1. The aims of this study were twofold: (i) to investigate the ability of a recently described [2H5]phenylalanine method for quantifying whole-body protein turnover during acute physiological perturbation; (ii) to determine specifically whether the previously observed increase in protein synthesis on insulin withdrawal in insulin-dependent (type 1) diabetic patients seen when employing the [13C]leucine technique could be corroborated by using [2H5]phenylalanine.

2. Whole-body protein turnover was measured by both the [2H5]phenylalanine and [13C]leucine primed continuous infusion methods applied simultaneously to six type I post-absorptive diabetic patients during insulin withdrawal and infusion.

3. Values were determined by the [13C]leucine method by measuring either [13C]leucine (primary pool) or α-[13C]ketoisocaproic acid (reciprocal pool) enrichment in plasma.

4. Values of whole-body protein breakdown during insulin withdrawal derived from the [2H5]phenylalanine and primary and reciprocal pool [13C]leucine models respectively were 3.54 ± 0.43, 3.85 ± 0.41 and 4.62 ± 0.44 g day−1 kg−1 (means ± SD). Insulin infusion resulted in a significant reduction (P <0.02) to 3.07 ± 0.34, 3.05 ± 0.26 and 3.82 ± 0.4 g day−1 kg−1, respectively. Synthesis values fell significantly but by a smaller amount than breakdown, resulting in increased (P <0.05) net protein deposition, regardless of the model used.

5. These data demonstrate that the [2H5]phenylalanine and [13C]leucine methods generate similar results both in absolute and relative terms in response to short-term insulin infusion.

6. The confirmation of increased whole-body protein synthesis during insulin withdrawal by two independent methods supports the validity of this observation.

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