Study on Lutein Use in Aquafeed

With the state paying more and more attention to food safety, the Catalogue of Feed Additives (2013) issued by the Ministry of Agriculture stipulates that the feed additive product used to meet the xanthophyll needs of aquaculture animals must be a natural xanthophyll product derived from marigolds. Natural lutein (derived from marigolds) products have formed a complete industrial chain and are widely used in poultry feed and aquatic feed. They are also used in large quantities as food additives and health products. The active ingredients of natural lutein (derived from marigolds) are lutein and its isomer zeaxanthin, with a molecular formula of C40H56O2.

1 Functions of natural lutein in aquatic animals

1.1 Coloring function

Natural lutein is bright orange-yellow in color and has strong coloring power. It is currently widely used to color the skin, yolks and feet of poultry. Since animals cannot synthesize lutein themselves, they can only digest and absorb lutein and its esters in feed for their own pigment deposition needs. After transfer and esterification of fatty acids in the body, it is finally deposited in tissues such as subcutaneous fat, egg yolk and shin. In aquaculture, the coloring function of natural lutein is mainly used.

1.2 Antioxidant function

Natural lutein has strong antioxidant properties, which can inhibit the activity of reactive oxygen species and prevent the damage to normal cells caused by reactive oxygen species. Experiments have shown that reactive oxygen species can react with DNA, proteins and lipids, weakening their physiological functions. Lutein can inactivate singlet oxygen through physical or chemical quenching, thereby protecting the body from damage and enhancing the body's immune capacity.

1.3 Other biological functions

In the farming of aquatic animals, lutein not only plays a role in coloring and antioxidant protection, but also has many other biological functions: it acts as a fertilization hormone to improve egg quality; boosts immunity, growth and maturation; enhances the tolerance of fish to high ammonia and low oxygen; acts as a precursor to vitamin A; inhibits lipid peroxidation in aquatic animals; provides protection against ultraviolet radiation; during the breeding season, mature males concentrate lutein on the skin to form the secondary sexual characteristics of the male – the marriage color; while females concentrate it on the ovaries to form the protective color of the young.

2 Research and application of natural lutein in aquatic animals

Color is one of the important indicators of the quality of aquatic products. The deposition of color often represents the time limit of the fish in the wild. Bright color often becomes the standard for people to judge the quality and flavor of aquatic products, and the market price will also increase. Aquatic animals cannot produce lutein themselves and must obtain it from their food. Under modern intensive farming conditions, artificial compound feeds are used in large quantities, and the lutein content of aquatic compound feeds is low. In addition, the short breeding cycle means that aquatic animals cannot obtain sufficient lutein, resulting in poor coloration on their surface or in their muscles.

Currently, natural lutein has been studied in special cultured fish such as yellow catfish, bearded catfish, big-mouth catfish, and yellow croaker, as well as shrimp, and has been successfully applied to many yellow varieties such as yellow catfish, bearded catfish, big-mouth catfish, golden pomfret (ovoid pomfret), yellow croaker, loach, eel, yellow turtle, eel, frog and other special economic aquatic animals, and it is also successfully used in crustaceans such as the whiteleg shrimp and Chinese mitten crab. The amount of natural lutein added to various feeds is 30-80 mg/kg, and the specific amount added depends on the farming stage, coloring time and feed intake.

2.1 In yellow catfish

Yellow barbs, commonly known as yellow barbs and yellow shads, are found in most rivers, lakes, ponds and swamps in China. Although they are small, they have a high yield, are highly adaptable, have tender flesh, no small spines, and are low in fat. Their protein content is 16.1%, and their fat content is 0.7%. They are not only nourishing, but also have certain medicinal value. They are a common food fish in China, and there is a huge potential market in Japan, South Korea, Southeast Asia and other countries. They are an excellent variety for generating foreign exchange through exports. Therefore, the yellow croaker has high development value and is one of the famous, high-quality species suitable for artificial breeding.

Under normal circumstances, the body color of wild yellow croakers varies from golden yellow to earth yellow to brownish yellow. However, under artificial breeding conditions, their yellow color gradually fades and turns greyish black, affecting their commercial value. Some studies have shown that when lutein is added in the range of 0–50 mg/kg, the coloring effect increases with the amount of lutein added, but it is not a linear relationship. When a certain level is reached, the utilization rate will decrease when the level of coloring agent is increased. At the same time, the coloring effect of a small dose of coloring agent for a long time is usually better and more stable than that of a large dose of coloring agent for a short time.

Wu Huachang et al. (2005) showed that lutein extracted from marigolds can effectively color yellow catfish. The experimental group with added lutein had a body color basically consistent with that of wild yellow catfish. The optimal amount of lutein added was 100 mg/kg. Each ton of yellow waxed-head fish feed with added lutein can bring an additional economic benefit of more than 500 yuan. Ding Xiaofeng et al. (2010) found that lutein products derived from marigolds have a very significant coloring effect on yellow barbels and that the coloring is stable. They are suitable coloring substances for yellow barbels.

