Clinicians Pocket Reference, 11th Edition

Interplay between Nutrition and Illness

Nutritional factors figure prominently in the pathogenesis of coronary heart disease, cancer, stroke, and type 2 diabetes diseases that account for more than half of all deaths in the United States. Dietary habits also play an extensive role in other causes of morbidity and mortality, including hypertension, obesity, and osteoporosis. This situation is compounded by the epidemic of obesity in the United States, which is expected to cause sharp increases in the incidence of chronic illness. In recognition of the serious health implications of obesity, current definitions of "malnutrition" include states of overnutrition as well as conditions marked by nutritional deficits.

The link between nutritional status and health risk extends beyond chronic disease to include acute illness. Surveys place the incidence of malnutrition among hospitalized patients between 30% and 55%. Malnutrition increases the risk of adverse clinical outcomes of hospital stays. In short, poor nutrition increases the risk of becoming ill, and when illness does strike, malnutrition complicates treatment and impairs recovery.

As the interplay between nutritional status and illness has become better understood, nutritional assessment has taken on greater importance in clinical care. By integrating nutritional assessment into the evaluation of all patients, clinicians not only identify malnutrition but also uncover risk factors for chronic disease and unfavorable clinical outcome, determine nutritional requirements, recognize people likely to benefit from nutritional support, and establish a framework for developing a therapeutic plan.

Nutritional Assessment

No single assessment technique has the validity to serve as the sole indicator of nutritional status. Nutritional assessment is a comprehensive process that combines objective data with relevant clinical information. Evaluate body composition, anthropometric measurements, and results of laboratory tests, and use the data in the context of the patient's history, physical examination findings, and clinical condition to make decisions concerning nutritional status.

Body Weight

Body weight is a reliable indicator of nutritional status. Details concerning body weight include deviation of weight from ideal level, change in weight over time, and relation between weight and height. Body weight 20% over or under the ideal level places a patient at nutritional risk. Numerous methods for determining ideal body weight exist, but the Hamwi formula is the most widely used in clinical settings because the calculation is simple and provides a reasonable estimate of ideal body weight:

Changes in body weight from baseline carry important prognostic value. Whether or not usual body weight is over or under the ideal, unintentional weight loss is cause for concern. Both the degree of weight loss and the timeframe in which it occurs are significant variables. In general, involuntary weight loss of 10% of usual weight over a period of 6 mo represents severe nutritional risk that warrants further investigation. Among children, a downward trend in percentile ranking on growth charts is cause for concern even if the child continues to gain weight. Growth charts can also reveal a tendency toward excessive weight gain, allowing early nutritional intervention and management of obesity.

Body Mass Index

Body mass index (BMI), a ratio of weight to height, is a value that eliminates the influence of frame size. BMI is a reliable indicator of adiposity. An elevated BMI is strongly correlated with the risk of development of cardiovascular disease, diabetes, cancer, hypertension, and osteoarthritis. The formula for calculating BMI is:

Table 11 1 shows the interpretation of BMI values. Each class of obesity represents a higher level of health risk. The BMI formula overestimates body fat in muscular athletes and underestimates fat stores in older persons, limiting the value of the formula in those populations. Online BMI calculators make quick work of the somewhat cumbersome equation.

Anthropometric Measurements

Anthropometric evaluations are body composition assessments derived through direct measurement. A summary of the techniques used to evaluate body composition appears in Table 11 2. In traditional anthropometric evaluations, the circumference of the midarm muscle and the thickness of the triceps skinfold were measured. These techniques have fallen out of favor because the reference tables lack validity and clinical applicability. The preferred method of nutritional assessment includes measuring waist circumference and waist-to-hip ratio to evaluate the distribution of body fat and the health risks associated with abdominal obesity. For men, a waist measurement > 40 in (102 cm) indicates higher risk of cardiovascular disease. Similar risk exists for women with a waist measurement > 35 in (89 cm). For both men and women, a ratio of waist circumference to hip circumference 1.0 represents increased risk of health problems related to obesity. A favorable waist-to-hip ratio is 0.90 for men and 0.80 for women. Abdominal obesity is one feature of metabolic syndrome, a cluster of risk factors, such as insulin resistance, dyslipidemia, hypertension, and prothrombotic and proinflammatory states, that increase risk of cardiovascular disease and type 2 diabetes. Table 11 3 details the identification of metabolic syndrome.

