Plants and Animals Store Energy Differently

As a rule, animals differ from plants in having extra options for storing energy. The previous section emphasized the body’s limited capacity to stockpile energy as glycogen. Beyond this threshold, the body stores energy as fat, whose structure as earlier section described. Although some plants have fat as oils, these lipids seem to be less reserve energy than enticements to tempt animals to eat fatty seeds, which may pass through the digestive tract and be excreted, a phenomenon that Chapter 14 applies to fruits. Excretion deposits them on the ground, which dung fertilizes for seedlings’ growth. The previous section—citing peanuts, cashews, walnuts, pistachios, and almonds as examples—noted that some seeds have many calories because of high oil content. Chapters 3 and 10 examine fat, permitting brief treatment here.

Lipids, Fat, and Oil

The previous paragraph introduced fat in the context of lipids and oils. Nutritionists and dieticians think of these as nested categories, the largest being lipids, inside of which is fat, which, in turn, contains oils. But chemists reject this type of nesting, instead defining lipids as the largest category, which houses both fat and oils as separate classes. In this scheme, fats are solid at room temperature, roughly 20 degrees Celsius (68 degrees Fahrenheit), whereas oils are liquid.50 This book follows nutritional convention in considering oils a type of fat, a simplification that permits treatment of all macronutrients that are neither carbohydrates nor protein as fat. Food labels follow this scheme, dividing macronutrients into fat, carbohydrate, and protein in that order, as shown in the sample.

Food label courtesy of FDA. (FDA public-domain statement https://www.fda.gov/about-fda/ about-website/website-policies.)

FIGURE 2.6 Food label courtesy of FDA. (FDA public-domain statement https://www.fda.gov/about-fda/ about-website/website-policies.)

Saturated Fat

This label introduces the principle of saturation. A saturated fat has carbon atoms forming single bonds, as in the following diagram.

Example of saturated fat. (Structure from ChemDraw.)

FIGURE 2.7 Example of saturated fat. (Structure from ChemDraw.)

Ignoring the carboxyl group mentioned earlier and within a rectangle in Figure 2.7, each carbon forms two bonds with hydrogen atoms and the other two with adjacent carbons. Aside from the carboxyl group, no carbon has a double bond with another atom.

Unsaturated Fat

In contrast to saturation, unsaturated fats have at least one carbon outside the carboxyl group with a double bond. When only one carbon, excluding the carboxyl group, is involved, the result is a mono- unsaturated fat. Unsaturated fats with more than one such carbon are polyunsaturated. Comparison of saturated, monounsaturated, and polyunsaturated fats yields the following diagram, whose arrows highlight double bonds apart from the carboxyl group.

Comparison of saturated, monounsaturated, and polyunsaturated fatty acids. (Structure from ChemDraw.)

FIGURE 2.8 Comparison of saturated, monounsaturated, and polyunsaturated fatty acids. (Structure from ChemDraw.)

Unsaturated fats promote health better than saturated ones, though the food industry prefers the latter in processed food because they lengthen shelf life and supply textures that consumers favor.51 Conversion of unsaturated fats into saturated, a process known as hydrogenation, creates foods that increase the risk of heart disease. Attention has focused on partial hydrogenation, which yields trans fats, a configuration uncommon in nature. The U.S. Food and Drug Administration (FDA) in 2015 declared them unsafe, requiring their elimination from foods by January 1, 2020.52

Lipids and Organic Solvents

Returning to the concept of nesting and to lipids, the observation that oil and water do not mix applies to all lipids, which dissolve not in water but in alcohols, gasoline, diesel, ether, chloroform, and other organic solvents. Lipids include oils, steroids (notably cholesterol), wax, and the fatty acids mentioned earlier. Oils, the smallest class, are triglycerides, whose structure a previous section illustrated. Their texture and consistency are greasy and thick.

Fat: The Energy Densest Macronutrient

An earlier section described fat as concentrated energy. In general terms, the body derives energy from breaking chemical bonds, a process that may be thought the reverse of photosynthesis, mentioned earlier, whereby plants store energy from sunlight in bonds. Humans and other animals break these bonds through oxidation, a reaction that requires oxygen. Because the body uses more oxygen to break bonds in fat molecules than in carbohydrate or protein molecules, it derives the most energy from fat.

The place of fat in the diet receives treatment from an evolutionary perspective in Chapter 3 and from the vantage point of prehistory and history in Chapter 10. This chapter confines commentary to the observation that throughout the past, food tended to be scarce rather than plentiful. This condition made fat important because it is energy dense. Amid modernity’s abundance, however, fatty foods provide too many calories. Of course, excess of any food—excepting the spartan fare of celery, cabbage, cucumber, strawberries, mushrooms, and other austerities mentioned earlier— presents this danger.

