Infants and children possess a

much higher fractional water content, which then decreases progressively with age.

Individuals with a greater percentage of body fat, such as women or obese patients,

also tend to have less water for a given weight.

The total aqueous volume in the body can be divided into the intracellular and

extracellular compartments. Because the intracellular fluid is the site of major

metabolic activity, homeostatic mechanisms are therefore in constant execution to

provide an environment of optimal ionic strength. The primary function of the

extracellular fluid is to serve as a conduit between cells and between organs.

Substantial ionic alterations within extracellular fluid can occur without a clinically

significant impact on body function. This extracellular compartment can be further

divided into three fractions: the interstitial volume, plasma volume, and transcellular

water volume. Interstitial fluid flows around cells, allowing the total surface area of

a cell to serve as an area of exchange. Plasma is the route for rapid transit within the

body. Transcellular water is the smallest component of extracellular fluid and is the

portion of total body water that can be found contained within epithelial-lined

spaces. Transcellular water includes the luminal fluid of the gastrointestinal (GI)

tract, the fluids of the central nervous system, and the fluid in the eye, as well as the

lubricating fluids at serous surfaces.

The basal requirement of water for a given individual is dependent on the sensible

(urinary) and insensible losses of water. Urine osmolality and the total amount of

solute excreted from the body dictate the volume of water comprising urine. Fever

can promote dehydration by increasing one’s basal metabolic rate in addition to

raising the vapor pressure of expired air and sweat, resulting in higher respiratory

and skin water losses, respectively. In the absence of fever and sweating, water loss

through the skin is relatively fixed; however, urinary water excretion can vary

greatly.

Essential Nutrients—Micronutrients

ELECTROLYTES

A subtle and complex balance of electrolytes exists between the intracellular and

extracellular milieu. The precise maintenance of these electrolyte gradients is critical

as such gradients regulate hydration and pH and ultimately have an impact on nerve

and muscle function. Sodium, chloride, and bicarbonate are the main solutes in the

extracellular fluid, whereas potassium, magnesium, phosphate, and proteins are the

dominant solutes inside the cell.

Although water can freely travel across the membrane of a cell, the cell membrane

itself is only selectively permeable to solutes. Osmotically active solutes are those

that are impermeable and thereby exert an osmotic pressure by which the distribution

of water between fluid compartments is determined.

Electrolyte balance is traditionally maintained by the oral intake of substances

containing electrolytes. Foods such as fruit juices, sports drinks, milk, and many

fruits and vegetables are replete with electrolytes. In oral rehydration therapy,

electrolyte drinks containing sodium and potassium salts replenish the water and

electrolyte levels in the body after dehydration caused by exercise, excessive alcohol

consumption, diaphoresis, diarrhea, vomiting, intoxication, or starvation. Hormones,

such as antidiuretic hormone, aldosterone, and parathyroid hormone, regulate

electrolytes once in the body, and the kidneys function to flush out those excess ions.

VITAMINS

Vitamins are organic compounds that cannot be biologically synthesized in sufficient

quantities by human beings and yet remain a vital requirement for the sustainment of

life. The biochemical functions of vitamins are diverse. Some vitamins assist in the

regulation of electrolyte metabolism, whereas others participate in the control of cell

and tissue growth and differentiation. Most vitamins function as cofactors, which are

molecules that bind to enzymes to promote their catalytic activities.

Vitamins are classified by their biologic and chemical activity. Vitamins are

designated as either water-soluble or fat-soluble. There are four fat-soluble vitamins

(A, D, E, and K) and nine water-soluble vitamins (eight B vitamins and vitamin C) in

humans. Water-soluble vitamins are used by the body quite rapidly, and amounts in

excess are readily excreted from the body in urine. The fat-soluble vitamins are

absorbed by the body using processes that closely parallel the absorption of lipids.

Fat-soluble vitamins are more likely to lead to toxicity, or hypervitaminosis. Fatsoluble vitamin regulation is also of particular significance in cystic fibrosis.

