Effect of dietary cholesterol, phytosterol, and docosahexaenoic acid on astaxanthin absorption and retention in rainbow trout

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Abstract Astaxanthin (Ax) determines the flesh redness of a salmonid fish which is the most desirable quality indicator by consumers. Fish cannot synthesize Ax de novo, therefore the only way to increase flesh redness is to increase dietary input or the absorption and retention rate of dietary Ax. As a hydrophobic carotenoid, the absorption of Ax can be modulated by other lipid molecules in the diet. The present study explored the effect of three lipids, cholesterol (CH), phytosterol (PS) and docosahexaenoic acid (DHA) on Ax absorption, transport, and retention in rainbow trout. Dietary CH significantly improved Ax absorption by elevating plasma Ax levels (p < 0.05), however it had no effect on whole body Ax or flesh color. Dietary PS appears to inhibit Ax absorption since fish had significantly (p < 0.05) reduced whole body Ax. Dietary DHA appeared to have no effect on Ax absorption or retention. By comparing intestinal transcriptomes, we have identified the gene low density lipoprotein receptor (ldlr) which was significantly downregulated in fish fed the cholesterol diet as compared to the control diet. Since ldlr plays a major role in plasma lipoprotein turnover, we hypothesized that the inhibition of ldlr by high dietary cholesterol resulted in higher retention of plasma astaxanthin. The elevation of plasma astaxanthin was not reflected in higher flesh coloration, which suggest other limiting factors governing astaxanthin retention in muscle. However, the transcriptomic and proteomic analyses found no changes of genes or proteins involved in Ax absorption, transport, or excretion in fish fed PS or DHA diets as compared to the control. In conclusion, our study has suggested that CH promotes Ax absorption by regulating lipoprotein retention and provide evidence for improving Ax absorption via dietary modulation.

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