Over the past 20 years, research has clearly identified the adverse health effects of trans fatty acids, most notably on coronary heart disease.

Epidemiologic estimates attribute upward of 100,000 deaths annually to trans fats. It is unknown what levels of trans fats are clinically significant, yet after two decades a research group at Indiana University concluded, "On a per-calorie basis, trans fats appear to increase the risk of (coronary heart disease) more than any other macronutrient, conferring a substantially increased risk at low levels of consumption (1 percent to 3 percent of total energy intake)."

There are only a handful of naturally occurring trans fats.

Trans fats are created by hydrogenation, which bubbles hydrogen through unsaturated oils to stabilize them, to prevent them from becoming rancid and to keep them solid at room temperature.

Fatty acid molecules are chains of 12 to 24 carbon atoms with hydrogen atoms attached like the legs of a centipede and a head of carbon, oxygen and hydrogen atoms, abbreviated "COOH."

Fat and oil molecules make up different combinations of three fatty acids with their heads bonded to a molecule of glycerin.

Carbon atoms can each form four bonds. In a carbon chain, two of the bonds attach a carbon atom to its neighbors. Hydrogen atoms usually occupy the other two bond sites. In some chains, one or more bonding sites on two adjacent carbon atoms has no hydrogen atom. When this happens a second bond forms with a neighboring carbon atom and leaves each of the two carbon atoms attached to only a single hydrogen atom. These are called unsaturated chains because each double bond represents a potential bonding site for a hydrogen atom.

The viscosity and melting temperature of fats are higher when there are fewer double bonds, so monounsaturated fats have a higher melting point than polyunsaturated fats.

Isomers are two molecules that have the same chemical formula but different structures. The two hydrogen atoms can be either on the same side of the chain or on the opposite side. It is a "cis" isomer if they are on the same side or a "trans" isomer when on opposite sides.

This difference affects the shape of the isomer. Trans fatty acid chains are linear, whereas the cis chains are bent because of the mutually repulsive hydrogen atoms crowded onto the same side of the chain.

Trans fats are easy to use, inexpensive to produce, last a long time and give foods a good taste and texture. Many restaurant and fast-food establishments choose trans fats to deep-fry because oils with trans fats resist rancidity and can be used many times in commercial fryers.

Fatty acids of both types cannot be metabolized directly because they are not soluble in blood. A sequence of metabolic events packages them into blood soluble packets of cholesterol and other proteins. In that form they may either be stored as fat or used as energy, depending on metabolic needs.

Although little is known about the mechanisms by which trans fatty acids actually function at the cellular level, it is becoming increasingly clear that they are bad news.

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Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.