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Twenty-four–hour oral tracer studies with L-[1-¹³C]lysine at a low (15 mg • kg^-1 • d^-1) and intermediate (29 mg • kg^-1 • d^-1) lysine intake in healthy adults^1,2,3

¹ From the Laboratory of Human Nutrition and Clinical Research Center, Massachusetts Institute of Technology, Cambridge, MA; Shriners Burns Hospital, Boston; and MassTrace Inc, Woburn, MA.

Background: We proposed previously that the mean lysine requirement value is 30 mg • kg^-1 • d^-1 rather than the proposed 1985 FAO/WHO/UNU estimate of the upper range of the requirement, which is 12 mg • kg^-1 • d^-1.

Objective: Our objective was to explore the 24-h pattern and rate of whole-body lysine [l-¹³C]oxidation and status of whole-body lysine balance in healthy, young adults given an L-amino acid diet supplying either a low lysine intake (14–15 mg • kg^-1 • d^-1) or an intermediate lysine intake (29 mg • kg^-1 • d^-1) for 6 d before a continuous tracer study with L-[1-¹³C]lysine.

Design: Five subjects received the low lysine intake, 6 subjects received the intermediate intake, and all were studied by using a standard 24-h oral tracer protocol that was described earlier for studies at a generous lysine intake.

Results: The rate of lysine oxidation was not significantly different between the 12-h fasted and 12-h fed states. The daily oxidation rate ( ± SD) was 27.9 ± 8.8 and 27.3 ± 17.6 mg lysine • kg^-1 • d^-1 for the low- and intermediate-intake groups, respectively (NS). Daily lysine balance was –12.4 ± 92 and 1.8 ± 17.7 mg • kg^-1 • d^-1, respectively (P < 0.025), for the low and intermediate intakes. The balance was significantly less than zero (P < 0.001) for the low intake.

Conclusion: The FAO/WHO/UNU lysine requirement value is not sufficient to maintain lysine homeostasis in healthy adults. From the results of this and tracer studies done by others, the mean lysine requirement of healthy adults was determined to be 30 mg • kg^-1 • d^-1.

Key Words: Lysine • oral tracer studies • amino acids • oxidation • healthy adults

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