5.13: Fatty Acids

A fatty acid is a chain of carbon atoms with an acid group at one end (the end with the oxygens in Figure 5.2). Fatty acids differ in the numbers of carbons and double bonds in the chain. Those with 16 or 18 carbons are most common.

Palmitic acid* [16:0 (16 carbons, 0 double bonds)], stearic acid (18:0), oleic acid (18:1), and linoleic acid (18:2) make up more than 90% of the fatty acids in the U.S. diet (Table 5-3) and are mostly in triglycerides (Figure 5.2). Only tiny amounts are “free” (unattached) fatty acids.

^{*Palmitic acid is often called palmitate; -ate replaces -ic when an acid is bound to something else. Palmitic acid bound to vitamin A when added to food or supplements is listed as vitamin A palmitate. Sodium bound to ascorbic acid (vitamin C) is sodium ascorbate. Monosodium glutamate (flavor enhancer MSG) is one (mono-) atom of sodium bound to glutamic acid (an amino acid)}

Saturated vs. Unsaturated Fatty Acids

A saturated fatty acid doesn’t have any double bonds in its carbon chain—it’s saturated with hydrogen. No more hydrogen atoms can be added to the chain because there’s no place for them to attach. Stearic acid (18:0) doesn’t have any double bonds between any of its 18 carbons (Figure 5.2). It’s a saturated fatty acid.

A monounsaturated fatty acid has one double bond in its carbon chain—there’s one (mono) place (double bond) where more hydrogen can be added. A polyunsaturated fatty acid has two or more (poly) double bonds in its chain.

The number of double bonds in the fatty acids affects whether a fat is solid or liquid. The more double bonds, the more liquid the fat. Because fat has a mix of fatty acids (Table 5-3), “majority rules.” Beef fat is solid—it’s mostly saturated fat. Corn oil is rich in polyunsaturated fatty acids (18:2), and olive oil is rich in monounsaturated fatty acids (18:1), and both are liquid at room temperature. If you refrigerate them, olive oil partially solidifies, whereas corn oil (with more double bonds) doesn’t.

Fish have fatty acids with many double bonds; their tissues must be somewhat fluid (soft) in their cold environment (one such fatty acid has 5 double bonds). Fish that migrate to warmer or colder water can change the number of double bonds in their fatty acids to keep the same softness in their tissues. If fish had fatty acids like beef, they’d be too stiff to swim.

Fat from plants (e.g., olive or corn oil) is usually liquid because the fatty acids are mostly unsaturated. Tropical oil (e.g., coconut oil, palm oil) is an exception. They have more saturated fat to give them similar fluidity in the warmer tropical climate.

Hydrogenated Fat

Food manufacturers can change double bonds in fatty acids to single bonds by adding hydrogen, making fat more saturated and more solid. This is called hydrogenation (hydrogen gas, heat, pressure, and a catalyst are used). Partially hydrogenated corn oil means that some, but not all, of the double bonds in corn oil have had hydrogen added to them, converting them to single bonds. Fully hydrogenated means that all of the double bonds have been converted to single bonds.

Shortening (e.g., Crisco) is made by either partially hydrogenating vegetable oil, which turns the clear liquid into a white solid, or by fully hydrogenating the oil and mixing it with unhydrogenated oil. Margarine is made the same way from vegetable oil, and carotene and vitamin A are added to simulate butter’s color and vitamin content.

Using hydrogenated oil in peanut butter solidifies it so it won’t rise to the top. This makes the jar of peanut butter smooth and consistent from top to bottom. Sometimes, more hydrogenated oil is added—more peanut butter without more peanuts. The oil in “old-fashioned” peanut butter isn’t hydrogenated; the oil floats to the top, to be stirred each time it’s used.

Figure 5.2: Types of Fat

Table 5-3: Common Fatty Acids in the U.S. Diet

When vegetable oils are hydrogenated, the fatty acids aren’t the same as before, e.g., completely hydrogenated linoleic acid (18:2) becomes stearic acid (18:0) (Figure 5.2). When you choose between margarines made from different oils (e.g., corn, safflower), choose by price, flavor, Nutrition Facts on label, etc.; margarines similar in softness are similar in how saturated they are.

Because oils can be hydrogenated to be physically similar (e.g., hard/soft, melts in the mouth), it’s economical for food companies to buy whatever oil is least expensive at that time. The ingredient list will then say, contains one or more of the following partially hydrogenated oils: soybean, cottonseed... so that labels won’t have to be changed when different oils are used. If one oil is more unsaturated than another, it can be hydrogenated more to get the same physical property desired in the food product.

Bacteria in the rumen of cows and sheep can hydrogenate fat; fat in beef and lamb is more saturated than fat from animals without a rumen (e.g., chickens). An animal’s diet also affects fatty acid content of the animal fat.

