Protein

Importance of Protein

Protein, whose structure was illustrated earlier, joins carbohydrates and fat as the third macronutrient. Like carbohydrates and fat, the body can metabolize protein for energy, though not ideally in large quantities. When carbohydrate and fat intake is adequate, the body derives only about 2 percent of energy from protein. In the nineteenth century, a formative period in nutrition, protein was inconspicuous as energy. In 1842, German nobleman and professor Justus von Liebig (1803-1873), a pioneer in nutritional and agricultural chemistry, announced that the body’s energy came entirely from carbohydrates and fat, though protein was far from inconsequential.60 He esteemed it muscles’ basic unit. Moreover, protein contained nitrogen, which he considered the primary nutrient for all plants and animals. Although his ideas combined fact and falsehood, Liebig’s renowrn deterred criticism. His work and that of others elevated protein, a word that derived from the Greek protos, meaning “first” such that protein has priority among nutrients. Such thinking, whose implications Chapter 4 explores, made meat and other protein-rich foods dietary cornerstones. The nineteenth- century mania for protein led the USDA in the 1890s to counsel men to consume over 110 grams (3.9 ounces) daily.61

Recommended Protein Intake

Current recommendations vary, with estimates between 0.3 and 1.2 grams of protein per kilogram (2.2 pounds) of body mass.62 Variations result from different methods of calculation, which in turn depend on divergent assumptions about digestive efficiency and protein loss through excretion and other processes. At a minimum, age, sex, and dietary quality shape protein requirements. Bodybuilders and athletes who prize strength have long championed protein-rich foods.

The 1890s USDA recommendation may be expressed in grams per kilogram of body mass for an average American man alive then. An absolute is elusive, though the USDA published ranges that vary by height and age. From 1885 to 1900, men averaged between 64 and 89 kilograms (140 and 195 pounds).63 A recommendation of 110 grams of protein daily translated into 1.7 grams per kilogram of body mass for a 64-kilogram man and just over 1.2 grams per kilogram for an 89-kilogram man. These numbers show that 1890s’ advice exceeded the current maximum.

Protein Quantity versus Quality

Preoccupation w'ith numbers may ignore more salient issues. The tussle over how much protein to consume might be compared to a debate over the amount of air to breathe. If it is full of soot and other pollutants, a person should inhale as little as possible and move to a pristine area. In a similar vein, nutritional quality is at least as crucial as quantity because not all proteins are identical. Their components, amino acids, determine quality. Amino acids are fundamental molecules with amine and carboxyl groups, both defined earlier. The body can manufacture from other compounds at least half the twenty amino acids that comprise all life.64 But it cannot make eight to ten, depending on whom the reader consults, which the diet must supply, and which are therefore essential.65 In addition, premature newborns and adults with liver damage may require amino acids that are normally unessential.66

Protein Sources

Animal products like meat, poultry, eggs, fish, and dairy have all essential amino acids and are regarded complete sources. Some plants are in this category, including potatoes despite seldom being valued in this capacity. As noted, Irish laborers subsisted on them. Chapter 13 examines potato eaters’ healthfulness in prehistory and history. This section restricts commentary to the observation that the tuber sustained commoners because of adequate amino acids and other nutrients. Another food seldom linked with amino acids is mushrooms, which also have all essentials.

Traditionally esteemed for protein and amino acids have been legumes and nuts. Chapters 8 and 9 focus on their roles in diet and nutrition, leaving this section to note that, excepting soybeans (Glycine max), neither supplies all essential amino acids.67 Another incomplete source is grains, treated in Chapter 12. They have some protein and amino acids, though taste buds sometimes counter the need for protein and other nutrients. For example, the types of wheat desirable for pastries, cookies, and similar items have starch at the expense of nutrients, including protein. Seeds, root and tuber crops, and kindred foods employ zero-sum properties. Increases in starch diminish space for proteins, vitamins, minerals, phytochemicals, and water. Of course, concentration on protein or appeal to a single criterion provides insufficient breadth for evaluating foods because those deficient in one nutrient may have others.

Inequalities and Protein Intake

Protein quality and quantity require more than an impartial selection of foods. Economic, social, racial, and political inequalities have dietary and nutritional consequences. Developing nations’ poor and minorities often cannot afford the complete proteins available in animal products. Later chapters emphasize the pervasiveness of inequalities throughout time and place and their nutritional effects. In modernity as in the past, consumption of meat and allied foods has risen with incomes. This trend yields more and better protein for elites but only by threatening others and the environment as cropland is converted into pasture and as innumerable cattle emit staggering amounts of greenhouse gas methane.

