8.3: What is a Fat?

Fats are a group of chemical compounds which do not dissolve in water, but do dissolve in organic solvents (solvents which have a chemical backbone of carbon, such as ether or chloroform). To put it simply, fat added to water floats to the top, as on the surface of chicken soup (thus the classic statement that oil and water don’t mix). Similarly, if we dribble gravy on our clothing, water will not sponge it away. Instead, we use cleaning fluid (an organic solvent) to remove it.

Figure 8-2: Structure of Triglycerides.

Triglycerides

There are various kinds of fats, but when we speak of fatty foods, or body fat, or salad oil—or just plain fat—we are generally referring to triglycerides. Triglycerides comprise 98% of fat in food and are the storage form of fat in our body. A triglyceride derives its name from its structure—three fatty acids linked to glycerol (see Fig. 8-2).

Glycerol is a fairly familiar substance, with the household name of glycerin. Indeed, it’s probably more common in the lives of most consumers than they suspect, since glycerol/glycerin is one of the most basic ingredients of the creams and lotions that are rubbed into so many hands and faces, mixed with colors and spread on the lips, or worked into hair for “styling.”

Fatty acids make up the bulk of triglycerides. Because glycerol is always the same, differences between triglycerides are accounted for by differences among the fatty acids. Since all triglycerides have three fatty acids in their structure, the differences in triglycerides are the result of different combinations of fatty acids.

Fat and oil are both triglycerides, but we think of fat as oil when it is liquid (e.g., salad oil). But some oils (e.g., palm and coconut oils, some fish oils) are solid. As a rule, fats from plants and sea animals (e.g., fish, seal) are called oil, and fats from land animals are called fat. Contrary to popular belief, liquid oil has as many calories—and as much fat—as solid fat.

Chemically, the structures of fatty acids are somewhat similar to carbohydrates—made of carbon (C), hydrogen (H), and oxygen (O). Let us compare glucose (a carbohydrate) and caproic acid, a fatty acid found in butter and coconut oil.

Look carefully, and we see some distinguishing differences. The most striking difference is that the fatty acid has very little oxygen. (The “acid carbon” at the one end of the fatty acid is the only part that carries oxygen. This is the part that attaches to the glycerol in a triglyceride.)

Fat as a Concentrated Source of Calories

Since it’s the carbon portion of fat or carbohydrate that’s “burned” as fuel, the lesser amount of oxygen in the fatty acid’s structure makes fat a concentrated source of fuel (concentrated carbon). Witness the crude oil lamps used in so many cultures for millennia, really nothing much more than dishes of oil and a wick. From Aladdin, to the Roman catacombs, to Colonial America, these were the basic lighting utensils.

Recall that the burning of fuel is a process of oxidation—the addition of oxygen. With so little oxygen in its structure to begin with, the carbons in fat can take on more oxygen before becoming fully oxidized to carbon dioxide. In contrast, a carbohydrate starts out with more oxygen in its structure—its carbons are already partially oxidized.

The fact that fat excludes water makes fat an even more concentrated source of calories. In other words, water doesn’t mix with fat to dilute its caloric value, as it does with protein and carbohydrate.

Salad oil, for example, is 100% fat, and thus has an energy value of 9 calories per gram (120 calories/tablespoon). In contrast, carbohydrate and protein hold about three times their weight in water. As examples, potatoes, bananas, and the non-fat portion of meat are about 75% water and 25% carbohydrate or protein. The 4 calories per gram of dry carbohydrate and protein have thus been diluted to only 1 calorie/gm of potato:

Thus in realistic terms, the caloric differences between fats on the one hand, and proteins and carbohydrates on the other, are much greater than the 9 calories/gm for fats and 4 calories/gm for protein and carbohydrate. As normally found in food (and in our own tissues), water-holding carbohydrate and protein actually have an energy value of 1 cal/gm compared to fat’s 9 cal/gm. This striking difference has important implications.

If you add just 1 tablespoon of butter to ½ cup cooked rice, you will double the calories, since each contains about 100 calories (see Table 8-1). One can see why a high-fat diet easily becomes a high-calorie diet—and why many people in developing countries, where fat is a luxury item, suffer from a lack of calories.

