Olive oil is 100% fat; there's nothing else in it. Pancake mix, on the other hand, is only about 11% fat. And, yet, olive oil is good for you, and pancake mix is not. Why is that? As it turns out, the amount of fat we eat doesn't impact our weight or our cholesterol or our risk of heart disease nearly as much as what kind of fat we eat. But let's back up: What is fat? If we were to zoom in on a salmon, which is a fatty fish, past the organs, past the tissues, into the cells, we would see that the stuff we call fat is actually made up of molecules called triglycerides, and they are not all alike. Here's one example. Those three carbons on the left, that's glycerol. Now, you can think of that as the backbone that holds the rest of the molecule together. The three long chains on the right are called fatty acids, and it's subtle differences in the structures of these chains that determine whether a fat is, let's say, solid or liquid; whether or not it goes rancid quickly; and, most importantly, how good or how bad it is for you. Let's take a look at some of these differences. One is length. Fatty acids can be short or long. Another, more important difference is the type of bond between the carbon atoms. Some fatty acids have only single bonds. Others have both single and double bonds. Fatty acids with only single bonds are called saturated, and those with one or more double bonds are called unsaturated. Now, most unsaturated fats are good for you, while saturated fats are bad for you in excess. For saturated fats, the story pretty much ends there but not for unsaturated fats. The double bonds in these molecules have a kind of weird property; they're rigid. So, that means there are two ways to arrange every double bond. The first is like this, where both hydrogens are on same side and both carbons are on the same side. The second way is like this. Now the hydrogens and carbons are on opposite sides of the double bond. Now, even though both of these molecules are made up of exactly the same building blocks, they are two completely different substances, and they behave completely differently inside of us. The configuration on the left is called CIS, which you've probably never heard of. The one of the right is called TRANS, and you probably have heard of trans fats before. They don't go rancid, they're more stable during deep frying, and they can change the texture of foods in ways that other fats just can't. They're also terrible for your health, by far worse than saturated fat, even though technically they're a type of unsaturated fat. Now, I know that seems crazy, but your body doesn't care what a molecule looks like on paper. All that matters is the 3-D shape where the molecule fits, where it doesn't, and what pathways it interferes with. So, how do you know if a food has trans fat in it? Well, the only sure way to know is if you see the words, "partially hydrogenated" in the ingredients list. Don't let nutrition labels or advertising fool you. The FDA allows manufacturers to claim that their products contain "0" grams of trans fat even if they actually have up to half a gram per serving. But there are no hard and fast rules about how small a serving can be, and, that means, you'll have to rely on seeing those key words, partially hydrogenated, because that's how trans fats are made, by partially hydrogenating unsaturated fats. So, let's go back to our olive oil and pancake mix from before. Olive oil is 100% fat. Pancake mix is only 11% fat. But olive oil is mostly unsaturated fat, and it has no trans fat at all. On the other hand, more than half the fat in pancake mix is either saturated or trans fat. And, so, even though olive oil has 10 times as much fat as pancake mix, it's healthy for you, whereas pancake mix is not. Now, I'm not trying to pick on pancake mix. There are lots of foods with this type of fat profile. The point is this: It's not how much fat you eat, it's what kind of fat. And what makes a particular fat healthy or unhealthy is its shape.
橄欖油是 100% 的脂肪 除此之外,沒有別的 鬆餅,相反地 只有大約 11% 的脂肪 然而,橄欖油對你有益 而鬆餅則不然 為什麼? 因為 我們所吃的脂肪量 並不會影響我們的體重 或是我們的膽固醇 又或是我們得到心臟病的機率 只和我們吃進哪一種脂肪有關 不過我們先回過頭來 什麼是脂肪? 如果我們放大檢視鮭魚 鮭魚是一種脂肪很多的魚 不看內臟、不看組織 不看內臟、不看組織 只看細胞 我們就會看到我們稱為脂肪的東西 其實是由一種稱為 三酸甘油酯的分子所組成 而且它們還不見得都一樣 舉個例子 左邊的那三顆碳,是甘油 你可以把它想像成支柱 能夠把其它的分子結合在一起 右邊的那三條長鏈 稱為脂肪酸 這些鏈有些微的不同 由此決定它們是什麼樣的脂肪 應該說,固體或是液體的 會不會很快腐臭 最重要的是,對你來說有多好或多壞? 讓我們看看有什麼不同 第一,是長度 脂肪酸可以是長的或短的 另外,最大的不同是 在碳原子之間是哪一種鍵 有些脂肪酸只有單鍵 也有同時有單鍵和雙鍵的脂肪 只有單鍵的 稱為飽和的脂肪酸 而有單鍵或更多雙鍵的 則稱為不飽和脂肪酸 大部份的不飽和脂肪酸 對你是有益的 而過多的飽和脂肪酸 對你則是有害的 對飽和脂肪酸來說,大概就是這個樣子 但是對不飽和脂肪酸來說 卻不是這麼一回事 雙鍵在這些分子中 有一種奇怪的特性 它們很堅固 因此,有兩種方式可以 連結雙鍵 第一種就像這樣 兩顆氫都在同一邊 兩顆碳也在同一邊 第二種方式像這樣 氫和碳,在雙鍵對立的兩端 氫和碳,在雙鍵對立的兩端 即使這兩種分子 都由同樣的東西組成 它們都還是會成為不同的物質 它們在我們的體內也會有不同的表現 左邊的這個結構稱為順式異構體 (CIS) 你可能從來也沒聽過 另一種在右邊的稱為反式異構體 (TRANS) 你以前可能有聽過反式脂肪 它們不會腐臭 它們在油炸時比較穩定 而且它們可以改變食物的口感 這是其它種類的脂肪無法做到的 而且它們也很不益健康 顯然比飽和脂肪酸要差得多 即使嚴格說來 它們也是不飽和脂肪中的一種 我知道聽起來很誇張 但是你的身體並不在乎 分子在紙上長的樣子 重要的是它在三度空間的形態 分子在哪裡結合 分子在哪裡結合 還有它所造成的影響 那麼,你要怎麼知道 食物裡有沒有反式脂肪呢? 嗯,唯一能夠確定的方法是 如果你在食品內容物中 看到「 部分氫化」的字眼 如果你在食品內容物中 看到「 部分氫化」的字眼 別讓營養標籤或廣告騙了你 食品藥物管理局 (FDA) 准許製造商 聲稱他們的產品包含 「0」克的反式脂肪 即使事實是每份的含量超過 0.5 克 但是沒有嚴格的規定 限制每一份最少要是多少 也就是說,你得依賴關鍵字 「部份氫化」 因為那是反式脂肪製成的方式 透過部份氫化製成反式脂肪 因此,讓我們回到先前 橄欖油和鬆餅的話題 橄欖油是 100% 的脂肪 鬆餅只有 11% 的脂肪 但是橄欖油幾乎都是不飽和脂肪 而且它完全不包含反式脂肪 相反地,鬆餅中有超過一半的脂肪 是飽和或反式脂肪 因此,即使橄欖油的脂肪是 鬆餅的十倍 它對你還是很健康 而鬆餅則不然 我並不是要找鬆餅的碴 有很多的食品 含有這種類型的脂肪 重點是 吃多少脂肪不重要 重要的是,你吃了什麼樣的脂肪 還有,是脂肪的形態 決定它是否健康