Most of us just don’t know the difference between fatty acids and fats. And there is indeed a difference.
Every molecule of fat incorporates three molecules of fatty acids. The fatty acids may be either saturated or unsaturated, and they thereby impart those qualities to the fat as a whole.
First, let’s see what a fatty acid is.
Fatty acids are the acids that are found as components of fats. They are members of a larger family that chemists call carboxylic acids. As acids go, they are very weak, unlike sulfuric acid, for example, which is the highly corrosive battery acid in your car.
A fatty acid molecule consists of a long chain of as many as sixteen or eighteen (or more) carbon atoms, each one of which carries a pair of hydrogen atoms. (Techspeak: The chain is made up of CH2 groups.) If the chain contains its full complement of hydrogen atoms, the fatty acid is said to be saturated (with hydrogen).
But if somewhere along the chain one pair of hydrogen atoms is missing, the fatty acid is said to be monounsaturated. If two or more pairs of hydrogen atoms are missing, it is said to be polyunsaturated. (Actually, one hydrogen atom is missing from each of two adjacent carbon atoms, but let’s not quibble.)
Some common fatty acids are stearic acid (saturated), oleic acid (monounsaturated), and linoleic and linolenic acids (polyunsaturated).
To chemists, and apparently to our bodies as well, the exact positions of the unsaturated parts of the fatty acid molecules (Techspeak: the double bonds) matter. You’ve heard that the “omega-3” fatty acids found in fatty fish may play a role in preventing coronary heart disease and strokes? Well, “omega-3” is the chemist’s way of telling exactly how far the first missing pair of hydrogen atoms (the first double bond) is from the end of the polyunsaturated molecule: it is three places from the end. (Omega is the last letter, the end, of the Greek alphabet.)
Fatty acids are generally bad-tasting and foul-smelling chemicals. Fortunately, they don’t usually exist in foods in their free, yucky forms. They are tamed by being chemically fastened to a chemical called glycerol, in the ratio of three fatty acid molecules to each glycerol molecule. Three fatty acid molecules tied to a glycerol molecule constitute one molecule of fat.
Chemists draw the fat molecule’s structure schematically on paper as a short flagpole (the glycerol molecule) with three long banners (the fatty acids) flying from it. They call the resulting molecule a triglyceride (triindicating that it contains three fatty acids), but its common name is simply a “fat” because by far the majority of natural fat molecules are triglycerides.
The fatty acids (I’ll call them FA’s) in any given fat molecule can be all of the same kind or any combination of different kinds. For example, they might be two saturated FA’s plus one polyunsaturated FA, or they might be one monounsaturated FA plus one polyunsaturated FA plus one saturated FA, or all three might be polyunsaturated FA’s.
Any real-life animal or vegetable fat is a mixture of many different fat molecules containing various combinations of FA’s. In general, shorter-chain and less saturated FA’s make softer fats, while longer-chain and more saturated FA’s make harder fats. That’s because in an unsaturated FA, wherever a pair of hydrogen atoms is missing (Techspeak: wherever there is a double bond), the FA molecule has a kink in it.
As a result, the fat molecules can’t pack together as tightly to make a hard, solid structure, and the fat is likely to be more liquid than solid. Therefore, predominantly saturated animal fats tend to be solids, while predominantly unsaturated vegetable fats tend to be liquids. When you read that a certain olive oil, for example, is 70 percent monounsaturated, 15 percent saturated and 15 percent polyunsaturated, it means that those are the percentages of the three kinds of FA’s, added up over all the different fat molecules in the oil.
We don’t care how the FA’s are distributed among the fat molecules, because it is only the relative amounts of the three kinds of FA’s, added up over the whole mixture of fat molecules, that determine the healthful or unhealthful qualities. The glycerol portions of all the fat molecules aren’t nutritionally important and just go along for the ride. The socalled essential fatty acids are those FA’s that the body needs in order to manufacture the important hormones called prostaglandins.
While we’re talking about fatty acids and triglycerides, let’s straighten out some other fat-related terms you may have heard.
Monoglycerides and diglycerides are like triglycerides but, as you may guess, have only one (mono-) or two (di-) FA molecules attached to the glycerol molecule. They exist in very minor amounts along with the triglycerides in all natural fats, and their FA’s are incorporated into the saturation/unsaturation profiles of the fats. They are also used as emulsifiers (substances that help oil and water to mix) in many prepared foods. But are they considered fats themselves? Sort of. Triglycerides are broken down into mono-and diglycerides during digestion, so their nutritional effects are essentially the same.
Finally, there is the word lipid, from the Greek lipos, meaning fat. But we use the word much more broadly than that.
Lipid is a catchall term for anything and everything in living things that’s oily, fatty, or oil-loving, including not only mono-, di-and triglycerides but such other chemicals as phosphatides, sterols, and fat-soluble vitamins. When your blood chemistry report comes back from the medical lab it may contain a lipid panel, listing not only the amount of triglycerides (fat blood isn’t good) but also the amounts of the various forms of cholesterol, which is a fatty alcohol.
What can be done to minimize the confusion between “fats” and “fatty acids” in food writing?
First of all, we have to recognize that although the word fat strictly means a specific kind of chemical, a triglyceride, as distinguished from a protein or a carbohydrate, in common usage the word fat is used to refer to mixtures of fats, such as butter, lard, peanut oil, and so on. (Each of these products is referred to as “a fat” in the diet.) There is little a reader can do about that ambiguity, except to try to determine whether the word is being used in the context of a specific chemical substance or a category of food.
Second, we can implore food writers to be more careful about switching indiscriminately back and forth between “fat” and “fatty acid.” Here are some suggestions:
The relative saturation and unsaturation of a fatty food can be expressed without using either term. For example, we can just say that it is x percent saturated, y percent monounsaturated, and z percent polyunsaturated, without adding the object (fatty acid) that these adjectives in truth modify.
Instead of saying, as I have seen many times, “a saturated (or unsaturated) fat,” which is meaningless, we should say “a fat high in saturates (or high in unsaturates)” or “a highly saturated (or highly unsaturated) fat.” Those are shorthand ways of saying “high in saturated (or unsaturated) fatty acids.”
In general, the less often the term fatty acid is used the better, because people already understand the term fat (or think they do), and that word is less intimidating. But if individual fatty acids must be discussed, the term should be defined the first time it is used as something like “the building blocks of fats.”