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Fat has 9 cal/gm (calories per gram) versus 4 cal/gm for carbohydrate and protein.

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When a diet is low in fat, it is “bulky.” Consider a child who needs 1,800 calories a day and only has low-fat plant food to eat. That child would have to eat about 26 cups of carrots or 17 bananas or 15 boiled potatoes or 8 cups of rice to get those 1,800 calories.

When American school children are asked, “When is chocolate candy nutritious?,” they are stumped. They aren’t accustomed to thinking of candy this way. And when they dawdle over the vegetables at dinner, and their parents say, “think of the poor starving children in the world,” chances are they’d be glad to send their vegetables overseas. But in fact, chocolate bars (rich in fat, a chocolate bar provides about 150 calories per ounce) would be better to send—these are more nutritious to starving children than vegetables (a dilute source of calories).

Table 8-1: Fat, Water, Calorie of Some Foods.

	Calories	% Water*	% Fat*	Cal/gram
One Tablespoon:** Salad oil Butter, margarine,mayonnaise Table sugar (sucrose) Honey Jam, jelly Sour cream Catsup Mustard	120 105 45 60 55 25 15 10	0 15 0 15 30 70 70 80	100 80 0 0 0 20 0 5	9 7 4 3 3 2 1 1
2 oz. almonds	355	0	55	6
2 oz. chocolate bar	300	0	35	5
1 saltine cracker	15	5	10	4
1 slice bread	70	35	5	3
1/2 cup cooked rice	105	75	0	1
1 banana	105	75	0	1
1 orange	65	80	0	0.5
1 egg white	15	90	0	0.5
1/2 cup watermelon	25	95	0	0.3

* % water by weight and % fat by weight, rounded to the nearest 5%
** 1 tablespoon (T) of foods has different weights, e.g., 1 T sugar weighs less than 1 T honey or jam.

Fat serves as a concentrated source of energy in our body as well. It enables us to store excess calories without adding much bulk. A 150-pound person of normal weight might have about 15 pounds of stored expendable fat. To store this many calories (more than 60,000) as carbohydrate would require about 120 pounds.

Birds that migrate long distances accumulate fat to store the large amount of energy needed for the flight. If they stored it as carbohydrate, they would have trouble even getting off the ground. Plants—“stuck” in the ground as they are—can effectively store the bulk of their energy as carbohydrate. Note, however, that many seeds of plants are rich in fat (e.g., sesame seeds). This gives the seeds a lightness (a mobility of sorts) that allows them to be more easily dispersed, and provides a compact source of the initial energy the seed needs to sprout.

Fat Substitutes: Fat being such a concentrated source of calories, the calorie-conscious among us may wonder: How can we indulge in the pleasures of fat-laden foods without taking in so much fat and calories? It is a billion-dollar question that food companies have raced to answer.

Simplesse, the fat substitute used in the 1990s to make the imitation ice cream Simple Pleasures, is based on protein (from egg and milk) shaped into tiny spheres. Because protein holds water, it’s a low-calorie (1 cal/gm) substitute for fat (9 cal/gm). The protein sphere was designed to give the slippery “mouth feel” of fat. Simplesse doesn’t work in products that must be heated (e.g., cookies and crackers) because this changes its shape (i.e., denatures the protein) and thus its feel.

What’s “juicy” about a hamburger patty is its fat. A lean hamburger patty tends to be quite dry. How does one get a hamburger that is both lean and juicy? In McDonalds’ McLean Deluxe burger (dropped from the menu because it didn’t sell), lean beef was mixed with carrageenan, a soluble fiber derived from seaweed. Carrageenan, like all dietary fibers, holds water—and thus provided “juiciness” to the lean hamburger patty.

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The juiciness in McDonalds’ Mclean Deluxe burger came mainly from water rather than fat. More fat in a food means less water in the food—and more calories.

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What about the condiments that can add to the fat content of the burger? The McLean Deluxe came without cheese and without the mayonnaise-type dressing. It was dressed only with low-fat condiments of lettuce, tomato, ketchup, mustard, onions, and pickles. Although the McLean Deluxe weighed about the same as a Big Mac and weighed more than a Quarter Pounder, it had fewer calories (320) and much less fat (10 gm) than either the Big Mac (540 cal, 29 gm fat) or the Quarter Pounder (410 cal, 19 gm fat).

