What Are the Benefits and Side Effects of Astaxanthin?

To date, more than 600 natural carotenoids have been discovered [1]. Among them, the antioxidant carotenoid astaxanthin is extremely beneficial to health. Its chemical name is 3,3'-dihydroxy-4,4'-dione-beta, and its chemical formula is C40H52O4. It is a bright red color. Astaxanthin, also known as shrimp yellow protein, is widely found in nature, especially in aquatic plants.

Small amounts are also synthesized by yeast and microorganisms [2-3]. The efficacy of astaxanthin has been confirmed by numerous experiments. It has strong antioxidant properties and is recognized as the only carotenoid that can penetrate the blood-brain barrier. It can effectively reduce the damage caused by ultraviolet light to the retina of the eye and the skin [4]. Therefore, the application of the natural active substance astaxanthin in dietary supplements can effectively improve human health problems. This article focuses on the scientific knowledge of the origin, structure, function, absorption and metabolism of astaxanthin, and combines the necessary information and literature works to help the future development and utilization of astaxanthin.

1 Structural characteristics, sources and forms of astaxanthin

1.1 Structural characteristics of astaxanthin

The chemical structure of astaxanthin contains a long conjugated unsaturated double bond, with four isoprene units connected to the central part of the molecule by conjugated double bonds, and two α-hydroxy keto hexa-cyclic structures at both ends [5]. There are two chiral carbon atoms in the astaxanthin molecule, and each carbon atom produces two conformations, which produces three corresponding optical isomers. The asymmetry of the hydroxyl carbon chain in the astaxanthin molecule and the structure of multiple conjugated long chains make it prone to cis-trans isomerization, forming a variety of optical isomers [6].

The cis structure has a large steric hindrance between the hydrogen atoms near the double bond, which is not conducive to the stable existence of astaxanthin. Therefore, all-trans astaxanthin is widely found in nature. The methyl structure is relatively stable, and its existence does not threaten the spatial position [7]. However, all-trans astaxanthin is sensitive to external factors such as light, heat, and oxygen, and is easily affected by ultraviolet radiation, which causes isomerization reactions and the formation of various cis-configuration isomers, thereby reducing the biological activity (such as antioxidant properties) of astaxanthin [8-9]. All-trans astaxanthin is currently the most stable form of astaxanthin, while cis-astaxanthin has better biological activity than other astaxanthin structures [10].

The cis structure is mostly chemically synthesized astaxanthin, while the naturally occurring free astaxanthin in nature is mostly all-trans. Although the structures are similar, the optical configuration of astaxanthin varies significantly in different organisms. Combining relevant literature [11-12], the structure of the optical isomers of astaxanthin in different species is summarized, as shown in Figure 1. Astaxanthin in Haematococcus pluvialis and Antarctic krill mainly exists in the 3S,3'S configuration, while astaxanthin in red fuchsia yeast does not have the 3S,3'S and 3S,3'R configurations. The relative content of the 3S,3'S and 3S,3'R configurations in swimming crab and Japanese shrimp is relatively high [13-14]. We can see that the content varies between different species.

1.2 Source of astaxanthin

Astaxanthin cannot be synthesized by higher organisms and is generally ingested through food. Natural astaxanthin is mainly synthesized in microalgae and phytoplankton, and is passed through the food chain, entering higher organisms gradually. In addition, some yeasts and bacteria can also synthesize astaxanthin autonomously, but the astaxanthin they synthesize differs greatly in structure. In experiments measuring the astaxanthin content in different species, it is not difficult to find that Haematococcus pluvialis is a microalga that contains a large amount of natural astaxanthin [15]. It is regarded as a concentrated product of astaxanthin, but the process of enriching astaxanthin is extremely demanding on the water source. The total astaxanthin content of several other common aquatic products is shown in Figure 2. The data show that Antarctic krill has the highest astaxanthin content, about 120 mg·kg-1, and salmon has the lowest astaxanthin content among several aquatic products, about 15 to 20 mg·kg-1 [16-17] (all calculated on a dry weight basis).

1.3 Forms of astaxanthin

Naturally occurring astaxanthin mainly exists in two forms: free and esterified. A large amount of data shows that the state of astaxanthin also differs in different species. Scientific studies have shown that the main form of astaxanthin in salmon and trout is the free form [18-19], while in shrimp and crab it is the esterified form [20-22]. The free form of astaxanthin is unstable and easily oxidized, so the free form of astaxanthin rarely occurs in animals and plants. The percentage of free and esterified astaxanthin in several common aquatic resources is shown in Figure 3.

As can be seen in the figure, the type of seafood can lead to differences in the form of astaxanthin, and the content and percentage of astaxanthin monoesters, astaxanthin diesters, and free astaxanthin are also different. Salmon and red yeast both contain free astaxanthin.

2 Research on the biological function of astaxanthin and its absorption and metabolism

2.1 Biological function of astaxanthin

The special structure of astaxanthin enables it to effectively degrade reactive oxygen species and has a strong ability to quench molecular oxygen. Related reports have shown that astaxanthin is a powerful natural antioxidant. Its mechanism of action is mainly to resist free radicals and accelerate the removal of free radicals [23]. Studies have shown that astaxanthin can resist tumors [24], inflammation [25], treat diabetes [26], improve the body's immune function [27], and prevent the occurrence of cardiovascular and cerebrovascular diseases [28].

2.2 Current research on the absorption and metabolism of astaxanthin

Whether astaxanthin can be utilized or stored in the body after ingestion depends mainly on its molecular structure and the fat content of the diet. There has been relatively little research on the absorption and metabolism of astaxanthin in the body, and the absorption process of astaxanthin in various structures in the body requires further investigation. According to a large number of existing experiments, adding a small amount of fat during eating can improve the bioavailability of astaxanthin.

Later, ØSTERLIE and CORAL et al. [29-30] found that after oral administration of astaxanthin, free astaxanthin binds to lipoproteins in human serum and is directly absorbed and utilized by the body. Later, ØSTERLIE continued to explore and compare the absorption of free astaxanthin and astaxanthin esters in the human body. A large amount of data showed that free astaxanthin is unstable, and esterified astaxanthin has higher stability and fat solubility. This conclusion laid the foundation for proving that astaxanthin esters have the same biological efficacy as free astaxanthin in the body.

3 Conclusion

Studying the mechanism of action of astaxanthin with different structures is not only important for revealing the factors affecting the efficacy and bioavailability of astaxanthin, but also provides guidance for examining the existence of astaxanthin with high strength and bioavailability. Although the cost of natural astaxanthin production is high, and supply has fallen short of demand in recent years, resulting in synthetic astaxanthin dominating the market, as the market demand for natural astaxanthin gradually increases, and it has broad research and development prospects in the health food, cosmetics, sun protection products and biomedicine industries, it is believed that natural astaxanthin resources will achieve high-value utilization and large-scale production.

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