Thank you for a very good question and one that's confusing to many people. In the world of food science, there is one very large umbrella category that includes both cholesterol and fat. That category is called "lipids." Unfortunately, this word isn't used much in everyday conversation. "Lipids" are defined as organic compounds that do not dissolve in water and that include fats, oils, waxes, sterols (including cholesterol), and triglycerides. Even though "lipids" is the correct scientific term for describing both fats and cholesterol, it's fairly common for people to say "fats" when they actually mean "lipids." So from this perspective, we think it would be fine to say that cholesterol is a type of fat.
Cholesterol is a very unique type of fat, however. Many types of fats have a fairly simple chemistry. For example, fatty acids (one very important kind of fat) are basically straight chemical chains. Cholesterol is more chemically complicated. Not only does it have chemical rings in its structure, but these ring structures have to occur in a very particular configuration. It's fairly complicated for our bodies to make this unique type of fat.
In a practical, dietary sense, there is one very basic difference between cholesterol and other fats. Most of the time when we talk about fat in food, we are talking about fairly large amounts of food parts that contain large amounts of calories. It's not uncommon for a fast food meal to contain over 50 grams of fat. These 50 grams of fat provide about 450 calories. This situation is very different from cholesterol. We almost never consume a food that has more than 1 gram of cholesterol, and we never get any significant number of calories from cholesterol. So in this respect, cholesterol is very different that other types of food fat. However, just because we don't need to worry about cholesterol in terms of excess grams and excess calories doesn't mean it isn't a type of fat with potential health risk. Excess intake of cholesterol can be a health problem in terms of our cardiovascular health, even though we now know that there is no simple relationship between dietary cholesterol and risk of cardiovascular disease, and that it takes more dietary cholesterol than we once thought to pose significant problems.
译文:
胆固醇和脂肪的区别
谢谢你提了一个很好而且很多人都会混淆的问题。在食品科学界里，有一个很广的类别包括了胆固醇和脂肪，这个类别称为“脂类”，不幸的是这个词很少用在日常对话中。“脂类”被定义为不能溶解在水中的有机混合物，它包括脂肪、油、蜡、固醇（包括胆固醇）和甘油三酯。即使“脂类”是用来描述脂肪和胆固醇的正确科学术语，人们时常用“脂肪”这个词，但实际上他们说的是“脂类”。如果从这个角度分析，我们把胆固醇说成是一种脂肪也没什么。
很多脂肪都有一种较简单的化学成份，比如，脂肪酸（一种很重要的脂肪）基本上是直化学链。但是胆固醇是一种很独特的脂肪，它的化学成份更复杂。不光是其化学结构包括了环形，而且这些环形结构必须以一定的排列形式出现，对我们的身体来说制造这种独特的脂肪是相当复杂的。
尤其从膳食方面来讲，在胆固醇和其它脂肪之间有一个最基本的区别。大多数时候当我们谈论食物中的脂肪，我们谈的是那些含有大量热量的食物。对于一顿快餐来说含50克的脂肪是很正常的事，这50克的脂肪可以提供450卡卢里的热量，这种情况和胆固醇是完全不同的。我们几乎从来吃不到胆固醇含量超过1克的食物，而且我们从不会从胆固醇中吸收到很多的热量。从这一角度来说，胆固醇和其它种类的食物脂肪是完全有区别的。但是，虽然我们不用担心过量的胆固醇摄入带来过多的热量，这不代表胆固醇不是一种可以潜在危害健康的脂肪。我们现在不了解在膳食胆固醇和心血管疾病之间的简单联系，而且会造成重大问题的膳食胆固醇量比我们曾想象的要多，但显然过量的胆固醇摄入会给我们的心脏血管健康带来影响。