Laboratory Tests

In primary care, the lipid profile is perhaps the most frequently ordered laboratory test with nutritional implications. Many other routine laboratory tests, such as CBC, blood glucose, electrolyte levels, creatinine, and BUN, also provide information relevant to nutritional status. Although the lipid profile is important during acute illness, nutritional assessment of hospitalized patients also emphasizes evaluation of serum protein concentration.

Visceral protein markers, such as albumin, transferrin, and prealbumin, are routinely measured in hospitalized patients. Because of its short half-life, prealbumin is the preferred marker in clinical settings. Visceral protein levels are frequently presented as a key part of nutritional assessment. These measurements, however, have limited value as nutritional indicators in acutely ill patients. Studies show a strong link between low visceral protein levels and increased risk of morbidity and mortality, but numerous clinical factors other than nutrition can influence visceral protein levels during acute illness. Visceral protein concentration often reflects hydration status, organ function, or an inflammatory response to injury more than the nutritional state of the patient. In inflammatory states, the liver reprioritizes protein synthesis in favor of acute-phase proteins, causing visceral protein levels to fall. Some investigators have suggested routinely measuring C-reactive protein in conjunction with visceral protein as a way to differentiate low visceral protein levels caused by nutritional factors from those related to the presence of an inflammatory process.

Nutritional assessment does not routinely include assays of specific nutrient levels unless clinical circumstances raise concern about potential imbalances. For instance, order iron studies as part of the assessment of a patient with microcytic anemia, but order vitamin B₁₂ and folate levels in the evaluation of macrocytosis.

Health History

The health history obtained in the evaluation of every patient is an indispensable source of information regarding nutritional status. The patient interview not only provides an additional opportunity to detect risk factors related to nutrition but also often reveals the mechanisms underlying nutritional problems. The medical history, for example, indicates the impact of disease or previous surgery on nutrient intake, absorption, and metabolism. A review of the patient's current medication profile may reveal drug nutrient interactions or GI side effects that affect appetite. Symptoms such as nausea, pain, fatigue, dry mouth, and shortness of breath often have negative effects on food intake. Aspects of the patient's social history, such as the presence of substance abuse, financial difficulties, or lack of support systems, also are causes of concern about nutritional status. Whenever possible, request a complete nutritional history by a registered dietitian to obtain complete information about nutritional deficits that may exist.

Physical Examination

Use standard physical assessment procedures to evaluate nutritional status. Physical signs of malnutrition are often nonspecific and require correlation with the patient's history, clinical condition, and results of diagnostic studies. As Table 11 4 shows, muscle wasting, poor skin integrity, and loss of subcutaneous fat are typical findings associated with long-standing deficits in protein and energy intake. Patients rarely exhibit the classic signs of vitamin or mineral deficiency that characterize conditions such as scurvy and beri-beri. Fortification of the food supply in the United States and widespread use of multivitamin supplements have made physical manifestation of nutrient deficiency an uncommon occurrence.

Assessing GI Function

Although not part of nutritional assessment per se, appraisal of GI function often provides insight into the mechanisms underlying nutritional problems and helps to pinpoint specific nutrient deficiencies that may exist. Any alteration in the key GI functions associated with eating appetite, chewing, swallowing, digestion, absorption, and elimination can have profound effects on nutritional status. An understanding of GI function is also essential for determining the most appropriate route of nutritional support.

The functional status of the GI tract is a key element in determining when to initiate feeding in postoperative patients. In most cases, patients can safely begin oral intake once they recover consciousness sufficiently to protect their airway. Laparotomy, however, can delay GI motility in the postoperative period. In general, delayed motility affects the stomach and colon, sparing the small intestine. The stomach regains motility about 24 h postoperatively, and the colon typically recovers 72 96 h after surgery. It is generally believed that by the time a patient reports flatus, oral intake can be resumed.

Establishing Protein and Energy Requirements

Caloric Expenditure

Establishing target ranges for energy and protein intake helps not only to ensure that the patient receives adequate nutrition but also to avoid overfeeding. Numerous studies have shown the deleterious effects of overfeeding, including unwanted weight gain, hyperglycemia, hepatic dysfunction, electrolyte imbalances, azotemia, hyperlipidemia, and elevated respiratory quotient. These effects are especially important in patients fed by vein. Several methods for determining energy expenditure exist, including weight-based calculations, formulas such as the Harris Benedict equations, and indirect calorimetry techniques. None of these methods is ideal for all situations. Each requires a degree of judgment to account for clinical variables that affect energy needs. The following simple weight-based system is the most practical way to establish goals for caloric intake.

