Fundamentals
of Nutrition

What is a
Food Portion?

Your Nutrition Style

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Physiology
of Nutrition
Lecture Series
Lecture 1
Lecture 2
Lecture 3
Lecture 4
Lecture 5
Lecture 6
Lecture 7
Lecture 8

Lecture 4 continued

Clinical and Laboratory Assessment of Protein Nutrition
A nutritionally-oriented history should inquire as to the patient's pre-illness weight, height, rate of weight loss prior to presentation, nausea, vomiting, anorexia, and specific ingestive, metabolic or absorptive problems which could impair nutritional status. Based on these assessments, the percent ideal body weight from standard tables, and the percent usual weight at presentation can be calculated. Body weight changes may be misleading in patients with fluid overload including those with congestive heart failure, liver disease, and renal failure. In uncomplicated starvation, there is an increase in extracellular fluid volume which tends to maintain weight despite loss of metabolically active tissues.

The sensitivity of nutritional evaluation is enhanced by including certain assessments of the functional indices of the body cell mass including certain proteins synthesized in the liver and the status of host immune function.

Albumin is the major protein synthesized in the liver and carries out significant functions as a carrier protein and to provide oncotic pressure. Its half-life is approximately 20 days, and it does not reflect recent changes in nutritional status. Transferrin has a half-life of only 8.8 days and so can reflect more recent changes in nutritional status. However, transferrin levels are increased in iron deficiency reducing the specificity of this measurement for nutritional status. Prealbumin has a half-life of 24 hours, and can be used to reflect changes in nutritional status over the short-term as patients receive nutritional support to assess response to therapy. Biochemical assessment should include measurement of albumin and prealbumin. An albumin level of greater than 3.5 g/dl is normal. Albumin levels of 3.0 to 3.5 g/dl indicate significant hypoalbuminemia, while levels below 3.0 g/dl indicate severe albumin deficiency.

Immune function is impaired in malnutrition. The quantitation of absolute lymphocyte counts derived from a complete blood cell count and differential, and the assessment of delayed hypersensitivity using skin test antigens are techniques used to assess the impact of nutritional status on immune function. The routinely utilized skin test antigens include tuberculin (as purified protein derivative or PPD), mumps, streptokinase-streptodornase (SKSD), candida albicans, and trichophyton. These tests were chosen on the basis that most normal individuals are exposed to them and would be expected to have a positive skin test reaction. In uncomplicated starvation or protein-energy malnutrition, skin test reactivity can be restored with renutrition. Anergy is not specific to malnutrition and can be a feature of certain diseases such as Hodgkin's disease, while decreased white blood cell counts can be depressed transiently in the postoperative period and following infection with human immunodeficiency virus. Therefore, these estimations of immunocompetence are not simply specific to malnutrition.

Given the variety of nutritional assessment techniques available, most clinicians will have to select a small group of routinely available tests to use on a regular basis. Most clinical centers will have available skin testing, albumin and transferrin for routine use. These tests should make it possible to assess whether patients are mildly, moderately or severely malnourished, and whether marasmic, kwashiorkor-like, or combined severe malnutrition is present.

The status of the lean body mass can be assessed by measuring urinary creatinine excretion over 24 hours. Creatinine production in most individuals is directly related to skeletal muscle mass, provided that there is no rapid catabolism of muscle in progress as with severe sepsis or trauma, and that large amounts of dietary creatinine found in animal skeletal muscle are not being ingested.

A creatinine-height index or CHI is calculated based on the measured 24 hr excretion of creatinine and that expected in a normal adult of the same height as the patient. This index has limited sensitivity with values between 60 and 80 percent of ideal representing moderate skeletal muscle depletion, and 40 to 50 percent representing severe skeletal muscle depletion. For purposes of estimation the ideal creatinine excretion for adults is taken as 23 mg/kg/24 hr for males and 18 mg/kg/24 hr for females.

Assessment of Risk of Nutritional Depletion
Assessment of risk of nutritional depletion can be used to determine which patients require consideration for nutritional therapy as described below. The presence of any of the following criteria should motivate the physician to conduct a complete nutritional assessment with consideration of appropriate forms of nutritional therapy:

1) Recent involuntary weight loss of greater than 5 percent in one month or over 10% in six months, especially when associated with anorexia, fatigue or weakness;

2) History of recent significant physiological stresses such as organ dysfunction, major surgery, infection, or illness within the last 3 months;

3) Absolute lymphocyte count < 1200 cells/cubic mm; or

4) Serum albumin less than 3.2 g/dl.

Evaluation of Response to Nutrition Support
Since the goal of nutritional support is the attainment of an anabolic state or reduction of nitrogen losses, the assessment of nitrogen balance is the most useful clinical assessment to determine whether nutritional therapy is effective. Nitrogen balance is defined as the difference between nitrogen intake and nitrogen excretion. Nitrogen intake is taken as the protein intake determined from dietary records divided by 6.25 gm Nitrogen per gm of "average" protein ingested. Nitrogen excretion is taken as the urinary nitrogen excreted per 24 hours plus a fixed estimate of 4.0 g/24 h for unmeasured nitrogen losses from cellular sloughing into the feces (1 g) , losses from the skin (0.2 g) and non-urea nitrogen losses in the urine (2 g). Since nitrogen balance is most usefully applied in a serial fashion in the same patient, the particular constants used to estimate unmeasured excretion are only important for comparison of published results.

At any given level of nitrogen intake, nitrogen balance improves with increased administration of non-protein calories, to a maximum achieved at a ratio of 150:1 of non-protein calories per gram of nitrogen. Proteins vary in their biological value based on the mixture of essential and non-essential amino acids which they contain. Albumin has the ideal mixture of amino acids for optimal utilization of protein and is assigned a biological value of 100. Casein is close to albumin in its biological quality followed by meat proteins such as those found in steak or tuna which have a biological value of 80. Corns and beans each with biological values of 40 or less can be combined in a protein mixture with a biological value of 80, because the amino acid patterns of the two proteins are complementary. The protein requirement for normal individuals is 0.55 gm/kg protein for a high biological value protein such as milk or albumin, but is 0.8 gm/kg for the mixture of proteins found in the average American diet.

The tests of protein nutrition already discussed can be employed on a serial basis to follow clinical responses in individuals given nutritional support. As discussed above, adequate time should be allowed for the rapid and more slowly responding proteins to change. A failure of response in the face of adequate nutriture suggests an ongoing neoplastic, infectious or immune process impairing the response to nutritional therapy.

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