ELDER TIP A person’s protein, vitamin, and mineral requirements usually remain the same as he ages, although calorie needs decline. Diminished activity may lower energy requirements by almost 200 calories per day for men and women ages 51 to 75, 400 calories per day for women older than age 75, and 500 calories per day for men older than age 75.
Carbohydrates: Primary energy source
The body gets most of its energy by metabolizing carbohydrates, especially glucose. Glucose catabolism proceeds in three phases:
❑ Glycolysis, a series of chemical reactions, converts glucose molecules into pyruvic or lactic acid.
❑ The citric acid cycle removes ionized hydrogen atoms from pyruvic acid and produces carbon dioxide.
❑ Oxidative phosphorylation traps energy from the hydrogen electrons and combines the hydrogen ions and electrons with oxygen to form water and the common form of biologic energy, adenosine triphosphate (ATP).
Other essential processes in carbohydrate metabolism include glycogenesis — the formation of glycogen, a storage form of glucose — which occurs when cells become saturated with glucose-6-phosphate (an intermediate product of glycolysis); glycogenolysis, the reverse process, which converts glycogen into glucose-6-phosphate in muscle cells and liberates free glucose in the liver; and gluconeogenesis, or “new” glucose formation from protein amino acids or fat glycerols.
A complex interplay of hormonal and neural controls regulates glucose metabolism. Hormone secretions of five endocrine glands dominate this regulatory function:
❑ Alpha cells of the islets of Langerhans secrete glucagon, which increases the blood glucose level by stimulating phosphorylase activity to accelerate liver glycogenolysis.
❑ Beta cells of the islets of Langerhans secrete the glucose-regulating hormone insulin, which assists in glucose transport across cell membranes and storage of excess glucose as fat.
❑ The adrenal medulla, as a physiologic response to stress, secretes epinephrine, which stimulates liver and muscle glycogenolysis to increase the blood glucose level.
❑ Corticotropin and glucocorticoids also increase blood glucose levels. Glucocorticoids accelerate gluconeogenesis by promoting the flow of amino acids to the liver, where they’re synthesized into glucose.
❑ Human growth hormone (hGH) limits the fat storage and favors fat catabolism; consequently, it inhibits carbohydrate catabolism and thus raises blood glucose levels.
❑ Thyroid-stimulating hormone and thyroid hormone have mixed effects on carbohydrate metabolism and may raise or lower blood glucose levels.
Fats: Catabolism and anabolism
The breaking up of triglycerides — lipolysis — yields fatty acids and glycerol. Betaoxidation breaks down fatty acids into acetyl coenzyme A, which can then enter the citric acid cycle; glycerol can also undergo gluconeogenesis or enter the glycolytic pathways to produce energy. Conversely, lipogenesis is the chemical formation of fat from excess carbohydrates and proteins or from the fatty acids and glycerol products of lipolysis. Adipose tissue is the primary storage site for excess fat and thus is the greatest source of energy reserve. Certain unsaturated fatty acids are necessary for synthesis of vital body compounds. Because the body can’t produce these essential fatty acids, they must be provided through diet. Insulin, hGH, catecholamines, corticotropin, and glucocorticoids control fat metabolism in an inverse relationship with carbohydrate metabolism; large amounts of carbohydrates promote fat storage, and deficiency of available carbohydrates promotes fat breakdown for energy needs.
Proteins: Anabolism
The primary process in protein metabolism is anabolism. Catabolism is relegated to a supporting role in protein metabolism — a reversal of the roles played by these two processes in carbohydrate and fat metabolisms. By synthesizing proteins — the tissue-building foods — the body derives substances essential for life (such as plasma proteins) and can reproduce, control cell growth, and repair itself. However, when carbohydrates or fats are unavailable as energy sources, or when energy demands are exceedingly high, protein catabolism converts protein into an available energy source. Protein metabolism consists of many processes, including:
❑ Deamination: a catabolic and energy-producing process occurring in the liver with the splitting off of the amino acid to form ammonia and a keto acid
❑ Transamination: anabolic conversion of keto acids to amino acids
❑ Urea formation: a catabolic process occurring in the liver, producing urea, the end product of protein catabolism.
The male hormone testosterone and hGH stimulate protein anabolism; corticotropin prompts secretion of glucocorticoids, which, in turn, facilitate protein catabolism. Normally, the rate of protein anabolism equals the rate of protein catabolism — a condition known as nitrogen balance (because ingested nitrogen equals nitrogen waste excreted in urine, feces, and sweat). When excessive catabolism causes the amount of nitrogen excreted to exceed the amount ingested, a state of negative nitrogen balance exists — usually the result of starvation and cachexia or surgical stress.