Uses of Fat besides Energy

This caveat aside, fat provides more than energy. All the body’s cells use fatty acids to construct their membranes, without which no cell can function. The membrane separates animal cells (plant cells have a wall) from their surroundings, preserving their integrity. The membrane’s indispensability and the cell’s antiquity as the basic unit in microbes, plants, and animals imply that fatty acids are among the earliest organic molecules as life’s precursors. Fats are important to the central nervous system (CNS), where they form neural membranes and the myelin sheath that surrounds each neuron’s axon. Made of neurons, the brain is roughly 70 percent fat.53 Cognition, a hallmark of humanity, would be impossible without it.

Fat is the storage medium for the fat-soluble vitamins, which receive treatment later. Its role as storehouse is crucial in another context. Earlier was noted the body’s meager ability to store energy as glycogen. As noted, only about 1 percent of body mass is glycogen whereas roughly 25 percent is fat, stored in a type of tissue known as adipose, which holds most of the body’s reserve energy.54 Each adipose cell, known as an adipocyte, harbors a fat droplet. The body retains primarily triglycerides in adipose, possibly an adaptation to the reality that they constitute around 95 percent of edible fats.55 More than a reserve, adipose insulates the body—an asset in cold but not in heat—cushions organs and helps transmit signals between cells (extracellular) and within a cell (intracellular).

Cholesterol and Its Regulation

Among fats, attention focuses on cholesterol, mentioned earlier, as contributor to heart disease. In this context, medical practitioners distinguish between good and bad cholesterol. This language causes confusion because “good cholesterol” is more than cholesterol—also having protein, triglycerides, and phospholipids (a lipid with phosphorus)—and is known as high-density lipoproteins (HDL).56 Doctors praise HDL for removing excess cholesterol from the blood whereas low-density lipoproteins (LDL)—having different ratios of the same components—are deemed bad because they carry cholesterol to cells. Cells depend on LDL for it, though problems arise when LDL bring too much. Criticisms of cholesterol should not discount its role in manufacturing cell membranes, acids that help digest food, vitamin D from sunlight, and five types of steroid hormones: progestins, glucocorticoids, mineralocorticoids, estrogens, and androgens, all of which the body requires.

Essential Fatty Acids

The body also needs fatty acids it cannot manufacture: omega-3 and omega-6, both polyunsaturated. Chemists number fatty acids to identify them, analogous to urban planners who number addresses and streets. By convention, chemists begin at the carboxyl group, whose carbon atom is designated alpha to honor the Greek alphabet’s first letter. The last carbon, at the methyl end, is omega, the Greek alphabet’s last letter. Attending to the methyl (omega) end, nutritionists note the position of the first double bond. For example, an omega-3 fatty acid has its first double bond three carbons from omega. Chapters 3 and 10 treat the nutritional roles and dietary composition of fatty acids, especially omega-3 and omega-6.

Fatty Foods

The body derives fat from foods. Earlier sections mentioned several fatty foods. To this list may be added meat given that livestock fatten as they approach market mass. Sausages, pepperoni, salami, and similar items have abundant fat. Oily fishes include sardine (Sardina pilchardus), herring (Clupea harengus), anchovy (commercial species are in the genus Engraulis), salmon (Oncorhynchus species), tuna (Thunnus thynnus and T. albacares), trout (Oncorhynchus mykiss), swordfish (Xiphias gladius), and mackerel (Scomber scombrus). Dairy products are another option, milk fat having hundreds of fatty acids. Butter, cream, and ghee are primarily fat. Many cheeses, ice creams, and some yogurts have plentiful fat. An ingredient in chocolate, theobroma oil in cacao (Theobroma cacao) seeds’ cocoa butter has saturated fat, much of it as triglycerides. Nutritionists and dieticians debate the merits of chocolate and cocoa butter. Manufacturers add fat to many processed foods. For example, a serving of Hostess twinkie (77 grams) has 8 grams of fat, most coming from tallow (beef fat).57 More than one-quarter of its calories are fat. Carbohydrates supply the rest. Another food with processed ingredients, pizza combines cheeses, meats, and vegetable oils, providing more fat and calories than might be desirable. People add fat to foods through frying. In an era of abundance, fat is too plentiful. Chapters 3 and 10 examine fatty foods in diets over time and space.

As noted, fatty foods—nuts were cited as examples—are caloric because fat is energy dense. For this reason, dieters may want to minimize them, though not all promote obesity. Mentioned above, oily fishes are not awash in calories. For example, 100 grams of salmon have 4 grams of fat and 139 calories.58 These numbers contrast salmon with twinkies, 100 grams of which provides roughly 9 grams of fat and 200 calories. This contrast extends to the big mac, introduced earlier, 100 grams of which supplies about 12 grams of fat and 225 calories.59 These differences reinforce the fact that not all sustenance is equal. Indeed, readers might doubt that twinkies and similar junk merit designation as sustenance. In addition to Chapters 3 and 10, those on meat (Chapter 4), fish (Chapter 5), dairy (Chapter 7), and nuts (Chapter 9) illuminate fatty foods’ health effects.

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