What about French fries? As yet, there are no fat-substitutes that are approved for frying foods other than chips. Olestra (Olean®) is a very versatile fat substitute because it is, in fact, a fat—yet calorie-free. How can this be?

Fig 8-3: Structure of Olestra. Olestra adds the “juicy” quality of fat, but is not digested.

Instead of having 3 fatty acids attached to a glycerol backbone (as a triglyceride has), Olestra has a sucrose (table sugar) backbone with 6 to 8 fatty acids attached (see Fig. 8-3). This makes olestra indigestible by humans—and calorie-free. Olestra is approved for use in salty snacks (e.g., “Light” or “Wow” potato chips), but not for use in frying foods like French fries, or in foods like salad dressing or cookies.

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The technical name of olestra is sucrose polyester. The properties of sucrose polyester can be changed by changing the number and kinds of fatty acids attached to the sucrose backbone.

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One concern was that olestra might act as a laxative. Being indigestible, it becomes part of the stool and might make it “slippery.” (Some people take mineral oil as a laxative—to make the stool “slippery.”) Many consumers complained of diarrhea and such, but a double-blind study showed diarrhea, stomachache, “gas,” etc., to be the same, whether or not olestra was consumed.²

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At a Chicago Cinemaplex, 1123 volunteers ages 1388 were randomly given regular or olestra potato chips. Followed-up for 4-10 days, symptoms and episodes of GI upsets were similar in both groups, with no relationship to the number of chips eaten.²

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Because olestra is chemically a fat, fat-soluble substances dissolve in olestra (but only when eaten about the same time), and can be lost along with olestra in the stool. To compensate for this, the fat-soluble vitamins A, D, E, and K are added to products made with olestra. Developing olestra, Simplesse, and the McLean burger was difficult and expensive, and each failed in the marketplace.

Saturated vs. Unsaturated Fat

Back to triglycerides. At room temperature, certain fats are liquid and others are solid. This reflects a basic difference in the kinds of fatty acids that are attached to glycerol in a triglyceride. Solid fats, as in bacon, are predominantly saturated fatty acids. Liquid fats, such as salad oil, are predominantly unsaturated fatty acids.

Of course, every shopper knows about unsaturated fatty acids—or was it polyunsaturated? And that there’s something good—or was it bad?— about them. They are why, if we really love our families, we buy corn oil—or was it canola? Anyway, they have something to do with the oil floating on top of old-fashioned peanut butter—or does it?

Happily, the basic chemistry of saturated and unsaturated fatty acids isn’t so complicated. A saturated fatty acid is one in which all the positions for hydrogen (H) are filled: the carbons in the fatty acid are saturated with hydrogen. Stearic acid, common in beef (steer), is an example (see Fig. 8-5a).

An unsaturated fatty acid is one which doesn’t hold all the possible hydrogen—it’s unsaturated with respect to hydrogen. Oleic acid, common in olive oil, is an example (see Fig. 8-5b).

Let’s look at these two structures carefully. We see that both molecules have 18 carbons, and in most other ways are exactly alike. The key difference is that the unsaturated oleic acid is missing two hydrogens. This seemingly trivial difference is important in its effects in the body (as we shall see in the next chapter). It also determines whether the fat in food is liquid or solid. If the oleic acid (with two missing hydrogens) predominates in the triglyceride—as it does in olive oil—it is liquid at room temperature. But if stearic acid (with all its hydrogens) predominates—as in beef fat—the fat is solid.

In unsaturated fatty acids, hydrogens are missing in twos, causing the adjoining carbons to form an extra bond—a double bond. Thus, oleic acid is called monounsaturated because it has one double bond—one (mono) place where it is unsaturated (can take on more hydrogen).

Figure 8-4: Unsaturated fats do not stack compactly and are liquid at room temperature. Saturated fats stack tightly and form solids.

