Protein-Energy Interactions (IDECG, 1991, 437 p.): The metabolic basis of amino acid requirements: 9. Stable isotope studies

Protein-Energy Interactions (IDECG, 1991, 437 p.)

The metabolic basis of amino acid requirements

		(introduction...)
		Abstract
		1. Introduction: The nature of the problem
		2. Nutrient requirement models
		3. The Millward & Rivers requirement model: Qualitative aspects
		4. The variable extrinsic component of the maintenance requirement
		5. The anabolic drive
		6. Hormonal components of the anabolic drive
		7. Protein requirements: Formal statement
		8. The issue of protein quality
		9. Stable isotope studies
		10. Practical experience of biological values of dietary protein
		11. Urea salvage
		12. Indispensable amino acid requirements for the anabolic drive
		13. Conclusions
		References

9. Stable isotope studies

YOUNG et al. (1989) support their new scoring pattern with stable isotope studies which point to considerable needs for leucine and lysine However, I have great difficulties with these studies, both in terms of their design, and in terms of the technical problems (MILLWARD and RIVERS, 1988). One particular technical problem, not previously considered and relating to all stable isotope studies, is the magnitude of the tracer.

Figure 13. Lysine balance studies as reported by MEREDITH et al. (1986) and recalculated taking into account the tracer infusion in the balance.

Consider lysine studies, which are the most important in practical terms. In these studies, lysine oxidation was measured with ¹³C lysine in individuals as lysine intakes were lowered and a 24-hour lysine balance was calculated from the measured fed-state oxidation rate and estimated fasted losses. The studies appeared to show that intakes of about 20 mg/kg were required for a positive balance. In fact these results were obtained by calculations which largely ignored the impact of the tracer. If, in contrast, the tracer had been included as input, in the way in which Young and colleagues have calculated subsequent balances (PELLETIER et al., 1991), then a much lower balance point would have been obtained (Figure 13). Because of this, these results in my opinion do not offer experimental support for the high lysine requirement which YOUNG et al. (1989) are proposing.