The body requires nutrients (carbohydrates, fats, proteins, vitamins, and minerals) to maintain homeostasis. Nutrition is the process by which the body metabolizes and uses the nutrients from food. The processes of nutrition include digestion, absorption, and metabolism.

Digestion, the process by which ingested foods are broken down in the GI tract to smaller segments in preparation for absorption, begins with mastication, the chewing, tearing, and grinding of food by the teeth and the mixing of food with enzymes contained in the saliva. The particles of food pass through the esophagus through peristalsis, a coordinated, rhythmic, serial contraction of the smooth muscle lining of the GI tract, to the lower esophageal sphincter (gastroesophageal constrictor muscle), which relaxes to allow food to go into the stomach. This sphincter contracts to prevent regurgitation (reflux) of stomach contents back into the esophagus. Digestion continues in the stomach and small intestines, with the aide of pancreatic enzymes, bile, and intestinal enzymes. Carbohydrates are acted upon by ptyalin, an enzyme in the mouth, and pancreatic amylase, maltase, dextrinase, lactase, and sucrase in the intestines. Proteins are acted upon by pepsin in the stomach, as well as trypsin, chymotrypsin, carboxypolypeptidase, and peptidase in the intestines. Fats are acted upon by bile, agitation, and pancreatic lipase in the intestines.

Absorption is the process by which the end products of digestion (monosaccharides, amino acids, glycerol, fatty acid chains, vitamins, minerals, and water) pass through the epithelial membranes of the small and large intestines, through osmosis, diffusion, and active transport. Water is absorbed in the large intestines, where food residue or fiber is collected and eliminated as a bowel movement. Excretion also occurs through the rectum, kidneys, sweat glands, skin, and lungs.

Metabolism includes the chemical reactions and processes in every body cell—the generation of energy, elimination of wastes, and other bodily functions. Nutrients are converted into energy production in the liver. Sugars are converted into glucose through a process known as glycolysis; excess amino acids to are converted to urea, carbohydrates, or fats; and excess fats are converted first to glycerol and fatty acids, then to acetyl coenzyme A (acetyl-CoA). Anabolism is the constructive phase of metabolism in which smaller molecules are combined to form larger molecules. Catabolism is the destructive phase of metabolism in which larger molecules are broken down into smaller molecules. The metabolic rate refers to the rate of heat liberation during chemical reactions, which is expressed as a calorie, the quantity of heat required to raise the temperature of 1 gram of water 1oC. Calories are expressed as kilocalories (kcal), the equivalent of 1000 calories. The basal metabolic rate (BMR) represents the energy needed to maintain essential physiological functions (respiration, circulation, and muscle tone) when the person is completely at rest.

Good nutrition requires choosing the healthiest forms of macronutrients (carbohydrates, fats, and proteins), the right amounts of micronutrients (vitamins and minerals), and the right amount of water. Water makes up 60% to 70% of the adult’s weight and 77% of the infant’s weight. Water and electrolytes are acquired from the diet. The amount of water taken in should balance the amount excreted. Body water decreases as body fat increases and the elderly have a reduced proportion of water in their bodies. Vitamins, organic compounds that aid in the regulation of cellular metabolism and assist in energy production, are either fat-soluble or water-soluble. Fat-soluble vitamins (A, D, E, and K) are stored in the fat and the liver and are toxic when too high. Water-soluble vitamins (C and B-complex) are not stored.

Minerals are inorganic elements that serve as catalysts in chemical reactions. For example, copper and iron are needed for hemoglobin formation, fluoride is required for the formation of teeth and prevention of cavities, and iodine is a component of thyroid hormones. Antioxidants are vitamins, minerals, and enzymes that block or inhibit destructive oxidative reactions. A by-product of oxidation is the free radical, an unstable atom that can cause cell damage. Common antioxidants are vitamins C and E, beta-carotene, selenium, germanium, coenzyme Q10, some amino acids, and phytochemicals (plant foods that provide antioxidant protection).

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen. They include monosaccharides (glucose, galactose, and maltose), or simple sugars; disaccharides (sucrose, lactose, and maltose), or double sugars; and polysaccharides (glycogen, cellulose, and starch), or complex sugars. Carbohydrates are the body’s primary source of energy. They are protein-sparing, i.e., the body uses carbohydrates before it uses proteins. Hyperglycemia is defined as a blood glucose greater than 110 mg/dl. Hypoglycemia is defined as a blood glucose of less than 70 mg/dl.

