周 洋 彭 艷 周小秋,3*

(1.四川農業(yè)大學動物營養(yǎng)研究所，成都 611130；2.上海美農生物科技股份有限公司，上海 201807；3.動物抗病營養(yǎng)教育部重點實驗室，成都 611130)

在集約化養(yǎng)殖過程中，動物易受到各種不良因素，如飼糧中致病微生物、抗營養(yǎng)因子的刺激，發(fā)生應激反應，導致消化道氧化損傷，引起炎癥反應，發(fā)生腹瀉等腸道疾病，最終導致動物生長受阻甚至死亡。因此，增強動物抗氧化功能和腸道健康，提高動物機體的免疫力對保證其健康生長至關重要。植物精油(essential oil，EO)是植物次生代謝產物，可由植物各個器官合成，具有一定揮發(fā)性。近年的研究發(fā)現植物精油具有抗氧化、緩解炎癥和促進動物生長發(fā)育的作用。研究發(fā)現，飼糧中添加植物精油能夠提高小鼠腎臟[1]和肉雞空腸[2]抗氧化能力，提高虹鱒(Oncorhynchusmykiss)機體免疫能力[3]，降低仔豬腹瀉[4]，促進動物生長。本文就植物精油在動物體內的消化吸收和代謝、促進動物生長及增強機體免疫力的機制進行綜述。

1 植物精油的種類、吸收與代謝

1.1 種類

植物精油是一類植物重要的次生代謝產物，分子質量較小，有特殊芳香味，常溫下多為油狀液體，具有一定揮發(fā)性。已知的植物精油有3 000多種，成分復雜，多為幾十種物質的混合物，主要包括萜烯類化合物(terpenes)、芳香族化合物(aromatic derivatives)、脂肪族化合物和含氮含硫類化合物4類基本成分，目前研究較多的幾種主要的植物精油見表1。

1.2 吸收途徑及代謝產物

少量研究發(fā)現植物精油在動物胃腸道和皮膚都可被迅速吸收，在體內的代謝迅速，主要經腎臟代謝后24 h內以尿液形式排出[6-7]。研究發(fā)現，在仔豬和大鼠采食含植物精油的飼糧2 h內，仔豬血漿中百里香酚、香芹酚和丁香酚含量以及大鼠腎臟和肝臟中D-檸檬烯含量均達到峰值[8-9]。進一步研究發(fā)現，飼糧中百里香酚和香芹酚主要在斷奶仔豬小腸前端吸收[9]。植物精油快速在胃腸道前端吸收，并且迅速代謝出體外，但微囊包被香芹酚2 h內在模擬胃液中僅有低于20%的部分被降解；在模擬腸道液體中2 h釋放量超過60%，在6 h內幾乎完全被釋放[10]。以上結果說明，植物精油能快速被動物胃腸道前端吸收，而包被的植物精油在胃腸道后端釋放。另外，研究發(fā)現植物精油在哺乳動物皮膚也可被迅速吸收。在人[11]和鼠[12]上的研究發(fā)現，涂抹植物精油10 min后血漿中的植物精油含量達到最大值，且經皮膚吸收的量與植物精油濃度、皮膚特性、精油接觸時間和接觸面積有關[8]。

表1 幾種主要的植物精油

不同植物精油在不同動物上的代謝產物有一定差異(表2)。在大鼠上的研究發(fā)現，百里香酚與香芹酚在體內代謝后以硫酸鹽、葡萄糖苷酸鹽和不發(fā)生降解的原始形態(tài)等形式代謝出體外[6]。在兔和人上百里香酚的代謝產物則為硫酸鹽或百里氫醌[7]。反式肉桂醛在小鼠上的代謝產物為馬尿酸和巰基尿酸[13]。茴香腦在大鼠和小鼠上代謝產物則是以與硫酸根、葡萄糖醛酸、甘氨酸和谷胱甘肽結合的形式代謝出體外[14]。

表2 幾種植物精油的代謝產物

2 植物精油對動物生長的影響及作用途徑

2.1 促進動物生長

2.2 促進動物生長的作用方式及初步機制

植物精油可能通過提高消化吸收能力促進動物生長。動物生長與消化吸收能力密切相關，消化酶活性能夠反映其消化能力。研究發(fā)現，飼糧中添加適宜水平的百里香酚和肉桂醛提高了肉雞腸道胰蛋白酶和淀粉酶活性[21-22]。腸道生長發(fā)育與其消化能力密切相關。研究發(fā)現，飼糧中添加牛至和柑橘精油提高了肉雞腸絨毛高度[18]，減少了凡納濱對蝦腸絨毛脫落[17]。以上研究說明，植物精油可能通過促進腸道的生長發(fā)育促進其消化吸收能力。Tiihonen等[23]的研究發(fā)現，飼糧中添加百里香酚和肉桂醛能夠增加肉雞盲腸丁酸的相對含量；而Tian等[24]的研究表明，丁酸能夠提高草魚腸道消化酶活性并促進腸絨毛生長。以上結果表明，飼糧中添加植物精油可能通過提高腸道中丁酸的含量促進動物腸道發(fā)育，提高消化吸收能力，從而促進生長。

