Astaxanthin isomers: Selective distribution and isomerization in aquatic animals

Astaxanthin, a predominant carotenoid mainly distributed in aquatic animals, has been widely utilized as a food additive in modern aquaculture. Astaxanthin has many optical and geometric isomers. Investigating the accumulation characteristics of astaxanthin isomers with different functional characters in aquatic animals could provide a valuable reference for achieving the efficient utilization of astaxanthin in aquatic feeds. Many studies have provided information about the composition of astaxanthin isomers in aquatic animals, but lack a systematic analysis and summary. This review summarizes the available data on the distribution of astaxanthin isomers in fish and crustaceans, and compares the differences in isomer composition between aquatic animals and their dietary source, with an aim to investigate the conversion and accumulation mechanism. Aquatic animals convert dietary astaxanthin to body astaxanthin through the processes of absorption, transport, metabolism and deposition. Three optical isomers (3S,3'S, 3R,3'R and 3R,3'S) and the main geometric isomers (all-trans, 9-cis, 13-cis and 15-cis) of astaxanthin are distributed in aquatic animals. Accumulation of these astaxanthin isomers in aquatic animals is related to, but not completely dependent on, the isomer configuration of dietary astaxanthin, but rather is specific for various species and tissues. The differences of astaxanthin isomers between dietary source and body indicate the complexity of the conversion of astaxanthin in aquatic animals, and the possibility that there is a selective mechanism as well as isomerization of astaxanthin.