Carbohydrates.

Carbohydrates, composed of carbon, hydrogen, and oxygen, are the main source of energy in the diet. Each gram of carbohydrate produces 4 kcal/g and serves as the main source of fuel (glucose) for the brain, skeletal muscles during exercise, erythrocyte and leukocyte production, and cell function of the renal medulla. People obtain carbohydrates primarily from plant foods, except for lactose (milk sugar). They are classified according to their carbohydrate units, or saccharides.

Monosaccharides such as glucose (dextrose) or fructose cannot be broken down into a more basic carbohydrate unit. Disaccharides such as sucrose, lactose, and maltose are composed of two monosaccharides and water. Both monosaccharides and disaccharides are classified as simple carbohydrates and are found primarily in sugars. Polysaccharides such as glycogen are made up of many carbohydrate units (i.e., complex carbohydrates). They are insoluble in water and digested to varying degrees. Starches are polysaccharides.

The body is unable to digest some polysaccharides because humans do not have enzymes capable of breaking them down. Fiber is a polysaccharide that is the structural part of plants that is not broken down by the human digestive enzymes. Because fiber is not broken down, it does not contribute calories to the diet. Insoluble fibers are not digestible and include cellulose, hemicellulose, and lignin. Soluble fibers dissolve in water and include barley, cereal grains, cornmeal, and oats.

Proteins.

Proteins provide a source of energy (4 kcal/g), and they are essential for synthesis (building) of body tissue in growth, maintenance, and repair. Collagen, hormones, enzymes, immune cells, deoxyribonucleic acid (DNA), and ribonucleic acid (RNA) are all made of protein. In addition, blood clotting, fluid regulation, and acid-base balance require proteins. These proteins transport nutrients and many drugs in the blood. Ingestion of proteins maintains nitrogen balance.

The simplest form of protein is the amino acid, which is made up of hydrogen, oxygen, carbon, and nitrogen. The body does not synthesize indispensable amino acids; thus these need to be provided in the diet. Examples of indispensable amino acids are histidine, lysine, and phenylalanine. The body synthesizes dispensable amino acids. Examples of amino acids synthesized in the body are alanine, asparagine, and glutamic acid. Amino acids can link together. Albumin and insulin are simple proteins because they contain only amino acids or their derivatives. The combination of a simple protein with a nonprotein substance produces a complex protein such as lipoprotein, formed by a combination of a lipid and a simple protein.

A complete protein, also called a high-quality protein, contains all essential amino acids in sufficient quantity to support growth and maintain nitrogen balance. Examples of foods that contain complete proteins are fish, chicken, soybeans, turkey, and cheese. Incomplete proteins are missing one or more of the nine indispensable amino acids and include cereals, legumes (beans, peas), and vegetables. Complementary proteins are pairs of incomplete proteins that, when combined, supply the total amount of protein provided by complete protein sources.

Nitrogen balance is achieved when the intake and output of nitrogen are equal. When the intake of nitrogen is greater than the output, the body is in positive nitrogen balance. Positive nitrogen balance is required for growth, normal pregnancy, maintenance of lean muscle mass and vital organs, and wound healing. The body uses nitrogen to build, repair, and replace body tissues. Negative nitrogen balance occurs when the body loses more nitrogen than it gains (e.g., with infection, burns, fever, starvation, head injury, and trauma). The increased nitrogen loss is the result of body tissue destruction or loss of nitrogen-containing body fluids. Nutrition during this period needs to provide nutrients to put patients into positive balance for healing.

Protein provides energy; however, because of the essential role of protein in growth, maintenance, and repair, a diet needs to provide adequate kilocalories from nonprotein sources. When there is sufficient carbohydrate in the diet to meet the energy needs of the body, protein is spared as an energy source.

Fats.

Fats (lipids) are the most calorie-dense nutrient, providing 9 kcal/g. Fats are composed of triglycerides and fatty acids. Triglycerides circulate in the blood and are composed of three fatty acids attached to a glycerol. Fatty acids are composed of chains of carbon and hydrogen atoms with an acid group on one end of the chain and a methyl group at the other. Fatty acids can be saturated, in which each carbon in the chain has two attached hydrogen atoms; or unsaturated, in which an unequal number of hydrogen atoms are attached and the carbon atoms attach to each other with a double bond. Monounsaturated fatty acids have one double bond, whereas polyunsaturated fatty acids have two or more double carbon bonds. The various types of fatty acids have significance for health and the incidence of disease and are referred to in dietary guidelines.

Fatty acids are also classified as essential or nonessential. Linoleic acid, an unsaturated fatty acid, is the only essential fatty acid in humans. Linolenic acid and arachidonic acid (also unsaturated fatty acids) are important for metabolic processes but are manufactured by the body when linoleic acid is available. Deficiency occurs when fat intake falls below 10% of daily nutrition. Most animal fats have high proportions of saturated fatty acids, whereas vegetable fats have higher amounts of unsaturated and polyunsaturated fatty acids.

Water.

Water is critical because cell function depends on a fluid environment. Water makes up 60% to 70% of total body weight. The percent of total body water is greater for lean people than obese people because muscle contains more water than any other tissue except blood. Infants have the greatest percentage of total body water, and older people have the least. When deprived of water, a person cannot survive for more than a few days.

Vitamins.

Minerals.