For adults in most clinical settings, a range of 25 30 kcal/kg of body weight is a reasonable estimate of daily energy expenditure. No attempt is made to account for variations in age, sex, body composition, or acuity of illness, hence the need for clinical judgment. Concerns about overfeeding and unfavorable outcome have eliminated the once common practice of providing as much as 35 40 kcal/kg/d to critically ill patients. The weight-based system has a wide margin of error for obese patients. When a patient's BMI falls into an obese category, many clinicians use adjusted body weight (ABW) to determine energy needs. The formula for ABW takes into account that not all of a person's excess weight is adipose tissue but that a portion is metabolically active, lean body mass:

Consensus does not exist regarding the optimal level of energy intake for obese patients. Studies in which patients received as little as 50% of estimated energy expenditure or 20 kcal/kg of ABW have shown positive outcomes. Further research is needed, however, for accurate prediction of optimal levels of energy intake for obese patients.

Infants and growing children need much higher energy intake per kilogram of body weight than adults. Infants may need as much as 110 kcal/kg/d. Energy intake remains elevated to support growth through the teenage years, but wide variation occurs. Satisfactory growth is the best indication that a child's energy intake is adequate.

Protein Requirements

Healthy persons with normal renal function need 0.8 g of protein per kilogram of body weight per day, but illness and injury can dramatically increase protein needs. For example, postoperative patients need 1.0 1.5 g/kg/d. Sepsis increases protein needs to 1.2 1.5 g/kg/d. Daily protein intake for patients with multiple trauma should fall within 1.3 1.7 g/kg, and burn victims may need 1.8 2.5 g/kg/d. With the exception of patients with burn injuries, guidelines set the upper limit for protein intake at 2.0 g/kg/d. Research suggests that doses of protein above this level exceed the patient's utilization capacity and can lead to azotemia. As with energy intake, protein intake for obese patients should be based on ABW.

Protein needs for children vary with age. The requirement is greatest in the first year of life and then gradually declines. Healthy infants need 2 3 g/kg/d, and children up to age 10 need 1.0 1.2 g/kg/d. The protein requirement of critically ill children is approximately 1.5 g/kg/d.

Nitrogen Balance

Nitrogen balance studies indicate the adequacy of protein intake by comparing nitrogen intake to nitrogen excretion. Positive nitrogen balance, a state in which nitrogen intake exceeds losses, implies that the amount of protein being administered is sufficient to promote anabolism and prevent erosion of lean body mass. In general, negative nitrogen balance indicates the need to increase protein intake and possibly energy intake as well. Urinary nitrogen losses of 8 12 g/d indicate a mild stress condition; 14 18 g/d, moderate stress; and 20 g/d, severe stress. The extremely high nitrogen losses that characterize critical illness can impede efforts to achieve positive nitrogen balance. In this situation, increasing protein intake to 2.0 g/kg/d may help to achieve nitrogen equilibrium.

Steps to determine nitrogen balance are as follows:

1. Measure urine urea nitrogen (UUN). This step requires 24-h urine collection to quantify urinary nitrogen loss. Because this study shows only nitrogen excretion that occurs as urea, a "fudge factor" of 4 g of nitrogen is added to the urinary loss to account for nonurea nitrogen losses through routes such as skin and feces. Therefore 24-h UUN + 4 g = 24-h nitrogen loss. 2. Determine nitrogen intake. Calculating nitrogen intake for the 24-h period of the UUN collection is relatively easy for patients who receive a prescribed amount of protein through parenteral or enteral nutrition. For patients who eat orally, a dietitian must work with the patient to maintain accurate records of all food consumed during the study period. To determine 24-h nitrogen intake, divide protein intake (g/24 h) by 6.25. 3. Formula for calculating nitrogen balance:

Therapeutic Diets

The term therapeutic diet refers to dietary changes that play a role in the management of a medical condition. The most commonly ordered therapeutic diets and their indications appear in Table 11 5. These dietary modifications, which usually require a physician's order, typically call for a change in the consistency of the food served or an adjustment in the quantity of one or more nutrients in the diet. Most hospitals have diet manuals available for reference, and registered dietitians are usually on staff for consultation in clinical situations that necessitate a therapeutic diet.

