Circadian urinary characteristics of adolescents: sensitive dynamic indices complement mean values as new physiologic endpoints

To introduce the study of physiologic urinary rhythms into secondary education, six girls and 11 boys, 14-18 years of age, collected urine at about 3-hr intervals for 24 hr. The volume and the excretion in urine of creatinine, potassium, and sodium were determined. Blood pressure was measured during the same 24-hr span to teach the students some elements of chronobiologic sampling and analysis in the context of evaluating the risk of developing a high blood pressure later in life. Herein, we examine on urinary excretory rates whether dynamic chronobiologic endpoints such as the amplitude (A) and/or acrophase (phi) may complement the more static mesor (M) in distinguishing groups of adolescents. In a comparison of the two sexes, dynamic characteristics of the urinary excretion of sodium and of the ratio of sodium/potassium do not separate the two groups, while the M does so. The reverse holds true for the excretion of potassium. In the case of urinary creatinine, the circadian A in itself is an index suggesting a "sex" difference; whereas in urinary volume, the M alone, and to a lesser extent the combination of (M, A, phi), yields a P value below the 5% level. The groups are rather small and heterogeneous; a study of ethnicity is beyond our scope. These qualifications notwithstanding, results indicate the need for testing multiple chronobiologic characteristics in comparing groups whether one's interest in the future relates to ethnicity, sex, or other factors. Such studies of urinary rhythms of high-school students serve for instruction and research and to instill responsibility for self-help in preventive health care.