dependent upon vitamin D, but was also modulated by dietary calcium levels in support of the early conclusions of Nicolaysen (Kimberg et al., 1961). Harrison and Harrison (1965) confirmed the everted intestinal sac work of Schachter and his colleagues but introduced the concept that vitamin D might not function at the active transport site but might rather increase the permeability of small intestine to calcium.