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Olive oil is 100% fat; there's nothing else in it.
橄榄油含有100%的脂肪，除此之外没有别的。
Pancake mix, on the other hand, is only about 11% fat.
相反，煎饼只有约11%的脂肪。
And, yet, olive oil is good for you, and pancake mix is not.
然而，橄榄油对你的身体有益，煎饼则不然。
Why is that?
为什么会这样呢？
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.
原来，我们摄入的脂肪含量并不影响我们的体重、胆固醇，或得心脏病的机率，这些只跟我们摄入的脂肪种类有关。
But let's back up: What is fat?
不过我们先回过头来看看：什么是脂肪？
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.
如果我们把三文鱼放大，它是一种富含脂肪的鱼，忽略掉内脏和组织只看细胞的话，我们就会看到脂肪，它其实是是由一种叫做三酸甘油酯的分子组成的，而这些分子并不完全相似。
Here's one example. Those three carbons on the left, that's glycerol.
举个例子。左边的那三颗碳分子是甘油。
Now, you can think of that as the backbone that holds the rest of the molecule together.
你可以把它想像成骨干，能够把其它分子紧紧连在一起。
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.
右边的那三条长链被称为脂肪酸，这些分子链结构的微妙差别决定了脂肪种类。比如，它们是固体还是液体，是否会很快变质，最重要的是，对你是有益还是有害。
Let's take a look at some of these differences.
让我们来看看这些差别。
One is length. Fatty acids can be short or long.
一方面是长度。脂肪酸可长可短。
Another more important difference is the type of bond between the carbon atoms.
另一个更重要的区别是连接碳原子的碳键类型。
Some fatty acids have only single bonds.
有些脂肪酸只有单键。
Others have both single and double bonds.
另外一些则同时拥有单键和双键。
Fatty acids with only single bonds are called saturated, and those with one or more double bonds are called unsaturated.
只有单键的脂肪酸叫做饱和脂肪酸，有单个或多个双键的脂肪酸则为不饱和脂肪酸。
Now, most unsaturated fats are good for you, while saturated fats are bad for you in excess.
大部分的不饱和脂肪酸对人体有益，而过多的饱和脂肪酸则是有害的。
For saturated fats, the story pretty much ends there but not for unsaturated fats.
就饱和脂肪酸而言，结果很简单，但不饱和脂肪酸不同。
The double bonds in these molecules have a kind of weird property; they're rigid.
双键在这些分子中有一个奇怪特性，它们有十分坚固的刚性结构。
So, that means there are two ways to arrange every double bond.
这意味着双键排列有两种。
The first is like this, where both hydrogens are on same side and both carbons are on the same side.
第一种就像这样，两颗氢都在同一边，两颗碳也在同一边。
The second way is like this.
第二种方式像这样。
Now the hydrogens and carbons are on opposite sides of the double bond.
氢和碳在双键对立的两端。
