Carbohydrates

Sugars

The previous section noted that simple sugars build complex carbohydrates like starch and glycogen (C6Hl0O5)n, where n is a positive integer. The most basic sugar, glucose (also known as dextrose), is fundamental to life. Through photosynthesis, plants use sunlight to manufacture glucose from carbon dioxide and water according to the reaction

Glucose is a carbohydrate because it fits the formula mentioned in the last section. For completeness, plants construct glucose by taking carbon dioxide (CO,) from the air and water from the soil. In addition to glucose, photosynthesis produces the oxygen (02) necessary for aerobic respiration. Without oxygen, life would consist of only plants and anaerobic microbes. All other organisms, including humans, could not have evolved without the oxygen that plants supply.

Glucose is one of several sugars available in foods. All cannot receive treatment here, but a handful has played important dietary and historical roles. Fructose, along with glucose, is the sugar in honey, high fructose corn syrup (HFCS), and many fruits and vegetables. Fructose has the same formula as glucose but a different structure and a unique metabolic pathway in the body. Chemists categorize the two as isomers because of these properties. Among natural sugars, fructose tastes sweetest. Sweetness is potent in HFCS, examined in Chapter 11, a product of corn starch. In the 1960s, American and Japanese chemists treated corn starch w'ith enzymes to generate corn syrup. Addition of the enzyme xylose isomerase converted some of the resulting glucose into fructose such that HFCS is typically 45 percent glucose and 55 percent fructose, though the latter may be up to 90 percent.28 By comparison, sucrose, discussed below, is roughly half glucose and half fructose.

Not all the body’s organs can metabolize fructose whereas cells throughout the body can metabolize glucose. In the case of fructose, the liver is the primary engine of metabolism, using the sugar to rebuild its store of glycogen. The liver can also use fructose to manufacture a class of fats known as triglycerides, whose structure the last section illustrated. Unlike glucose, the body can metabolize fructose without the hormone and protein insulin. This property allows many diabetics to tolerate fructose better than other sugars. Nonetheless, research implicates fructose in insulin resistance and type 2 diabetes. Additionally, fructose may not prod the brain to signal satiety. Absence of fullness may cause a person to overeat, thereby gaining fat and increasing risk of type 2 diabetes, heart disease, and some cancers.

Among sugars, sucrose has inordinately shaped history. In sugarcane stems and sugar-beet taproot, sucrose has stimulated taste buds since prehistory. Sugarcane fueled the rise of plantations and slavery in the Americas, lands that still grapple with the racism spawned by unequal and unjust economic, social, and political systems. Chemically, sucrose contains molecules of both glucose and fructose, as noted. By mass, sucrose has the same calories (roughly 3.6 per gram) as glucose or fructose and, Chapter 11 notes, has come under scrutiny for undermining health.29 Like other sugars, sucrose is empty calories, supplying nothing beyond energy. No more helpful is manufacturers’ addition of it to processed foods, which have too many sugars and artificial ingredients and too much fat and salt. In such arrangements, sucrose is not the lone villain and attempts to assign it special blame may be misguided (see Chapter 11) given that manufacturers cluster it among allied rubbish, all deserving condemnation.

For completeness, the word “salt” is imprecise because there are many salts, all forming from reactions between acids and bases. Nutritionists and dieticians use the term to mean table salt, the ionic compound sodium chloride (NaCl). It forms a crystal from union of the metal sodium (Na) and the nonmetal chlorine (Cl) in a 1:1 ratio. Such arrangements typify ionic compounds. Although sodium has a dismal reputation among nutritionists, it and chlorine are nutrients. In excess, however, both weaken health. The section on minerals examines them.

