Porth's Essentials of Pathophysiology, 4e

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Integrative Body Functions

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are essential to the body. These are leucine, isoleucine, methionine, phenylalanine, threonine, tryptophan, valine, lysine, and histidine. The foods that provide these essen- tial amino acids in adequate amounts are milk, eggs, meat, fish, and poultry. Dried peas and beans, nuts, seeds, and grains contain all the essential amino acids but in less than adequate proportions. These proteins need to be combined with each other or with complete proteins to meet the amino acid requirements for protein synthesis. Unlike carbohydrates and fats, which are composed of hydrogen, carbon, and oxygen, proteins contain nitrogen; therefore, nitrogen excretion is an indicator of protein intake. The average protein contains about 16% nitrogen. About 90% of this is excreted in the urine as urea, uric acid, creatinine, and other less important nitrogen break- down products, with the rest being excreted in the feces. 1 Therefore, the rate of protein breakdown can be estimated by measuring the amount of nitrogen in the urine. If the amount of nitrogen taken in by way of protein is equiva- lent to the nitrogen excreted, the person is said to be in nitrogen balance. A person is in positive nitrogen balance when the nitrogen consumed by way of protein is greater than the amount excreted. This occurs during growth, pregnancy, or healing after surgery or injury. A negative nitrogen balance often occurs with fever, illness, infection, trauma, or burns, when more nitrogen is excreted than is consumed. 1 It represents a state of tissue breakdown. Fats. Dietary fats are composed primarily of triglyc- erides (i.e., a compound consisting of three fatty acid chains attached to a glycerol backbone). Saturated fatty acids have no double bonds between carbon atoms in the chains, whereas monounsaturated fatty acids have one double bond in the chain, and polyunsaturated fatty acids have two or more double bonds. The saturated fatty acids elevate blood cholesterol, whereas the mono- unsaturated and polyunsaturated fats lower blood cho- lesterol. Saturated fats are usually derived from animal sources and remain solid at room temperature. With the exception of coconut and palm oils (which are satu- rated), unsaturated fats are found in plant oils and usu- ally are liquid at room temperature. Trans fatty acids are produced when unsaturated oils are partially hydroge- nated and are called artificial trans fats . They are found primarily in vegetable shortenings and some margarines and foods. Small amounts of natural trans fatty acids are found in dairy products, some meats, and other animal- based foods. Although trans fatty acids tend to increase low-density lipoprotein (LDL) cholesterol and decrease high-density lipoprotein (HDL) cholesterol, the natu- rally occurring trans fats may have a beneficial effect. Dietary fats provide energy, function as carriers for the fat-soluble vitamins, serve as precursors of prosta- glandins, and are a source of the essential fatty acids, linoleic and alpha-linolenic acids. 7 Because vegetable oils are rich sources of linoleic acid, this level can be met by including two teaspoons per day of vegetable oil in the diet. Alpha-linolenic acid is found primar- ily in dark green, leafy vegetables, certain plant oils, soybeans, and walnuts. The ADMR for fat is 20% to 35% to prevent the fall of HDL cholesterol associated

with very–low-fat diets. 7 Guidelines from the National Cholesterol Education Program recommend that 25% to 35% of the calories in the diet should come from fats. 8 The daily dietary recommendation for cholesterol is less than 300 mg. The American Heart Association recommends limiting saturated fat to less than 7% and trans fatty acids to less than 1% of calories daily. 9 Carbohydrates. Dietary carbohydrates include simple sugars, such as glucose, sucrose, and fructose, as well as complex carbohydrates, which are commonly called starches. Fiber is an indigestible form of complex carbo- hydrate. It is recommended that the majority of carbohy- drates consumed in the diet be complex carbohydrates, which are rich in fiber and a variety of vitamins and minerals. In contrast, simple sugars do not provide fiber and, unless fortified, usually contain few vitamins and minerals. Moreover, sucrose (i.e., table sugar) is impli- cated in the development of dental caries, and excessive dietary fructose (e.g., from high-fructose corn syrup in soft drinks and other sweetened foods and beverages) has been increasingly associated with childhood and adult obesity, diabetes, and cardiovascular disease. The current RDA for adults is 130 g of carbohydrates per day. Some tissues, such as the nervous system, require glucose as an energy source. Although this need can be met by gluconeogenesis, in which amino acids are con- verted to glucose, a carbohydrate-deficient diet usually results in the loss of tissue proteins. The fatty acids from triglycerides can be converted to ketones and used for energy by other body tissues when glucose is inadequate; however, excessive ketone production can prompt the development of ketosis. Because protein and fat metabo- lism increases the production of osmotically active meta- bolic wastes that must be eliminated through the kidneys, there is also danger of dehydration and electrolyte imbal- ances. The minimum amount of carbohydrates needed to prevent tissue wasting and ketosis is 50 to 100 g/day. In practice, most of the daily energy requirement should be from dietary carbohydrate sources. This is because pro- tein is an expensive source of calories, and it is recom- mended that dietary fat not exceed 35% of the calorie intake. The AMDR for carbohydrates intake is 45% to 65% of total calories to prevent a high intake of fat. 7 Vitamins and Minerals Vitamins are a group of organic compounds essential for numerous body functions. A compound cannot be classified as a vitamin unless it is shown that a deficiency causes disease. Contrary to popular belief, vitamins do not provide energy directly. Instead, many act as coen- zymes, components of the enzyme systems required for the release of energy from protein, fat, and carbohy- drates. Vitamins also are necessary for the formation of red blood cells, hormones, genetic materials, and the nervous system. They are essential for normal growth and development. There are two types of vitamins: fat soluble and water soluble. The four fat-soluble vitamins are vitamins A, D,