Modifying the Consistency of Food

Changing the consistency or texture of the diet is a simple way to make food easier to chew, swallow, and digest. For instance, patients with poor dentition may benefit from a pureed diet. Another use of this type of therapeutic diet is the postoperative diet progression that begins with clear liquids and advances to regular food as tolerated. Although doubt exists concerning the need to step through slow diet advancement postoperatively, the practice remains common after some types of surgery. Patients who have undergone elective colonic resection, for example, can usually receive a regular diet after they tolerate one meal of clear liquids, whereas those who have undergone surgery involving the esophagus, stomach, or small intestine may benefit from a more conservative approach.

Patients with dysphagia frequently need a change in texture or consistency of food to enhance the safety of eating. An evaluation of swallowing function by a speech language pathologist is essential in determining the appropriate diet for patients with impaired swallowing. Dysphagia occurs most often as a result of neurologic conditions, but many medical and surgical problems can compromise swallowing. A swallowing evaluation is warranted before the start of oral intake in any situation that increases the risk of dysphagia, including cognitive or functional decline, surgery or radiation of structures involved in swallowing, prolonged intubation, and recent tracheostomy.

Modifying Nutrient Content of the Diet

Because illness frequently alters nutrient requirements or nutrient tolerance, diet modification is a common therapeutic intervention in the management of many chronic diseases. In some cases, the therapeutic diet may simply limit a single nutrient, such as sodium. At other times diet prescription may require broad changes in eating habits. The dietary changes recommended for prevention and treatment of cardiovascular disease fall into the latter category. Any diet that restricts one or more nutrients poses nutritional risks. One concern is that in adhering to a dietary restriction, patients may unintentionally omit other essential nutrients. In addition, patients frequently find restrictive diets unpalatable, a problem that leads to poor intake or noncompliance. A patient with a prescription for a therapeutic or modified diet need instruction by a clinical dietitian before discharge or as an outpatient.

Oral Nutritional Supplements

For patients unable to tolerate sufficient food to maintain adequate nutritional status, oral nutritional supplements can halt or reverse nutritional decline and improve clinical outcome. For elderly patients, for example, the use of oral supplements improves nutritional status and reduces mortality. The nutritional products, which are available without a prescription, come in a variety of forms, including high-protein, high-calorie beverages, puddings, snack bars, and soups. Ensure (Ross Laboratories) and Boost (Novartis) are two common examples of liquid oral supplements. These products are flavored for oral consumption, but they are also appropriate for administration through a feeding tube. Depending on the circumstances, patients can consume these products in addition to regular meals or as a meal replacement. Most oral supplements on the market are lactose-free, an important consideration for persons who cannot tolerate milk-based supplements. Unlike most snack foods, commercially prepared oral supplements provide a balanced mix of nutrients, including vitamins and minerals. Encourage patients to try a variety of supplements to avoid taste fatigue, a common problem among patients who consume only one supplement over an extended period. Adding flavoring such as chocolate or coffee syrup to oral supplements can improve palatability. Sustained success with oral supplements frequently requires the creative support of the entire health care team.

Infant Feeding

Breast Feeding

Clinical practice guidelines consistently endorse breast feeding as the sole source of infant nutrition for the first 6 mo of life. Breast milk is uniquely suited to the nutritional needs of growing infants, supporting optimal nutritional and reducing the risk of childhood obesity. Research findings suggest that the presence of long-chain polyunsaturated fatty acids in human milk may also enhance neurocognitive development. Breast-fed infants gain protection against infectious disease early in life from immunoglobulins present in the milk and may have fewer infantile allergies than their formula-fed counterparts. In addition to these physiologic advantages, breast feeding offers psychological benefits to both mother and infant and reduces the cost of infant feeding.

Commercial Infant Formulas

Despite compelling evidence of the benefits of breast feeding to both infant and mother, commercial infant formulas continue to play a prominent role as a source of infant nutrition in the United States. Commercial infant formulas serve as an appropriate substitute for breast milk in the presence of medical contraindications to breast feeding, when the mother decides against breast feeding, or if maternal milk production is inadequate. Because manufacturers have refined the nutrient profile of commercial infant formulas to more closely resemble the composition of breast milk, homemade infant formulas are no longer considered an acceptable substitute. Most commercial infant formulas have a cow's milk base, but soy-based formulas are also available. Soy formula does not prevent colic or allergy, as once thought, and has no role as a primary method of infant feeding.

