Is Vitamin D Redundant in an Aquatic Habitat?

Abstract

SummaryCertain fish are very rich sources of vitamin D as compared to most of the higher vertebrates which have insignificant amounts of this vitamin. Not only the teleosts, which possess a calcified skeleton, but also the elasmobranchs, which lack calcified skeleton, contain extremely high concentrations of this vitamin, leading to the speculation that the function of vitamin D in fish may be different from its known classical functions in terrestrial animals. Interestingly, the two most common calcemic hormones associated with Ca and P homeostasis in higher vertebrates are either missing [parathyroid hormone (PTH)] or inactive [calcitonin (CT)] in fish. In fact, these hormones appear to have developed after transition of life from water (Ca-P rich environment) to land (environment poor in Ca and P). Thus, living in an aquatic environment with a continuous rich supply of Ca and P, do fish need vitamin D? If so, does it need to be converted to its polar forms? Additionally what are the functions of vitamin D and its metabolites in fish? Since fish stand between the invertebrates and higher vertebrates in evolution, they serve as a unique model for the study of the evolutionary and physiological significance of vitamin D. Investigations have demonstrated that the source of a high amount of vitamin D in them is primarily through their food-chain (plankton). In addition, it appears from the studies in fish that vitamin D perhaps had no physiological function in the calcium-rich aquatic environment, and its metabolism was essentially for catabolic purposes. During the course of evolution, when life started on calcium poor terrestrial environment, vitamin D became functional and its metabolism, an anabolic one, was concerned with calcium homeostasis. Key Words fish, calcium, evolution, metabolism, vitamin D Calcium homeostasisNature has in a way ensured that calcium and vitamin D are inextricably interlinked. Vitamin D, as it is understood today, carries out the policing job of regulating serum calcium levels in highly evolved mammals. And this regulating mechanism is done with astounding efficiency.

Effects of dietary vitamin D3 administration on innate immune parameters of seabream (Sparus aurata L.)

“…Certain fish (not only teleosts, which possess a calcified skeleton) contain extremely high concentrations of vitamin D in comparison with most higher vertebrates, leading to speculation that the function of this vitamin and VDR in fish could be different from its known classical functions in terrestrial animals [15]. Furthermore, the two most common calcemic hormones (parathyroid hormone and calcitonin) associated with Ca and P homeostasis in higher vertebrates are either missing or inactive in fish and they appear to have developed after the transition of life from water (Ca and P rich environment) to land (environment poor in Ca and P).…”

“…Those that do exist have demonstrated that teleosts, unlike other vertebrates, have large hepatic stores of this vitamin [9], the highest concentrations being found in the livers of marine fish [10]. Although physiological studies have suggested a role for the vD 3 system in teleosts similar to that in other vertebrates [11][12][13][14], it has been speculated that the functions of this vitamin in fish may differ from its known classical functions in terrestrial animals [15].…”

Vitamin D: calcium and bone homeostasis during evolution

“…Of course these organisms need(ed) access to sunlight for their photosynthesis using different wavelengths than the ones responsible for UVB damage and vitamin D synthesis. Vitamin D is therefore regularly found in phytoplankton 5,13,15,16 as well as in zooplankton. 17 Plankton is a major part of the food chain of many fish, and their vitamin D content may be about 0.08-0.27%.…”

“…17 Plankton is a major part of the food chain of many fish, and their vitamin D content may be about 0.08-0.27%. 15 This high nutritional supply is therefore considered to be the main reason why fish (liver) has such a high vitamin D content, especially as fish like cod are deep water fish. 13,16,17 Others, however, found that oral cholecalciferol is rapidly metabolized in fish so that this food chain origin of massive vitamin D accumulation is questionable (D Fraser, personal communication).…”

“…The 15 Little is known about the effects of vitamin D deficiency on calcium or bone homeostasis in basic or late teleosts but vitamin D metabolites are unlikely to have had a major role in calcium homeostasis of fish living permanently in a seawater environment. Gilthead sea bream (Sparus aureus) fed a vitamin D-deficient diet for 22 weeks were found to have undetectable serum 1,25(OH) 2 D 3 levels but still remained normocalcemic albeit with reduced growth and calcium fluxes.…”