Wang Lubo et al. (2012) studied the effect of the addition level of natural lutein (derived from marigolds, with a content of 4.64%) on the growth and skin pigmentation of yellow catfish. The results showed that 24.2-1,700 mg/kg of natural lutein significantly improved the growth performance of yellow catfish with an initial body weight of 21 g. The optimal dosage of natural lutein as a skin colorant for yellow catfish is 76.25 mg/kg feed.

2.2 In terms of bearded catfish

The bearded catfish is a freshwater fish with small individuals and high economic value that grows in the water bodies of southern China. It is loved by consumers in Guangdong, Guangxi and Hong Kong for its tender meat, rich nutrition and delicious taste. In addition to being eaten, it also has good medicinal properties and is one of the common tonic foods. Wild mandarin fish resources are small and have long been unable to meet market demand. In recent years, artificial breeding has developed rapidly, and mandarin fish has become one of the main freshwater cultured species in the Guangdong and Guangxi regions. However, the body color of artificially bred mandarin fish is often a thorny issue, lacking the healthy yellow color of natural mandarin fish, which greatly reduces its commercial value.

Leng Xiangjun et al. (2002) found that there are significant differences in the body color of wild, semi-artificially cultured and fully artificially cultured mandarin fish. The difference in body color between cultured and wild mandarin fish mainly comes from the difference in the lutein content of the food. Adding lutein products to the feed can effectively improve the body color of farmed mandarin fish. The recommended amount of lutein to be added to the feed is 100 mg/kg (adult fish) or 50 mg/kg (fingerlings).

2.3 In terms of large yellow croaker

Yellow croaker belongs to the Carangidae family, the genus Carangidae, commonly known as yellow croaker, large yellow croaker, etc., is a warm-temperate schooling cave-dwelling fish. It was once one of the “four major seafoods” in China, and is now one of the highest-yielding marine cultured fish in China, known as the “national fish”. Yellow croaker farming is mainly concentrated in Fujian Province and Zhejiang Province. The large yellow croaker has a golden body color that is very pleasing to the Chinese aesthetic sense. It is also very nutritious, rich in protein, vitamins and trace elements, and has tender meat and a good taste, so it is very popular with consumers.

Yi Xinwen et al. (2012) studied the effect of natural lutein on the body color of yellow croaker. The study used yellow croaker with an initial body weight of 33.33±1.67 g as the research object, and added 37.5 mg/kg and 75 mg/kg lutein to the feed. The results showed that the yellow value of the skin on the back and abdomen of the yellow croaker in the lutein group continued to increase with time. The yellow values of the back and abdomen of the big yellow croaker were linearly correlated with the pigment content, which proves that lutein can improve the body color of big yellow croaker.

Han Xingxing et al. (2018) studied the effect of feed pigments on the body color and antioxidant capacity of adult yellow croaker. The results showed that adding a mixture of lutein and astaxanthin (1:1) to the feed can improve the body color and antioxidant capacity of adult yellow croaker. It is recommended that the amount of mixed pigment added be 100-200 mg/kg.

2.4 Research and application of natural lutein in shrimp and crab

It has been reported that almost all crustaceans (such as shrimp and crab) can convert lutein and zeaxanthin into astaxanthin, changing the composition of their pigments and thus exhibiting a bright color. Tanaka et al. (1976) believe that the metabolic pathway of lutein in Japanese tiger prawns is zeaxanthin → 4-oxo-zeaxanthin → astaxanthin. Therefore, to achieve the desired coloring effect in farmed shrimp and crab, high zeaxanthin-type lutein can be added.

Lutein (lutein, zeaxanthin) not only makes the shrimp body color bright, but also cures the “blue body disease” of farmed shrimp, improves the quality of shrimp, makes shrimp tolerant to adverse environments, and improves survival rate. Yanada et al. (1990) used carotenoids of different sources, different contents and different feeding methods, and concluded that zeaxanthin in carotenoids is effective in coloring shrimp.

Mustafa et al. used shrimp with an initial body weight of 11.10±0.26 g as the research object and studied the effect of adding marigold flowers (total lutein content 100 mg/kg) on shrimp color. The results showed that it significantly increased the total carotenoid content of the shrimp. Eduardo et al. (2012) reported that the addition of 75 mg/kg and 150 mg/kg of natural lutein (derived from marigolds) to the feed could increase the astaxanthin content and total carotenoid content of juvenile Penaeus vannamei, and the results were better than those of the feed group with 75 mg/kg synthetic astaxanthin. Penaeus vannamei can metabolize lutein into astaxanthin in vivo.

