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Effect of Histidine Intake on Plasma and Urine Histidine Levels, Nitrogen Balance and N⁷-Methylhistidine Excretion in Normal and Chronically Uremic Men¹

Medical and Research Services, Veterans Administration Wadsworth Medical Center and Schools of Medicine and Public Health, University of California, Los Angeles, Los Angeles, CA 90073

This study assessed whether changes in plasma histidine or nitrogen balance could indicate the dietary histidine requirement in short-term feeding studies. Five normal and two chronically uremic men were fed diets varying in histidine content in a metabolic research unit. Subjects received amino acid diets for 71 ± 12 (SD) days during which time the histidine intake was varied between 60 and 2,800 mg/day at 8.0 ± 0.5 day intervals. The results indicated that both postabsorptive plasma histidine and urinary histidine correlated with the dietary histidine intake. However, in individual patients the response curve of plasma histidine to the dietary histidine intake did not demonstrate a consistent breakpoint which could be used to indicate the dietary histidine requirement. Rather, the histidine intake above which the plasma levels increased rapidly seemed to be influenced by the previous dietary histidine. Urinary histidine excretion also correlated with plasma histidine. N⁷-methylhistidine was increased in plasma and reduced in urine of the uremic patients as compared to normal subjects. Nitrogen balances were less positive with histidine intakes of 2 mg/kg/day or lower. These data support the finding that histidine is an essential amino acid in normal and chronically uremic man. However, clinical and metabolic studies of greater duration may be necessary to determine the daily histidine requirement.

KEY WORDS: • histidine • essential amino acids • nitrogen balance • 3-methylhistidine

¹ Supported in part by DHEW NIH Contract AM 32210.

Manuscript received 13 October 1980.