What’s the Skinny on Fats?

How trans and saturated fats differ.

The Food and Drug Administration announced last Wednesday that by 2006 all nutrition labels—which currently list both total fat and saturated fat—must also detail the amount of trans fatty acids any serving contains. But as recently as 1999, the agency was considering lumping trans and saturated fats together, listing both on the “Sat. Fat” line. How different are the two fats, and is one worse than the other? 

The distinction lies in the number of hydrogen atoms each fat molecule contains. All fat molecules consist primarily of strings of carbon atoms to which hydrogen atoms can link; in a saturated fat, every carbon in the chain has as much hydrogen attached to it as possible (the fat is “saturated” because no more hydrogen will fit). Unsaturated fats have less hydrogen; trans fats fall somewhere in the middle and are created when unsaturated fats undergo partial hydrogenation, a process which adds some hydrogen without fully saturating the fat. (The procedure also bends fat molecules into the strange shapes, called trans configurations, that give the fats their name.)

Partially hydrogenated and saturated fats have longer shelf lives than their unsaturated peers. That’s because the extra hydrogen raises the fats’ melting points, making them more stable at room temperature. Trans fats are useful because they’re slightly softer than saturated fats (think margarine vs. butter). And food producers (well aware that they’d have to list any saturated fats on the label) also sometimes opt to use trans fats instead so their products appear more healthful.

Now that the loophole has been closed, snackers will know what they’re eating, although nutritionists are still debating whether saturated or trans fat is worse for you. Saturated fats—which you’ll find in steak, ice cream, and butter—have been studied for decades, while trans fats—present in doughnuts, fries and margarine—have been under scrutiny for only the last 10 years. Both have been proven to increase low-density lipoprotein, your “bad cholesterol” indicator. LDL transportscholesterol—a waxy substance that helps rebuild cell membranes and create hormones, among other things—from the liver to the rest of the body, where it can accumulate in arteries and cause heart disease.

One thing that helps keep LDL in check is the “good cholesterol” indicator, high-density lipoprotein, which carries cholesterol back to the liver. This is where saturated fat starts looking better: It increases cholesterol indicators across the board, so HDL levels rise as well. Trans fat, however, raises LDL while reducing HDL levels, and this dangerous double whammy has set nutritionists on alert.

Trans fats may also be guilty of numerous secondary sins: There are some indications that they could increase your risk for cancer, diabetes, and even cause pregnancy complications. That’s why the FDA will not put a recommended daily allowance next to the new trans statistic—any amount of this stuff is bad for you.

Next question?

Explainer thanks Dr. Frank Hu of the HarvardSchool of Public Health; Biochemistry,fifth edition, Berg, Stryer, and Tymoczko, W.H. Freeman and Co., 2001; and Lippincott’s Illustrated Reviews: Biochemistry, second edition, P. Champe and R. Harvey, J.B. Lippincott Co., 1994.