Essential Nutrient Requirements in Health and Illness
The essential nutrient requirements of adults in health and illness vary. These requirements will be briefly addressed from the perspective of energy, carbohydrate, fat, protein, fluid, electrolyte, vitamin, and trace elements based on the recommendations of the American Society of Enteral and Parenteral Nutrition (A.S.P.E.N.).2
The Institute of Medicine defines estimated energy requirement
(EER) for healthy adults as the dietary energy intake predicted to maintain energy balance in a healthy adult of a defined age, gender, weight, height, and level of physical activity consistent with good health.3
In calculating EER age, physical activity, height, and intensity of activity are included. Energy requirements can also be measured with indirect calorimetry that results in the resting energy expended (REE). This indirect method of estimating REE requires special equipment that measures the ratio of carbon dioxide expired to the amount of oxygen inspired. The values collected are used in a
mathematical equation to determine nutritional needs. Indirect calorimetry is not widely used because it is very expensive and requires both specialized machinery and highly trained staff to complete the estimation of needs.
Other options to estimate REE in the hospitalized patient are the Harris-Benedict equations, and more recently, the Ireton-Jones equations.4
The advantage of the Ireton-Jones calculations is that it is normed to sick populations so that the stress of illness has already been calculated into the equation. The Harris-Benedict equations must be adapted to sick patients. There are also two calculation methods to estimate energy needs for the critically ill patient: the Swinamer and Penn State equations. These equations are more dynamic than the previously mentioned equations and are often used in intensive care units because they consider factors that can change daily such as body temperature and minute ventilation.5
A few basic rules are useful for calculating energy requirements as outlined in the A.S.P.E.N guidelines.2
Energy requirements are based on kilocalories per kilogram of body weight with a range of 20 to 35 total kcal/kg body weight/day. Energy comes from carbohydrates and fats. Carbohydrates should be provided at a rate not to exceed 7 g/kg/day, while fat should not exceed a rate of 2.5 g/kg/day. For critically ill patients fat intake is limited to 1 g/kg/day.
Carbohydrates are defined as starches and sugars that are used by the body for energy. When metabolized, 1 gram (g) of carbohydrate yields 4 cal. Carbohydrates are classified as monosaccharides, disaccharides, or polysaccharides. For carbohydrates to be used by the body, they must be broken down into glucose (a monosaccharide), the simplest form of sugar. Glucose is oxidized to release energy and is the source of energy for cerebral cell metabolism. Glucose may also be stored as a reserve in the liver (and in muscle tissue to a lesser degree) in the form of glycogen through a process called glycogenesis. Hydrolysis of glycogen to glucose is called glycolysis (the anabolic enzymatic conversion of glucose to lactate or pyruvate, resulting in energy stored in the form of adenosine triphosphate [ATP], such as occurs in muscles). In addition, excess glucose can be converted into fat and stored in the body as adipose tissue.
occurring as organic substances in the body are called lipids
and include triglycerides (fats and oils), phospholipids (e.g., lecithin), and sterols (e.g., cholesterol).6
When 1 g of fat is oxidized, 9 cal are generated. The major function of fats is energy production, although they are also important for the manufacture of other fat-related compounds, such as cholesterol, triglycerides, phospholipids, and lecithin. The major sources of fat in the normal diet are butter, margarine, oil, bacon, meat, fats, egg yolks, nuts, and legumes.
are the basic units of structure in lipids; they can be divided into essential fatty acids (EFA) and nonessential fatty acids. An EFA
is one that cannot be manufactured in the body. There are two EFAthat are polyunsaturated; they are linoleic acid (omega-6 fatty acid) and α-linoleic acid (omega-3 fatty acid). EFA play a role in maintaining skin and growth in children. As part of phospholipids, EFA are a component of cell membranes and are precursors of eicosanoids
, a group of hormone-like substances involved in inflammation and blood clotting. Prostaglandins, thromboxanes, and leukotrienes are types of eicosanoids related to EFA.6
Although the body cannot make EFA, it does store them so that deficiencies are rare under normal conditions. Essential fatty acid deficiency (EFAD) was first documented in animals in the 1920s. This deficiency was first seen in humans when parenteral nutrition (PN) was developed 4 decades later. EFAD is not usually seen in patients with adequate oral intake or in patients requiring enteral nutrition support as most enteral nutrition formulas contain EFA along with the other nutrients (vitamins, minerals) needed to maintain health. In patients requiring total parenteral nutrition (TPN), EFAD can be prevented by proper lipid administration. If EFAD is present, it is usually seen after 4 or more weeks of fat-free (lipid-free) PN administration.5
Under normal circumstances, nonessential fatty acids
do not cause specific deficiency disorders if not ingested in sufficient amounts because they can be manufactured in the body.
