How Does Lutein Help Eyesight?

Tagetes erecta L., also known as marigold, is a herbaceous annual plant in the Asteraceae family. Native to Mexico and the Americas, it has been cultivated in recent years in Shandong, Jilin, Shanxi, Inner Mongolia, Yunnan and other provinces in China. Its flowers have strong resistance to and absorption of harmful gases such as hydrogen fluoride and sulfur dioxide. The flowers, leaves and roots can all be used in medicine, with the effects of clearing away heat and phlegm, nourishing the blood and regulating menstruation, removing blood stasis and promoting regeneration, and detoxifying and reducing swelling [1]. Lutein is an asymmetric dihydroxy carotenoid widely found in vegetables and fruits, and is most abundant in flowers such as marigolds [2].

Lutein's scientific name is 3,3'-dihydroxy-alpha-carotene, molecular formula: C₄₀H₅₆O₂, molecular weight 568.85, structural formula as shown in Figure 1, melting point 190°C, insoluble in water, soluble in oils and fatty solvents [3,4]. Lutein is present in the dried flowers of marigolds in the form of an ester, and has the dual effect of enhancing color and providing nutrients. Marigolds are therefore an ideal raw material for the production and development of lutein. Lutein plays a variety of important disease-preventing and disease-treating functions in the human body, and has good application prospects in functional foods and drugs [5]. This paper reviews the research progress on the extraction methods and health-promoting functions of lutein from marigolds.

1 Extraction methods for lutein from marigolds

At present, the main methods for extracting lutein from marigold flowers include organic solvent extraction, enzymatic treatment, ultrasonic and microwave-assisted extraction, and supercritical fluid extraction.

1.1 Organic solvent extraction

The extraction effect of lutein is mainly affected by factors such as the size of the raw material particles, the extraction temperature, the extraction time, the type of extraction agent and the flow rate of the extraction agent. Currently commonly used extraction agents include hexane, petroleum ether, acetone, ethanol, tetrahydrofuran, chloroform, 6# solvent oil, etc. The extraction effect of 6# solvent oil, chloroform and tetrahydrofuran is better[6]. Xia Shulin et al. studied the process conditions for extracting lutein from marigold flowers. The results showed that the process flow of mixing marigold flowers, tetrahydrofuran, ethanol, water and KOH at room temperature, and simultaneously carrying out extraction and saponification treatment is optimal. The lutein obtained using this process has a purity of over 97.6%.

Cui Zhenhai et al. [8] studied the effect of changing the type of solvent and extraction time on the lutein extraction rate of dried marigold pollen. The results showed that the best conditions for the extraction of lutein from marigold were as follows: the extraction solvent was a mixture of chloroform and ethanol in a volume ratio of 3:2, the extraction was carried out twice, the material ratio was 1:10, the extraction time was 2 h each time, the concentration of NaOH during saponification was 5%, and the saponification time was 8 h.

1.2 Enzyme treatment method

Enzymes can break down the integrity of cell structures, exposing more substances inside the cells during extraction and increasing the permeability of the oil. After enzymatic treatment, the lutein extraction rate can be significantly increased. Enzyme degradation of marigold flowers does not cause lutein isomerization, and the lutein content of the enzymatically treated marigold powder is the highest. However, due to the long reaction time of the enzymatic treatment method, the large amount of moisture in the enzymatic treatment process needs to be removed before solvent extraction, which limits the application and promotion of this method in practical work.

Using marigold flower powder as raw material, researchers have developed a new method for degrading the cell wall with cellulase and extracting lutein, with an extraction rate of over 90% [9]. In addition, in terms of the selection of hydrolase, Navarrete-Bolanos et al. [10,11] studied the effect of a non-commercial enzyme preparation on the extraction of lutein from marigold flowers. This enzyme was synthesized by an endophytic microorganism produced during the storage process, and it had high cellulase activity and a good extraction effect.

1.3 Ultrasonic method

In the ultrasonic-assisted extraction of marigold lutein, ultrasound breaks down cells, allowing lutein to quickly and efficiently enter the extraction solvent, thereby shortening the extraction time and increasing the extraction efficiency [12]. Li Dajing et al. [13] studied the process of ultrasonic-enhanced organic solvent extraction of lutein from marigold flower powder. The results showed that the optimal extraction conditions for lutein from marigold flower powder using ultrasonic-enhanced n-hexane were a liquid-to-solid ratio of 1:20 (g/mL), an ultrasonic power of 400 W, and an ultrasonic time of 30 min, with a lutein extraction rate of up to 98.77%. Using n-hexane as the solvent, the ultrasonic extraction conditions were a liquid-to-solid ratio of 1:20, an ultrasonic power of 300 W, and an ultrasonic action time of 30 min, and the lutein extraction rate was up to 93.65%.

Yang Yunshang et al.¹ 4] studied the optimal process conditions for ultrasonic extraction of lutein using an L9(34) orthogonal test with lutein content as the evaluation index. The results showed that the optimal process conditions were: extraction temperature 40 °C, extraction time 60 min, ascorbic acid dosage 7.5%, and ultrasonic frequency 100 kHz. This process is reasonable, simple, reliable, and has a high extraction rate of effective ingredients. Wang Jingyu et al. [15 studied the application of cyclic ultrasound technology in the extraction of lutein from marigolds. The results showed that the optimal extraction solvent was acetone: petroleum ether = 1:1 (V/V); the more ideal extraction conditions were a liquid-to-material ratio of 1:70, an ultrasonic action time of 20 min, and an ultrasonic power of 800 W; under these conditions, the extraction rate of the ultrasonic cycle extraction method was 98%, while the solvent static extraction method was only 42%.

