What Are the Uses of Astaxanthin in Layer Feeding?

Eggs are an important source of protein for humans and are important for health. The color of the egg yolk is an important indicator of egg quality, and a bright yolk color is more appealing to consumers. Carotenoids can improve the color of the egg yolk. Astaxanthin is a keto-type carotenoid that cannot be synthesized independently in animals and is widely found in algae, microorganisms and marine animals.

Astaxanthin has coloring, antioxidant and other effective biological and pharmacological functions [1]. Astaxanthin was approved as a feed additive in 2009 in the “Feed Additive 10% Astaxanthin” (GB/T23745-2009). Adding astaxanthin to the diet of laying hens can significantly improve the color and luster of the egg yolk, increase the astaxanthin content in the egg, and enhance the antioxidant properties of the egg [2]. This paper reviews the sources, mechanisms of action and application of astaxanthin in laying hen production, with a view to providing a theoretical basis for the further development and application of astaxanthin in laying hen production.

1 Sources of astaxanthin

1.1 Synthetic

Astaxanthin is a carotenoid with extremely strong antioxidant activity. Synthetic astaxanthin is obtained by converting β-carotene with the addition of two ketone groups and two hydroxyl groups. Synthetic astaxanthin is a mixture of three optical isomers: a pair of enantiomers (3S,3'S; 3R,3'R) and a racemic form (3R,3'S or 3S,3'R). It has lower antioxidant activity [3]. Chemically synthesized astaxanthin has a lower biological potency than natural astaxanthin and has issues with food safety. Its addition to foods and feeds is subject to certain restrictions [4].

1.2 Sources from algae, microorganisms and seafood

Floating plants Algae are one of the sources of natural astaxanthin, including Haematococcus p luvialis, Chlorella zofingiensis, etc. [5]. Microorganisms such as Phaffia rhodozyma, Phodotorala rubra and Agrobacterium aurantiacum are also rich in astaxanthin [6]. Marine animals such as shellfish, shrimp and crabs are an indirect source of astaxanthin, as astaxanthin is deposited in their bodies, especially in their shells, after they eat floating plants. The algae Haematococcus pluvialis is considered a promising microalgae for the commercial production of astaxanthin. Environmental conditions such as temperature and light are artificially controlled to promote astaxanthin accumulation in Haematococcus pluvialis [7-8]. Studies have found that the accumulation of astaxanthin in Rhodopseudomonas palustris is lower than that in Haematococcus pluvialis [9], but the addition of carbon sources or mixed carbon sources during culture can promote the growth of Rhodopseudomonas palustris and the synthesis of astaxanthin [10-11]. It can be seen that Haematococcus pluvialis and Rhodopseudomonas palustris are ideal sources of natural astaxanthin.

2 Astaxanthin catabolism and deposition

2.1 Astaxanthin catabolism

Yuan Chao et al. [12] used thermogravimetric/differential thermal analysis to study the thermal stability and degradation kinetics of astaxanthin under aerobic and anaerobic conditions. It was found that the temperature at which astaxanthin begins to decompose is about 250°C. The presence or absence of oxygen has no significant effect on the thermal decomposition temperature of astaxanthin, but heat-resistant degradation products are formed at high temperatures. Regarding the catabolism of astaxanthin in the body, early studies found that astaxanthin is metabolized into (rac)-3-hydroxy-4-oxo-β-ketone and its reduced form (rac)-3-hydroxy-4-oxo-7,8-dihydro-β-ketone in rat liver cells, and this metabolic process is not affected by toxic metabolic enzymes [13]. There are no reports on the metabolism of astaxanthin in livestock.

2.2 Astaxanthin deposition in the body

In animals, the main sites of carotenoid deposition are the liver, fat, shell and skin. After carotenoids are released from food in the gastrointestinal tract, they form chylomicrons with bile acids, cholesterol, fatty acids, etc., which are absorbed by the small intestinal mucosa and enter the bloodstream and liver via the lymphatic vessels. They are then transported to the deposition site by very low density lipoprotein (VLDL) from the liver. Very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL) are all involved in the absorption and transport of carotenoids. Studies have found that astaxanthin is more efficiently deposited in laying hens than β-carotene and canthaxanthin [14].

