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Among the essential unsaturated fatty acids, the balance of omega-6 fatty acids and omega-3 fatty acids is crucial to the health of the body. The World Health Organization (WHO) recommends the golden ratio of 4:1. A serious imbalance in the intake of omega-6 fatty acids and omega-3 fatty acids is a vital cause of many modern diseases, including cardiovascular disease, cancer, inflammation and autoimmune diseases (Simopoulos 2008). We visited 200 residents in the community and researched their choices of cooking oil in daily lives (Figure 2). We found that soybean oil accounted for 52%, more than 1/2 of the total. However, according to the nutrient table of soybean oil, it is obvious that the content of omega-6 fatty acids is about 7.4:1 of omega-3 fatty acids. This ratio is higher than the WHO recommended ratio of 4:1. Therefore, eating soybean oil alone cannot satisfy the health of the body, and it needs to be supplemented by exogenous sources of omega-3 fatty acids.

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Currently, the main source of omega-3 fatty acid supplementation for the body is deep-sea fish oil or certain nutraceuticals. However, the low content of omega-3 fatty acid in deep-sea fish, which leads to more expensive extraction and purification costs (Wang et al. 2022). In addition, this type of production capacity is unstable and easily affected by seasonal and geographical factors. Multiple factors lead to an increase in the cost of omega-3 fatty acid intake by the body, which may make it unaffordable for the general population over a long time. Eggs are usually used by the general public as a dietary necessity. Assuming that we get eggs rich in omega-3 fatty acids, this will promote the body's replenishment of omega-3 fatty acids and alleviate the imbalance between omega-3 and omega-6 fatty acids.

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Docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) are two typical omega-3 fatty acid (Yee et al. 2010). DHA is vital for the development of the brain and eyesight of infants, and is colloquially known as "golden brain". The main effect of EPA is to reduce cholesterol and triglyceride levels to prevent cardiovascular diseases. In order to obtain eggs rich in DHA, it is often necessary to add omega-3 rich fatty acids to chicken feed. The main source of omega-3 fatty acids added to chicken feed is flaxseed or linolenic acid (ALA) (Mattioli et al. 2017). However, the conversion rate of ALA to DHA in chickens is low, which is not conducive to obtaining eggs with high DHA content. In addition, it is common to find DHA-rich eggs on the market, while EPA eggs are relatively rare. Therefore, the preparation of eggs containing both DHA and EPA can help increase consumer choice.

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In this study, the metabolic pathways for EPA and DHA synthesis were constructed by using the homologous recombination gene editing technology in Yarrowia lipolytica. We further applied the "push-pull strategy" to unblock the metabolic pathways. Meanwhile, weakening the β-oxidation of fatty acids can reduce the fatty acid consumption. In addition, we adopted promoter engineering strategies to balance metabolic flow and increase the yield of target compounds. Our goal is to construct Y.lipolytica with high yield of DHA and EPA, so as to provide theoretical basis for industrial production of polyunsaturated fatty acids by microorganisms in the future. Finally, the fungus is sprayed into powder by spray drying tower. The chicken feed with the added powder is supplied to the chickens to obtain eggs rich in DHA and EPA.

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Mattioli S, Ruggeri S, Sebastiani B, Brecchia G, Dal Bosco A, Cartoni Mancinelli A, Castellini C (2017) Performance and egg quality of laying hens fed flaxseed: highlights on n-3 fatty acids, cholesterol, lignans and isoflavones. Animal 11(4):705-712 doi:10.1017/S175173111600207X

Simopoulos AP (2008) The importance of the omega-6/omega-3 fatty acid ratio in cardiovascular disease and other chronic diseases. Exp Biol Med (Maywood) 233(6):674-88 doi:10.3181/0711-MR-311

Wang K, Shi TQ, Wang J, Wei P, Ledesma-Amaro R, Ji XJ (2022) Engineering the Lipid and Fatty Acid Metabolism in Yarrowia lipolytica for Sustainable Production of High Oleic Oils. ACS Synth Biol 11(4):1542-1554 doi:10.1021/acssynbio.1c00613

Yee LD, Lester JL, Cole RM, Richardson JR, Hsu JC, Li Y, Lehman A, Belury MA, Clinton SK (2010) Omega-3 fatty acid supplements in women at high risk of breast cancer have dose-dependent effects on breast adipose tissue fatty acid composition. Am J Clin Nutr 91(5):1185-94 doi:10.3945/ajcn.2009.29036