A polyunsaturated fatty acid has more than one double bond. An example is the essential fatty acid linoleic acid, with two double bonds (see Fig. 8-5c). The other essential fatty acid linolenic acid also has 18 carbons, but contains three double bonds (see Fig. 8-5d).

As said before, the number of double bonds in the fatty acids affects whether a fat is solid or liquid at a given temperature. Triglycerides contain mixtures of fatty acids (see Table 8-2), so whether a fat is solid or liquid depends on which ones predominate. For example, corn oil is richest in linoleic acid (2 double bonds), and olive oil is richest in oleic acid (1 double bond), and both are liquid at room temperature. But if you put these oils in the refrigerator, olive oil partially solidifies, whereas corn oil (with more double bonds) does not.

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All fats and oils contain a variety of fatty acids.

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Beef fat is solid at room temperature because its fatty acids are mainly saturated. In contrast, fats from plants are usually liquid because their fatty acids are mainly unsaturated.

Figure 8-5: Chemical Structure of Some Fatty Acids

Fish contain fatty acids that are particularly polyunsaturated (contain many double bonds) since their fat-containing tissues need to be limber in their cold environment. For example, one fatty acid found in fish contains five double bonds. When fish migrate to waters of different temperature, they change the number of double bonds in their fatty acids to keep the same degree of fluidity in their tissues. If fish had the fatty acids of cows, they would be too stiff to swim. On the other hand, if cows had the fatty acids of fish, more than their tails would swish.

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It’s warmer in the Tropics, and the oil in tropical plants—such as coconut and palm oils—are more saturated, keeping the same degree of fluidity at a warmer temperature.

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Omega Double Bond: Unsaturated fatty acids not only differ in the number of double bonds, but differ also in the location of the double bonds.

In order to describe the location of double bonds, the carbons in the fatty acid are numbered starting from the far end (the CH₃ end)—the omega end—of the fatty acid. The number of the carbon holding the first double bond is the “omega number.” Linoleic acid, for example, has its first double bond between the 6^th and 7^th carbon (omega-6). This makes linoleic acid an omega-6 fatty acid. Linolenic acid is an omega-3 fatty acid since its first double bond is located between the 3^rd and 4^th carbon.

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Omega is the last letter—the far end—of the Greek alphabet. The omega number designates the location of the first double bond, counting from the far end. Omega-3 and omega-6 fatty acids are essential fatty acids.

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	% of Total Fatty Acids
	Saturated	Monounsaturated	Polyunsaturated
Plant source: Canola oil Safflower oil Corn oil Soybean oil Olive oil Peanut oil Palm oil Cocoa butter Coconut oil	6 8 14 15 16 17 51 60 91	62 13 28 23 72 49 39 35 6	32 78 58 61 11 34 9 3 2
Animal Source: Salmon fat Chicken fat Lard (pork) Tallow (beef) Tallow (lamb) Butterfat	19 31 41 52 59 65	55 48 47 44 35 31	26 20 12 4 5 4

Table 8-2: Fatty Acid Composition of Fats Common in the American Diet

The location of the first double bond—the omega number—is important because the body can add double bonds or more carbon atoms only beyond that first double bond. In other words, the structure of a fatty acid up to that first double bond is fixed. The body needs linoleic acid for its omega-3 structure, and linolenic acid for its omega-6 structure—making them essential nutrients.

Hydrogenated Fat

Food manufacturers can take polyunsaturated salad oils and add hydrogen to them, making them more saturated. (Hydrogen gas is used along with high temperature, high pressure, and a catalyst to speed the reaction.) This process is called hydrogenation (see Fig. 8-6).

When the list of ingredients on a food label includes partially hydrogenated oil, this means that some, but not all, of the double bonds in the fatty acids in the oil have had hydrogen added to them (converting double bonds to single bonds).

Hydrogenating the oil in peanut butter solidifies the oil so it won’t float to the top. This makes the peanut butter smooth and consistently spreadable from the top to the bottom of the jar. More hydrogenated oil can be added—more peanut butter without more peanuts. The oil in “old-fashioned” peanut butter is not hydrogenated, and the oil floats to the top.

