All fats fall into three categories. Saturated, polyunsaturated, and monounsaturated fats.
I had never noticed the funny arithmetic you mention, but as soon as I received your question I ran to my pantry and grabbed a box of Nabisco Wheat Thins. Here’s what I saw in the Nutrition Facts panel for the amounts of fat per serving: “Total Fat 6g. Saturated Fat 1g. Polyunsaturated Fat 0g. Monounsaturated Fat 2g.”
I got out my calculator. Now, let’s see. One gram of saturated fat plus zero grams of polyunsaturated fat plus two grams of monounsaturated fat makes three grams of total fat, not six. What happened to the other three grams?
Next, I grabbed a box of Premium Original Saltine Crackers. Worse yet! The two grams of total fat are supposedly made up of zero grams of saturated fat, zero grams of polyunsaturated fat, and zero grams of monounsaturated fat. Since when does zero plus zero plus zero equal two? I didn’t even need my calculator to know that something was wrong with that one.
Something very strange was going on here. I hurried to my computer and pulled up the Web site of the FDA, the agency that made the rules for the nutrition labeling of prepared foods. The FDA’s site has a page that answers frequently asked questions about food labeling. Here’s what I found.
“Question: Should the sum of saturated, monounsaturated, and polyunsaturated fatty acids equal the total fat content?
“Answer: No. The sum of the fatty acids generally will be lower than the weight of total fat, because the weights of components of fat such as trans fatty acids and glycerol are not included.” Aha! So that’s it!
Still not clear? Lemme ’splain it to ya.
A fat molecule consists of two parts, a glycerol part and a fatty acid part. Although the number of grams of “Total Fat” on the label is indeed the weight of the whole fat molecules, glycerol parts and all, the amounts of “Saturated Fat,” “Polyunsaturated Fat,” and “Monounsaturated Fat” are the weights of the fatty acid parts alone. Part of the missing weight is the combined weights of the glycerol parts of all the fat molecules. (I’ll get to the trans fatty acids later.)
Why, then, are these amounts called “fats” on the labels instead of what they really are: fatty acids? According to Virginia Wilkening, deputy director of the FDA’s Office of Nutritional Products, Labeling and Dietary Supplements, there are two reasons: (1) the general public wants to know only the relative amounts of saturated and unsaturated stuff in their fats, and it’s the fatty acid parts alone that determine that; (2) space is at a premium on food labels, and the words “fatty acids” take up more space than “fats.”
Okay, I guess, but the inaccurate wording still annoys nitpickers like me.
As the FDA’s Q&A page admits, there is even more fudging in the Nutrition Facts panel, because the weights of the trans fatty acids are not included in the list. In fact, they usually account for even more of the missing weight than the glycerols do.
Trans fatty acids are the latest villains to appear in the Frightening Fat Follies; they seem to raise the levels of LDL (“bad”) cholesterol in the blood just about as much as naturally saturated fatty acids do.
Trans fatty acids don’t occur naturally in vegetable oils, but are formed when they are hydrogenated. The two added hydrogen atoms may attach themselves to opposite sides of the carbon chain (Techspeak: in the trans configuration) instead of both on the same side (Techspeak: in the cis [pronounced sis] configuration). That changes the molecular shape of the fatty acid from kinked to straight, thus making them resemble and behave like saturated fatty acids.
Partially hydrogenated vegetable oils may contain substantial amounts of trans fatty acids but, largely because of difficulties in determining their amounts, they are not currently reported separately on food labels.
In your own pursuit of longevity, you will still want to pay attention to the amount of “Total Fat” listed on the label. But to learn whether it is primarily “good fat” or “bad fat,” disregard the exact numbers of grams and pay attention to the relative amounts of saturated, polyunsaturated, and monounsaturated fat(ty acids). That’s what counts. And remember that at this writing the villainous trans fatty acids are still lurking somewhere off the label. The FDA is considering listing them together with the saturated fatty acids.
Oh, and what about those “zero grams of fat(ty acids)” in my Premium Crackers that mysteriously add up to 2 grams of total fat? Are there some kinds of fat that have no fatty acids attached to them at all? No. Then they wouldn’t be fats. It’s that the FDA permits manufacturers to list “zero grams” of either a fat or a fatty acid when the amount is less than 0.5 gram per serving.
The rules of arithmetic that we learned in first grade are not in jeopardy.