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想像一個溫熱、黏膩的餅乾

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爽脆的糖果

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細緻可口的蛋糕

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甜筒上堆起的冰淇淋球

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讓你口水直流了嗎？

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讓你想吃點心了嗎？

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為什麼？

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是什麼讓你的腦子

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無法抗拒含糖食物呢？

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糖是對

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碳水化合物分子的

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總稱

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而它們存在許多食物、飲料中

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只要看看你買甜食上的標簽

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葡萄糖

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果糖

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蔗糖

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麥芽糖

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乳糖

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右旋糖

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和澱粉

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都是不同形態的糖類

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果糖含量高的糖漿

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果汁

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粗糖

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和蜂蜜

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而且糖類不只在糖果、點心中

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它也被加到番茄醬

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優格

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果乾

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調味水

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和燕麥棒中

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既然糖類無所不在

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我們更要了解

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糖對大腦的影響

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當糖沾上舌頭時
會造成什麼反應呢？

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吃一點點糖

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會不會讓你想吃更多？

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你吃一點麥片

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其中所含的糖

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刺激了舌頭味蕾上的

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甜味受器

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受器向大腦傳送訊號

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在那訊號分支傳送到

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前腦中的不同區域

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其中一個就是大腦皮層

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大腦皮層中的不同分區

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處理不同味覺訊號

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苦味

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鹹味

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鮮味

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和我們討論的甜味

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訊號在這裡活化

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大腦的報償系統

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這個報償系統是一系列

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電子與化學的傳遞鏈

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橫跨大腦不同區域

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它是一個複雜的網絡

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但是它可以回答一個簡單
潛意識裡的問題

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我要重複這個動作嗎？

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在品嘗奶奶巧克力蛋糕食

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你這種溫暖、放鬆的感覺

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這就是你的報償系統說：

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「我還要！」

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而且它不只可由食物啟動

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社交

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性行為

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和藥物

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只是其中幾件

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事情或經驗

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可以啟動報償系統的例子

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但是過度活化報償系統

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會造成許多不良後果：

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失控

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渴望

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和對糖類耐受性增加

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讓我們回到剛所講的麥片

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它向下進入到你的胃

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再到達你的腸子

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誰料的到呢？

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腸子也有糖類受器

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它們不是味蕾

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但也能傳送訊號

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告訴你的腦說
你已經吃飽了

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或告訴身體需要製造更多胰島素

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去處理你多吃的糖

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報償系統中的

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主要貨幣是多巴胺

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它是重要的神經傳導物質

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前腦中有很多多巴胺受體

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但是分佈並不平均

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特定地方有密集、叢聚的受體

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這些是多巴胺的熱點

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也是報償系統的一部份

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藥物像是酒精

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尼古丁

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或海洛因

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會造成多巴胺受體超載

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造成一些人持續追求這種快感

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也就是成癮

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糖經由一種較藥物和緩的機制

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促成多巴胺的釋放

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有些食物含糖量較低

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例如：花球甘藍就沒有作用

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這或許可以解釋

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為什麼很難叫小孩吃青菜

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說到健康食物

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假設你餓了

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而且決定要吃一餐

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你這麼做後
多巴胺量激增

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刺激報償系統的熱點

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但如果你一再吃同一道菜

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多巴胺量會越來越低

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最後和平常無異

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因為當刺激來自食物時

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大腦演化出特別的機制

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關注新、或不同的味道

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為什麼呢？

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有兩個原因

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一、確定食物是否過期

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二、因為如果我們的

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飲食越多樣化

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我們就更有可能

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得到所有所需營養

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為了保持多樣性

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我們需要辨別新食物的能力

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更重要的是

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我們必須想吃新食物

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這就是為什麼食物相同時

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多巴胺變化就不明顯

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回到我們所說的那一餐

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如果將健康、平衡的飲食

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以高糖份的食物代替

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會發生什麼事呢？

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如果你很少吃糖

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或一次吃的不多

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它的影響與均衡飲食相同

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如果你吃過量

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多巴胺改變並不會逐漸下降

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換句話說，吃大量的糖

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會繼續使人
有被回饋的感覺

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這麼說來，糖就像毒品一樣

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這是為什麼有人會對

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高糖食物成癮的原因

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想想各種不同的糖

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每種都各有不同

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但是吃進不同的糖

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都會刺激腦中的多巴胺釋放

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造成被回饋的感覺

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吃太多或太頻繁

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都會造成超載

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所以過量攝取糖類

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也會造成大腦的成癮

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但是偶而吃塊蛋糕無傷大雅