As the US population migrated from the farms to the cities, but largely maintained the same diet, the frequency of heart disease increased. This was linked to the consumption of large amounts of animal fat -- cooking with lard, frying eggs and toast in bacon grease, etc. Other possibilities may not have been entirely ruled out, but it was logical, and so began the campaign against animal fat. We now refer to saturated fats, but a half-century ago, the first-named culprit was "animal" fat.
Plant oils are also technically fats, but liquid at room temperature. These were thought to be healthier, but their cooking properties were different from those of solid animal fat. To modify them so that they could be used in similar ways to animal fats, food chemists used -- and still used -- the process of of hydrogenation. In this process, hydrogen is combined with the unsaturated plant oils, converting some of the molecules into saturated fats.
Hydrogenation of Fats
Complete hydrogenation converts healthful unsaturated fats into fully saturated fats, making them entirely equivalent to regular, saturated, animal fat. This would not do as a substitute for animal fat, since it is essentially the same. The solution seemed to be to partially hydrogenate the plant oil. This way, some unsaturated molecules would remain, and keep the product healthy.
Thus was born partially hydrogenated vegetable oil.
cis and trans Configurations
But partial hydrogenation has a down-side. Natural unsaturated fatty acids are in configuration that is called cis, but partial hydrogenation can flip the chemical bonds into a different configuration that is called trans. The difference between cis and trans is that the two H atoms are on the same side of the double bond (cis), compared to being on opposite sides (trans).
This may not seem like much of a difference, but it affects the shapes of the molecules. In a cis configuration, the double bond creates a kink in the fatty acid. In the trans configuration, there is no kink. Therefore, a trans-fatty acid is structurally similar to a saturated fatty acid (which has no kinks, either). The kinks in the molecules determine whether the material is solid or liquid at room temperature.
In recent decades, as more has been learned about the biology of different forms of fats, the story has become much more complicated than it seemed in the 1950's. The following are a few of the issues:
- It is less clear (and to some researchers, not clear at all) whether saturated fat is responsible for the ailments attributed to it, particularly heart disease.
- Although nutritional advice gradually moved toward the recommendation that we banish all fat from our diets, there is compelling evidence that some fats are good. Indeed, ω-3 and ω-6 fatty acids are essential for survival.
- Both saturated fats and cis-unsaturated fats are natural. We produce the necessary enzymes to metabolize these. However, there is no evidence that we produce the enzymes to metabolize trans-fatty acids.
- Fat-free and low-fat prepared foods typically replace the fat with something else that tastes good to us. Given our evolutionary history, the really-tasty things tend to be fat, sugar, and salt. Removing fat often makes things taste "flat." Sugar and salt make it palatable again. These have their own health problems when consumed in large amounts, as it typical of the Western Diet.
- Even among unsaturated fats, there are complexities. The ω-3 and ω-6 fatty acids are not equivalent medically, and have distinct differences with respect to chronic inflammatory diseases.
Avoid foods containing trans-fatty acids.
Unfortunately, this is not as straightforward as reading the Nutrition Facts label on the package. US law permits manufacturers to establish the "serving size" of their product at will, and to round to zero grams the amount of any component that is present in less than 0.5 grams per serving. If the Nutrition Facts label claims "zero grams of trans-fat," this does not mean the food contains no trans-fat. The only way to be certain is to look for "partially hydrogenated vegetable oil" in the list of ingredients.