The Skinny on Fats

UNSATURATED FATTY ACID An unsaturated FA is one whose molecules contain one or more sharp bends or kinks. If an unsaturated FA contains only one kink, it is said to be monounsaturated; if more than one, polyunsaturated. Unsaturated FAs are found mostly in vegetable oils. They do not raise cholesterol levels. Monounsaturated FAs are even considered healthful.

Olive oil's oleic acid is a monounsaturated FA.

OMEGA-3 and OMEGA-6 FATTY ACIDS These are polyunsaturated FAs that have kinks in their chains at either the third or the sixth carbon atom from the end. Examples are alpha-linolenic acid, an omega-3 FA and linoleic acid, an omega-6 FA.

Omega-3 and omega-6 FAs are "essential" FAs because our bodies can't produce them. We have to get them from our foods, such as fish oils, walnut oil, flaxseed oil and canola oil. Omega-3 FAs actually reduce our risk of coronary disease.

HYDROGENATION is a high- temperature, high-pressure process that changes kinky, unsaturated FAs into straight-chain saturated FAs, by forcing a pair of hydrogen atoms into each kink as a splint would support a broken bone.

The straightened-out FA molecules can now pack together more compactly, like a bundle of twigs that have had all their crooked twiglets trimmed off. Fats containing these compacted FAs therefore become harder and less fluid. If your soybean- or sunflower-oil margarine hadn't been hydrogenated, you would be pouring it, rather than spreading it, on your toast. Manufacturers use hydrogenated oils in virtually every cake, cookie or cracker in your supermarket to make it smooth, rather than oily.

Partial hydrogenation straightens out only some of the kinks in the unsaturated FA molecules. The rest are left in their bent, unsaturated shapes. Complete hydrogenation straightens out all the kinks in the FA molecules, making the fats completely saturated, and that's not good.

TRANS FATTY ACID A trans FA is an unsaturated FA that has been subjected to hydrogenation, but was able to evade the hydrogen atoms and remain defiantly kinky and unsaturated. However -- and this is the crucial distinction -- in the fiery cauldron of hydrogenation, its kink has been twisted into an angle different from that of a normal unsaturated FA. Chemists call this shape of FA a "trans" fatty acid.

Trans FAs are rarely found in nature; they are unnatural. When we eat a trans FA, our metabolism is baffled by its alien kinkiness and doesn't know what to do with it. So it vents its frustration not only by raising our LDL ("bad") cholesterol level, as saturated FAs do, but also by lowering our HDL ("good") cholesterol level. That double whammy is just about the worst thing a fat can do to us.

Complete hydrogenation, by the way, as distinguished from partial hydrogenation, does not form trans FAs, because when all of the kinks in an unsaturated FA are straightened out -- no escapees -- there are no unsaturated kinks left, either natural or trans. Not that the resulting saturated fats aren't harmful, but at least they are not as harmful as trans FAs.

Trans fatty acids are commonly but erroneously referred to as "trans fats," but they are not the entire fat molecules. They are merely the fatty-acid parts.

The Food and Drug Administration has declared that the only acceptable amount of trans FAs in our diets is zero. Manufacturers don't "add trans fats" to foods, as I have seen stated. What they do is use partially hydrogenated fats for their desirable properties, and trans FAs are an inevitable result. When you see "partially hydrogenated" on a food label, you can bet that trans FA villains are in there. But if the product contains less than half a gram of trans FAs per serving, the FDA permits the label to say "No trans fats."

On the labels of manufactured foods, the amounts of trans FAs have customarily been incorporated into the total amounts of unsaturated FAs. But beginning on Jan. 1, 2006, the FDA will require the amounts of trans FAs to be stated separately. Manufacturers are engaged in a frantic scramble to achieve the benefits of partial hydrogenation without generating the concomitant trans FAs.

As they begin to succeed, we will see more and more food labels flaunting the words "no trans fats," just as they have flaunted the words "low calorie," "low fat," "low sodium" and "low carb."

I wonder how long it will be before we see labels reading "contains nothing at all"?

Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh. He can be reached at wolke@pitt.edu.