Every day a typical American eats a lot of fat. We eat things like french fries that are deep fried in hot liquid fat. We eat meat that contains saturated animal fats. We use things like butter and mayonnaise that contain a lot of fat. We add liquid fats like corn oil to dishes and dressings.

Which brings up several good questions, like, what is fat? And why are certain fats “good” while other fats are “bad”? And what is an Omega-3 fatty acid, and why do we constantly hear about the importance of Omega-3s? Let’s find out.

A typical fat molecule looks like three molecules of diesel fuel hooked together by a little piece called glycerol (This is why fats are called triglycerides). Diesel fuel? That’s no joke. A molecule of diesel fuel is a hydrocarbon made up, say, of 12 carbon atoms in a chain and 26 hydrogen atoms arranged around the carbons. Triglycerides have three pieces that look almost exactly like diesel fuel, like this:

You may have heard of a fuel called biodiesel. It is made by stripping off the glycerol to free the three diesel fuel molecules. Biodiesel is just like diesel fuel made from crude oil, except that biodiesel does not contain any of the impurities (like sulfur) that make diesel fuel smell funny.

Bottom line: when we eat fat, we are eating something that is an awful lot like diesel fuel. Our bodies can use the fat immediately as fuel, or store the fat as fuel for future use. If someone is 30 pounds overweight, they essentially have 30 pounds of diesel fuel – roughly four gallons – in storage for future use.

Those three diesel fuel molecules in the triglyceride are called “fatty acids” because they have a little knob at one end that makes them acidic. And those fatty acid molecules come in several different forms. A fatty acid is said to be “saturated” if it has the maximum number of hydrogen atoms allowed along the carbon chain. Saturated fats are solid at room temperature and are commonly found in meat. Saturated fats are said to be “bad” because they have a greater tendency to stick to artery walls and clog them.

If a fatty acid does not contain a full complement of hydrogen atoms, it is said to be “unsaturated”. If it is missing just one hydrogen, it is said to be “mono-unsaturated”. Olive oil is a good example, and is said to be one of the healthiest fats you can eat. If a fatty acid is missing more than one hydrogen atom, the fatty acid is said to be “poly-unsaturated”. Unsaturated fats are liquid at room temperature, so we refer to them as oils.

If you have corn oil, which is poly-unsaturated, and you chemically add extra hydrogen to it, you create “partially hydrogenated vegetable oil”. This is a solid at room temperature, and is the main ingredient in margarine.

Now, with all this knowledge under our belts, we can talk about Omega-3 fatty acids. There are several different kinds of fatty acids that are called “essential fatty acids”. They are “essential” because our bodies has no way to make them internally. The only way for your body to get the essential fatty acids it needs is by eating them. Some of these essential fatty acids are called Omega-6 fatty acids, and the rest are called Omega-3 fatty acids. Both Omega-3 and Omega-6 fatty acids are poly-unsaturated, meaning that they are missing several hydrogen atoms. The 3 and the 6 refer to the position on the carbon chain where the first hydrogen atom is missing.

The reason why Omega-3s are important is because they are rare. Omega-6 fatty acids can be found in any oil, including corn oil, soybean oil, etc. To get Omega-3s, on the other hand, you have to work at it. Walnuts contain Omega-3, as do flax seeds and certain kinds of fish (e.g. salmon). In addition, you would like to have an even balance between Omega-3s and Omega-6s. But because Omega-3s are so rare, a typical American gets 20 times more Omega-6s than Omega-3s, and that is bad.

Heart disease is a leading killer in the United States. One cause of heart disease is the wrong kind of fats. The lack of Omega-3 fatty acids in the typical American diet is harmful. The reason why you hear people talking about Omega-3 fatty acids so much is because, if you get enough of them, it means better heart health.

See also: How Fats Work and: