Validation of the tracer-balance concept with reference to leucine: 24-h intravenous tracer studies with L-[1-13C]leucine and [15N-15N]urea

The validity of tracer-derived estimates of whole-body leucine balance was investigated. Seven healthy young adult subjects received an adequate protein diet for 6 d; at 1800 on the last day, L-(l-'3Cjleueine and ('5N-'�N)urea were given as primed, continuous intravenous infusions for 24 h. Sub- jeets were in a fasting state for the first 12 h and at 0600 on day 7 they then received hourly 10 equal meals to achieve a fed state. Total leucine intake (diet plus tracer) was 89.4 mgkg�' d'. Mean daily leucine oxidation was equivalent to 89.5 ± 3.3 mg leueinefkg. The predicted daily oxidation rate. from measure- ments made during the last hour of the fast and the fifth hour of the fed period, was 91.2 ± 5.8 mg/kg (P = 0.25 from measured). Measured and predicted whole-body leucine balances were 0.76 ± 2.99 and -0.98 ± 5.54 mg/kg, respectively (P = 0.25). Urea production exceeded urea excretion by 20%; daily protein oxi- dation was the same when estimated from leucine oxidation or nitrogen excretion. Thus, the tracer-balance concept is valid, and reliable predictions of total daily leucine oxidation and whole- body leucine balance can be obtained from short-term measure- ments of leucine oxidation during fasted and fed states. Am