Trans fatty acids: When fatty acids are partially hydrogenated—whether by food companies or in an animal’s rumen—the arrangement of atoms around the remaining double bonds can change from cis (normal) to trans (Figure 5.3). This makes the fat more solid (a fatty acid with one double bond is more solid in a trans arrangement than in a cis arrangement).

Figure 5.3: Cis and Trans Forms of Unsaturated Fatty Acids

In the body, trans fat raises the risk of heart disease more than does saturated fat (see Chap. 8). The advice is to avoid trans fat in processed food. Trans fat in processed foods has gone down drastically due to less use of partial hydrogenation.

To get the desired consistency, polyunsaturated oils have been replaced by the more solid/ saturated (and more expensive) fats like palm and coconut oils. Also, a liquid fat can be completely hydrogenated (making it a trans-free saturated fat) and mixing it with a more liquid fat.

As said earlier, trans fat occurs naturally in foods that come from ruminants, e.g., beef, lamb, goat, and their milk. Food labels can say 0g trans fat when trans fat is less than 0.5 g/serving. Thus, 0 g trans fat, doesn’t mean trans-fat free. It just means there’s less than 0.5 g trans fat/serving.

Oxidation of Double Bonds

Fat’s double bonds are fragile. Unlike single bonds, they can take on other atoms (as in hydrogenation). Double bonds are thus prone to oxidation (taking on oxygen), which breaks the fatty acid’s carbon chain at that site (Figure 5.4). This forms undesirable breakdown products—the fat has “gone bad” (become rancid). Fat in fish has so many double bonds, fish can develop an undesirable “fishy” odor and taste in a short time, especially if not kept at their normally cold temperature or colder.

Figure 5.4: Oxidation of Unsaturated Fatty Acids

Using partially hydrogenated fat, or coconut or palm oil (naturally rich in saturated fat) extends the shelf life of food products—there are fewer double bonds that can be oxidized. Antioxidants (e.g., BHT and BHA)* are often added to help prevent oxidation of the remaining double bonds.

Vitamin E in food (and the body) is an antioxidant that protects the double bonds in plant oil (and body tissue).† Vitamin E is found in a wide variety of foods, such as salad oil, margarine, nuts, fish, olives, fruits, vegetables, eggs, muffins, beans—even chocolate—though it’s richest in polyunsaturated oils.

^{*BHT (butylated hydroxytoluene) and BHA (butylated hydroxyanisole) are then listed as preservatives.
†We need more vitamin E when our diet is rich in polyunsaturated fat, but foods rich in polyunsaturated fat naturally have more E (a fat-soluble vitamin). Vitamin E deficiency is unusual, but symptoms (anemia and edema) have been seen in premature infants who are born with low levels of E and have difficulty absorbing fat (and thus fat-soluble vitamins).}

Omega Double Bond

Besides differing in the number of double bonds, fatty acids also differ in the location of the double bonds. To describe this location, the carbons in the chain are numbered starting from the far end (the CH₃ end) of the fatty acid, called the omega end (omega is the last letter—the far end—of the Greek alphabet).

The omega number gives the location of the first double bond. This location is important because the body can add double bonds (or more carbons) only beyond this first double bond, and makes different and important substances from these fatty acids.

Omega-6 fatty acids have their first double bond between the 6th and 7th carbon. The omega6 fatty acid linoleic acid has 18 carbons and 2 double bonds: 18:2/omega-6 (Figure 5.2).

Omega-3 fatty acids have their first double bond between the 3rd and 4th carbon. The omega-3 fatty acid linolenic acid has 18 carbons and 3 double bonds: 18:3/omega-3 (Figure 5.2).

Fatty Acids Essential in the Diet

Linoleic acid (18:2/omega-6) and linolenic acid (18:3/omega-3) are essential nutrients. See Table 5-3 for food sources (18:2 for linoleic acid, 18:3 for linolenic acid).

Deficiencies have been seen only in patients with conditions that severely affect fat intake or absorption. Long-term hospital patients intravenously fed a fat-free formula developed a linoleic acid (omega-6) deficiency; symptoms include retarded growth in infants and appearance of scaly skin lesions. Symptoms of linolenic acid (omega-3) deficiency include blurred vision and numbness and pain in the legs.

Cells use linoleic and linolenic acids to make other important substances, e.g., prostaglandins (so called because the first one was found in prostate gland secretions).*

Long-chain omega-3 fatty acids (see >18:3 fatty acids in Table 5-3) have important functions. They can be made from linolenic acid, so aren’t essential nutrients per se. But we only make small amounts, which is why fish is recommended in the diet. Fish such as salmon and tuna contain docosahexenoic (22:6) and eicosapentaenoic (20:5) fatty acids that are found in the brain, eye, and other tissues.

^{*Prostaglandins are potent hormone-like substances affecting such things as immunity, blood pressure, and inflammation. Aspirin hampers one of the enzymes needed to make (from linoleic acid) a prostaglandin involved in inflammation; aspirin lessens inflammation in arthritis.}