It should be noted that when vegetable oils are hydrogenated, they don’t contain the same fatty acids they had originally. If, for example, linoleic acid (2 double bonds) is completely hydrogenated, it becomes stearic acid instead. In other words, hydrogenated oils no longer have the fatty acid composition shown in Table 8-2.

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Hydrogenation adds hydrogen to double bonds, making fat more saturated and more solid.

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Because different vegetable oils can be hydrogenated to be similar in physical properties (e.g., hard/soft, “melts in the mouth”), companies that make cookies, crackers, etc., could buy whatever oil happened to cost the least at the time. The ingredient list would then say, for example, “contains one or more of the following hydrogenated oils: soybean, cottonseed…” so that the label wouldn’t have to be changed depending on which oil was used. Buying the least expensive oil for this purpose made economic sense. If one oil is more unsaturated than another, it could simply be hydrogenated more to achieve the same physical properties (e.g., firmness) desired in the food product.

By hydrogenating the oils, food companies could extend the shelf-life of their products. This is because double bonds in fatty acids are more easily broken by oxidation (see Fig. 8-6), making the oil disagreeably rancid (“go bad”). By hydrogenating the oil used in a food product, the product would taste fresh longer (has a longer shelf-life) because there would be fewer of the susceptible double bonds.

Bacteria in the rumen of cows and sheep can hydrogenate fatty acids. For this reason, fat in beef and lamb is more saturated than fat from animals without a rumen (e.g., chickens). The extent of saturation of the fat can also be affected by the saturation of the fat in the animal’s diet.

Figure 8-6: Oxidation and Hydrogenation of Unsaturated Fatty Acids. Hydrogenation lessens the chance of oxidation, increasing the product’s shelflife.

Trans fatty acids are a by-product of partially hydrogenating unsaturated fatty acids—whether by food companies or in an animal’s rumen. This relates to the “shape” of a fatty acid wherever there’s a double bond.

A double bond in an unsaturated fatty acid creates a distinctive bend in the fatty acid—a cis arrangement (see Fig. 7). The process of partially hydrogenating a fat can flip the arrangement around the remaining double bonds—a trans arrangement. Removing that distinctive bend makes it resemble a saturated fatty acid (see Fig. 8-4). Trans fatty acids are more solid than a cis fatty acid of the same length and with the same number of double bonds. Also, as we shall see in the next chapter, “trans fat”—like saturated fat—can raise our risk of heart disease. For this reason, our FDA (Food and Drug Administration) has banned artificial trans fat (i.e., created by manufacturers) in processed foods.

Figure 8-7: Cis vs. Trans Fatty Acids

Before 2006, partially hydrogenated fat was used extensively in food products (crackers, cookies, etc.) and to fry foods (french fries, fried chicken, etc.). The listing of trans fat on food labels has been required since 2006. Before, only the amounts of total fat and saturated fat were required. Because consumers recognized trans fat—like saturated fat—as “bad fat,” marketers knew that products labeled as having trans fat could lose market share, and that “trans fat free” touted on labels would have market value.

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In 2003, Frito-Lay proactively started frying their Lays and Ruffles potato chips in corn oil instead of partially hydrogenated oil, to eliminate trans fat (but this also shortened the shelf-life of the chips). In 2006, they started using NuSun™ sunflower oil, which has less polyunsaturated fat (so extends shelf life), less saturated fat, and more monounsaturated fat.

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Food manufacturers found ways to eliminate partially hydrogenated fat (and its trans fat) by, for example, substituting tropical oils such as palm or coconut oil. Tropical oils are higher in saturated/ solid fat and lower in polyunsaturated/liquid fat.

Why didn’t they do this before trans fat was required on the label? For one, tropical oil costs more than partially hydrogenated oil. For another, the trans fat in partially hydrogenated oil made the fat more solid, yet wasn’t a saturated fat—handy when the amount of saturated fat, but not trans fat, was required on the label.

Not All Fats Are Triglycerides

Although triglycerides are what we generally speak of when we speak of fats, there are two other fats—phospholipids and cholesterol—that play essential roles in the body. These represent a very small part of dietary fat and aren’t required in the diet because the body can make them. In other words, phospholipids and cholesterol are required in the body but not in the diet.