Nutritional Requirements
Nutritional
Requirements
The food of animals must include carbohydrates, proteins, fats, water, mineral salts, and vitamins. Carbohydrates and fats are required as fuels for energy and for the synthesis of various substances and structures. Proteins (actually the amino acids of which they are composed) are needed for the synthesis of specific proteins and other nitrogen-containing compounds. Water is required as the solvent for body chemistry and as a major component of all fluids of the body. Inorganic salts are required as the anions and cations of body fluids and tissues and form important structural and physiological components throughout the body. Vitamins are accessory factors from food that are often built into the structure of many enzymes.
A vitamin is a relatively simple organic compound that is not a carbohydrate, fat, protein, or mineral and that is required in very small amounts in the diet for some specific cellular function. Vitamins are not sources of energy but are often associated with the activity of important enzymes that serve vital metabolic roles. Plants and many microorganisms synthesize all the organic compounds they need; animals, however, have lost certain synthetic abilities during their long evolution and depend ultimately on plants to supply these compounds. Vitamins therefore represent synthetic gaps in the metabolic machinery of animals.
Vitamins are usually classified as fat soluble (soluble in fat solvents such as ether) or water soluble. The watersoluble vitamins include the B complex and vitamin C (Table 34-1). Vitamins of the B complex, so grouped because the original B vitamin was subsequently found to consist of several distinct molecules, tend to be found together in nature. Almost all animals, vertebrate and invertebrate, require B vitamins; they are “universal” vitamins. The dietary need for vitamin C and the fat-soluble vitamins A, D, E, and K is mostly restricted to vertebrates, although some are required by certain invertebrates. Even within groups of close relationship, requirements for vitamins are relative, not absolute. A rabbit does not require vitamin C, but guinea pigs and humans do. Some songbirds require vitamin A, but others do not.
The recognition years ago that many human diseases and those of domesticated animals were caused by or associated with dietary deficiencies led biologists to search for specific nutrients that would prevent such diseases. These studies eventually yielded a list of essential nutrients for human beings and other animal species studied. Essential nutrients are those needed for normal growth and maintenance and that must be supplied in the diet. In other words, it is “essential” that these nutrients be in the diet because the animal cannot synthesize them from other dietary constituents. Nearly 30 organic compounds (amino acids and vitamins) and 21 elements are essential for humans (Table 34-1). Considering that the body contains thousands of different organic compounds, the list in Table 34-1 is remarkably short. Animal cells have marvelous powers of synthesis, enabling them to build compounds of enormous variety and complexity from a small, select group of raw materials.
In the average diet of North Americans approximately 50% of the total calories (energy content) comes from carbohydrates and 40% comes from lipids. Proteins, essential as they are for structural needs, supply only a little more than 10% of the total calories of the average diet of North Americans. Carbohydrates are widely consumed because they are more abundant and cheaper than proteins or lipids. Actually humans and many other animals can subsist on diets devoid of carbohydrates, provided sufficient total calories and essential nutrients are present. Eskimos, before the decline of their native culture, lived on a diet that was high in fat and protein and very low in carbohydrate.
Lipids are needed principally to provide energy. However, at least three fatty acids are essential for humans because we cannot synthesize them. Much interest and research have been devoted to lipids in our diets because of the association between fatty diets and the disease atherosclerosis. The matter is complex, but evidence suggests that atherosclerosis may occur when the diet is high in saturated lipids (lipids with no double bonds in the carbon chains of the fatty acids) but low in polyunsaturated lipids (two or more double bonds in the carbon chains).
Atherosclerosis (Gr. atheroma, tumor containing gruel-like matter,+ sclerosis, to harden) is a degenerative disease in which fatty substances are deposited in the lining of arteries, resulting in narrowing of the passage and eventual hardening and loss of elasticity.
Proteins are expensive foods and limited in the diet. Proteins, of course, are not themselves the essential nutrients but rather contain essential amino acids. Of the 20 amino acids commonly found in proteins, eight and possibly 10 are essential to humans (Table 34-1). We can synthesize the rest from other amino acids. Generally, animal proteins have more of the essential amino acids than do proteins of plant origin. All eight of the essential amino acids must be present simultaneously in the diet for protein synthesis. If one or more is missing, the use of the other amino acids will be reduced proportionately; they cannot be stored and are broken down for energy. Thus heavy reliance on a single plant source as a diet will inevitably lead to protein deficiency. This problem can be corrected if two kinds of plant proteins having complementary strengths in essential amino acids are ingested together. For example, a balanced protein diet can be prepared by mixing wheat flour, which is deficient only in lysine, with a legume (peas or beans), which is a good source of lysine but deficient in methionine and cysteine. Each plant complements the other by having adequate amounts of those amino acids that are deficient in the other.
Because animal proteins are rich in essential amino acids, they are in great demand in all countries. North Americans eat far more animal proteins than do Asians and Africans. In 1989 the annual per capita consumption of red meat was 76 kg in the United States, 27 kg in Japan, 12 kg in Egypt, and 1 kg in India.* Seventy percent of the protein in the diet of Americans comes from animal products and 30% from plants. By comparison, in China only 11% comes from animal sources and 89% from plants. North Americans consume approximately one-quarter of all beef produced in the world. The high consumption of meat in North America and Europe carries the price of a high death rate from so-called diseases of affluence: heart disease, stroke, and certain kinds of cancer.
