2.13: Fats are tasty molecules too plentiful in our diets.

All fats have two distinct components: they have a “head” region and two or three long “tails” (FIGURE 2-29). The head region is a small molecule called glycerol. It is linked to “tail” molecules known as fatty acids. A fatty acid is simply a long hydrocarbon—that is, a chain of carbon atoms, often a dozen or more, linked together and with one or two hydrogen atoms attached to each carbon atom.

The fats in most foods we eat are triglycerides, which are fats having three fatty acids linked to the glycerol molecule. For this reason, the terms “fats” and “triglycerides” are often used interchangeably. Triglycerides that are solid at room temperature are generally called “fats,” while those that are liquid at room temperature are called “oils.”

Fat molecules contain much more stored energy than do carbohydrate molecules. That is, the chemical breakdown of fat molecules releases significantly more energy. A single gram of carbohydrate stores about 4 calories of energy, while the same amount of fat stores about 9 calories—not unlike the difference between a $5 bill and a $10 bill. Because fats store such a large amount of energy, a strong taste preference for fats over other energy sources has evolved in animals (FIGURE 2-30). Organisms evolving in an environment of uncertain food supply will build the largest surplus by consuming molecules that hold the largest amount of energy in the smallest mass. This feature helped the earliest humans to survive, millions of years ago, but today puts us in danger from the health risks of obesity now that fats are all too readily available.

An important distinction is made between “saturated” and “unsaturated” fats (FIGURE 2-31). These terms refer to the hydrocarbon chain in the fatty acids. If each carbon atom in the hydrocarbon chain of a fatty acid is bonded to two hydrogen atoms, the fat molecule carries the maximum number of hydrogen atoms and is said to be a saturated fat. Most animal fats, including those found in meat and eggs, are saturated. They are not essential to your health and, because they accumulate in your bloodstream and can narrow the vessel walls, they can contribute to heart disease and strokes.

An unsaturated fat is one in which some of the carbon atoms are bound to only a single hydrogen (and are connected to each other by a double bond). Most plant fats are unsaturated. Unsaturated fats may be mono-unsaturated or polyunsaturated. A mono-unsaturated fatty acid hydrocarbon chain has only one pair of neighboring carbon atoms in an unsaturated state—that is, has only one double bond. A polyunsaturated fatty acid hydrocarbon chain has more than one pair of carbons in an unsaturated state—there’s more than one double bond. Unsaturated fats are still high in calories, but because they can lower cholesterol, they are generally preferable to saturated fats. Foods high in unsaturated fats include avocados, peanuts, and olive oil. Relative to other animals, fish tend to have less saturated fat.

The shapes of unsaturated fat molecules and saturated fat molecules are different. When saturated, the hydrocarbon tails of the fatty acids all line up very straight and the fat molecules can be packed together tightly. The tight packing causes the fats, such as butter, to be solid at room temperature. When unsaturated, the fatty acids have kinks in the hydrocarbon tails and the fat molecules cannot be packed together as tightly (see Figure 2-31). Consequently, unsaturated fats, such as canola oil and vegetable oil, do not solidify as easily and are liquid at room temperature.

The ingredient list for many snack foods includes “partially hydrogenated” vegetable oils. The hydrogenation of an oil means that hydrogen atoms have been added to a liquid, unsaturated fat so that it becomes more saturated. Adding hydrogen atoms can create a food with a more desirable texture, since increasing a fat’s degree of saturation changes its consistency and makes it more solid at room temperatures. By attaining just the right degree of saturation, it is possible to create foods, such as chocolate, that are near the border of solid and liquid and “melt in your mouth” (FIGURE 2-32). Unfortunately, hydrogenation also makes the food less healthful because saturated fats increase the risk of heart disease. They are less reactive—your body is less likely to break them down—and so are more likely to accumulate in your blood vessels.

Hydrogenation of unsaturated fats is doubly problematic from a health perspective because it also creates trans fats, the “trans” referring to the unusual orientation of some or all of the double bonds that remain following the addition of hydrogen atoms. This orientation differs from that in other unsaturated dietary fats—which have their double bonds in an orientation called “cis.” Trans fats in your diet cause your body to produce more cholesterol, further raising the risk of heart disease, and they also reduce your body’s production of a type of cholesterol that protects against heart disease.

Because of the well-documented links between dietary fats and heart disease, many people are trying to reduce their fat intake. “Fake fats” make this possible. They are designed to be similar to fats in taste and texture, but have one big difference: they cannot be digested by humans. One such “fake fat” is olestra. Olestra, instead of being a triglyceride fat with three fatty acid tails linked to a glycerol molecule, has eight separate hydrocarbon fatty acids attached to a molecule of sucrose. The fatty acids in this octopus-like molecule stimulate their usual taste buds on your tongue, telling your brain that you are eating a fat. The complex shape of the molecule, however, prevents your body’s digestive chemicals from grabbing onto it and breaking it down. As a consequence, it passes through your digestive system without being digested. It’s not a perfect solution, however. Olestra reduces absorption of some vitamins, and in some people causes abdominal cramping.