What Is the Use of Lutein in Ruminant Feeding?

Lutein is an oxygenated carotenoid that is widely found in plants in nature (such as pasture grass, algae and fruits and vegetables) [1] and can be synthesized into vitamin A in the human body [2]. Lutein has a variety of biological functions. Its strong antioxidant function helps to enhance the body's immune system [3], and it has an inhibitory effect on many types of cancer and cardiovascular disease [4-5], while also preventing lipid peroxidation caused by oxidation [6]. Lutein is a major component of the macula region of the human eye's retina.

The human body cannot synthesize lutein on its own, and external food is the only source of lutein intake [7]. In addition, lutein has a significant coloring function, and researchers have conducted a large number of studies on this function as a feed and food additive [8-9]. Some researchers have evaluated lutein-added poultry and aquatic feeds [10-11]. Supplementing lutein in dairy cattle rations can directly affect the nutritional quality of dairy products [12]. In addition, Al⁃ varez et al. [13] showed that lutein is only present in the plasma of grazing lambs, which can help to distinguish grazing lambs from housed lambs. The results show that there are significant differences in the fat tissue and meat color of ruminants when comparing the feeding of concentrate-based feed to the feeding of exclusively green forage [14-15].

The main sources of lutein in ruminant feeding are green forage and lutein preparations. Although there is a lot of lutein in grass, the amount of lutein in grass varies greatly due to differences in cultivation management methods, sunshine and rainfall, and processing methods [16]; in addition, the conversion rate of lutein in feed to meat, adipose tissue and dairy products is low, and supplementing lutein preparations can effectively increase the lutein content in livestock products. Lutein preparations are mainly extracted from marigold petals [17].

Jeon et al. [18] found that a large amount of lutein is also present in chlorella. With the increasing demand, researchers have begun to focus on the use of in vitro culture methods to obtain free lutein [19]. In recent years, the efficient development of large-scale rearing of ruminants has increased the input of grain feed, which has placed a huge pressure on the environment. Therefore, expanding the utilization efficiency of roughage resources has a positive effect on the ecological environment. In addition, making full use of the pigment resources in pasture grass is of great significance for improving the utilization methods of roughage, enhancing the value of roughage, healthy feeding of animals, and obtaining high-quality livestock products [20-21]. In summary, this paper reviews the changes in lutein content in forage and the impact on the feeding of ruminants, focusing on lutein and lutein preparations in roughage, and combining the latest research progress at home and abroad.

1. Dynamic changes in lutein content in green forage

1.1. Structure and characteristics of lutein

Lutein is widely found in plants. Its chemical formula contains two keto rings, three chiral centers, and eight stereoisomers. It can capture light energy in photosynthesis and regulate plant growth and development [22]. Lutein is poorly soluble in water and has poor stability, being susceptible to factors such as oxygen, light, heat, metal ions and pH [23]. As an antioxidant, lutein has strong antioxidant capacity, can eliminate the activity of reactive oxygen species and prevent normal cell damage, thereby protecting the body from metabolic damage [24-25]. Lutein exists in both free and esterified forms in different plants [26]. During the preparation of lutein preparations, the esterified lutein needs to be purified by saponification [27].

1.2 Changes in lutein content in green forage

The lutein content in green forage is affected by the intensity of photosynthesis [28]. Nitrogen is involved in plant photosynthesis, so applying nitrogen fertilizer to grass can significantly increase the lutein content [29]. Lv et al. [16] measured the lutein content in Italian ryegrass under different fertilization conditions and at different harvest stages. found that the lutein content in early-harvested samples was significantly higher than that in late-harvested samples, and that both increased linearly with the amount of fertilizer applied. In particular, the lutein content in early-harvested samples under the condition of 120 kg/hm2 nitrogen fertilizer application was as high as 1,003 mg/kg.

Elgersma et al. [30] determined the lutein content in various grasses and found that the lutein content in centipede root was the highest, at 206 mg/kg, and that the lutein content in chicory, small ground elder, caraway, plantain, Luteolin content in yellow herb, wood hyssop and alfalfa was 129, 174, 152, 149, 131 and 129 mg/kg, respectively. In addition, studies have shown that the luteolin content in most forages is negatively correlated with forage yield [16, 31]. Rey-Noso et al. [32] determined the lutein content of a mixture of Pennisetum purpureum and Digitaria sanguinalis from dry and wet regions in Mexico and found that the average lutein content of the mixed forage in the wet region was 185 mg/kg, while the average lutein content of mixed pasture in dry areas was only 64 mg/kg. The research results prove that geographical and climatic conditions also affect the lutein content of pasture.

Li Jianhua [33] investigated the law of variation of lutein in clover and elephant grass under different drying conditions and processing methods. The results showed that the loss of pigment in forage grass after high-temperature drying was much higher than that after low-temperature drying. In addition, compared with the preparation of grass powder, the production of compressed grass blocks can significantly reduce the loss of lutein. 1.3 Changes in lutein content during ensiling Lv et al. [29] investigated the dynamic changes in lutein content during ensiling and found that lutein content did not change throughout the ensiling process and was not affected by ensiling quality. Lutein can be well preserved in a low pH ensiling environment. Therefore, the lutein content in the silage is almost the same as the lutein content in the raw material before silage, and it is also affected by the fertilization level and harvest stage [16]. Kara et al. [34] added maleic acid to silage corn, and the lutein content in corn silage was significantly increased, which shows that silage is an effective means of preserving the lutein content in roughage.