Undernourishment and malnourishment rank as two of the world’s oldest problems and remain major health problems today, afflicting an eighth of the human population. Growing children and pregnant and lactating women are especially vulnerable to the devastating effects of malnutrition. Cell proliferation and growth in the human brain are most rapid in the terminal months of gestation and the first year after birth. Adequate protein for neuron development is a requirement during this critical time to prevent neurological dysfunction. The brains of children who die of protein malnutrition during the first year of life have 15% to 20% fewer brain cells than those of normal children (Figure 34-16). Malnourished children who survive this period suffer permanent brain damage and cannot be helped by later corrective treatment (Figure 34-17). Recent studies suggest that poverty, with attendant lack of educational and medical resources, and lowered expectations, exacerbates the effects of malnutrition by delaying intellectual development.†
Two different types of severe food deficiency are recognized:marasmus, general undernourishment from a diet low in both calories and protein, and kwashiorkor, protein malnourishment from a diet adequate in calories but deficient in protein. Marasmus (Gr.marasmos, to waste away) is common in infants weaned too early and placed on low-calorie–low-protein diets; these children are listless, and their bodies waste away. Kwashiorkor is a West African word describing a disease a child gets when displaced from the breast by a newborn sibling.This disease is characterized by retarded growth, anemia,weak muscles, a bloated body with typical pot belly, acute diarrhea, susceptibility to infection, and high mortality.
The world’s precarious food supply is threatened by rapid population growth. The world population was 2 billion in 1927, reached 4 billion in 1974, passed 6 billion in October of 1999 (Figure 34-18), and is expected to reach 8.9 billion by the year 2030, several years ahead of earlier estimates. Approximately 78 million people are added each year. The equivalent of the total 1999 United States population of 274 million people is added to the world every 42 months. Yet today, as the demand for food increases, the world per capita production of grain and the world fish catch are in decline‡. Furthermore, the world each year loses billions of tons of topsoil and trillions of gallons of groundwater needed to grow food crops. In the view of many, the exploding human population is a major force driving the global environmental crisis.
The food of animals must include carbohydrates, proteins, fats, water, mineral salts, and vitamins. Carbohydrates and fats are required as fuels for energy and for the synthesis of various substances and structures. Proteins (actually the amino acids of which they are composed) are needed for the synthesis of specific proteins and other nitrogen-containing compounds. Water is required as the solvent for body chemistry and as a major component of all fluids of the body. Inorganic salts are required as the anions and cations of body fluids and tissues and form important structural and physiological components throughout the body. Vitamins are accessory factors from food that are often built into the structure of many enzymes.
A vitamin is a relatively simple organic compound that is not a carbohydrate, fat, protein, or mineral and that is required in very small amounts in the diet for some specific cellular function. Vitamins are not sources of energy but are often associated with the activity of important enzymes that serve vital metabolic roles. Plants and many microorganisms synthesize all the organic compounds they need; animals, however, have lost certain synthetic abilities during their long evolution and depend ultimately on plants to supply these compounds. Vitamins therefore represent synthetic gaps in the metabolic machinery of animals.
Vitamins are usually classified as fat soluble (soluble in fat solvents such as ether) or water soluble. The watersoluble vitamins include the B complex and vitamin C (Table 34-1). Vitamins of the B complex, so grouped because the original B vitamin was subsequently found to consist of several distinct molecules, tend to be found together in nature. Almost all animals, vertebrate and invertebrate, require B vitamins; they are “universal” vitamins. The dietary need for vitamin C and the fat-soluble vitamins A, D, E, and K is mostly restricted to vertebrates, although some are required by certain invertebrates. Even within groups of close relationship, requirements for vitamins are relative, not absolute. A rabbit does not require vitamin C, but guinea pigs and humans do. Some songbirds require vitamin A, but others do not.
The recognition years ago that many human diseases and those of domesticated animals were caused by or associated with dietary deficiencies led biologists to search for specific nutrients that would prevent such diseases. These studies eventually yielded a list of essential nutrients for human beings and other animal species studied. Essential nutrients are those needed for normal growth and maintenance and that must be supplied in the diet. In other words, it is “essential” that these nutrients be in the diet because the animal cannot synthesize them from other dietary constituents. Nearly 30 organic compounds (amino acids and vitamins) and 21 elements are essential for humans (Table 34-1). Considering that the body contains thousands of different organic compounds, the list in Table 34-1 is remarkably short. Animal cells have marvelous powers of synthesis, enabling them to build compounds of enormous variety and complexity from a small, select group of raw materials.
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In the average diet of North Americans approximately 50% of the total calories (energy content) comes from carbohydrates and 40% comes from lipids. Proteins, essential as they are for structural needs, supply only a little more than 10% of the total calories of the average diet of North Americans. Carbohydrates are widely consumed because they are more abundant and cheaper than proteins or lipids. Actually humans and many other animals can subsist on diets devoid of carbohydrates, provided sufficient total calories and essential nutrients are present. Eskimos, before the decline of their native culture, lived on a diet that was high in fat and protein and very low in carbohydrate.
Lipids are needed principally to provide energy. However, at least three fatty acids are essential for humans because we cannot synthesize them. Much interest and research have been devoted to lipids in our diets because of the association between fatty diets and the disease atherosclerosis. The matter is complex, but evidence suggests that atherosclerosis may occur when the diet is high in saturated lipids (lipids with no double bonds in the carbon chains of the fatty acids) but low in polyunsaturated lipids (two or more double bonds in the carbon chains).
Atherosclerosis (Gr. atheroma, tumor containing gruel-like matter,+ sclerosis, to harden) is a degenerative disease in which fatty substances are deposited in the lining of arteries, resulting in narrowing of the passage and eventual hardening and loss of elasticity.
Proteins are expensive foods and limited in the diet. Proteins, of course, are not themselves the essential nutrients but rather contain essential amino acids. Of the 20 amino acids commonly found in proteins, eight and possibly 10 are essential to humans (Table 34-1). We can synthesize the rest from other amino acids. Generally, animal proteins have more of the essential amino acids than do proteins of plant origin. All eight of the essential amino acids must be present simultaneously in the diet for protein synthesis. If one or more is missing, the use of the other amino acids will be reduced proportionately; they cannot be stored and are broken down for energy. Thus heavy reliance on a single plant source as a diet will inevitably lead to protein deficiency. This problem can be corrected if two kinds of plant proteins having complementary strengths in essential amino acids are ingested together. For example, a balanced protein diet can be prepared by mixing wheat flour, which is deficient only in lysine, with a legume (peas or beans), which is a good source of lysine but deficient in methionine and cysteine. Each plant complements the other by having adequate amounts of those amino acids that are deficient in the other.
Because animal proteins are rich in essential amino acids, they are in great demand in all countries. North Americans eat far more animal proteins than do Asians and Africans. In 1989 the annual per capita consumption of red meat was 76 kg in the United States, 27 kg in Japan, 12 kg in Egypt, and 1 kg in India.* Seventy percent of the protein in the diet of Americans comes from animal products and 30% from plants. By comparison, in China only 11% comes from animal sources and 89% from plants. North Americans consume approximately one-quarter of all beef produced in the world. The high consumption of meat in North America and Europe carries the price of a high death rate from so-called diseases of affluence: heart disease, stroke, and certain kinds of cancer.
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| Figure 34-16 Effect of early malnutrition on cell number (measured as total DNA content) in the human brain. This graph shows that malnourished infants (purple oval) have far fewer brain cells than do normal infants (green growth curve). |
Undernourishment and malnourishment rank as two of the world’s oldest problems and remain major health problems today, afflicting an eighth of the human population. Growing children and pregnant and lactating women are especially vulnerable to the devastating effects of malnutrition. Cell proliferation and growth in the human brain are most rapid in the terminal months of gestation and the first year after birth. Adequate protein for neuron development is a requirement during this critical time to prevent neurological dysfunction. The brains of children who die of protein malnutrition during the first year of life have 15% to 20% fewer brain cells than those of normal children (Figure 34-16). Malnourished children who survive this period suffer permanent brain damage and cannot be helped by later corrective treatment (Figure 34-17). Recent studies suggest that poverty, with attendant lack of educational and medical resources, and lowered expectations, exacerbates the effects of malnutrition by delaying intellectual development.†
Two different types of severe food deficiency are recognized:marasmus, general undernourishment from a diet low in both calories and protein, and kwashiorkor, protein malnourishment from a diet adequate in calories but deficient in protein. Marasmus (Gr.marasmos, to waste away) is common in infants weaned too early and placed on low-calorie–low-protein diets; these children are listless, and their bodies waste away. Kwashiorkor is a West African word describing a disease a child gets when displaced from the breast by a newborn sibling.This disease is characterized by retarded growth, anemia,weak muscles, a bloated body with typical pot belly, acute diarrhea, susceptibility to infection, and high mortality.
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| Figure 34-17 Biafran refugee child suffering severe malnutrition. |
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| Figure 34-18 Portion of a graph for human population growth since A.D. 800, as it appeared in the 1979 when the earth’s population had passed 4 billion five years earlier, and updated to show the 1999 figure of 6 billion. |
The world’s precarious food supply is threatened by rapid population growth. The world population was 2 billion in 1927, reached 4 billion in 1974, passed 6 billion in October of 1999 (Figure 34-18), and is expected to reach 8.9 billion by the year 2030, several years ahead of earlier estimates. Approximately 78 million people are added each year. The equivalent of the total 1999 United States population of 274 million people is added to the world every 42 months. Yet today, as the demand for food increases, the world per capita production of grain and the world fish catch are in decline‡. Furthermore, the world each year loses billions of tons of topsoil and trillions of gallons of groundwater needed to grow food crops. In the view of many, the exploding human population is a major force driving the global environmental crisis.
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