Why is some vegetable oil "partially hydrogenated"?

Dear Cecil: As a longtime reader of ingredients labels, I beg you to clarify the ubiquitous phrase, “partially hydrogenated.” I assume it has something to do with hydrogen, but what does that highly flammable gas have to do with food? And why partially hydrogenated? Why not get down, go crazy and hydrogenate to the max? Mr. Sinister, Chicago

Cecil replies:

Ah, the mad impetuousness of youth. We dasn’t totally hydrogenate or we’d all die of heart failure inside of a year. At least that’s the impression I get from the extremists on the subject.

Hydrogenation involves cramming hydrogen gas into vegetable oil under pressure. It’s what’s you do to make the oil semisolid at room temperature rather than liquid, which is obviously useful in the case of products like margarine. Hydrogenation also retards spoilage and prevents baked goods from winding up too greasy. Worthy though these goals may be, they involve converting unsaturated (good) fats into saturated (bad) fats, which have been linked with heart disease.

The solution is partial hydrogenation, in which you create no more saturated fat than necessary to accomplish the task at hand. Done properly, partial hydrogenation results in only a minor increase in saturated fats. Margarine made from soybean oil, for example, starts out around 15 percent saturated and winds up 17-20 percent. Soft margarine, the kind that comes in tubs, does better than the stick variety, but both compare quite favorably with butter, which is 66 percent saturated.

Prior to 1990 or so hydrogenation was thought to be harmless. Today scientists aren’t so sure. The process produces something known as “trans” fats, trans referring to a certain configuration of hydrogen atoms in the fat molecules. There’s an increasing body of evidence linking trans fats with coronary heart disease. Although trans fats occur naturally to some extent, their major source in the typical Western diet is hydrogenated vegetable oil. For example, trans fats reportedly account for more than 40% of total fat in some margarines.

Right now it’s looking like trans fats are something you really want to avoid. But who knows? Scientists have changed their advice about fats several times over the last 20 years, and even those sounding the alarm over trans fats concede the subject is not well understood. Rather than switch from one form of fat to another (as occurred when people replaced saturated animal fat with hydrogenated, and often trans-laden, vegetable oil), I advise dialing back on the fat altogether, eating more fruits and vegetables, avoiding processed foods, and all that other granola kinda stuff, which is looking smarter all the time.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.