This week’s big diet and nutrition news has been the news that the FDA has proposed removing the “generally recognized as safe” label from trans-fats. Now, if you’re like me, the first question you might want to ask is “what’s a trans-fat?”
A fat is composed of long carbon-hydrogen chains called fatty acids. A saturated fat has one hydrogen for every possible bond to a carbon; an unsaturated fat has some places where the possible hydrogen bond is replaced by a double-bond between carbon atoms. The terms trans- and its opposite cis- mean “on opposite sides” and “on the same side” respectively, and a trans-fat includes fatty acids that have those carbon-carbon bonds on opposite sides. The effect is that you can have two fats with the same chemical formula, but different geometric structures, like in these pictures cribbed from Wikipedia.
In general, the more hydrogenated, or saturated, a fat is, the higher its melting point. Butter, lard, and beef tallow all have lots of saturated fats, so they’re more waxy and solid at room temperatures; most vegetable oils are much less saturated, and so are liquid at room temperature. But around 1900, the French chemist Paul Sabatier discovered that hydrogen could be combined with carbon dioxide through the use of a nickel catalyst, producing methane or methanol; a German chemist named Wilhelm Normann applied the same process to hydrogenate fatty acids to make fats with higher melting points from vegetable oils. This process is the basis for making both margarine and old-fashioned vegetable shortening like Crisco.
It happens that most — although not all — of the less saturated fats from natural sources are in the cis-configuration, but artificially hydrogenated fats have a much higher concentration of the trans-configuration. So, products like margarine have relatively high amounts of trans-fats compared to natural fats.
Okay, so wake up, the chemistry lesson is over. Those of you who are old enough — most of my readers, I think — still get a little bit of a chill at the phrase “saturated fat”. For most of my lifetime, saturated fats were considered to be unhealthy; advertisements for margarine made a big point about how they were “lower in cholesterol” and therefore more “heart healthy”. In the 80′s, the conventional wisdom, pushed by groups like Ralph Nader’s Centers for Science in the Public Interest (CSPIRGs), was that fats were bad and saturated fats were really really bad.
So manufacturers reacted by replacing more saturated fats with hydrogenated fats that had lots of trans-fats.