Fats: Required, but not too Much

Crossposted at DailyKos.com

There is lot of confusion about fats (technically, esters of glycerol and three fatty acids, hence the term “triglycerides”). All true fats are esters of glycerol with three fatty acids, but mono and diglycerides are encountered as emulsifying agents in many processed foods. The reason is that glycerol can hydrogen bond, and fully substituted glycerol can not. Thus, mono and diglycerides are emulsifying agents. There is nothing inherently wrong with them, it is the fatty acids that present the problem. Typical fatty acids are from around 16 to 22 carbons long, and usually even numbers of carbon are involved due to the biosynthetic pathways.

Fatty acids occur in two main forms, saturated and unsaturated. Unsaturated fats have several variants that I will discuss further. First, though, the difference between saturated and unsaturated fats: saturated fats contain zero carbon-carbon double bonds, whilst unsaturated ones contain at least one. All fats contain a carbon-oxygen double bond, and a carbon-oxygen single bond.

Saturated fats tend to come from animal products, such as butter, lard, tallow, and so forth. However, two common vegetable fats are highly saturated: palm oil and palm kernel oil. Saturated fats are more stable towards air oxidation than are unsaturated ones, and tend to be solid or semisolid at room temperature. The problem with saturated fats is that they tend to elevate the blood levels of triglycerides and bad cholesterol, so should be consumed in moderation. Actually, all fats should be eaten in moderation, but that is another story.

Unsaturated fats come in several varieties. There are the monounsaturated ones, with olive oil being the prime example. That means that the fatty acids esterified to the glycerol have one carbon-carbon double bond. These are thought to be healthy, especially when they crowd out less healthful fats. Olive oil is about 14% saturated, 14% polyunsaturated, and 72% monounsaturated. I will describe polyunsaturated fats presently.

A polyunsaturated fat in a glyceryl ester of fatty acids with predominately two or more carbon-carbon double bonds. For instance, my bottle of soy oil is about 14% saturated, 21% monounsaturated, and 57% polyunsaturated. Soy oil gets a bad rap. It is really one of the more healthy oils, with no more saturated fat than olive, and lots of polyunsaturated fat.

My bottle of canola oil (also known as rapeseed oil, from a mustard) is about 7% saturated, 64% monosaturated, and about 29% polyunsaturated. It is a healthful oil, but it just has a “whang” about it that I do not like. I generally use a mixture of half and half canola and soy.

The degree of unsaturation can go even further. For instance, linseed oil and tung oil have on average three carbon-carbon double bonds and react with atmospheric oxygen to form a stiff, hard coating used on wood as a preservative. These are amongst the so called “drying oils” in that they harden up very quickly once exposed in a thin film to air. They are also healthful, well at least linseed is, but go rancid very fast unless stabilized. Since those are most often used in wood finishing, I would not recommend consuming them except for gel caps from the health food aisle.

I will not consider the omega three oils tonight, because they are a topic of their own. I will make a fair treatise on the so called “trans” fatty acids. First, I will say that they occur in trace amounts in many animal fats, but in very low amounts indeed. You would have to eat a whole lot of butter or suet to get a gram of trans fat from it.

The fatty acid component of natural fatty acids, except for the little bit in animal products, are what is called “cis” in chemistry. Carbon-carbon double bonds have no free rotation along the axis, so it is possible for two isomers otherwise identical in formulae can have two different configurations, with either to two big groups on the same side of the plane of the bond (cis, or “Z”, from the German “zusamman”, on the same side) of the big groups can be on opposite sides of the double bond (it has a spatial direction of 120 degrees) called “trans” or, in German, “entgegen”, on opposite sides.

It was not recognized until quite recently why this is important. It turns out that our enzyme systems evolved with the “cis” fats (interestingly, the less thermodynamically stable one) and metabolize them well. The “trans” ones are more difficult to process, and tend to be incorporated into the system that produce the bad cholesterol, the LDL (low density lipoproteins). Why do we consume them, then?

It turns out that what was thought to be enlightened science came to our rescue in the 1920′s or so. They found that if soy, or other oil, was subjected to high pressure under hydrogen with a finely divided nickel catalyst, the liquid oil would take on some of the hydrogen and thus convert carbon-carbon double bonds into single ones, thus increasing the stability and shelf life of the product. It was only the pioneering work of folks like Professor Samuel Siegel at the University of Arkansas was it discovered that there was a byproduct, and that is that not all of those “cis” fats were saturated, and the more stable thermodynamic state is the unhealthy “trans” configuration. I had the honor of taking a course or two from Professor Siegel, and I wish him good rest.

I will stick around a bit for Q and A. Sorry not to respond yesterday, I was extremely ill all day. Warmest regards, Doc.

Tags: Fats, Oils, Saturated, Teaching, Trans fat, Unsaturated