Astaxanthin (Ax), a natural carotenoid, is the primary pigment responsible for the intense red hue of salmonid flesh. This highly desirable quality is in high demand by consumers. However, despite its inclusion in the diet, Ax retention in salmonid muscle is generally low, ranging from 3% to 18% in rainbow trout and from 3.9% to 12% in Atlantic salmon. To achieve the desired pink-red color in the flesh, salmonid farmers must add high levels of Ax to the diet, which can be costly and inefficient.
However, rainbow trout cannot produce astaxanthin on its own, so the color of its flesh is directly influenced by the amount of Ax present in its diet. Researchers from the Norwegian University of Life Sciences, BioMar AS, and the Norwegian University of Science and Technology explored the effect of three lipids—cholesterol (CH), phytosterol (PS), and docosahexaenoic acid (DHA)—on the absorption, transport, and retention of astaxanthin (Ax) in rainbow trout.
The influence of dietary lipids on astaxanthin absorption
Astaxanthin, a hydrophobic xanthophyll, shares structural similarities with cholesterol (CH) and phytosterol (PS). These lipids compete with Ax for absorption via the NPC1L1 pathway and excretion via ABC transporters. Additionally, omega-3 fatty acids, such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), may inhibit NPC1L1, further affecting Ax absorption.
Intestinal mechanisms that limit Ax retention
The intestine plays a key role in regulating astaxanthin retention. Enzymatic cleavage by β-carotene 15,15′-oxygenase (BCO1) and efflux by the adenosine triphosphate-binding cassette subfamily G member 2 (ABCG2) are key mechanisms limiting the release of Ax into circulation.
Previous studies have explored the impact of various lipids on Ax absorption and retention in salmon. This new research focused on three specific lipids: cholesterol (CH), phytosterol (PS), and docosahexaenoic acid (DHA).
Cholesterol: a key factor in Ax absorption
The study found that adding cholesterol to the diet of rainbow trout significantly improved Ax absorption. This was evident from elevated astaxanthin levels in the fish’s blood. However, while cholesterol improved Ax absorption, it did not lead to a corresponding increase in whole-body Ax levels or a more pronounced red color in the flesh.
The role of LDL receptors
Further analysis revealed that cholesterol may influence Ax absorption by regulating the activity of low-density lipoprotein receptors (LDLR). LDLR proteins play a crucial role in the turnover of plasma lipoproteins, which can transport Ax. When fish were fed a cholesterol-rich diet, LDLR gene expression was significantly reduced. This suggests that inhibition of LDLR activity by cholesterol may result in Ax retention in the bloodstream.
Other factors limiting Ax retention in muscle
Despite the increased levels of astaxanthin in the blood, the study did not observe a corresponding increase in flesh coloration. This indicates that other factors likely limit Ax retention in muscle tissue. Future research will be needed to identify these factors and develop strategies to enhance astaxanthin retention.
The impact of phytosterol and DHA
Unlike cholesterol, phytosterol appeared to inhibit Ax absorption. Fish fed a diet containing phytosterol had lower whole-body astaxanthin levels compared to the control group. DHA, on the other hand, had no significant effect on Ax absorption or retention.
Implications for rainbow trout aquaculture
The findings of this study highlight the importance of understanding the mechanisms underlying Ax absorption and retention in salmonids. By modulating the dietary inclusion of lipids and optimizing the intestinal environment, rainbow trout farmers can enhance astaxanthin retention and achieve the desired pink-red flesh color without adding excessive amounts of Ax to the feed.
However, further research is needed to fully elucidate the complex interactions between astaxanthin, dietary lipids, and intestinal proteins, but the results of this study provide valuable insight into potential strategies for improving Ax retention in salmon.
Conclusion
“In conclusion, this study has suggested that CH promotes astaxanthin absorption by regulating lipoprotein retention and provides evidence of improved Ax absorption through dietary modulation,” the researchers concluded.
The study’s findings provide valuable insight into the factors influencing Ax absorption and retention in rainbow trout. Cholesterol appears to be a promising dietary component for enhancing astaxanthin absorption by regulating lipoprotein retention.
Future research should focus on identifying the factors limiting Ax retention in muscle tissue and developing strategies to improve the overall red coloration of rainbow trout flesh.
The study was funded by BioMar AS and the Research Council of Norway. Open access funding was provided by SIKT GOLD.
Contact
Rolf Erik Olsen
Department of Biology, Norwegian University of Science and Technology
Trondheim, Norway
Email: rolf.e.olsen@ntnu.no
Reference (open access)
Jin, Y., Li, K., Vik, J. O., Hillestad, M., & Olsen, R. E. (2024). Effect of Dietary Cholesterol, Phytosterol, and Docosahexaenoic Acid on Astaxanthin Absorption and Retention in Rainbow Trout. Aquaculture Nutrition, 2024(1), 8265746. https://doi.org/10.1155/2024/8265746