Minerals are inorganic elements essential to the body as catalysts in biochemical reactions. They are classified as macrominerals when the daily requirement is 100 mg or more and microminerals or trace elements when less than 100 mg is needed daily. Macrominerals help to balance the pH of the body, and specific amounts are necessary in the blood and cells to promote acid-base balance. Interactions occur among trace minerals. For example, excess of one trace mineral sometimes causes deficiency of another. Selenium is a trace element that also has antioxidant properties. Silicon, vanadium, nickel, tin, cadmium, arsenic, aluminum, and boron play an unidentified role in nutrition. Arsenic, aluminum, and cadmium have toxic effects.

Digestion.

Most enzymes have one specific function. Each enzyme works best at a specific pH. For example, the enzyme amylase in the saliva breaks down starches into sugars. The secretions of the GI tract have very different pH levels. For example, saliva is relatively neutral, gastric juice is highly acidic, and the secretions of the small intestine are alkaline.

The mechanical, chemical, and hormonal activities of digestion are interdependent. Enzyme activity depends on the mechanical breakdown of food to increase its surface area for chemical action. Hormones regulate the flow of digestive secretions needed for enzyme supply. Physical, chemical, and hormonal factors regulate the secretion of digestive juices and the motility of the GI tract. Nerve stimulation from the parasympathetic nervous system (e.g., the vagus nerve) increases GI tract action.

Digestion begins in the mouth, where chewing mechanically breaks down food. The food mixes with saliva, which contains ptyalin (salivary amylase), an enzyme that acts on cooked starch to begin its conversion to maltose. The longer an individual chews food, the more starch digestion occurs in the mouth. Proteins and fats are broken down physically but remain unchanged chemically because enzymes in the mouth do not react with these nutrients. Chewing reduces food particles to a size suitable for swallowing, and saliva provides lubrication to further ease swallowing of the food. The epiglottis is a flap of skin that closes over the trachea as a person swallows to prevent aspiration. Swallowed food enters the esophagus, and wavelike muscular contractions (peristalsis) move the food to the base of the esophagus, above the cardiac sphincter. Pressure from a bolus of food at the cardiac sphincter causes it to relax, allowing the food to enter the fundus, or uppermost portion, of the stomach.

The chief cells in the stomach secrete pepsinogen; and the pyloric glands secrete gastrin, a hormone that triggers parietal cells to secrete hydrochloric acid (HCl). The parietal cells also secrete HCl and intrinsic factor (IF), which is necessary for absorption of vitamin B₁₂ in the ileum. HCl turns pepsinogen into pepsin, a protein-splitting enzyme. The body produces gastric lipase and amylase to begin fat and starch digestion, respectively. A thick layer of mucus protects the lining of the stomach from autodigestion. Alcohol and aspirin are two substances directly absorbed through the lining of the stomach. The stomach acts as a reservoir where food remains for approximately 3 hours, with a range of 1 to 7 hours.

Food leaves the antrum, or distal stomach, through the pyloric sphincter and enters the duodenum. Food is now an acidic, liquefied mass called chyme. Chyme flows into the duodenum and quickly mixes with bile, intestinal juices, and pancreatic secretions. The small intestine secretes the hormones secretin and cholecystokinin (CCK). Secretin activates release of bicarbonate from the pancreas, raising the pH of chyme. CCK inhibits further gastrin secretion and initiates release of additional digestive enzymes from the pancreas and gallbladder.

Bile is manufactured in the liver and concentrated and stored in the gallbladder. It acts as a detergent because it emulsifies fat to permit enzyme action while suspending fatty acids in solution. Pancreatic secretions contain six enzymes: amylase to digest starch; lipase to break down emulsified fats; and trypsin, elastase, chymotrypsin, and carboxypeptidase to break down proteins.

Peristalsis continues in the small intestine, mixing the secretions with chyme. The mixture becomes increasingly alkaline, inhibiting the action of the gastric enzymes and promoting the action of the duodenal secretions. Epithelial cells in the small intestinal villi secrete enzymes (e.g., sucrase, lactase, maltase, lipase, and peptidase) to facilitate digestion. The major portion of digestion occurs in the small intestine, producing glucose, fructose, and galactose from carbohydrates; amino acids and dipeptides from proteins; and fatty acids, glycerides, and glycerol from lipids. Peristalsis usually takes approximately 5 hours to pass food through the small intestine.

Absorption.

Carbohydrates, protein, minerals, and water-soluble vitamins are absorbed by the small intestine, processed in the liver, and released into the portal vein circulation. Fatty acids are absorbed in the lymphatic circulatory systems through lacteal ducts at the center of each microvilli in the small intestine.

Metabolism and Storage of Nutrients.

Metabolism refers to all of the biochemical reactions within the cells of the body. Metabolic processes are anabolic (building) or catabolic (breaking down). Anabolism is the building of more complex biochemical substances by synthesis of nutrients. Anabolism occurs when an individual adds lean muscle through diet and exercise. Amino acids are anabolized into tissues, hormones, and enzymes. Normal metabolism and anabolism are physiologically possible when the body is in positive nitrogen balance. Catabolism is the breakdown of biochemical substances into simpler substances and occurs during physiological states of negative nitrogen balance. Starvation is an example of catabolism when wasting of body tissues occurs.

Nutrients absorbed in the intestines, including water, are transported through the circulatory system to the body tissues. Through the chemical changes of metabolism, the body converts nutrients into a number of required substances. Carbohydrates, protein, and fat are metabolized to produce chemical energy and maintain a balance between anabolism and catabolism. To carry out the work of the body, the chemical energy produced by metabolism converts to other types of energy by different tissues. Muscle contraction involves mechanical energy, nervous system function involves electrical energy, and the mechanisms of heat production involve thermal energy.

Dietary Reference Intakes.

Food Guidelines.

Daily Values.

Environmental Factors.

Developmental Needs