Proteins are organic compounds that contain carbon, hydrogen, oxygen, nitrogen, and (occasionally) sulfur. They responsible for growth and replacement, clotting, immunity, fluid balance, sodium and potassium balance, buffering the body’s fluids, and energy production. Proteins are made up of amino acids. Nonessential amino acids can be synthesized; Essential amino acids must be ingested in the diet. High-biological-value (complete) proteins, found in animal proteins and soy, contain all of the essential amino acids. Low-biological-value (incomplete) proteins lack one or more of the essential amino acids, usually lysine, methionine, and tryptophan. Vegetables are low-biological-value proteins.

Fats (lipids) are used for energy; cell membrane integrity; absorption of fat-soluble vitamins; maintaining body temperature; and synthesis of bile salts, steroid hormones, and vitamin D. Lipids are made up carbon, hydrogen, and oxygen, with higher hydrogen concentrations than carbohydrates. Saturated fatty acids form glycerol esters of organic acids whose carbon atoms are joined by single bonds. Unsaturated fatty acids form glycerol esters of organic acids whose carbon atoms are joined by double or triple bonds. Polyunsaturated fatty acids form esters that have many carbons unbounded to hydrogen atoms. The most important lipids are triglycerides, phospholipids, and cholesterol (formed from saturated fats). Triglycerides can be used for energy. They are transported to the liver by chylomicrons (lipoproteins), which are synthesized in the intestines. Low-density lipoproteins (LDLs) are responsible for the formation of atherosclerosis (the formation of atheromatous plaques inside the walls of the arteries).

The recommended dietary allowances (RDAs) for essential nutrients are set by the national Nutrition Board of the National Academy of Sciences—National Research Council. The Dietary Reference Intake (DRI) classification has been established to assure that the diet is balanced with adequate amounts of the following groups: calcium, vitamin D, phosphorus, magnesium, and fluoride; folate and other B vitamins; antioxidants; macronutrients; trace elements; electrolytes and water; and other food components. The Food Pyramid, with recommended amounts of each of six groups of food, is used in practice when teaching clients about good nutrition. Among the objectives for Healthy People 2010 is the goal to reduce the number of people who are overweight.

The factors that affect nutrition are age, lifestyle, ethnicity, culture, religion, and various other factors. Very small children have erratic eating patterns. They need a high calorie diet with adequate protein, vitamin D, calcium, and phosphate for growth. School-age children and adolescents also need adequate nutrients but are at risk for poor food choices and obesity. Caloric requirements decrease into adulthood, except during pregnancy and lactation. Older adults are less active and need further calorie reductions, plus adequate fats and proteins; vitamin D, calcium, and phosphorus for bone integrity; and high fiber foods.

Nursing assessment of nutrition is important for identification of malnutrition, an alteration related to inadequate intake, disorders of digestion or absorption, and overeating. The components of nutritional assessment are the nutritional history, physical examination, and diagnostic and laboratory data. Tools for nutritional history include the 24-hour recall, food-frequency questionnaire, a three-day record of food consumed, and the diet history. The physical examination includes an inspection of the skin, hair, and mucous membranes; intake and output; anthropometric measurements (measurement of the size, weight, and proportions of the body); skinfold measurement; mid-upper-arm circumference; and abdominal-girth measurement. Diagnostic and laboratory data include protein indices, such as serum albumin; pre-albumin; serum transferring, a blood protein in combination with iron and indicator of protein deficiency; hemoglobin; total lymphocyte count, a measure of protein depletion; blood urea nitrogen; and urine creatinine excretion.

Nursing diagnoses for nutrition problems include Imbalanced Nutrition: Less Than Body Requirements, Imbalanced Nutrition: More Than Body Requirements or Risk for More Than Body Requirements; and High Risk for Impaired Skin Integrity: Related to Inadequate Intake of Proteins, Vitamins, and Minerals. Outcomes should be set with the client, considering age, lifestyle, culture, likes and dislikes, and so on. Implementation of the plan includes monitoring weight and intake, initiating diet therapy, assisting with feeding if necessary, enteral nutrition (oral or feeding tube feeding), and parenteral nutrition (feeding through a peripheral or central line).