Fluid and electrolyte balance
A critical component of metabolism is fluid and electrolyte balance. Water is an essential body substance and constitutes almost 60% of an adult’s body weight and more than 75% of a neonate’s body weight. In both older and obese adults, the ratio of water to body weight drops; children and lean people have a higher proportion of water in their bodies.
Body fluids can be classified as intracellular (or cellular) or extracellular. Intracellular fluid constitutes about 40% of total body weight and 60% of all body fluid; it contains large quantities of potassium and phosphates but very little sodium and chloride. Conversely, extracellular fluid (ECF) contains mostly sodium and chloride but very little potassium and phosphates. Incorporating interstitial, cerebrospinal, intraocular, and GI fluids and plasma, ECF supplies cells with nutrients and other substances needed for cellular function. The many components of body fluids have the important function of preserving osmotic pressure and acid-base and anion-cation balance.
Homeostasis is a stable state — the equilibrium of chemical and physical properties of body fluid. Body fluids contain two kinds of dissolved substances: those that dissociate in solution (electrolytes) and those that don’t. For example, glucose, when dissolved in water, doesn’t break down into smaller particles; but sodium chloride dissociates in solution into sodium cations (+) and chloride anions (–). The composition of these electrolytes in body fluids is electrically balanced so the positively charged ions (cations: sodium, potassium, calcium, and magnesium) equal the negatively charged ions (anions: chloride, bicarbonate, sulfate, phosphate, proteinate, and carbonic and other organic acids). Although these particles are present in relatively low concentrations, any deviation from their normal levels can have profound physiologic effects.
ELDER TIP Institutionalized older people are at particularly high risk for dehydration because of their diminished thirst perception and any combination of physical, cognitive, speech, mobility, and visual impairment.
In homeostasis — an ever-changing but balanced state — water and electrolytes and other solutes move continually between cellular and extracellular compartments. Such motion is made possible by semipermeable membranes that allow diffusion, filtration, and active transport. Diffusion refers to the movement of particles or molecules from an area of greater concentration to one of lesser concentration. Normally, particles move randomly and constantly until the concentrations within given solutions are equal. Diffusion also depends on permeability, electrical gradient, and pressure gradient. Particles, however, can’t diffuse against any of these gradients without energy and a carrier substance (active transport). ATP is released from cells to aid particles needing energy to pass through the cell membrane.
The diffusion of water from a solution of low concentration to one of high concentration is called osmosis. The pressure that develops when a selectively permeable cell membrane separates solutions of different strengths of concentrations is known as osmotic pressure, expressed in terms of osmols or milliosmols (mOsm). Osmotic activity is described in terms of osmolality — the osmotic pull exerted by all particles per unit of water, expressed in mOsm/kg of water — or osmolarity, when expressed in mOsm/L of solution.
The normal range of body fluid osmolality is 285 to 295 mOsm/kg. Solutions of 50 mOsm above or below the high and low points of this normal range exert little or no osmotic effect (isosmolality). A solution below 240 mOsm contains a lower particle concentration than plasma (hypo-osmolar) while a solution over 340 mOsm has a higher particle concentration than plasma (hyperosmolar).
Rapid I.V. administration of isosmolar solutions to patients who are debilitated, are very old or very young, or have cardiac or renal insufficiency could lead to ECF volume overload and induce pulmonary edema and heart failure because particulate concentration is the same as plasma, so fluid shifting into and out of cells will occur.
Continuous I.V. administration of hypo-osmolar solutions decreases serum osmolality and leads to excess intracellular fluid volume (water intoxication), whereas continuous I.V. administration of hyperosmolar solutions results in intracellular dehydration, increased serum osmolality and, eventually, ECF volume deficit due to excessive urinary excretion. These states occur because of fluid diffusion and the cell’s attempt to balance the particulate concentrations inside and outside the cell.
Regulation of pH
Primarily through the complex chemical regulation of carbonic acid by the lungs and of base bicarbonate by the kidneys, the body maintains the hydrogen ion concentration to keep the ECF pH between 7.35 and 7.45. Nutritional deficiency or excess, disease, injury, or metabolic disturbance can interfere with normal homeostatic mechanisms and raise pH (acidosis) or lower it (alkalosis).