All food contains a mixture of saturated, monounsaturated, and polyunsaturated fatty acids. The saturation characteristic is important because it influences the fat’s physical traits and its impact on health. Discussion about the role of omega-3 and omega-6 fatty acids in health is of interest. Omega-3 polyunsaturated oils are found in fish such as salmon, herring, trout, mackerel, and swordfish and also in some plant oils such as canola oil, flaxseeds, walnuts, and hazelnuts whereas omega-6 polyunsaturated oils are found in plant oils such as sunflower, corn, soybean, and cottonseed oils. Both omega-6 and omega-3 fatty acids have immunemodulating properties and both have an effect on inflammation, blood clotting, and lipid levels. However, these are often opposing functions; therefore, it is important that both omega-6 and omega-3 fatty acids are present in the diet in a balanced ratio.
Protein is a class of energy-yielding nutrients composed of carbon, hydrogen, and oxygen plus nitrogen, which is unique to protein. When metabolized, 1 g of protein yields 4 cal. The building blocks of protein are amino acids, the primary function of which is to build and repair body tissue. Almost all nitrogen ingested comes from protein, and most of the nitrogen lost from the body is in the form of nitrogenous end-products found in the urine as urea, creatinine, uric acid, and ammonium salts. A small amount of nitrogen loss occurs through the stool and skin. Nitrogen balance is defined as when protein synthesis and protein breakdown occur at the same rate. Positive nitrogen balance is present when protein synthesis exceeds protein breakdown whereas negative nitrogen balance occurs when protein breakdown exceeds protein synthesis. In the clinical setting, nitrogen balance is determined by calculating protein intake and urine urea nitrogen (UUN) excretion for the same 24-hour period and comparing findings.
Proteins are composed of amino acids. There are 20 common amino acids that are classified as either essential or nonessential. Essential amino acids are necessary for normal growth and development and cannot be manufactured by the body. Nonessential amino acids are defined as amino acids that are not necessary for normal growth and development and can be manufactured by the body. Protein can also be classified as either complete or incomplete. A complete protein is one that contains all essential amino acids in sufficient quantity and appropriate proportions to supply the body’s needs. Proteins of animal origin, such as milk, meat, cheese, and eggs, are examples of complete proteins. An incomplete protein is defined as one that is deficient in one or more essential amino acids. Incomplete proteins are of plant origin and include grains, legumes, and nuts.
The RDA of protein for healthy adults is 0.8 g/kg regardless of gender or age.3
For a patient who is in a catabolic state, the requirement is higher, ranging from 1.2 to 2 g/kg/day.2
In addition, adequate kilocalories must be provided with the protein to ensure proper protein utilization. Note that patients with certain diseases, such as renal failure and end-stage liver disease, may require a protein-restricted diet.
Fluid, electrolytes, vitamins, and trace elements
are all essential to health. The typical fluid
requirement for adults is 20 to 40 mL/kg/day or 1 to 1.5 mL/kcal of energy expended.7
In the clinical setting, there are great variations dependent on disease (e.g., heart failure, ascites), which may require fluid restriction, and hypermetabolic states (e.g., perspiration, diarrhea), which may require an increased fluid intake. Fluid supplement is individualized based on intake and output measurements and clinical condition. Relying on the thirst mechanism to trigger drinking is unreliable, especially in elderly persons, because there tends to be impairment of thirst, which leads to underhydration. When adequate fluid intake cannot be achieved by the oral route, the enteral or parenteral routes are alternative options that will be discussed later in the chapter. See Chapter 10
for further discussion of fluid imbalance. The major electrolytes
in the body are sodium, potassium, chloride, calcium, magnesium, and phosphorus. Electrolytes are involved in metabolic activities and are essential to the normal cellular function. Electrolyte imbalances are common in disease states and require careful physical and laboratory monitoring and correction as part of therapeutic intervention.
cannot be stored in the body, so that deficiencies can develop if an adequate diet is not consumed daily. Other vitamins can be stored in the body so that deficiencies are not apparent for weeks to months of inadequate vitamin intake. Vitamins are classified as either water soluble or fat soluble. Water-soluble vitamins
are vitamin C and the B-complex vitamins (i.e., thiamine, riboflavin, niacin [nicotinic acid], pyridoxine, pantothenic acid, biotin, folic acid, and cobalamin). The fat-soluble vitamins
are vitamins A, D, E, and K. Vitamins are needed for recovery and maintenance of cellular and system functions. Although there are recommendations for vitamin and trace mineral requirements for a healthy person on an oral diet, the recommendations can be extrapolated with caution for patients who are ill.2
In summary, this brief review provides a backdrop to consider the nutritional requirements and the impact of neurological conditions and nutritional modifications needed for recovery and maintenance.