Vitamins are procured through the diet or supplements. The body can manufacture

only three vitamins from nondietary sources: vitamins D and K, and the B vitamin,

biotin. Critically ill patients experiencing metabolic stress possess vitamin needs that

may increase dramatically. Many disease states such as inflammatory bowel disease,

liver and renal disease, short-bowel syndrome, cancer, and acquired

immunodeficiency syndrome-associated wasting can also result in a higher demand

for vitamins. A parenteral formulation of multivitamins combining both fat- and

water-soluble vitamins into an aqueous solution designed for incorporation into

intravenous infusions is available for these classes of patients.

TRACE ELEMENTS

Appropriate intake levels of certain dietary minerals have been demonstrated to be

required in small amounts to maintain optimal health. These dietary minerals are

known as trace elements or ultratrace elements. Iron, zinc, copper, manganese, and

fluoride are classified as trace elements. These minerals are required in amounts

between 1 and 100 mg/day by adults. Ultratrace elements, or those dietary minerals

that are required in quantities less than 1 mg/day, include arsenic, boron, chromium,

iodine, selenium, silicon, nickel, and vanadium.

Consuming specific foods rich with the dietary mineral of interest is the

recommended method for satisfying these micronutrient requirements. Many trace

elements are naturally present in foods; however, some are added to foods to prevent

nutrient deficiencies—such as fortifying salt with iodine to

p. 748

p. 749

prevent development of hypothyroidism and goiter. When dietary intake is

insufficient to meet the daily nutrient requirements of an individual or when chronic

or acute deficiencies arise from pathology and injury, dietary supplements remain a

viable option. Supplements can be formulated to include multiple trace elements, a

combination of vitamins, or a single trace element.

MALNUTRITION

Malnutrition may occur when there is any disruption of nutritional status, including

disorders resulting from overfeeding or underfeeding or through impaired nutrient

metabolism. Clinically, a more useful definition of malnutrition is the state induced

by alterations in dietary intake, which results in subcellular, cellular, or organ

function changes that expose the individual to increased risks of morbidity and

mortality and can be reversed by adequate nutritional intervention.

8 An incidence of

malnutrition as high as 30% to 50% has been reported among hospitalized

patients.

9,10

In these hospitalized patients, the risk of acute malnutrition development

is greater because nutrient intake is often inadequate, nutrient stores can be depleted,

or the patients may experience concurrent injury or stress (e.g., trauma, infection,

major surgery). The presence of acute stress or injury increases energy requirements

to repair tissues. Breakdown of skeletal muscle to release amino acids for energy

production by conversion to glucose occurs if exogenous energy is not provided to

stressed patients. Even patients who were well nourished before the stressful event

may quickly become at risk for this type of iatrogenic malnutrition. Usually

conditional, acute malnutrition resolves once the illness or injury improves and

normal nutrient intake is resumed.

In stark contrast to stress-induced malnutrition, patients in starvation or

semistarvation states slowly adapt to inadequate nutrient intake. In this scenario,

endogenous fat stores are used for energy and a slow loss of muscle proteins ensues.

Nevertheless, energy and protein stores are not unlimited, and death occurs in

previously normal-weight individuals after about 60 to 70 days of starvation.

11,12

Patients with a history of chronic malnutrition who are faced with stress or injury are

at the greatest risk of developing malnutrition.

The most common type of nutritional deficiency in hospitalized patients is proteincalorie malnutrition, which includes depletion of both tissue energy stores and body

proteins. Complications develop more frequently for hospitalized patients who are

malnourished as a result of organ wasting and functional impairments. These

complications may include weakness, decreased wound healing, altered hepatic

metabolism of drugs, increased respiratory failure, decreased cardiac contractility,

and infections such as pneumonia and abscesses. Complications often increase the

length of hospital stay and costs of care, and may even ultimately reduce

reimbursement for institutions.

13–15

Malnutrition or its risk may occur in patients with inadequate intake for 7 to 14

days or in patients with an unintentional weight loss of 10% before their illness. For

these patients, nutritional intervention is appropriate and should be considered.

16,17

Patients who cannot meet their nutritional needs by consuming enough food orally

should be considered for an alternative nutrition mechanism. Specialized nutritional

support is the provision of parenteral or enteral nutrients, which are specifically

formulated or delivered to maintain or restore nutritional status.

18 For those who

cannot eat by mouth but have a functional GI tract, the first line of nutritional

intervention to be considered should be enteral feeding through an appropriate access

device (see Chapter 37, Adult Enteral Nutrition, and Chapter 38, Adult Parenteral

Nutrition).

