Changes in the amount of urinary protein and β₂-microglobulin (β₂-MG) were observed after middle-distance running in 46 primary school boys, 46 junior high school boys, 74 high school boys and 51 male college students, aged between 6 and 21 years.
The running distance was varied according to age : 800 m for 6- to 7-year-olds, 1, 200 m for 8- to 9-year-olds, 1, 700 m for 10- to 11-year-olds and 1, 500 m for 12- to 21-year-olds.
Urine was collected from each subject before and 30 min after running, and total urinary protein was analyzed by the Lowry method and β₂-MG by the reverse passive hemagglutination method.
For all subjects at rest, total urinary protein was 14.2-19.1 mg/dl on average and increased to 24.6-96.2 mg/dl at 30 min after running, while β₂-MG at rest was 3.10-7.12 μg/dl and increased to 30.53-1202.87 μg/dl at 30 min after running.
Urinary protein originating in blood plasma and that in non-blood plasma was calculated on the basis of the study of Poortmans (1968) . Urinary protein originating in blood plasma after running was 2.54-6.58 times higher than that before running, whereas non-blood plasma after running was 1.04-1.92 times higher than that before running. This suggests that the increased urinary protein after running mainly originates from blood plasma. In terms of age, urinary protein from non-blood plasma in boys aged 6 to 11 years showed a greater increase than that in boys over 12 years old.
The ratio of β₂-MG to urinary protein after running was 7.0-80.6 times higher than that before running. The correlation coefficient between urinary protein and β₂-MG became higher after running than that before running. These findings indicate that low-molecular-weight urinary protein is predominantly reabsorbed in the proximal renal tubule. In terms of age, the increase in the above ratio in 12-14-year-olds was lower than that in 1521-year-olds, suggesting that the reabsorption ability of the proximal renal tubule matures with age.