Protein’s Roles in the Body

Clamor for animal products cannot be ignored given protein’s roles in the body. After water, it is the body’s most abundant constituent at roughly 16 percent total mass.68 Of this amount, about 43 percent is muscle, 16 percent blood, and 15 percent skin, with smaller quantities in other structures. Of the body’s proteins, four are most numerous: collagen in skin, tendons, ligaments, and muscles; hemoglobin in blood; and myosin and actin in muscles. Beyond them, every cell in the body has proteins because of genes. DNA, mentioned earlier and present in a cell’s nucleus and mitochondria, comprises genes and contains large molecules known as nucleotide bases. Through the intermediary RNA, also mentioned previously, a sequence of three nucleotide bases (codon) directs production of one amino acid outside the nucleus, an area known as the cytoplasm. The order of these bases defines an amino acid. In the cytoplasm, structures known as ribosomes assemble amino acids into proteins.

Proteins perform crucial functions. For example, insulin, mentioned earlier and having fifty- one amino acids, is the hormone that tells cells to admit glucose.69 In this way, insulin regulates the amounts of glucose inside and outside cells. Insulin also regulates glucose by telling the liver to store excess for release when the sugar becomes scarce in blood. Heme proteins, defined by the presence of iron (Fe), shuttle molecules and electrons throughout the body. Hemoglobin, a component of red blood cells, brings oxygen to cells and removes carbon dioxide for transport to the lungs. Carbon dioxide is a greenhouse gas, though human respiration emits little compared to factories and automobiles. The protein keratin helps form hair and skin. Proteins known as enzymes catalyze the body’s reactions. For example, enzymes pepsin and trypsin aid protein digestion by catalyzing cleavage of amino acid peptide bonds, mentioned earlier. Integral to the immune system, proteins that combat pathogens are known as antibodies. Attention has focused on the protein interferon, which targets viruses.

Minerals

Need for Minerals

Liebig believed that the macronutrients carbohydrates, fats, and proteins supplied all the body needed for health. Not only was this supposition wrong, but his research helped lay the groundwork for exposing this error. In his role as agricultural chemist, Liebig focused on another nutritional triad, emphasizing that the elements nitrogen, potassium, and phosphorus were plants’ most important constituents. Earlier was noted nitrogen’s role in amino acids and proteins. Potassium and phosphorus, however, need not be part of organic molecules and are known as minerals, another class of animal nutrient.

Characteristics of Minerals

Minerals differ from carbohydrates, fats, proteins, and phytochemicals (discussed in a later section) in being inorganic compounds. They are solid at room temperature and occur as crystals or other regular structures in rocks, soils (which contain pulverized rocks), meteorites, and dust. From soils, minerals enter roots, thereafter ascending trophic levels to become ubiquitous across flora and fauna and essential to life. Crystalline structure is evident in sodium chloride, mentioned previously and discussed in this section. Nutritionists and dieticians focus on elements within minerals. Having atoms of a single type, each element is unique in nature and a fundamental unit of matter. Shown below, the Periodic Table organizes the elements.

Of the table’s 118 elements, ninety-four occur in nature. All heavier than plutonium (Pu) are artificial, being derived in the laboratory.

Left of the zigzag line known as the staircase are metals, notable in electrons’ ability to move through them. To the right are nonmetals, which resist electron flow'. For completeness, elements on the staircase have characteristics of metals and nonmetals, do not neatly fit either category, and are known as metalloids. Most mineral elements are metals. The major minerals are sodium, calcium, potassium, phosphorus, and magnesium. Necessary in smaller amounts are the minor minerals: manganese, iron, zinc, iodine, fluorine, selenium, copper, chromium, cobalt, and molybdenum. Of these, only fluorine is a nonmetal. Although many nutritionists and dieticians do not consider chlorine a mineral, it appears to fulfill the criteria. It too is a nonmetal. Being in the column (group) of the Periodic Table known as the halogens, fluorine and chlorine share properties. The body needs chlorine as the electrolyte chloride and to make hydrochloric acid, which aids digestion.

