Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients)

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Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids

TABLE 10-6 Use of Serum Proteins to Assess Protein Nutritional Status

	Half-Life	Clinical Use	Limitations
Albumin	18 d	Severe malnutrition	Affected by protein losing enteropathy, renal loss, burn, and by liver disease
Transferrin	8–9 d	Limited—chronic deficiency	Affected by iron deficiency and by infection
Pre-albumin	2 d	Acute depletion	Affected by vitamin A deficiency
Retinol-binding protein	12 h	Acute depletion	Affected by vitamin A deficiency
SOURCE: Adapted from Young et al. (1990).

tissues. In protein malnutrition, the skin becomes thinner and appears dull; the hair first does not grow, then it may fall out or show color changes (Pencharz, 1985). Over a longer period of time, assessment of changes in lean body mass reflects protein nutritional status. The clinical tools most available to assess lean mass are dual emission x-ray absorptiometry and bioelectrical impedance (Pencharz and Azcue, 1996).

SELECTION OF INDICATORS FOR ESTIMATING THE REQUIREMENT FOR PROTEIN (NITROGEN)

In the framework for Dietary Reference Intakes, as described in Chapter 1 and Appendix B, adequacy of requirements is defined as the lowest daily intake value for a nutrient that will meet the need, as defined by the specified indicator or criterion of adequacy, of apparently healthy individuals. This section reviews some of the possible indicators used or proposed for use in analyses estimating human protein requirements.

Factorial Method

The factorial method is based on estimating the nitrogen (obligatory) losses that occur when a person is fed a diet that meets energy needs but is essentially protein free and, when appropriate, also relies on estimates of the amount of nitrogen that is accreted during periods of growth or lost to mothers during lactation. The major losses of nitrogen under most conditions are in urine and feces, but also include sweat and miscellaneous losses, such as nasal secretions, menstrual losses, or seminal fluid. In this

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