The sugar in milk, lactose has the same formula as sucrose; both are disaccharides, though their structures differ. Producing the enzyme lactase, the small intestine digests lactose. People without this enzyme suffer distress upon consuming milk, a phenomenon that Chapter 7 discusses. This condition may exist at birth. In other cases, people can digest lactose as infants only to reduce lactase production with age, impairing milk tolerance. In regions where dairying has a long history— parts of northern Europe, for example—natural selection has equipped most people to produce lactase throughout life and thereby to tolerate milk. Those intolerant need not eschew all dairy products. Being concentrated in milk solids, lactose is largely absent from butter. Separation of curd from whey and removal of the lactose-rich whey from cheese lower the lactose content. Moreover, enzymes that help make many types of cheese break down lactose. A similar process occurs to produce many yogurts.

Chapter 12 mentions that the addition of grains, first harvested as wild plants at least 22,500 years ago, to diets in southwestern Asia and their subsequent spread to many parts of the world introduced humans to the sugar maltose, another disaccharide with the formula C12H22On.30 The advent of grain cultivation heralded the beginnings of agriculture in southwestern Asia. Among grains, barley (Hordeum vulgare) attracted attention as much for making beer as bread. Although ancient peoples were not chemists in any modern sense, they understood how to manipulate barley’s maltose content to produce the alcohol ethanol (C2H5OH). Through a process known as malting, they soaked seeds in water to hasten germination. Drying these seeds after germination halted growth, stabilizing the amount of maltose for conversion into ethanol.

Starch

Sugars may be stored as starch in plants or as glycogen or fat in animals. Storage holds energy that may be used during lean times in the way that a person might put money in a bank to cover expenses should he lose employment. Sunlight is the ultimate source of energy. Recall that plants use sunlight to manufacture glucose, whose energy fuels their growth and maintenance and whose excess is stored as starch by linking together glucose molecules. The simplest starch molecule, known as amylose, comprises 1,000-600,000 such units.31 The formula (C6H10O5)#, where # is a positive integer, characterizes starch, which is found in plant structures that evolved as energy depots. For example, the edible tubers of the potato plant, being storehouses, hold roughly 87 percent of their carbohydrates as starch.32 Seeds also tend to have starch to fuel germination. For instance, by mass oats are 45-62 percent starch but under 1 percent sugars.33

Due to their structure, not all starches affect the body the same. Being large, straight, and helical, amylose slows digestive enzymes from breaking it apart and is classified as resistant starch. The more amylose food has, the harder it is to digest. Although this characteristic might seem undesirable, foods that resist digestion do not inundate the bloodstream with glucose and so do not cause the pancreas to overproduce insulin. The body needs insulin to metabolize glucose, but too much produced too quickly and too often creates a condition known as insulin insensitivity, whereby cells lose their ability to respond to the hormone. This problem contributes to chronic diseases, notably type 2 diabetes, a malady linked to heart disease, stroke, and kidney disease. In 2017, the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, estimated that 100 million Americans have this kind of diabetes or are on its threshold.34 In contrast to amylose, the starch amylopectin, although larger, arrays its glucose molecules in branches that digestive enzymes readily dismantle. The more amylopectin in food, the more rapid is digestion, causing the above cascade of perils.

Fiber

Unable to produce enzymes to cleave them, the body cannot digest carbohydrates termed fiber, roughage, or bulk. An example is cellulose, which has the same formula as starch and which plants use to make rigid cell walls. Crucifers, all in the Brassicaceae or Cruciferae family, contain abundant cellulose and include broccoli (Brassica oleracea var. italica), kale (Brassica oleracea var. sabellica), Brussels sprouts (Brassica oleracea var. gemmifera), cauliflower (Brassica oleracea var. botrytis), and cabbage (mentioned earlier). Packed with cellulose, these vegetables do not expand the waistline, demonstrating that indigestibility makes fiber a dieter’s friend. In addition to controlling mass, fiber-rich foods diminish constipation, the risk of type 2 diabetes, and blood levels of low-density lipoproteins (LDL or “bad cholesterol”), whose properties are discussed later.

