9. Relation between leucine oxidation and nitrogen excretion

It would help to validate estimates of leucine oxidation if parallel changes were found in N excretion as a measure of total amino acid oxidation. Prediction of the oxidation of total N from that of leucine has traditionally relied on the assumption that leucine represents 8% of body protein. As far as I can make out, this value was introduced into the isotope literature by Golden & Waterlow (1977), based on the old analyses of Block & Weiss (1956). It corresponds also to the leucine content of beef muscle (FAO, 1970) and that of the human fetus (Widdowson et al, 1979). Since the body contains about 20% of structural protein that includes very little leucine, the value of 6.5% proposed by Reeds & Harris (1981), based on carcass analysis, is probably not appropriate, although Young has used it in a recent paper (1991)

The data of Reeds & Harris (1981) in pigs show excellent correspondence between measured balances and those predicted from leucine. However, in man Clugston & Garlick (1982) were able to account for only 68% of total N excretion from their measurements of the oxidation of leucine. Young et al (1987) measured N balances in the third week on diets providing, 7, 14 and 30 mg leucine/kg/d and again after repletion. There was qualitative agreement with the leucine balances, but no attempt was made at a quantitative comparison. In the experiments of Marchini et al (1993) N balance as well as leucine balance was measured on the FAO, MIT and egg diets. No significant differences were found between 1 and 3 weeks on the diets (cf. Young et al, 1987), so the results have been averaged in Table 15. The leucine balances put the results in the expected order, which the N balances hardly do, so this may justify the authors' claim for the 'poor reliability' of N balances. However, the s.ds of both sets of balances are so large that the difference between the FAO and MIT diets is significant in only one comparison. Furthermore, there are disturbing quantitative discrepancies between predicted and observed balances.

Price et al (1994) investigated this point in some detail, using the figure of 8% for the leucine content of body protein. Their results are shown in Figure 5. The ratio of predicted to measured N loss was the same over the whole range of protein intakes, with a mean of 0.79. The measured N loss included estimated faecal and miscellaneous losses. It could be argued that in this context it would have been more appropriate to include only the amounts of urea and ammonia N excreted. This would bring the ratio closer to 1. The predicted loss was calculated from leucine oxidation after correcting for any difference between the leucine content of body protein and of food. The important point is that correspondence, even if not exact, is the same over a wide range of intakes. These data, therefore, provide valuable confirmation of the validity of the leucine balances.

El Khoury et al (1994a), during their 24 h infusions, measured urea production with ¹⁵N¹⁵N urea, as well as urea excretion. The results in Table 16 show a remarkable correspondence between N intake, observed N output and predicted N oxidation, calculated on the basis that leucine is 8% of body protein. One might have expected a better correspondence between predicted oxidation and urea production (which does not include urea recycled from the gut) rather than urea excretion, since urea production is presumably a measure of the total amino-N oxidized. However this is a controversial point on which Young and Millward are in agreement and I differ (see section 13). As with Price's data, if the faecal and non-urea N are excluded from the calculation, leucine oxidation comes into good agreement with urea production.

Table 15 Average of leucine and nitrogen balances after 1 and 3 weeks on egg, MIT or FAO diets. Data of Marchini et al, (1993)

Diet	Leucine balance (mg/kg/d)	Measured N balance (mg/kg/d)	Predicted N balance (mg/kg/d)
egg	+164	+5	+33
MIT	+2.5	+7	+5
FAO	-10.9	- 0.5	- 22

Predicted N balance = leucine balance × 100/8 × 1/6.25.

Figure 5 Predicted/measured nitrogen excretion.