Free amino acids, urea, and creatinine were analyzed in venous blood and urine of 11 trained (28–81 years old) male subjects before, immediately after, and 1 day after a 100 km running competition.

The urinary excretion per minute of all amino acids was lowered after the contest. The renal clearance of creatinine was reduced from 116 to 60 ml/min and the clearance of most amino acids was reduced to a similar extent. However, for the amino acids with a resting clearance under 1 ml/min (x), a high relative clearance ratio (y in % of x) was seen post-exercise: y = -92.3 (log₁₀ x) +23.1, r= -0.83, showing that their high reabsorption capacity had been impaired.

Serum concentrations of most free amino acids, including the branched-chain amino acids and alanine, were reduced to 35–85% of the pre-race values. The sulfur amino acids were elevated either at the end of (cystine, to 180%) or 24 h after (methionine, to 155%) the race. Urea production increased by 44% while creatinine production tended to decrease. The production of 3-methylhistidine remained unchanged. These findings are compatible with a stimulation of gluconeogenesis at the expense of the amino acid pool without induction of muscle protein catabolism.