Nutritional Requirements and Assessment
I. METABOLISM
A. Normal energy metabolism.
Every day normal adults need roughly 25 to 30 kcal of fuel/kg of body weight. For a 70-kg person, this is about 2,100 kcal per day. A typical American derives 40% to 60% of daily calories from carbohydrates, 20% to 45% from lipids, and 10% to 20% from protein. Glucose is the body’s preferred source of immediate energy. The healthy body manufactures glucose from carbohydrates, protein, and the glycerol backbone of triglycerides (TGs). Since very little carbohydrate can be stored in the body, most of the body’s reserve energy stores are made up of lipids. There is no storage form of protein. Even though protein can be metabolized for energy, this results in wasting of lean body mass and negative nitrogen balance.
B. Metabolism of carbohydrates
1. Glucose.
Most of the ingested carbohydrate is broken down to glucose, which enters the circulation. Glucose is taken up by all cells of the body and burned for immediate energy. Under normal, well-fed circumstances, the cells of the central nervous system depend on glucose for energy. However, neurons of the brain cortex and blood cells can use only glucose as fuel under all circumstances.
2. Glycogen.
Glucose, fructose, and galactose can be converted to glycogen, a polymer of glucose, which is stored mainly in the liver (200 mg) and muscle (300 mg) as a readily available energy reserve. Liver glycogen can be converted directly to glucose for systemic distribution. However, muscle glycogen is burned by muscle itself. Total glycogen stores of the body can meet the body’s energy needs for 36 to 48 hours.
The body protects its glycogen stores for emergencies. At times of temporary glucose insufficiency, the body manufactures glucose by gluconeogenesis, from protein and glycerol of lipids. Also free fatty acids (FFAs) and amino acids are burned for direct energy. Excess dietary carbohydrate is converted to TGs for storage in the adipose tissue.
C. Metabolism of lipids.
Lipids constitute the body’s main energy reserve. A nonobese, 70-kg man has 12 to 18 kg of fat stores. This figure is somewhat higher for women. Fat supplies 9 kcal/g compared to 4 kcal/g from glucose or protein.
Ingested lipids are hydrolyzed in the intestinal lumen, and then absorbed into enterocytes of the small intestine, where they are resynthesized into TGs. In the enterocytes, TGs made up of long-chain FFAs form chylomicrons by the addition of apoproteins. Chylomicrons are secreted into the intercellular space and are absorbed into the lymphatics. Short- and medium-chain FFAs are directly absorbed into the portal vein. Lipoprotein lipase of endothelial cells release FFAs from TGs, and FFAs enter cells of various tissues (e.g., heart and muscle), where they are oxidized for energy, and cells of adipose tissue to form TGs for storage. Adipose tissue can also convert carbohydrates and proteins into TGs by lipogenesis.
Mobilization of TGs for energy by lipolysis begins with the hydrolysis of TGs into FFAs and glycerol. Glycerol may be either converted into glucose by gluconeogenesis or directly oxidized further. FFAs enter some tissue cells, are broken down to acetyl coenzyme A, and oxidized through the Krebs cycle. At times of starvation and lack of glucose as fuel, large quantities of FFAs enter tissue cells. The Krebs cycle may become overloaded, and FFAs may be incompletely oxidized.
Intermediate products in the form of acids and ketone bodies accumulate in the blood, leading to ketosis and acidosis.
Intermediate products in the form of acids and ketone bodies accumulate in the blood, leading to ketosis and acidosis.
D. Metabolism of protein.
The body of a 70-kg man contains 10 to 14 kg of protein. Because there is no storage form of protein in the body, the protein compartment must be maintained by daily intake. A typical adult requires about 0.8 to 1.0 g of protein/kg of body weight per day. For a 70-kg man, this is about 65 to 70 g.
Ingested protein may be used for protein synthesis or fuel, especially if the body requires more energy than is supplied by carbohydrate and lipid intake. In this event, body protein may also be catabolized for energy. One third of the body’s total protein is potentially available as an energy source in case of dire need. Further protein catabolism, however, severely jeopardizes health.
If protein is ingested in amounts greater than needed for protein synthesis and energy production, it is stripped of its nitrogen and converted to glucose, glycogen, and TGs for storage. Protein, besides being needed for structural purposes, is also needed for replacement, repair, and growth of tissue (cell components) and maintenance of circulating proteins (e.g., albumin, transferrin, coagulation proteins, enzymes, and antibodies).
E. Nitrogen balance.
In a healthy adult, ingested protein must supply enough amino acids to maintain a constant level of body protein. Thus, the intake of protein must equal or exceed the breakdown of body protein. The effect of diet on the body’s protein compartment may be approximated by nitrogen balance. Nitrogen balance is the difference between nitrogen intake and output.
1.
When protein synthesis is equal to protein degradation, one has a neutral nitrogen balance. In adults, this is a sign of health.
2.
Positive nitrogen balance occurs when protein synthesis exceeds protein degradation. This suggests tissue growth. This state is normal and expected in children. In adults, it may mean rebuilding of wasted tissue.
3.
In negative nitrogen balance, the protein breakdown is in excess of protein synthesis. This catabolic state usually occurs in sepsis, trauma, and burns, and when the carbohydrate and lipid intake is less than the body’s energy needs, necessitating use of the body’s own protein for fuel.
