FAT! Yes, it is a damned scary word, and it should scare you too, because nobody knows what the hell it is. Fat, lipid count, triglycerides, HDL, cholesterol, LDL, fatty acids, omega-3, saturated and unsaturated...nobody has bothered to really explain what the hell all this meant to me, they are just numbers on a blood panel and plenty of people to tell you how bad some numbers are and how good others are, but no reasons why.
We'll start with the largest category, lipids. Lipids include all sorts of chemistry, fat, wax, alcohols in the sterol family, and a whole host of others. What we call "fat" is triglyceride. A triglyceride is three fatty acids bound together with something, usually a molecule of glycerol. Glycerol is an interesting compound in itself, having three dangling hydroxyls that allow the attachment of the three fatty acids, which makes triglycerides an ester molecule when fully formed. Glycerol is already hydrophobic, adding three more hydrophobic fatty acids works to make it even more, fatty like, and oily.
Almost any fatty acid will do to make a triglyceride, you only need three. And there are a bunch of different fatty acids broken up into, broadly, two groups, saturated and unsaturated. So what is saturation to begin with?
In organic chemistry, a molecule is said to be saturated when there are no double bonds in place for all the carbon atoms present. Any space that a double bond can occur is replaced with a hydrogen, so generally the saturation amount of a particular molecule describes how many hydrogens that a particular substance can pick up. Fatty acids are long chains of carbon to begin with, so unsaturated fats are triglycerides which have at least one double bond connecting the carbon atoms together that make up the fatty acids. Monounsaturated fats have one double bond, polyunsaturated fats have more than one double bond. Saturated fatty acids don't have any double bonds at all, they have all been taken up by hydrogen atoms and are connected only by single bonds.
Why the hell would the presence of hydrogen matter? Energy. Unsaturated fats contain less calories, and are easier to break up. Saturated fats are harder to break up and can persist longer, and release more energy (they are bound together with all those hydrogens pretty hard) when they finally get oxidized. Interestingly enough, this is the exact reason hydrogenation is used in oils and fats for foods in the first place. By adding all these hydrogens the chemistry of the fatty acids are changes, making them more resistant to oxidation and by changing the binding properties of the fats and oils themselves. Hydrogenation makes the oils firmer and gives them a higher smoke point, and stay fresh longer because of this resistance to oxidization. So you might think that you would want all your fat to be polyunsaturated, but there is one more problem with fats than just how many hydrogens and double bonds: stereochemistry.
Molecules are described not only by their constituent atoms, but also by how they are put together. Simple molecules like water and salt bind together the exact same way every time, but larger configurations can have multiple arrangements. These different substances, the same in composition, but different in structure, are isomers. There are different kinds of isomers, and just because it may be composed of the same atoms, they may have vastly different reactions because of the shape. In this case, we're concerned with cis-trans isomers, and I can explain trans fatty acids and why everyone is scared of them.
This is oleic acid, found in the fats of olive oil, and is a cis fatty acid.
This is elaidic acid, a trans isomer, generated during the hydrogenation process of making cooking oils: