Human Nutrition: science that deals with nutrients and other food substances and how the body assimilates them. The extremely complex processes that nutrients undergo in the body - how they affect one another, how they are broken down and released as energy, and how they are transported and used to rebuild countless specialized tissues and sustain the overall health of the individual - are understood only in part. Nevertheless, important nutrition decisions must be made for the health of individuals, groups such as the young and the elderly, and entire populations who suffer from malnutrition.

Essential Nutrients
Nutrients are classified into five major groups: proteins, carbohydrates, fats, vitamins, and minerals. These groups comprise between 45 and 50 substances that scientists have established, mostly through experiments with animals, as essential for maintaining normal growth and health. Besides water and oxygen, they include about eight amino acids from proteins, four fat-soluble and ten water-soluble vitamins, about ten minerals, and three electrolytes (the ions required by cells to regulate the electrical charge and flow of water molecules across cell membranes). Although carbohydrates are needed for the body's energy, they are not considered absolutely essential because protein can be converted for this purpose.

Energy
The body uses energy to conduct vital functions and maintain itself at a constant temperature. By using a calorimeter (a device for measuring the heat generated by a chemical reaction or change in a physical state), scientists have established the amounts of energy in the body's fuels - carbohydrates, fats, and proteins. About 4 calories each are yielded by 1g (0.035oz) of pure carbohydrate and 1g of pure protein; 1 gram of pure fat yields about 9 calories. (A kilogram calorie, in nutrition, is defined as the heat energy needed to raise the temperature of 1 kilogram of water from 14.5° to 15.5° C (58.1° to 59.9° F). Carbohydrates are the most abundant foods in the world, and fats are the most concentrated and easily stored fuels. If the body exhausts available carbohydrates and fats, it can use proteins directly from the diet or break down its own protein tissue for energy. Alcohol is also a source of energy, yielding 7 calories per gram. Alcohol cannot be oxidized by the body cells but must be processed by the liver into fat, which is then stored by the liver or the adipose tissue (a layer of fat just beneath the skin and around various internal organs).

Functions of Nutrients
The functions of the various categories of nutrients are described below.

Proteins
The primary function of protein is to build body tissue and synthesize enzymes, some hormones, such as insulin, that regulate physiological activity, and other complex substances that govern body processes. Animal and plant proteins are not used in the form in which they are ingested but are broken down by digestive enzymes called proteases into nitrogen-containing amino acids. Proteases disrupt the peptide bonds by which ingested amino acids are linked, so the amino acids can be absorbed through the intestine into the blood and recombined into the particular tissue needed. Proteins are usually readily available from both animal and plant sources. Of the 20 amino acids that make up protein, 8 are considered essential - that is, because the body cannot synthesize them, they must be supplied ready-made in foods. If these essential amino acids are not all present at the same time and in specific proportions, the other amino acids, in whole or in part, cannot be used for metabolizing human protein. Therefore, a diet containing these essential amino acids is vital for sustaining growth and health. When any of the essential amino acids is lacking, the remaining ones are converted into energy-yielding compounds, and their nitrogen is excreted.

When an excess of protein is eaten, which is often the case with heavy meat diets in the United States, the extra protein is similarly broken down into energy-yielding compounds. Because protein is far scarcer than carbohydrates and yields the same 4 calories per gram, the eating of meat beyond the tissue-building demands of the body becomes an inefficient way to procure energy. Foods from animal sources contain complete proteins because they include all the essential amino acids. In most diets, a combination of plant and animal protein is recommended: 0.8 grams of protein per kilogram of body weight is considered a safe daily allowance for normal adults. Many illnesses and infections lead to an increased loss of nitrogen from the body, which needs to be replaced by a higher consumption of dietary protein. Infants and young children also require more protein per kilogram of body weight. A protein deficiency accompanied by energy deficits results in a form of protein-energy malnutrition called kwashiorkor, which is characterized by loss of body fat and wasting of muscle.

Minerals
Inorganic mineral nutrients are required in the structural composition of hard and soft body tissues; they also participate in such processes as the action of enzymes, the contraction of muscles, nerve reactions, and the clotting of blood. These mineral nutrients, all of which must be supplied in the diet, are of two classes: major elements such as calcium, phosphorus, magnesium, iron, iodine, and potassium; and trace elements such as copper, cobalt, manganese, fluorine, and zinc.

Calcium is needed for developing and maintaining the rigidity of bones. It also contributes to the formation of intracellular cement and the cell membranes, and regulation of nervous excitability and muscular contraction. About 90 percent of calcium is stored in bone, where it can be reabsorbed by blood and tissue. Milk and milk products are the chief source of calcium. Phosphorus, also present in many foods and especially in milk, combines with calcium in the bones and teeth. It plays an important role in energy metabolism of the cells, affecting carbohydrates, lipids (fatty acids in the blood that also include cholesterol and triglycerides), and proteins. Magnesium, which is present in most foods, is essential for human metabolism and is important for maintaining the electrical potential in nerve and muscle cells. A deficiency in magnesium among malnourished people, especially alcoholics, leads to tremors and convulsions.

Sodium, which is present in small and usually sufficient quantities in most natural foods, is found in liberal amounts in salted prepared and cooked foods. It is present in extracellular fluid (the fluid between cells or cell layers), which sodium helps regulate. Too much sodium causes edema, an over-accumulation of extracellular fluid. Evidence now exists that excessive dietary salt contributes to high blood pressure.

Iron is needed to form hemoglobin (the pigment in red blood cells responsible for transporting oxygen), but the mineral is not readily absorbed by the digestive system. It exists in sufficient amounts in men, but women of menstrual age, who need nearly twice as much iron because of blood loss, often have deficiencies and must take absorbable iron supplements. Iodine is needed to synthesize hormones of the thyroid gland. A deficiency leads to goiter (swelling of the thyroid gland in the lower neck). Goiter, which used to be common in the U.S. population, remains prevalent in certain parts of Asia, Africa, and South America. Low iodine intakes during pregnancy may result in the birth of cretinous or mentally retarded infants. It is estimated that worldwide more than 150 million people suffer from iodine deficiency diseases.

Trace elements are other inorganic substances that appear in the body in minute amounts but are essential for good health. Little is known about how they function, and most knowledge about them comes from how their absence, especially in animals, affects health. Among the more important trace elements is copper, which is present in many enzymes and in copper-containing proteins found in the blood, brain, and liver. Copper deficiency is associated with the failure to use iron in the formation of hemoglobin. Zinc is also important in forming enzymes. A deficiency of zinc is believed to impair growth and, in severe cases, to cause dwarfism. Fluorine, which is retained predominantly in the teeth and bones, has been found necessary for growth in animals. Fluorides, a category of fluorine compounds, are important for protecting against demineralization of bone. Other trace elements include chromium, molybdenum, and selenium.

Vitamins
Vitamins are organic compounds that mainly function in enzyme systems to enhance the metabolism of proteins, carbohydrates, and fats. Without these substances, the breakdown and assimilation of foods could not occur. Certain vitamins participate in the formation of blood cells, hormones, nervous system chemicals, and genetic materials. Vitamins are classified into two groups, the fat-soluble and the water-soluble vitamins. Fat-soluble vitamins include vitamins A, D, E, and K. The water-soluble vitamins include vitamin C and the B-vitamin complex.

Fat-soluble vitamins are usually absorbed with foods that contain fat. They are broken down by bile (see Liver), and the emulsified molecules pass through the lymph vessels and veins for distribution through the arteries. Excess amounts of these vitamins are stored in the body's fat, the liver, and kidneys. Because fat-soluble vitamins can be stored, they do not have to be consumed every day. Vitamin A is essential for normal growth and for the health of epithelial cells, which form membranes that line body cavities. A deficiency of vitamin A leads to skin changes and night blindness, or failure of the eye to adapt to the dark because of this deficiency's effect on the retina. Later, xerophthalmia, an eye condition characterized by dryness and thickening of the surface of the conjunctiva and cornea, may develop. Untreated, xerophthalmia can lead to blindness, especially in children. Vitamin A can be obtained directly from foods of animal origin such as milk, eggs, and liver. In many developing countries, most vitamin A is obtained from carotene, which is present in green and yellow fruits and vegetables. Carotene is converted to vitamin A in the body.

Vitamin D acts much like a hormone and regulates calcium and phosphorus absorption and metabolism. Some vitamin D is obtained from such foods as eggs, fish, liver, butter, margarine, and milk, some of which might have been fortified with vitamin D. But humans get most of their vitamin D from exposure of the skin to sunlight. A deficiency leads to rickets in children or osteomalacia (a bone condition characterized by fractures, weakness, and weight loss) in adults. Vitamin E is an essential nutrient for many vertebrate animals, but its role in the human body has not been established. It has been popularly advocated for a great variety of afflictions, but no clear evidence exists that it alleviates any specific disease. Vitamin E is found in seed oils and wheat germ. Vitamin K is necessary for the coagulation of blood. It assists in forming the enzyme prothrombin, which is needed to produce fibrin for blood clots. Vitamin K is produced by bacteria in the intestine, but is also provided by leafy green vegetables such as spinach and kale, and egg yolk.

The water-soluble vitamins, C and B complex, cannot be stored and therefore need to be consumed daily to replenish the body's needs. Vitamin C, or ascorbic acid, is important in the synthesis and maintenance of connective tissue. It prevents scurvy, which attacks the gums, skin, and mucous membranes. It is mainly found in citrus fruits and rose hips. The most important B-complex vitamins are thiamine (B1), riboflavin (B2), nicotinic acid or niacin (B3), pyridoxine (B6), pantothenic acid, lecithin, choline, inositol, para-aminobenzoic acid (PABA), folic acid, and cyanocobalamin (B12). These vitamins serve a wide range of important metabolic functions and prevent such afflictions as beriberi and pellagra. They are found mostly in yeast and liver.

Carbohydrates
Carbohydrates provide a great part of the energy in most human diets. Foods rich in carbohydrates are usually the most abundant and cheapest when compared with foods high in protein and fat content. Carbohydrates are burned during metabolism to produce energy, liberating carbon dioxide and water. Humans also get energy less efficiently from fats and proteins in the diet, and also from alcohol. The two kinds of carbohydrates are starches, which are found mainly in grains, legumes, and tubers, and sugars, which are found in plants and fruits.

Carbohydrates are used by the cells in the form of glucose, the body's main fuel. After absorption from the small intestine, glucose is processed in the liver, which stores some as glycogen, a starchlike substance, and passes the rest into the bloodstream. In combination with fatty acids, glucose forms triglycerides, which easily can be broken down into ketones (complex acids that can be converted into energy). Glucose and triglycerides are carried by the bloodstream to the muscles and organs to be oxidized, and excess quantities are stored as fat in the adipose and other tissues, to be retrieved and burned at times of low carbohydrate intake. Carbohydrates containing the most nutrients are the complex carbohydrates, such as unrefined grains, tubers, vegetables, and fruit, which also provide protein, vitamins, minerals, and fats. A less beneficial source is foods made from refined sugar, such as candy and soft drinks, which are high in calories but low in nutrients and fill the body with what nutritionists call empty calories.

Fats
Although scarcer than carbohydrates, fats produce more than twice as much energy. As a compact fuel, fat is stored efficiently in the body for later use when carbohydrates are in short supply. Animals need stored fat for survival in dry or cold seasons, as do humans during times of scarce food supply. However, in industrialized nations, such as the United States, where food is readily available and machines have replaced human labor, the accumulation of excess body fat has become a serious health concern. Dietary fats are broken down into fatty acids that pass into the blood to form the body's own triglycerides. The fatty acids that contain as many hydrogen atoms as possible on the carbon chain are called saturated fatty acids and are derived mostly from animal sources. Unsaturated fatty acids are those with some of the hydrogen atoms missing. This group includes monounsaturated fatty acids, which have a single pair of hydrogens missing, and polyunsaturated fatty acids, which have more than one pair missing. Polyunsaturated fats are found mostly in seed oils. Saturated fats in the bloodstream have been found to raise the level of cholesterol and polyunsaturated fat tends to lower it. Saturated fats generally are solid at room temperature; polyunsaturated fats are liquid.

Food Types
Foods generally can be classified into breads and cereals; pulses, or legumes; tubers, or starchy roots; vegetables and fruits; meat, fish, and eggs; milk and milk products; fats and oils; and sugars, preserves, and syrups.

Breads and cereals include wheat, rice, corn, and millet. They are high in starches and are easily procured sources of calories. Although protein is not abundant in whole cereals, people tend to consume such large quantities of this food group that significant amounts of protein are acquired. However, these cereal proteins should be supplemented with other protein foods to supply all the essential amino acids. White wheat flour and polished rice are low in nutrients, but, as whole grains containing the germ and outer seed layer, wheat and rice supply the body with needed fiber; the B vitamins thiamine, niacin, and riboflavin; and the minerals zinc, copper, manganese, and molybdenum.

Pulses, or legumes, include a wide variety of beans, peas, lentils, and grains, and even peanuts. All are rich in starch but might provide considerably more protein than do cereals or tubers. Their amino acid patterns often complement those of rice, corn, and wheat, which are staples in many poor countries.

Tubers and starch roots include various kinds of potatoes, cassava, yams, and taro. They are rich in starch and relatively low in protein content, but provide a variety of minerals and vitamins.