4. Calculation of nitrogen balance.
Nitrogen balance can be calculated with reasonable accuracy. Nitrogen makes up 16% of ingested protein. The division of protein intake in grams by 6.25 (the reciprocal of 0.16) will give nitrogen intake. Most of the nitrogen output from the body is into the urine as urea, which can be measured. Other excreted nitrogen is in feces and urine as nonurea nitrogen, amounting to about 4 g per day. The addition of 4 g to the urine nitrogen measured will give the daily nitrogen output. Thus, nitrogen balance can be calculated by subtracting nitrogen output from nitrogen input.
Nitrogen balance | = | nitrogen in − nitrogen out |
= | protein intake − [daily urinary nitrogen + 4 g 6.25 (for nonurea nitrogen)] |
For example, if protein intake is 75 g, urine urea nitrogen is 500% or 5 g/L with 2,000-mL urine output per 24 hours.
Nitrogen balance | = | 75 g protein |
− [(5 g/L × 2 L) + 4]6.25 g protein/g nitrogen | ||
= | 12 g nitrogen intake − 14 g nitrogen output | |
= | −22 g per day |
F. Energy metabolism
in starvation. During periods of starvation, when ingested nutrients are unavailable, the body goes through different stages of metabolic adaptation. Energy requirements are met by metabolism of substrates from the energy reserves, which are drawn on simultaneously, but not equally, following a careful sequence.
1. For immediate use,
the glycogen in the liver is depolymerized to glucose for systemic use. Muscle glycogen is oxidized locally. The lactate produced may be converted to glucose in the liver for systemic use. If used up entirely, the glycogen reserves are depleted in 36 to 48 hours.
2. In early starvation,
glucose is produced from gluconeogenesis from amino acids, lactate, and glycerol, but within a week the amount available for fuel becomes severely limited. Its use is reserved exclusively for the central nervous system and glycolytic tissues: erythrocytes, leukocytes, and macrophages. Maintenance of this basal glucose production is essential. However, because its main source is amino acids derived from catabolism of the body’s own protein, it jeopardizes the body’s survival.
3. After the first few days of starvation,
an adaptive response takes effect. Metabolic rate decreases. FFAs become the main source of energy. The heart, kidneys, and muscle take up and oxidize FFAs directly. Twenty-five percent of the FFA released from the adipose tissue is partially metabolized in the liver to form ketone bodies, which are readily used in peripheral tissues. There is also a recycling of lactate and pyruvate back into glucose by the Cori cycle. These changes decrease the protein requirements to about one third that of the non-adapted state.
4. In prolonged starvation,
the brain also adapts to the use of ketone bodies for fuel. This further spares glucose and decreases protein breakdown. As the adipose tissue becomes depleted, the body is forced to use its own essential protein for energy, leading to loss of protein from muscle, liver, spleen, kidneys, gastrointestinal tract, and plasma. The heart, adrenals, and central nervous system are initially protected. However, weight loss in excess of 20% to 30% seriously increases mortality due to organ dysfunction, anemia, impaired immunity, ineffective wound healing, and decreased resistance to infection.
In contrast, the catabolic state in sepsis and injury induces an increase in protein requirements. Protein catabolism, as represented by urine urea nitrogen, may increase by 50% in patients with sepsis and may nearly double in those with severe trauma or burns. This increase in protein catabolism occurs without an adequate compensatory increase in protein synthesis. Mediators of this catabolic response include glucocorticoid, catecholamines, glucagon, and perhaps interleukin-1. These substances induce an increase in lipid mobilization and oxidation, skeletal muscle catabolism, and hepatic gluconeogenesis; they also induce a state of insulin resistance. The septic state is highly catabolic, with net catabolism even in the presence of abundant protein and calories.
II. ASSESSMENT OF NUTRITIONAL STATUS.
Protein-calorie starvation is the progressive loss of lean body mass and adipose tissue because of inadequate intake of amino acids and calories. Anemia, malabsorption, and hypermetabolism are some of the causes of malnutrition in patients with a variety of subacute and chronic illnesses. Protein-calorie malnutrition, which increases morbidity and mortality, can be reversed by appropriate nutritional support by enteral and parenteral routes.
A. Anthropometric measurements
1. Height, weight, triceps skinfold thickness
(for quantitative estimate of adipose tissue stores) and mid-upper arm circumference (for estimate of muscle mass [somatic protein]) are useful data in nutritional assessment. Each actual measurement can be compared to the standards (Tables 10-1, 10-2, and 10-3) as follows:
% standard = Actual measure/Standard × 100
2. Body weight
below 90% of ideal is considered as protein-calorie undernutrition. However, due to pre-illness obesity and presence of edema, body weight may not be subnormal in the malnourished patient.
3. Creatinine height index.
The most important laboratory test for detecting protein-energy undernutrition is the creatinine height index (CHI). The actual daily urinary creatinine excretion is compared with an ideal value to compute CHI.
Actual urinary creatinine/Ideal urinary creatinine × 100 = CHI
By expressing this index as a percentage of standards, the severity of the loss of muscle mass can be graded as mild, moderate, severe, or critical (Tables 10-4 and 10-5).
TABLE 10-1 Ideal Weight for Height* | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
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