As some of you know, I've been in biochemistry hell this summer, and TFAs are one of the things that were covered in the course. Here's what my notes say about them:

Fatty acids are long-chain carboxylic acids. If they are saturated it means they are saturated with hydrogens, and there are no double or triple bonds in the molecule. (this is basic organic chemistry). Palmitic acid, steric acid are examples of saturated fatty acids. Obviously, then, an unsaturated fat or acid is NOT completely saturated with hydrogen and has at least one double bond in it. Oleic acid would be an example of an unsaturated acid. Linoleic acid has two double bonds. The double bonds are usually "cis" in formation, and are often three units apart on the carbon chain of the molecule. When we partially hydrogenate an unsaturated acid, it leads to cis/trans isomerization, and we end up with trans conformations, which give us trans-fatty acids. As anyone who has taken basic o-chem knows, cis and trans molecules look similar but behave very differently. The melting point of a cis-isomer is lower than a trans isomer, because the cis conformation puts a "kink" in the molecule and prevents tight packing. The trans conformation packs more easily, more energy must be applied to disrupt this good packing, and hence, trans isomers are more solid and have higher melting points than cis isomers.

Basically, what I take from all of this is that it's not nice to fool Mother Nature -- when we partially hydrogenate, we screw up the conformation of the molecule in the aforementioned way and get a molecule that behaves very differently.

Can meat and dairy and oils have TFAs...? Yes, because animals can be fed foods with TFAs in them. And as I understand it, sometimes it just happens. (without our help) But if we avoid refined foods and fats, we should be in pretty good shape.

From this class and my reading, I tend to believe that going crazy on saturated fats is not a great idea. Cholesterol, for example, is a flat, rigid molecule that has little flexibility. When it gets into our cell membranes, this rigidity can cause problems with diffusion and fluidity in the membrane. The same tight packing mentioned before decreases membrane fluidity. Cholesterol molecules are big, so they slow things down...things kind of have to move around it!

OK, that's enough biochem for one day...I can see your eyes glazing over.