Fiber is soluble or insoluble depending on whether it dissolves in water. Soluble fiber, useful against LDL, is in many whole foods, including peas (Pisum sativum), beans (Phaseolus vulgaris, P. lunatus, P. acutifolius, and P. coccineus), oats, citrus fruits, apples (Malus domestica), and barley. Insoluble fiber promotes regularity and may be consumed in potatoes, nuts, beans, and wheat (Triticum monococcum, T. dicoccon, T. aestivum, and T. durum) bran among other foods.

Being a type of carbohydrate, fiber is absent from fat and protein. Consequently, diets rich in these macronutrients tend to lack fiber. Such fare pervades developed countries and affluence everywhere. As incomes rise, people buy meat, eggs, milk, cheese, and other animal products, which crowd plants from the diet. To be sure, animal products contain nutrients but not fiber.

Consider the hamburger, a fast-food staple. The ground beef lacks fiber. The bun has fiber, though the amount may be meager. If made from whole wheat, the bun has roughly 3 grams of fiber per around 50 grams of bread, the standard for a bun.35 But the fast-food hamburger comes on white bread, which reduces fiber below 1 gram per bun because milling, a process discussed in Chapter 12, strips most of the bran from flour, thereby removing fiber.36 The consumer might add cheese or bacon to enhance flavor but neither has fiber. Among condiments, 100 grams of mustard have 4 grams of fiber, but the same amount of ketchup or mayonnaise has none.37 Lettuce (Lactuca sativa), tomato (Solarium esculentum), and onion (Allium сера) are the most fibrous additions to the hamburger, but what the consumer buys has little roughage. McDonald’s counts 3 grams of fiber in its big mac and mushroom and swiss burger, 2 grams in its cheeseburger and bacon smokehouse burger, and 1 gram in its hamburger.38 McDonald’s computes just 6 percent of fiber’s daily value (DV) in its hamburger.

Table 2.9 shows that hamburgers deserve no special animus in a world full of low-fiber options. Snacks bulge from supermarket shelves and fill stomachs with little besides fat and sugars. For example, U.S. bakery Hostess Brands’ twinkies lack fiber, and ho hos at 85 grams per serving, cupcakes at 45 grams, and ding dongs at 72 grams have 1 gram of it.39 Being sucrose or HFCS and water, soft drinks and related beverages lack roughage. Depending on the brand, ice cream and chocolate may have some, but in small quantities. Dunkin’ Donuts lists a single glazed jelly donut at 1 gram of fiber.40 Kellogg’s tallies under 1 gram of fiber in one brown sugar cinnamon pop tart.41 These and kindred items are prevalent not because the food industry is particularly malevolent but because consumers demand them. Chapter 3 treats flavor preferences and cravings from an evolutionary perspective.

By contrast, Table 2.10 indicates that whole plant foods tend to have fiber.42

TABLE 2.9

Fiber in Processed Foods

Manufacturer

Product (100 g)

Fiber (g)

McDonald’s

Big mac

1.38

McDonald’s

Mushroom and swiss

1.4

McDonald’s

Cheeseburger

1.75

McDonald’s

Bacon smokehouse burger

N/A

McDonald’s

Hamburger

1

Hostess

Twinkie

0

Hostess

Ho Ho

1.18

Hostess

Cup cake

2.2

Hostess

Ding dong

1.39

Hostess

Soft drink

0

Dunkin Donuts

Glazed jelly donut

1

Kellogg

Brown sugar cinnamon pop tart

<2

TABLE 2.10

Fiber in Whole Foods

Whole Food (100g)

Fiber (g)

Dried apricots

18

Prunes

13

Dried figs

8

Oatmeal

7

Kidney beans

7

Whole grain bread

7

Mixed nuts

6

Peas

5

Dried dates

4

Whole grain pasta

4

Glycemic Index

Scientists use the glycemic index (GI), a scale between zero and 100, to quantify carbohydrates’ digestibility. Recall that sugars are easily digested, enter the bloodstream rapidly, and spike insulin production. Glucose provokes the fastest increase, signified by a value of 100. At the opposite pole, fiber does not raise insulin production and so rates zero. That is, numerical values correlate with insulin production such that the nearer a food comes to 100, the more it elevates the hormone. The Glycemic Index Foundation in New South Wales, Australia, ranks foods as low (under 46), medium (between 46 and 59), or high (more than 59) and cautions against consumption of foods medium or high on the index.43 These categories are not absolute because not all scientists partition numbers the same way.