When 7.1, 21.3 and 42.6 mg/kg astaxanthin was added to the diet, the 21.3 mg/kg group had the highest astaxanthin deposition rate based on the values at weeks 8 and 24 [15]. Astaxanthin is a non-provitamin A carotenoid. Supplementation with vitamin A in the diet reduced astaxanthin deposition in eggs, as vitamin A competes with astaxanthin and reduces its absorption [16]. The reverse effect of high-dose astaxanthin supplementation on the rate of astaxanthin deposition in eggs requires further investigation.

3 Application of astaxanthin in egg production

3.1 Astaxanthin and egg production performance

The effects of adding astaxanthin to laying hen diets on laying hen performance have been reported to vary widely. When laying hens were fed diets containing 20, 50 or 100 mg/kg of natural astaxanthin, there was no significant effect on laying hen performance [17]. There was no significant difference in laying hen performance between medium-dose (21.3,42.6 mg/kg) and high-dose (213.4 mg/kg) astaxanthin had no significant effect on the production performance of laying hens [18]. The addition of 0.6, 1.2, 2.4, and 3.6 g/kg astaxanthin compound powder to the diet of Jinghong chickens had no significant effect on production performance [19]. Adding 0.8, 1.2, and 1.6 g/kg astaxanthin to a corn-soybean meal diet tended to increase daily feed intake in 60-week-old Roman brown laying hens, but had no significant effect on egg shape index, egg shell strength, or egg shell thickness [20].

In a 24-week-old Hy-Line Brown chicken diet, adding 15 g/kg astaxanthin can significantly increase egg production and egg weight, and reduce the feed-to-egg ratio [21]. 510-day-old Taihang chickens fed astaxanthin 40, 60, 80, and 100 mg/kg, the results showed that the group fed 80 mg/kg astaxanthin had the highest daily feed intake, the lowest feed-to-egg ratio, and the highest egg production rate [16]. The differences in the results of adding different levels of astaxanthin to the laying hen diet on the laying hen's production performance may be related to factors such as the composition of the diet, differences in palatability, the duration of the breeding trial, the source and content of astaxanthin, the type of laying hen and its age. Therefore, the effect of adding astaxanthin to the laying hen diet on the laying hen's production performance deserves further research.

3.2 Astaxanthin and egg quality

Many studies at home and abroad have shown that adding astaxanthin to the diet can effectively improve the color and luster of the egg yolk. Within a certain range, the higher the added amount, the darker the egg yolk color, and there is no negative impact on other indicators [1,14-15]. Egg yolk color is a fat-soluble pigment that is deposited into the egg yolk during egg formation. Laying hens cannot synthesize it themselves, and can only ingest it from the diet and deposit it in the egg yolk [22]. Adding 20, 40, 80, or 160 mg/kg of Haematococcus pluvialis astaxanthin to the laying hen diet effectively delayed the decline in yolk index and yolk color after storage at 4°C and 25°C for 8 weeks [23].

The egg yolk color score increased significantly with the increase of the dietary astaxanthin supplement dose, and affected the redness of the egg yolk, satisfying consumer preferences for egg yolk color [14]. The reason for the darker egg yolk color may be that astaxanthin is a carotenoid, which exists in the form of a brown oil diester. It is absorbed by the body after binding to low-density lipoproteins in the digestive tract, and then enters the egg yolk and is deposited in the bloodstream in a free state via the carrier of lipoproteins, thus significantly increasing the egg yolk color.