腸道微生態(tài)系統(tǒng)是影響動物消化吸收能力的重要因素，腸道菌群平衡有助于提高動物的消化吸收能力[25]。研究發(fā)現，在仔豬[4]和雞[26]飼糧中添加適宜水平的百里香酚和肉桂醛等提高了腸道乳酸桿菌的數量，降低了大腸桿菌數量；牛至和大蒜精油降低了肉雞腸道梭狀芽孢桿菌和鏈球菌數量[27]。以上結果表明，植物精油能通過促進腸道有益菌生長，抑制有害菌生長，改善腸道菌群平衡，從而保證其消化吸收能力的發(fā)揮。

由此可見，植物精油可能通過提高動物腸道消化酶活性、促進腸道生長發(fā)育、維持腸道菌群平衡提高消化吸收能力并促進動物生長。

3 植物精油對動物免疫力的影響及其作用途徑

3.1 對動物免疫力的作用

3.2 影響動物免疫力的作用途徑

3.2.1 通過緩解炎癥反應提高動物免疫力

炎癥反應是一種重要的免疫應答，過度的炎癥反應會損害動物健康[29]。細胞因子是介導炎癥反應的主要因子，包括促炎細胞因子[如腫瘤壞死因子-α(TNF-α)、白細胞介素-1β(IL-1β)、白細胞介素-6(IL-6)等]和抗炎細胞因子[如白細胞介素-10(IL-10)]等，主要由免疫細胞分泌產生[30]。研究發(fā)現，飼糧中添加香芹酚等植物精油緩解了肉雞[31]及小鼠[32]腸道炎癥，并降低了斷奶仔豬腹瀉指數[4]。進一步的研究發(fā)現，百里香酚和肉桂酸能分別降低小鼠乳腺上皮細胞[33]和單核巨噬細胞[34]中促炎細胞因子TNF-α和IL-6的含量；濱艾精油能夠降低小鼠巨噬細胞IL-1β、TNF-α和IL-6的基因表達[35]。以上結果表明，植物精油能通過降低促炎細胞因子含量來緩解動物炎癥反應，促進動物健康。

核轉錄因子-κB(NF-κB)是調控細胞因子或其他炎癥反應相關基因表達的重要轉錄因子[36]。研究發(fā)現，百里香酚和佛手柑精油降低了小鼠乳腺上皮細胞[33]和小鼠巨噬細胞[37]NF-κB p65磷酸化水平，并可通過抑制小鼠乳腺上皮細胞中NF-κB的激活降低促炎細胞因子的產生[33]。在正常生理條件下，NF-κB在細胞質與其抑制因子核因子κB抑制因子(IκB)綁定，但是當受到刺激時，IκB發(fā)生磷酸化并降解，導致NF-κB p65磷酸化調控相關基因的表達[36]。在HeLa細胞上，IκB激酶(IKK)的激活使IκBα磷酸化，磷酸化后的IκBα與NF-κB脫離，使NF-κB由抑制變?yōu)榧せ頪38]。研究發(fā)現，百里香酚能夠降低小鼠乳腺上皮細胞[33]和小鼠巨噬細胞[37]IκBα磷酸化水平；檸檬醛能夠降低小鼠單核巨噬細胞IKK蛋白表達[39]。這說明植物精油可能通過IKK/IκBα/NF-κB p65信號途徑調控動物的炎癥反應，但具體機制有待進一步研究。

此外，絲裂原活化蛋白激酶(MAPK)家族包括細胞外信號調節(jié)激酶(ERK)、c-jun氨基末端激酶(JNK)和p38絲裂原激活蛋白激酶(p38)3個主要成員，在炎癥反應中能夠調控細胞因子產生[40]。研究表明，炎癥發(fā)生時百里香酚能夠降低小鼠乳腺上皮細胞JNK、ERK和p38的磷酸化水平[33]，肉桂醛能夠降低小鼠巨噬細胞炎癥發(fā)生時JNK、ERK和p38的磷酸化水平[34]，而佛手柑精油能夠降低脂多糖(LPS)誘導下小鼠巨噬細胞ERK和JNK的磷酸化水平[37]。這表明植物精油還可能通過MAPK信號途徑調控炎癥細胞因子產生，進而緩解炎癥反應，提高動物機體的免疫力，但還需要開展更加系統(tǒng)深入的研究。

3.2.2 通過增強抗氧化功能提高動物免疫力

4 小 結

綜上所述，植物精油能夠通過提高動物消化吸收能力和增強機體免疫功能促進動物生長。植物精油提高動物消化吸收能力與其促進腸道生長發(fā)育和改善腸道微生態(tài)平衡有關；動物免疫力的提高與其提高免疫物質含量、緩解炎癥反應以及提高抗氧化損傷能力相關。植物精油作為一種綠色安全的飼料添加劑已經被廣泛應用到生產中，具有廣闊的應用前景，但目前的研究較少且較缺乏具體作用機制研究，需要進一步開展更加系統(tǒng)深入的研究。

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*Corresponding author, professor, E-mail: zhouxq@sicau.edu.cn