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
Essential Nutrients—Micronutrients
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 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.
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.
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
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 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.
malnutrition as high as 30% to 50% has been reported among hospitalized
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.
Patients with a history of chronic malnutrition who are faced with stress or injury are
at the greatest risk of developing malnutrition.
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.
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.
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.
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
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.
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
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
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
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 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
determined as shown in Eq. 35-1:
Components of a Nutrition History
History of gastrointestinal problems (nausea, vomiting, or diarrhea)
Diet history, including diets for weight gain or loss
Food preferences and intolerances
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
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