Commonly used formulas are outlined in Table 11 6. Most infant formulas are isoosmolar (eg, Similac 20, Enfamil 20, and SMA 20 with and without iron). These formulas are used most often for healthy infants.

If possible, preterm infants should receive human milk, although breast milk can be fortified to meet the elevated requirements of a rapidly growing infant. Commercial formulas for premature infants contain 24 kcal/oz (eg, Similac 24, Enfamil 24, "preemie" SMA 24). Many other specialty formulas are available for infants with medical conditions such as inborn errors of metabolism, malabsorption syndromes, and milk and protein sensitivity.

Commercial formulas are available with and without iron, but current guidelines call for use of an iron-fortified formula for most infants. Many pediatricians recommend vitamin supplements with some formulas if the infant is taking < 32 oz/d. However, at this point most infants are beginning solid food that serves as an additional source of vitamins and minerals.

Oral Rehydration Solutions

Infants with mild or moderate dehydration, often due to diarrhea or vomiting, may benefit from oral rehydration formulas. These solutions typically include glucose, sodium, potassium, and bicarbonate or citrate. Common formulations include Pedialyte, Lytren, Infalyte, Resol, and Hydrolyte.

Initiating Infant Feeding

Most healthy term infants can begin breast feeding immediately after birth. The initial feeding for bottle-fed infants generally takes place within the first 4 or 5 h of life as long as the infant displays signs of readiness for feeding, such as alertness, active bowel sounds, and rooting and sucking behavior. For preterm and sick infants, conduct a detailed assessment before introducing feeding. In this setting, feedings should begin only if the infant has hemodynamic stability, no excessive oral secretions, no vomiting, no bile-stained gastric aspirate, normal bowel sounds, and a nondistended, soft abdomen and can coordinate breathing, sucking, and swallowing. Because tachypnea increases the risk of aspiration, verify that the infant's respiratory rate is within normal limits before offering a bottle for the first time. Infants who have been weaned from a ventilator should exhibit no evidence of respiratory distress for at least 6 h after extubation before feeding.

Feeding Progression

The initial feeding for bottle-fed infants is usually sterile water or D₅W. Do not use hypertonic solutions such as D₁₀W.

Controversy exists regarding the optimal way to introduce commercial infant formula after the initial feeding with water or D₅W. Some clinicians advocate diluting infant formula with sterile water and advancing the concentration as tolerated (eg, start with 1/4 strength, increase to strength, and then progress to strength before giving full-strength formula). Others believe this gradual progression is unnecessary and start with full-strength formula after the infant tolerates the initial feeding without difficulty. Breast-fed infants typically begin feeding without first receiving a water feeding, and breast milk is never diluted.

Considerations for Preterm Infants

Many preterm infants lack the coordination to take oral feedings safely. In this situation, provide nutrients through a feeding tube. Considerable controversy remains concerning the timing of initial enteral feeding of preterm infants. For larger (> 1500 g) premature infants in stable condition, give the first feeding within the first 24 h of life. Early feeding may allow the release of enteric hormones that exert a trophic effect on the intestinal tract. On the other hand, apprehension about necrotizing enterocolitis (mostly in very low birth weight infants) precludes initiation of enteral feeding in the following circumstances: perinatal asphyxia, mechanical ventilation, presence of umbilical vessel catheters, patent ductus arteriosus, indomethacin treatment, sepsis, and frequent episodes of apnea and bradycardia. Consensus does not exist regarding the optimal timing and method of introducing feeding to preterm infants with those conditions. In general, preterm infants begin enteral feeding in the first 3 d of life. The objective is reaching full enteral feeding by 2 3 wk of life. Start parenteral nutrition, including amino acids and lipids, at the same time as enteral feeding to provide adequate caloric intake.

Candidates for Nutritional Support

When nutritional assessment reveals evidence of poor or declining nutritional status, investigate the causes and develop a plan for intervention. This process includes management of underlying medical problems, management of symptoms that interfere with appetite and eating, and efforts to increase intake with dietary modification and oral supplements. Consider enteral or parenteral nutrition when nutritional deficits persist despite efforts to improve oral intake or when the patient's clinical condition precludes safe or adequate intake by mouth (see Chapter 12).