Plasma appearance and tissue distribution of astaxanthin isomers in male Sprague–Dawley rats after oral administration of Z‐isomer‐enriched astaxanthin esters through thermal treatment

Astaxanthin Z-isomers have attracted much attention because recent studies have demonstrated that they exhibit greater bioavailability and biological activity than the naturally predominant all-E-isomer. However, the plasma appearance and tissue distribution of astaxanthin isomers when administered with a diet rich in astaxanthin Z-isomers are largely unknown. To understand the health benefits and safety of astaxanthin Z-isomers, it is important to study the in vivo kinetics of the isomers. Z-Isomer-enriched astaxanthin (the total Z-isomer ratio = 58.7%) esters derived from Haematococcus lacustris were fed to male rats, and the in vivo kinetics using single- and 14-day repeated-dose oral tests were investigated. In a single-dose study, (9Z)-astaxanthin appeared to be the fastest among the Z-isomers in the plasma, but its ratio decreased over time. In contrast, the 13Z-isomer in plasma gradually increased and became the most dominant Z-isomer 3 h after administration. After a 14-day repeated-dose oral test, astaxanthin isomers were detected in various tissues such as liver, prostate, and adipose tissues. The total Z-isomer ratio in the liver was the highest (61.1% ± 3.1%). The 13Z-isomer ratio was the highest in all tissues, and small amounts of 9Z-, 15Z-, and a few unknown Z-isomers were observed. The composition of the isomers differed greatly between tissues and diet (i.e., the most dominant Z-isomer in the diet was 9Z-isomer). The absorption and accumulation efficiencies of astaxanthin differs between the Z-isomers, and several astaxanthin Z-isomers would be converted into other isomers or metabolites into other substances during the absorption and body circulation processes.