Rhythmicity of plasma amino acids and relation to dietary intake

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Abstract
1) The circadian rhythm in the concentration of free amino acids in blood appears to be related closely to the periodicity of many other metabolic processes. Absorption of amino acids contained in food often has been assumed to account for increasing blood amino acid values during hours of wakefulness. Such a concept, however, is both overly simplified and probably incorrect. 2) Well controlled studies have demonstrated the lack of a direct relationship between dietary protein intake and blood amino acid values. Blood amino acid periodicity in humans is fully evident on the day of birth and persists normally thereafter despite the round-the-clock pattern of neonatal feedings. Normal periodicity in adults is maintained despite starvation of several days duration or the virtually complete restriction of dietary protein intake. The normal evening-hour decline in most blood amino acid values is not prevented or reversed by large intakes of protein at that time of day. On the other hand, amino acid rhythmicity is easily altered by the onset of an acute illness or a change in an individual’s sleep—wakefulness pattern. 3) Available evidence indicates that a characteristic magnitude of rhythmic change exists for each amino acid. These changes appear to be related closely to rhythmic alterations in rates of protein synthesis and the activity of individual amino acid-degrading enzymes within tissues. Circadian amino acid periodicity may, in addition, be related to normal daily rhythmic changes in deoxyribonucleic or ribonucleic acid concentration, chromatin template activity, availability of intracellular energy stores, and hormonal activity. Any effect of diet on amino acid periodicity would appear to be interwoven with these phenomena. 4) Studies that include serial amino acid measurements over a 2 to 20 hour period require adequate control observations to document concomitant changes attributable to circadian rhythmicity. Such control observations are especially pertinent during any study of amino acid absorption after feeding a test meal or an amino acid load to a patient.