為了對抗有如恐怖份子的身體脂肪,美國人可歌可泣的戰鬥罊竹難書,而且還屢戰屢敗。為何一個人類成千上萬年求之難得的能量資源,今天卻這麼難以擺脫呢?雖然市面上已有汗牛充棟的書籍談論如何擺脫討人厭的體脂,但是有用嗎?或許我們迄今其實根本還未徹底瞭解這個敵人,所以才無法知己無彼而百戰全殆?生物化學博士席薇亞.塔拉(Sylvia Tara)的這本好書《脂肪的祕密生命:最不為人知的器官脂肪背後的科學與它對身體的影響》(The Secret Life of Fat),就是要帶領大家先來認識一下全民公敵,引用了大量最新的科學研究證據,讓你會對脂肪,有全新的認識。
來自馬來西亞,畢業於台灣國立清華大學生命科學系學士暨碩士班,以及美國加州大學戴維斯分校(University of California at Davis)遺傳學博士班,從事果蠅演化遺傳學研究。曾於台灣中央研究院生物多樣性研究中心擔任博士後研究員,現任教於國立清華大學分子與細胞生物學研究所,從事鳥類的演化遺傳學、基因體學及演化發育生物學研究。過去曾長期擔任中文科學新聞網站「科景」(Sciscape.org)總編輯,現任台大科教中心CASE特約寫手、Readmoo部落格【GENE思書軒】、關鍵評論網專欄作家;個人部落格:The Sky of Gene;臉書粉絲頁:GENE思書齋。
在棕色脂肪細胞中,也發生了和電影院一樣的離奇故事。棕色脂肪細胞中的粒線體就像電影院,在它的粒線體內膜上,存在許多特殊的蛋白 UCP1(Uncoupling protein 1),它是一種載體蛋白,可以將氫離子從膜間腔(Intermembrane space)運輸到粒線體基質(Mitochondrial matrix)中。一般來說,粒線體是細胞的發電廠,可以透過呼吸作用來產生能量 ATP,而這中間經歷了重重關卡。首先,葡萄糖經過糖解作用形成丙酮酸;丙酮酸轉變成乙醯輔酶 A 後進入檸檬酸循環,產生高能分子;最後由這些高能分子提供電子,進入電子傳遞鏈(Urry et al., 2017)。棕色脂肪產熱就是和最後一個步驟:電子傳遞鏈有關。
傳遞電子的過程發生在粒線體內膜上。過程中,粒線體內膜上的蛋白會將電子傳給能階較低的蛋白,逐步釋放高能電子中的能量,作為將氫離子從基質主動運輸至膜間腔的動力,膜間腔氫離子濃度因此升高,建立起氫離子的電化學梯度(Electrochemical gradient)。在一般狀況下,高濃度的氫離子就會從粒線體內膜上的 ATP 合成酶流入,產生 ATP。此時,ATP 合成酶就好像粒線體這間電影院的正門,顧客從正門進入,為電影院帶來收益,也就是 ATP。
當你現在有能力專注閱讀這篇電子報,我就必須告訴你:很遺憾的,你的棕色脂肪已經退化許多啦!一生中棕色脂肪最旺盛的時候是嬰幼兒時期,因為剛出生時,進入比媽媽身體的 37℃ 還要冷的世界中,需要產生大量熱量來維持體溫;再加上嬰兒的肌肉不發達,無法藉由顫抖來產生熱量,身體的表面積/體積比又遠高於成人,熱量易散失(Lidell et al., 2018)。因此,豐富的棕色脂肪是支持寶寶活下去的重要組織!成年人也有棕色脂肪,但比小時候還要少許多,分布在頸部、鎖骨上方、肩胛骨、脊椎旁和腎臟周圍(Zoico et al., 2019) (圖四)。
雖然我們的棕色脂肪漸漸退化了,但還是有些因素能刺激棕色脂肪產生,或是藉由提高粒線體與 UCP1 含量來增進棕色脂肪效能!像是長時間處在寒冷環境下,就能提升棕色脂肪效率、誘導米色脂肪產生!也有研究指出,食用特定食物也能提升棕色脂肪效能,例如辣椒中的辣椒素、茶葉中的兒茶素(Yoneshiro et al., 2017)都有類似效果!
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