Overview of vitamin D and C requirements in fish and their influence on the skeletal system

Vitamins D and C are essential in many physiological functions. Vitamin D, a fat soluble vitamin, is crucial to preserve calcium and phosphate homeostasis and to protect the skeletal integrity. This hormone functions through the vitamin D receptor (VDR) inducing the expression of various calcium binding and transport proteins in the intestine to stimulate active calcium uptake, thus preserving normocalcemia and, indirectly, maintaining bone mineralization. Besides, vitamin D also acts directly on osteoblasts, the resident bone-forming cells of the skeleton, to inhibit proliferation, modulate differentiation, and regulate mineralization of the extracellular matrix. Vitamin C, a water soluble vitamin, acts as a co-substrate for hydroxylase and oxygenase enzymes involved in the biosynthesis of pro-collagen, carnitine and neurotransmitters, among other numerous physiological functions such as antioxidant or pro-oxidant. Both vitamins should be supplied by the diet because fish are unable to synthesize them. However, their wide range of action makes it difficult to adjust the adequate amount of these vitamins to achieve an optimal fish performance. Besides, the dietary vitamin needs of fish depend on several factors such as developmental stage, physiological, environmental/ecological and genetic conditions. In this sense, vitamin requirements of flatfish do not necessarily meet those of pelagic fish and depends also on their feeding habits (carnivorous, planktivorous or detritivorous); the dietary vitamin demands of an adult fish differ from those of a larva; and even within the same fish species and developmental stage, the environmental conditions would also influence the vitamin needs (i.e., under stress conditions, high vitamin C levels have been demonstrated to improve stress resistance and, consequently, growth).The present paper gives a general overview about the requirements of vitamins D and C in fish and specifically reviews the role of these vitamins in fish skeletogenesis and their influence in the development of skeletal deformities. In addition, new insights on the molecular pathways involving these vitamins in the skeletal ossification process are provided.