The Facts About Fats

We all know we're not supposed to eat trans-fats, but the distinction between saturated fats, unsaturated fats, and trans-fats seem to be less understood.

Fats, or triglycerides, are three fatty acid carbon chains connected by a glycerol.

Glycerol, a fatty acid's carbon chain, and a triglyceride.

Saturated fats have a carbon chain with all single bond between the carbons. They're called "saturated" because they have the maximum number of hydrogens attached to the carbon chain. These chains look like zig-zags, and are easily packed close together. It is this property that makes saturated fats (for example butter and lard) solids at room temperature. The closer the chains are to each other, the more stable they are, and the more solid the fat is.

Three closely stacked stacked saturated fatty acid carbon chains.

If two neighboring hydrogens are taken away from the carbon chain, the fat is said to be unsaturated because it is missing hydrogens. The loss of hydrogens makes the molecule very slightly lower in calories. More importantly, the neighboring carbon atoms that are missing the hydrogens are now sharing a double bond. If this happens once in a carbon chain it is monounsaturated (meaning one, as in Monopoly or monotheistic), and if it happens multiple times in a chain it is said to be polyunsaturated (meaning multiple, as in polygon or polygamy).

Monounsaturated vs polyunsaturated.

Because these fatty acid chains have a double bond in them, they can be either cis (meaning on the same side) or trans (meaning across, as in transportation or transatlantic). Trans-fats maintain the zig-zag shape as seen above, but cis-fats appear to change direction, resulting in a kink in the carbon chain. This kink makes cis-fats harder to closely pack.

Three stacked fat carbon chains: saturated, trans-fats, & cis-fats.

The cis-monounsaturated fatty acid carbon chains are much further apart than either the saturated or trans-monounsaturated fatty acid carbon chains. Because of this, cis-fats are liquids at room temperature, while trans-fats (for example margarine) are solids or semi-solids. Trans-fats rarely occur in nature, and are instead produced by partially hydrogenating (or adding hydrogen atoms to) cis-fats in order to make the liquid fats solid. Partial hydrogenation removes some, but not all, of the cis double bonds, resulting in trans double bonds and a more solid product.

Cis-fatty acid carbon chains are partially hydrogenated into a mixture of
saturated & trans- fatty acid carbon chains.

Trans-fats were originally made because of shortages of naturally occurring solid fats such as butter, but due to lower costs, longer shelf lives, and use in baking, trans-fats have been made readily available since despite health concerns. Trans-fats have been linked to many health issues, such as obesity and coronary heart disease.

It is not completely understood why trans-fats are negatively effecting human health. Some researchers have suggested humans lack the necessary chemicals to break down trans-fats, while others have indicated they believe that while the chemicals are breaking down trans-fats they are then too busy to be breaking down other essential fats (such as Omega 3s), which changes the fats present in the heart, leading to heart disease. It has also been suggested that since trans-fat molecules can pack closely and form solids, trans-fats can form plaques in the heart.

A cross section of an artery (pink), with a plaque build-up (yellow). 
(photo from bbc.co.uk)

It is important to remember that all fats are a mixture of saturated and unsaturated, and that listing a product as a "saturated" or "unsaturated" fat indicates which type is in higher percentage. It is also important to note that a variety of types of fats are necessary for life, and that ideally a diet would consist of a mixture of types of fat. However, limiting the intake of trans-fats where possible is best, since they are already naturally present in small amounts in some products containing saturated fats (such as cow milk).