The Haugh unit is an important indicator of egg quality. The higher the Haugh unit, the better the egg quality. Astaxanthin has a significant effect on the Haugh unit of Hy-Line Brown laying hens, and the higher the astaxanthin content, the higher the Haugh unit [20]. However, some studies have shown that astaxanthin does not have a significant effect on the Haugh unit [17], which may be related to the age of the laying hens. Astaxanthin from natural sources has a significant effect on the lipid profile of egg yolk, and can improve its nutritional value by changing the ratio of fatty acids. In egg yolk from a diet supplemented with astaxanthin, the main unsaturated acids are oleic acid, linoleic acid and arachidonic acid [24]. Further research is needed to investigate the effect of astaxanthin on n-3 and n-6 polyunsaturated fatty acids.

3.3 Astaxanthin and egg incubation

Studies have shown that adding 0.5%, 1.0% or 1.5% astaxanthin to the diet of 24-week-old Hy-Line brown laying hens can improve the fertilization rate, hatchability and chick health rate of the eggs [25]. Astaxanthin has strong immunomodulatory activity, which can prevent the occurrence of poultry diseases and increase their adaptability to stressful environmental conditions such as high temperatures [26]. Studies have also found that oral administration of nano-astaxanthin 25 mg/kg can protect the testicular function and semen quality of male chickens induced by cadmium, and improve sperm motility and testicular weight [27]. Adding astaxanthin to the basic diet can improve animal body function, improve egg quality, and increase the number of healthy chicks. The mechanism of astaxanthin's protective effect on egg fertilization rate and hatchability needs to be further studied.

3.4 Astaxanthin and the antioxidant capacity of laying hens

The molecular structure of astaxanthin consists of multiple conjugated double bonds, and the α-hydroxy ketone is composed of a ketone group and a hydroxyl group at the end of the conjugated double bond chain. These molecular structural characteristics determine the active electron effect of astaxanthin, which can provide electrons to free radicals or attract electrons from inactive free radicals, thus playing a role in scavenging free radicals and providing antioxidant protection [28]. Adding astaxanthin to the diet of laying hens can significantly enhance the resistance of body tissues to oxidative stress, improve the health of laying hens and the quality of their eggs, and significantly increase the activity of superoxide dismutase and glutathione peroxidase (GSH-Px) in the liver and serum of laying hens [1]. Studies have also found that oral administration of nano-astaxanthin 25 mg/kg can protect cadmium-induced damage to the catalase, total antioxidant capacity and glutathione peroxidase activity of roosters [27].

3.5 Astaxanthin and immune function in laying hens

Astaxanthin can protect the integrity of immune cells, ensure normal immune response processes, promote cytokine secretion and increase immunoglobulin levels [29-30]. Studies have shown that adding the right amount of astaxanthin can significantly increase the serum immunoglobulin G (IgG) content of laying hens [20]. With increasing levels of astaxanthin added to the diet (0.5%, 1.0%, 1.5%), the immune enhancement effect becomes more pronounced. Adding 1.5% astaxanthin can significantly increase the antibody levels, T lymphocyte percentage and immune organ index of Hy-Line brown laying hens [30]. The increase in serum IgG content may be related to T cell proliferation, and astaxanthin can increase T cell activity and the ability of peripheral blood mononuclear cells to produce immunoglobulins [31]. Conversely, some studies have shown that astaxanthin has no significant effect on hen IgG, immunoglobulin M (IgM), tumor necrosis factor-alpha (TNF-α), and tumor necrosis factor-beta (TNF-β) [15].

4 Summary and outlook

The main sources of natural astaxanthin are Haematococcus pluvialis and Rhodopseudomonas palustris. Adding astaxanthin to the diet of laying hens can increase the yellow and red color of the egg yolk, improve the hatching performance of eggs, and enhance the body's antioxidant capacity and immune function. There have been many studies on the application of natural astaxanthin in laying hens, but the metabolic pathways and mechanisms of astaxanthin in the body, the evaluation of the biological activity of astaxanthin from different sources, and the efficiency and mechanism of astaxanthin deposition in egg yolks need to be studied in depth.

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