In practical applications, natural lutein can effectively solve the problem of “blue” shrimp color caused by industrial shrimp farming, and improve the color of the shrimp. Natural lutein can also meet the demand of Chinese mitten crab for color enhancement of the crab's roe, and improve the survival rate of the cultured animals.

3 Factors affecting the application effect of natural lutein in aquatic feed

The factors affecting the application effect of natural lutein in aquatic feed mainly include the following:

3.1 Type, size and physiological state of aquatic animals

Different fish and shrimp species have different abilities to deposit lutein, and individuals with larger bodies have a stronger ability to deposit lutein. Different strains of the same species also have different utilization of lutein. The ability and parts of fish and shrimp to deposit lutein also differ at different physiological and growth stages. The amount of lutein deposited in different parts also differs significantly, and genetic characteristics can also affect lutein deposition.

3.2 Lutein addition and coloring time

Lutein deposition is a gradual process, and simply increasing the lutein level cannot shorten the coloring time or enhance the coloring effect. Because there may be a certain “saturation” in the absorption and transport of lutein in the body, excess lutein that cannot be absorbed and utilized is wasted. The coloring effect of a smaller dose of lutein over a longer period of time is usually not as good or as stable as that of a larger dose of lutein over a shorter period of time.

3.3 Feed composition

3.3.1 Fat content in feed

Increasing the fat content in feed can help enhance the coloration effect in aquatic animals, because lutein is fat-soluble and needs to be dissolved in fat in the digestive tract before it can be effectively absorbed. In addition, different sources of fat also affect the deposition of carotenoids.

3.3.2 Toxic and harmful substances in feed

In recent years, there have been frequent occurrences of abnormal body colors in farmed fish, such as “banana color” and “whitening” in yellow catfish, “banana color” and “whitening” in bearded catfish, and “whitening” in golden pomfret and large yellow croaker, which seriously affect the commercial value of the fish. These are all caused by the presence of toxic and harmful substances in the feed, which leads to the “albinism” of the fish. According to reports, based on the extensive and serious impact of feed ingredients on the physiological functions of fish, the main feed ingredients at this stage are oxidized fats and oils and non-protein nitrogen such as melamine added to feed ingredients. Prevention and control measures: First, when formulating feed, try not to use oxidized oil ingredients such as corn oil, rice bran oil, low-quality lecithin oil (lecithin powder), oxidized fish oil, as well as corn DDGS and corn citric acid residue with high corn oil content, as well as oxidized rice bran and meat meal. You can choose lard, soybean oil, vegetable oil, butter and other oils to enter the feed formula. Second, after removing non-protein nitrogen feed ingredients containing melamine, use products such as liver protection products and vitamin C phosphate to quickly detoxify the fish, which will then rely on its own physiological recovery to gradually return to normal body color.

3.3.3 Antinutritional factors and antioxidants in feed

Plant protein sources such as soybean meal and rapeseed meal contain many antinutritional factors that can affect the utilization of nutrients by fish and shrimp. Lutein is easily oxidized by lipoxygenase, which affects the coloring effect. Therefore, a certain amount of antioxidants such as butylated hydroxytoluene (BHT) in the feed can protect lutein and improve its absorption and utilization in aquatic animals.

3.3.4 Levels of vitamin A and vitamin E in feed

Since fish and shrimp have the ability to convert astaxanthin, canthaxanthin, zeaxanthin, etc. into vitamin A, if the vitamin A in the feed is insufficient, the carotenoids will be partially converted into vitamin A to meet the nutritional needs of the animals, which will affect pigment deposition and lead to poor coloring. However, if the vitamin A content is too high, there will be competition for absorption between molecules with similar structures, and the carotenoids will not be fully absorbed, which will also reduce the coloring effect. Vitamin E (α-tocopherol) also has a certain effect on the absorption of carotenoids by fish. Carotenoids are chemically unstable and easily oxidized, so a certain amount of antioxidants such as vitamin E in the feed can also protect the carotenoids and improve their utilization.

3.3.5 Types, batches and storage time of feed ingredients

Feed ingredients of different types contain different amounts of lutein; the lutein content of feed ingredients of the same type but from different batches is also different, and the lutein content will change during processing and storage. In addition, factors such as feeding methods, water temperature, and light will also affect the coloring effect of aquatic animals.

In short, natural lutein (derived from marigolds) can meet the coloring needs of yellow-based farmed fish and crustaceans, safe and green; natural lutein also has strong antioxidant capacity and improves the immune function of aquatic animals; more importantly, natural lutein (derived from marigolds) has achieved a whole industry chain in China, from marigold seed breeding, marigold cultivation, and lutein deep processing, with sufficient sources and excellent cost performance. It has great value for use in aquaculture.