1.4 Microwave-assisted solvent extraction method

Compared with traditional methods for extracting lutein, microwave-assisted solvent extraction has the characteristics of short extraction time, low solvent consumption, and high extraction rate. Fan Jianfeng et al. [16] used single-factor experiments combined with orthogonal experiments to study the microwave-surfactant synergistic extraction of lutein from marigold flowers in order to improve the extraction rate of lutein from marigold leaves. The results showed that the optimal process for the microwave-surfactant synergistic extraction of marigold lutein was to use ethyl acetate as the extracting agent, Tween-20 as the best co-extracting agent (0.03% by mass), an extraction solid-liquid ratio of 1:60 (g/mL), a microwave power of 400 W, a microwave extraction temperature of 60°C, and extraction time was 2 min. Under this process, the amount of lutein extracted from marigold leaves was 3.209 mg/g.

1.5 Supercritical fluid extraction

In 1998, Qingdao University obtained a patent for the production of lutein using supercritical CO₂ extraction technology. The method for extracting lutein from marigold flowers using supercritical CO₂ is to ferment, dry and crush fresh marigold flowers to obtain the raw material, and then use supercritical CO₂ with ethanol as an entrainer to extract the marigold flower extract. The marigold flower extract is saponified with potassium hydroxide to obtain a water-soluble natural food colouring agent, lutein resin. Compared with the traditional organic solvent method, the use of supercritical CO₂ fluid extraction technology to obtain lutein has no solvent residue and no pollution. It can avoid the deterioration of the extract at high temperatures, protect the activity of the physiologically active substances in lutein, and maintain the natural flavor of the extract. This method has the advantages of simple process, low energy consumption, environmental friendliness, high product purity, positive hue, good heat and light resistance, and stable color [17-19]. Supercritical CO₂ fluid extraction technology has been successfully applied to the extraction and purification of natural food colors such as capsanthin, lycopene, β-carotene and gardenia yellow pigment.

2 Health benefits of lutein in marigolds

2.1 Anti-atherosclerotic effect

Recent research has shown that lutein has an anti-atherosclerotic effect on early stages of the process. This is mainly due to the relationship between changes in the thickness of the inner lining of the main artery and the amount of lutein in the blood. A low lutein level in the blood can easily lead to thickening of the arterial wall. As the lutein level gradually increases, the tendency for the arterial wall to thicken decreases, and arterial blockage is also significantly reduced. At the same time, lutein in the arterial wall cells can also reduce the oxidation of LDL cholesterol [20].

2.2 Anti-cancer and immune-enhancing effects

Studies have shown [21] that lutein has an inhibitory effect on various cancers such as breast cancer, prostate cancer, and rectal cancer. Lutein can inhibit the apoptosis of breast cancer mouse lymphocytes, while inducing the apoptosis of tumor cells, so that the mouse maintains a high immune status. A study on the proliferation of prostate cancer cells showed that lutein alone can reduce the growth rate of cancer cells by 25%, and if it acts in coordination with lycopene, the growth rate can be reduced by 32%. In vitro studies have found that lutein is more effective than β-carotene in inhibiting lipid peroxidation of cell membranes and inducing cell damage.

2.3 Antioxidant effect

As an antioxidant, lutein can inhibit the activity of reactive oxygen species and prevent it from damaging normal cells. Lutein can protect the body from harm by inactivating singlet oxygen through physical or chemical quenching. The latest research results show that lutein protects the skin. The natural antioxidant lutein can prevent skin damage caused by harmful solar radiation. The Riard Granstein research team found that lutein can prevent the negative effects of UV light on animal skin [22.23]. Therefore, adding a certain amount of lutein to food can prevent the aging of body organs. At the same time, lutein can be made into skin care products and cosmetics to protect the skin from damage.

2.4 Vision protection

Only lutein and zeaxanthin are specifically found in the human body in the macula lutea and lens. They are selectively deposited in the macula and throughout the retina, with the highest density around the fovea of the macula and gradually decreasing towards the periphery of the retina [24,25]. The main function of lutein in the eye is as an antioxidant and light protection. Studies have shown that increasing lutein intake can reduce the incidence of cataracts [26]. The German National Food Consumption Survey (NVS) stipulates that the total per capita consumption of carotenoids should be 5.33 mg/d, of which lutein accounts for 1.91 mg/d. Today, the domestic and foreign industries that extract lutein [27] use marigolds, which have a high lutein content, as raw materials and extract and refine them.

2.5 Coloring effect

Lutein powder  is non-toxic, has strong coloring power, a bright and vivid color, no peculiar smell, and a good taste, and is therefore widely used in health foods. In China, lutein is already used as a feed additive. Because of its good coloring power and stable and safe properties, lutein has been listed as a food coloring agent in Europe and the United States[2].

3 Development prospects

Lutein extracted from natural marigold flowers is a natural pigment with strong coloring power, high nutritional value and health-promoting properties. It is attracting increasing attention because of its various important physiological functions in the human body. Marigold, the main source of lutein, is also a precious plant with multiple effects. It also has very bright and broad application prospects in the pharmaceutical and food industries.

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