To mimic the normal physiologic state when possible, the GI tract should be used

for providing nutrients. Enteral nutrients may be more beneficial and are generally

less costly than those provided by the parenteral route.

19 Enteral nutrients stimulate

the intestine, thus maintaining the mucosal barrier structure and function. This has

been associated with decreased infectious morbidity in critically ill patients

compared with those receiving nutrients parenterally.

20–24 Parenteral nutrition is

therefore reserved for patients whose GI tracts are not functional or cannot be

accessed, or who do not absorb enough nutrients to maintain adequate nutritional

status.

16

NUTRITION SCREENING

According to The Joint Commission (http://www.jointcommission.org), hospitals

are required to screen patients within 24 hours of admission to determine whether

they are malnourished or at risk for developing malnutrition. The nutrition screening

process identifies needs for further nutritional intervention or monitoring based on

nutritional risk. The information collected in the screening process is dependent on

the patient population, the healthcare setting, and individual institution policy. A

number of screening tools have been described in the literature with varying

reliability, specificity, and sensitivity. Some parameters included in nutrition

screening may have wider applicability in the outpatient setting than in the inpatient

arenas.

1,9

Patient Assessment

Nutritional assessment of a patient incorporates a collection of historical data,

analysis of body composition, evaluation of physiologic function, and complete

physical examination. Proper patient assessment should include the examination of

multiple factors and should not rely on any one parameter. This assessment serves to

identify the presence and severity of malnutrition or the risk of developing

malnutrition. A complete patient assessment performed by a trained practitioner can

help determine the goals of therapy and specify the need for specialized nutritional

support. Goals of therapy may be maintenance of existing nutritional status, repletion

of fat and lean body mass, and prevention of complications associated with

malnutrition.

NUTRITION HISTORY

A nutrition history is crucial in an effective nutritional assessment. Practitioners may

gain valuable information by interviewing the patient or the patient’s family and by

reviewing the medical record to identify factors that can contribute to malnutrition or

increase the risk of developing malnutrition.

Multiple factors can contribute to the development of malnutrition, including the

patient’s underlying disease states, past medical history, and socioeconomic

circumstances. Medications can adversely affect nutritional status by decreasing the

synthesis of nutrients, minimizing food intake through alteration of appetite and taste,

changing the absorption or metabolism of nutrients, or increasing nutrient

requirements. A complete evaluation of present and past body weight habits

contributes significantly to an appropriate nutrition history.

The components of a nutrition history are summarized in Table 35-1, some of

which are expanded subsequently.

WEIGHT HISTORY

Weight history and influences are important in evaluating nutritional status. Weight

loss is a sign of negative energy and negative protein balance and is often associated

with poor outcome in hospitalized patients.

9,25 A patient’s current weight often is

compared with a standard for ideal body weight (IBW). Percentage of IBW9

is

determined as shown in Eq. 35-1:

p. 749

p. 750

Table 35-1

Components of a Nutrition History

Medical history

Chronic illnesses

Surgical history

Psychosocial history

Socioeconomic status

History of gastrointestinal problems (nausea, vomiting, or diarrhea)

Diet history, including diets for weight gain or loss

Food preferences and intolerances

Medications

Weight history

Increase or decrease

Intentional or unintentional

Time period for weight change

Functional capacity

The primary limitation of this method of assessing weight is that the patient’s

weight is compared with a population standard rather than using the individual as the

reference point. For example, a patient who is significantly overweight but has lost

large amounts of weight may still be more than 100% of IBW and therefore not

considered at risk for developing malnutrition. A more patient-specific method of

evaluating weight is to compare current weight with the patient’s usual weight. This

can be determined using Eq. 35-2:

Using this method, the obese patient who has lost weight may be determined to be

less than 90% of usual weight and therefore nutritionally at risk. It is also important

to assess over what time period the change has occurred. Involuntary weight loss is

considered severe if loss exceeds 5% of usual weight within 1 month, or 10% of

usual weight within 6 months. A nonvolitional weight loss of more than 10% is

considered significant for malnutrition.

18 Patterns of weight loss must be evaluated to

determine whether the loss is stabilizing or continual, the latter being a more serious

concern. Weight gain after a significant weight loss may be considered a positive

sign.