Assessing homeostasis
The goal of metabolism and homeostasis is to maintain the complex environment of ECF — the plasma — which nourishes and supports every body cell. This special environment is subject to multiple interlocking influences and readily reflects any disturbance in nutrition, chemical or fluid content, and osmotic pressure. Such disturbances can be detected by various laboratory tests. For example, measurements of albumin, prealbumin, and other blood proteins; electrolyte concentration; enzyme and antibody levels; and urine and blood chemistry levels (lipoproteins, glucose, blood urea nitrogen [BUN], creatinine, and creatinine-height index) accurately reflect the state of metabolism, homeostasis, and nutrition throughout the body. (See Laboratory tests: Assessing nutritional status, page 874.) Results of such laboratory tests, of course, supplement the information obtained from dietary history and physical examination — which offer gross clinical information about the quality, quantity, and efficiency of metabolic processes. To support clinical information, anthropometry, height-weight ratio, and skin-fold thickness determinations specifically define tissue nutritional status.
The following measures can help you maintain your patient’s homeostasis:
❑ Obtain a complete dietary history and nutritional assessment to determine if carbohydrate, fat, protein, vitamin, mineral, and water intake are adequate for energy production and for tissue repair and growth. Remember that during periods of rapid tissue synthesis (growth, pregnancy, healing), protein needs increase.
❑ Consult a dietitian about any patient who may be malnourished because of malabsorption syndromes, renal or hepatic disease, clear-liquid diets, or who may possibly receive nothing by mouth for more than 5 days. Planned meals that provide adequate carbohydrates, fats, and protein are necessary for convalescence. Supplementary carbohydrates are often needed to spare protein and achieve a positive nitrogen balance.
❑ Accurately record intake and output to assess fluid balance (this includes intake of oral liquids or I.V. solutions, and urine, gastric, and stool output).
❑ Weigh the patient daily — at the same time, with the same-type clothing, and on the same scale. Remember, a weight loss of 2.2 lb (1 kg) is equivalent to the loss of 1 L of fluid.
❑ Observe the patient closely for insensible water or unmeasured fluid losses (such as through diaphoresis). Remember, fluid loss from the skin and lungs (normally 900 ml/day) can reach as high as 2,000 ml/day from hyperventilation or tachypnea, thus increasing insensible water losses.
ELDER TIP Teach elderly patients and others vulnerable to fluid imbalances the importance of maintaining adequate fluid intake.
❑ Recognize I.V. solutions that are hyposmolar, such as 0.45% NaCl (half-normal saline solution). Isosmolar solutions include normal saline solution (0.9% NaCl), 5% dextrose in 0.2% NaCl, Ringer’s solutions, and 5% dextrose in water. (The latter acts like a hypotonic solution because dextrose is quickly metabolized, leaving only free water.) Hyperosmolar solutions include 5% dextrose in normal saline solution, 10% dextrose in water, and 5% dextrose in Ringer’s lactate solution.
❑ When continuously administering hypo-osmolar solutions, watch for signs of water intoxication: headaches, behavior changes (confusion or disorientation), nausea, vomiting, rising blood pressure, and falling pulse rate.
❑ When continuously administering hyperosmolar solutions, be alert for signs of hypovolemia: thirst, dry mucous membranes, slightly falling blood pressure, rising pulse rate and respirations, low-grade fever (99°F [37.2° C]), and elevated hematocrit, hemoglobin, and BUN levels.
❑ Administer fluid cautiously, especially to the patient with cardiopulmonary or renal disease, and watch for signs of overhydration: constant and irritating cough, dyspnea, moist crackles, rising central venous pressure, and pitting edema (late sign). When the patient is in an upright position, neck and hand vein engorgement is a sign of fluid overload.
ELDER TIP Many older patients take drugs to treat a variety of conditions. Remember that drugs can affect the patient’s nutritional status by altering nutrient absorption, metabolism, utilization, or excretion. Likewise, various foods, beverages, and mineral or vitamin supplements can affect the absorption and effectiveness of drugs. Be aware of these potential interactions when evaluating the patient’s medication regimen and nutritional status. Pictures
Book Source Details
- Book Title: Professional Guide to Diseases (Eighth Edition)
- Author(s): Springhouse
- Year of Publication: 2005
- Copyright Details: Professional Guide to Diseases (Eighth Edition), Copyright © 2005 Lippincott Williams & Wilkins.
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