Different forms of selenium supplementation in fish feed: The bioavailability, nutritional functions, and potential toxicity

Selenium (Se) is a trace element vital for humans and animals for normal growth, health and stress resistance. Organic and inorganic Se sources are widely explored in fish feed, their absorption, metabolism, and distribution mechanisms are different. Previous studies in fish mainly focused on estimating the optimal dietary Se requirements of different fish species using a single Se source for the best growth performance, antioxidative and immune status. However, systematic information of comparison among various Se sources is needed to estimate Se status in fish. The present review compares the bioavailability, toxicity, and nutritional functions of inorganic, organic, and novel Se sources in fish. Organic Se forms present higher bioavailability, better ability in improving fish immune status but relatively lower toxicity than inorganic Se forms. Dietary Se requirements should be estimated with different Se sources using multiple parameters for various fish species. This review will provide a better understanding of the selenium's functions in the promotion of growth, antioxidative and immune status, and stress resistance ability, as well as the differences in absorption, metabolism, bioaccumulation, and toxicity of different Se sources.