Nitrogen Balance

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The oldest (and most widely used) method of following changes in body N is the N balance method. Because of its simplicity, the N balance technique is the standard of reference for defining minimum levels of dietary protein and essential amino acid intakes in humans of all ages ( 44, 45). Subjects are placed for several days on a specific level of amino acid and/or protein intake and their urine and feces are collected over a 24-h period to measure their N excretion. A week or more may be required before collection reflects adaptation to a dietary change. A dramatic example of adaption involves placing healthy subjects on a diet containing a minimal amount of protein. As shown in Figure2,Z, urinary N excretion drops dramatically in response to the protein-deficient diet over the first 3 days and stabilizes at a new lower level of N excretion by day 8 (46).

Figure 2.7. Time required for urinary N excretion to stabilize after changing from an adequate to a deficient protein intake in young men. Horizontal solid and broken lines are mean ± 1 standard deviation for N excretion at the end of the measurement period. (Data from NS Scrimshaw, Hussein MA, Murray E, et al., J Nutr 1972;102:1595-604.)

The N end-products excreted in the urine are not only end products of amino acid oxidation (urea and ammonia) but also other species such as uric acid from nucleotide degradation and creatinine (Ta.ble 2,.9.). Fortunately, most of the nonurea, nonammonia N is relatively constant over a variety of situations and is a relatively small proportion of the total N in the urine. Most of the N is excreted as urea, but ammonia N excretion increases significantly when subjects become acidotic, as is apparent in T.a.b.l..e 2 9. when subjects have fasted for 2 days (47). Tabje..2.9. also illustrates how urea production is related to N intake and how the body adapts its oxidation of amino acids to follow amino acid supply (i.e., with ample supply, excess amino acids are oxidized and urea production is high, but with insufficient dietary amino acids, amino acids are conserved and urea production is greatly decreased).

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Table 2.9 Composition of the Major Nitrogen-Containing Species in Urine tsucionpwtïr noproer FinflBSl Pi Citafl |T»i)(iwc m Jf

Table 2.9 Composition of the Major Nitrogen-Containing Species in Urine

Nitrogen appears in the feces because the gut does not completely absorb all dietary protein and reabsorb all N secreted into the gastrointestinal tract ( Fig. 2.6). In addition, N is lost from skin via sweat as well as via shedding of dead skin cells. There are also additional losses through hair, menstrual fluid, nasal secretions, and so forth. As N excretion in the urine decreases in the case of subjects on a minimal-protein diet ( Fig, 2.7.), it becomes increasingly important to account for N losses through nonurinary, nonfecal routes (48). The loss of N by these various routes is shown in Tabje... „2.1.0. Most of the losses that are not readily measurable are minimal (<10% of total N loss under conditions of a protein-free diet when adaptation has greatly reduced urinary N excretion) and can be discounted by use of a simple offset factor for nonurinary, nonfecal N losses. The assessment of losses comes into play in the finer definition of zero balance as a function of dietary protein intake for the purpose of determining amino acid and protein requirements. As we shall see below, small changes in N balance corrections make significant changes in the assessment of protein requirements using N balance.

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Table 2.10 Obligatory Nitrogen Losses by Adult Men on a Protein-Free Diet

Although the N balance technique is very useful and easy to apply, it provides no information about the inner workings of the system. An interesting analogy for the N balance technique is illustrated in Figure 2.8 where the simple model of N balance is represented by a gumball machine. Balance is taken between "coins in" and "gumballs out." However, we should not conclude that the machine turns coins into gum, although that conclusion is easy to reach with the N balance method. What the N balance technique fails to provide is information about what occurs within the system (i.e., inside the gumball machine). Inside the system is where the changes in whole-body protein synthesis and breakdown actually occur (shown as the smaller arrows into and out of the Body N Pool in Fig, 2.8). A further illustration of this point is made at the bottom of Figure,2..8 where a positive increase in N balance has been observed going from zero (case 0) to positive balance (cases A-D). A positive N balance could be obtained with identical increases in N balance by any of four different alterations in protein synthesis and breakdown: a simple increase in protein synthesis (case A), a decrease in protein breakdown (case C), an increase in both protein synthesis and breakdown (case B), or a decrease in both (case D). The effect is the same positive N balance for all four cases, but the energy implications are considerably different. Because protein synthesis costs energy, cases A and B are more expensive, while cases C and D require less energy than the starting case, 0. To resolve these four cases, we have to look directly at rates of protein turnover (breakdown and synthesis) using a labeled tracer.

Figure 2.8. Illustration of the N balance technique. Nitrogen balance is simply the difference between input and output, which is similar to the introduction of a coin into a gumball machine resulting in a gumball being released. The perception of only the "in" and "out" observations is that the machine changed the coin directly into a gumball or that the dietary intake becomes directly the N excreted without consideration of amino acid entry from protein breakdown (B) or uptake for protein synthesis (S). This point is further illustrated with four different hypothetical responses to a change from a zero N balance (case 0) to a positive N balance (cases A-D). A positive N balance can be obtained by increasing protein synthesis (A), by increasing synthesis more than breakdown (B), by decreasing breakdown (C), or by decreasing breakdown more than synthesis (D). The N balance method does not distinguish among the four possibilities.

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