Sodium, Chlorine, and Table Salt

These functions may seem incongruous with chlorine’s presence in table salt, a villain to nutritionists and dieticians. Salt’s other element, sodium, w'as noted as another mineral. Sodium is in bones, where its function is unknown. Like chlorine, sodium becomes an electrolyte in the body. Sodium and chlorine’s roles made salt indispensable to people in hot climates because sweating depletes electrolytes. For this reason, American kinesiologists J. Luke Pryor and Deanna M. Dempsey in 2015 encouraged athletes w'ho perspired heavily to salt food.70

Salt’s dangers stem from overconsumption. In the body, it ionizes into sodium cations (Na+) and chloride anions (CL). In this state, attention focuses on sodium’s role in raising blood pressure by increasing the density of cations in blood relative to the corresponding density within cells. Through a process known as osmosis, water from cells enters blood to equalize cation density. The heart must work hard to pump this blood, full of extra w'ater, elevating blood pressure. Furthermore, the kidneys must labor to excrete excess sodium. Research also implicates chloride in high blood pressure.71

Minerals Interact with Other Nutrients

Exhaustive treatment of minerals is unnecessary. Instead emphasis is on their interactions with other nutrients. In this regard, the relationship between calcium and vitamin D has received study.

Periodic Table of the elements. (Adapted from National Institutes of Health [NIH].)

FIGURE 2.9 Periodic Table of the elements. (Adapted from National Institutes of Health [NIH].)

Chapter 1 mentioned vitamin D as an example of a nutrient under scientific, medical, and journalistic scrutiny. The interplay between calcium and vitamin D is wide ranging, affecting bones, teeth, the immune system, muscles including the heart, metabolism, the respiratory system, and retention and activity of phosphorus, magnesium, iron, selenium, copper, and zinc. None of these dynamics was apparent in the eighteenth century, when the rise of factories in Europe and North America lured people from countryside to city and from outdoors to indoors, increasing the number of children with rickets, a condition whereby weak leg bones bow under the upper body’s mass.72 An ailment related to rickets—osteomalacia—afflicted adults. These problems arose because existence indoors deprived skin of sunlight and the concomitant ability to manufacture vitamin D.

Only in the twentieth century did scientists discover this vitamin and begin to understand how it, calcium, and phosphorus prevented and cured rickets and osteomalacia.73 Bones and teeth require vitamin D to retain calcium, whose absorption improves in the presence of phosphorus and magnesium. Too much phosphorus, however, hinders bones and teeth from absorbing calcium, and too much calcium weakens the body’s absorption of phosphorus, zinc, and iron. Calcium also affects the body’s uptake of selenium and copper. Magnesium helps regulate vitamin D by reducing excess, increasing dearth, and thereby improving calcium absorption.74 This list need not be lengthened to underscore the complexity of relationships among minerals and between minerals and vitamins.

Sources of Minerals

A balanced diet gives the body adequate minerals. Nuts, particularly almonds and cashews, provide calcium, copper, iron, selenium, zinc, phosphorus, and magnesium. Seed legumes, notably beans, soybeans, chickpeas (Cicer arietinum), and lentils (Lens culinaris), supply copper, iron, potassium, phosphorus, zinc, and magnesium. Salmon, tuna, and mackerel have calcium, magnesium, selenium, potassium, and phosphorus. Oysters (species in genera Crassostrea and Ostrea), mussels (Mytilus edulis), clams (Mercenaria mercenaria and other edible species), scallops (Placopecten magellanicus and other edible species), and other shellfish deliver copper, iron, selenium, phosphorus, and zinc. Seeds from sunflower (Helianthus annuus), pumpkin (Cucurbita pepo) and other squashes (Cucurbita species), and flax (Linum usitatissimum) contain copper, iron, phosphorus, selenium, and zinc. Mushrooms have copper, potassium, selenium, and zinc. Milk and yogurt contribute calcium, magnesium, potassium, and phosphorus whereas cheeses tend to be high in calcium, copper, and phosphorus. Spinach, kale, Swiss chard (Beta vulgaris subsp. vulgaris), turnip greens (Brassica rapa ssp. rapa), and other dark green leafy vegetables contain calcium, copper, iron, potassium, magnesium, and zinc.

Plant foods, however, are sometimes suboptimal mineral sources. For example, whole grains and beans have the antioxidant phytic acid, which combines with iron, zinc, manganese, and calcium, reducing their absorption. Calcium and oxalate anions (C204)2-, prominent in leafy green vegetables and grains, compound this problem by worsening iron absorption. In contrast, vitamin C improves iron absorption.75 The body absorbs iron better from meat, poultry, and seafood than from plants. Animal iron is designated heme because it is in blood’s hemoglobin. Animals also harbor iron in muscles’ myoglobin. Plant iron is nonheme.

 
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