Glycogen

Like plants, animals store sugars, as noted, converting them into glycogen or fat. Like starch and cellulose in plants, animals construct glycogen from glucose molecules. Such duplication is unsurprising because these aggregates serve similar purposes in plants and animals and because all life shares a common evolutionary heritage such that the more closely related are two organisms the greater tends to be their chemistry. Muscles and the liver store glycogen but not in quantity, the body holding only about 1 percent of mass as it.44 Despite such parsimony, glycogen is important. Evolution equipped people to draw upon it for a burst of energy. In crises, the body prepares for battle or retreat, metabolizing glycogen, as well as fatty acids, in expectation of strenuous exertions. Muscles and the brain prefer to draw' energy from the glucose in glycogen.45 Physical activity, including athletics and vigorous recreation, demands carbohydrates. Even endurance athletes metabolize carbohydrates over fat. To be sure, the body burns fat during exertions but only by increasing the demand for oxygen over that required for carbohydrates.

Other Functions of Carbohydrates

Beyond being energy, carbohydrates enable the body to synthesize the molecule of heredity, deoxyribonucleic acid (DNA), and its messenger, ribose nucleic acid (RNA). No cell can exist much less function without them. As w'ith DNA and RNA manufacture, glucose allows the body to make the monosaccharide glucuronic acid, w'hich increases wastes’ solubility in urine or bile. Glucose, fructose, and the sugar galactose help the body form protective mucus. Through various pathways, carbohydrates increase cells’ efficiency and aid in creating humans’ four blood types.

Carbohydrate Critics

Carbohydrates’ importance has not spared them criticism, a topic that Chapters 10 and 11 also treat. An earlier paragraph implicated sugars in type 2 diabetes, but hostility goes farther. Nutritionists, dieticians, researchers, and physicians who left the 1980s low-fat bandwagon disparage carbohydrates. Several of the most strident critics have written best-selling books with trade publishers. The dust jacket to American neurologist David Perlmutter’s (b. 1954) 2018 edition of Grain Brain boasts “more than 1 million copies in print.” He targeted carbohydrates and gluten, a complex of over one hundred proteins in some grains, which makes dough sticky, elastic, and able to rise during baking. Because grains have starch and sugars, animus toward gluten provides ammunition against carbohydrates, which Perlmutter characterized as unnatural because the body supposedly prefers to metabolize fat for energy.46 Carbohydrates’ putative unnaturalness predisposes the body to several ailments, including overweight and obesity. He blamed the food industry, especially breakfast cereal companies, for deceiving Americans about carbohydrates. Profits trump health in a case where capitalism devours the masses.

Perlmutter is not a lone voice in the wulderness. Losing more than 14 kilograms (30 pounds) each by cutting carbohydrates, mother Laura Childs and daughter Veronica Childs faulted them for causing an “obesity epidemic” in The Complete Low Carb, High Fat, No Hunger Diet (2014).47 Carbohydrates cause overeating and fatness because they are addictive, prodding the brain to release pleasurable chemicals like neurotransmitter serotonin, believed American dietitian Sandra Woodruff.48 In the crusade against carbohydrates, American cardiac surgeon Steven R. Gundry excoriated dessert fruits: “The next time you ask for a fruit salad as a ‘healthy’ breakfast, I suggest that instead you order a bowl of Skittles candy. Go ahead—it’s the same poisonous stuff.”49 Readers must decide whether hyperbole enlightens or frightens.

 
Source
< Prev   CONTENTS   Source   Next >