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