Vegetables and fruits are a direct source of many minerals and vitamins lacking in cereal diets, especially vitamin C from citrus fruits and vitamin A from the carotene of leafy vegetables and carrots. Sodium, cobalt, chloride, copper, magnesium, manganese, phosphorus, and potassium are present in vegetables. The mostly indigestible cellulose of vegetables supplies the roughage needed to pass food through the digestive tract. Many of the more fragile, water-soluble vitamins exist in vegetables and fruit and can be easily destroyed by overcooking.

Meat, fish, and eggs supply all the essential amino acids that the body needs to assemble its own proteins. Meats usually contain about 20 percent protein, 20 percent fat, and 60 percent water. Organ meats are rich sources of vitamins and minerals. All fish are high in protein, and the oils of some are rich in vitamins D and A. Egg white is the most concentrated form of protein.

Milk and milk products include whole milk, cheese, yogurt, and ice cream, all of which are well-known for their abundant protein, phosphorus, and especially calcium. Milk is also rich in vitamins but contains no iron and, if pasteurized, no vitamin C. Although milk is essential for children, for adults too much can cause unsaturated fatty acids to build in the blood system.

Fats and oils include butter, lard, suet, and vegetable oils. They are all high in calories, but, apart from butter and such vegetable oils as red palm oil, they contain few nutrients.

Sugars, preserves, and syrups are heavily consumed in more affluent countries, where they make up a large portion of the carbohydrate intake. Americans eat their own weight in sugar every year. Honey and maple syrup are composed of more than 75 percent sugar and contain few nutrients. Sugar causes tooth decay.

Dietary Guidelines
The Food and Nutrition Board of the National Research Council, National Academy of Sciences, has set dietary standards called Recommended Dietary Allowances (RDA). These describe the daily amounts of energy, protein, minerals, and fat-soluble and water-soluble vitamins needed by normal healthy males and females from infancy to old age. A male, for example, 23 to 50 years old and weighing 70 kg (154 lb) has an RDA of 56 g of protein, 45 mg of ascorbic acid, and 10 mg of iron. A female 23 to 50 years old and weighing 58 kg (128 lb) has an RDA of 46 g of protein, 45 mg of ascorbic acid, and 18 mg of iron.

When a new version of the RDA list was prepared and presented to reviewers on the National Academy of Sciences in 1985, they repressed it and re-endorsed the 1980 version. They particularly opposed the 1985 lowering of minimum requirements for nutrients such as vitamins A and C, although the new list raised minimum requirements for some other nutrients such as calcium. Preparers of the new report objected to the repression as an unscientific buttressing of prevailing nutritional assumptions, and the National Institutes of Health planned to print the report.

Numerous nutrition experts of the U.S. Departments of Agriculture and of Health and Human Services have voiced concern that Americans generally eat too much food and, specifically, too much fat, cholesterol, sugar, and salt. The fat intake of Americans has risen from 32 to 42 percent of the total diet since 1900. Pointing to the incidence of obesity, diabetes, heart attacks, high blood pressure, and tooth decay, the Senate Select Committee on Nutrition and Human Needs has suggested that Americans eat 30 percent fewer calories from fats, 45 percent fewer calories from refined sugars, and 70 percent more calories from complex carbohydrates and naturally occurring sugars, and that they reduce cholesterol intake by one-half and salt intake by two-thirds.

As for individual dietary guidelines, scientists in the federal government's food and health services recommend that a person should eat a variety of foods; maintain ideal weight; avoid too much fat, saturated fat, and cholesterol; eat foods with adequate starch and fiber; avoid too much sugar; avoid too much sodium; and drink alcohol in moderation, if at all.

The science of nutrition is still far from explaining how foods affect certain individuals. Why some people can discontinue eating at a certain point and why others eat obsessively, for example, is still a mystery. Researchers have recently found that shortly after ingestion, foods influence the release of important brain chemicals and carbohydrate foods, in particular, trigger the release of serotonin, a neurotransmitter that stimulates involuntary muscles in the intestine and suppresses the desire for carbohydrates. Such a mechanism might have evolved to prevent people from glutting themselves on carbohydrates and failing to procure harder-to-find protein. Until recent times, carbohydrate foods were far more accessible than protein. Serotonin is believed to work in complex relationships with insulin and several amino acids, especially tryptophan, all of which participate in monitoring the appetite for various food types. In this same area of research, nutrition experts are trying to unravel the relationship between diabetes and obesity and the role that sweets play for people with these afflictions.

"Nutrition, Human," Microsoft(R) Encarta(R) 98 Encyclopedia. (c) 1993-1997 Microsoft Corporation. All rights reserved.