Plasma Amino Acid Levels of Men Fed Diets Differing in Protein Content. Some Observations with Valine-deficient Diets

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Abstract
The time course of alterations in plasma amino acid patterns developing during the ingestion of a low protein diet was investigated and attempts were made to correlate the changes with tissue nitrogen loss. Post-absorption plasma amino acid levels were measured at intervals during a 5-week period in 6 men aged 55 years or older who were receiving a diet containing 3.5 g of nitrogen/day. The essential amino acid values decreased throughout the period with valine showing a significant reduction after 1 week as compared with values obtained when the subjects were given a diet of 14 g of nitrogen/day. The total nonessential amino acid levels in plasma increased after 1 week of administering the low protein diet and showed no further change. The increase was due to higher values for glutamine-asparagine, glycine and alanine. During the 5-week period, the ratio of essential to nonessential amino acids decreased from a mean of 0.43 to 0.28 and this decrease was associated with a body nitrogen loss of from 30 to 75 g. Reducing the daily caloric content of the low protein diet from 2500 kcal to 900 kcal did not appear to alter the plasma amino acid response. When a valine-deficient diet was given to subjects, the plasma valine levels during post-absorption were greatly decreased within 5 days.