2 Lutein in ruminants

2.1 Lutein metabolism in ruminants

Mora et al. [35] investigated the mechanism of lutein degradation in the rumen. Although no definite results were obtained, these results indicate that the disappearance of lutein in the rumen may be due to the involvement of certain cell components, rather than the direct destruction of lutein molecules in the rumen or attack by rumen microorganisms. In addition, the metabolic mechanism of lutein may also differ in animals of different breeds or in different rumen environments. Cardinault et al. [36] believe that rumen microorganisms have the ability to release conjugated lutein.

Studies have shown that after adding chlorella to the diet of dairy cows, the lutein content of both serum and growing oocytes is significantly increased [37]. Jeon et al. [38] believe that after being ingested by ruminants, lutein in the diet enters the liver and mammary gland through the bloodstream and accumulates there. It has also been reported that carotenoids and retinol in ruminants are mainly deposited in the liver [39]. Mireia Blanco et al. [40] also found that the lutein content in the liver of grazing ewes was significantly higher.

Wang et al. [41] screened for lutein-related mammary proteins involved in mammary gland metabolism and found that 33 related proteins changed, 15 of which showed an upward trend. These proteins are related to glucose metabolism, fatty acid metabolism and immune function in dairy cows. Although there are differences in the fat color of different ruminant species [42], Dunne et al. [43] confirmed that the feeding method of beef cattle can be judged by comparing the color of their adipose tissue, and that the yellowing value of fat was significantly higher under grazing conditions. Reynoso et al. [32] monitored the lutein content of the adipose tissue of grazing cattle of different sexes in the dry and humid tropics, respectively. The results showed that the lutein content of the adipose tissue was not affected by climate, region or sex.

Prache et al. [44] showed that lutein was the only carotenoid deposited in the adipose tissue surrounding the kidneys of lambs, lambs, rams and castrated sheep, lutein was the only carotenoid deposited in the adipose tissue surrounding the kidneys. Yang et al. [45] showed that lutein was hardly detectable in the adipose tissue of lambs. Tucker et al. [46] already found in 1967 that lutein is present in relatively high amounts in the jejunum and faeces of sheep. The cecum and colon are not the main absorption sites for lutein, and due to the lipophilicity of lutein, ruminants may preferentially absorb lutein through the lymphatic vessels [36]. Zhou Limei et al. [47] discussed in detail the mechanism of lutein absorption in goats. In the small intestine of goats, the amount of lutein absorbed increases with perfusion time, reaching a peak after 2 h of perfusion. In addition, the addition of free fatty acids to the lutein perfusion solution can significantly promote absorption. The detailed mechanism of the above phenomenon remains to be further explored, and it is believed that there is a potential correlation with the intestinal flora and signal expression. The above research generally revealed the metabolic law of lutein in different species of ruminants and under different feeding conditions, but there was no report on the detailed metabolic pathway of lutein in ruminants, which needs to be clarified in future research.

2.2 Effect of lutein on ruminant animal products

Lutein powder not only indirectly affects the nutritional value of dairy products through its antioxidant activity [12], but also directly affects the sensory characteristics of dairy products, as it can make consumers perceive the yellowish color of dairy products in a positive way [48]. Ripoll et al. [49] found that under grazing conditions, the lutein content in bovine plasma increased significantly, but when the feeding conditions changed from grazing to hay feeding, the lutein content in plasma decreased significantly. It can be seen that the lutein content in the feed can effectively predict the lutein content in the plasma and livestock products of ruminants.

Generally, the lutein content in cow's milk accounts for 12% to 25% of the total carotenoids [50-51]. Mireia Blanco et al. [40] compared the lutein content in goat's milk after feeding goats fresh grass and hay. The results showed that the lutein content in goat's milk under fresh grass feeding conditions was significantly higher. Han Jiyu et al. [52] supplemented the diet of dairy cows with lutein preparations. The results showed that after 10 days of feeding, the lutein content in cow's milk was significantly higher than that in the control group, but it did not affect the milk yield, milk fat, milk protein and glucose content. Xu et al. [53] showed that in the diet of dairy cows, the optimal content of lutein preparations is 150-200 g/(d·head). Within this range, the proportion of lutein converted to milk is about 0.08%, and the content is 1. 2 ~ 1.5 μg/dL. The test results also showed that supplementing the dairy cow diet with lutein preparations can improve the antioxidant capacity of dairy cows, improve immunity, and prevent disease [53].

Jeon et al. [38] found that the highest dose of lutein fed to Holstein cows resulted in a lutein content of 71.9 μg/dL in milk, which is 40 to 50 times higher than the result of Xu et al. [53]. The reason for this difference may be due to the different sources of lutein, or it may be potentially related to the feeding environment, basal feed, etc. Mora⁃Gutierrez et al. [54] found that selecting the appropriate type of casein is important for improving the chemical stability of lutein in low-fat dairy drinks, and the results will help to improve the production process of lutein dairy products. In addition, similar to adipose tissue, grazing also increased the content of lutein and retinol in muscle [55], but no further reports revealed the mechanism of changes in lutein content in the muscle of ruminants.

3 Summary

Lutein is one of the most important functional substances in humans and plays an important role in human health. Green forage is rich in lutein and is also the main source of lutein in ruminant animals and livestock products. With the maturity of lutein extraction technology in recent years, supplementing lutein preparations in ruminant animal feed has become the main means of producing high-quality livestock products. The results of many studies have shown that there are many factors that affect the lutein content during the cultivation and growth of pasture. Formulating appropriate cultivation and management standards will help to improve the quality and nutritional stability of roughage. Differences in feeding methods, regions, and feedstuffs lead to differences in the quality of animal products. In future research, it is necessary to combine lutein resources in roughage with lutein preparations to formulate comprehensive large-scale feeding standards. In addition, in the processing and preservation of ruminant animal products, exploring the stability of lutein in animal products will help to further clarify the chemical properties of lutein and the production standards of animal products.

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