Introduction: Blood lactate levels have been used in many sports fields and laboratories. It is a useful test to evaluate the capacity for endurance performance and also for setting exercise intensity. However, the evaluation of capillary blood lactate can be affected by factors such as plasma volume and sampling site. Lactate can be also taken from urine; urine lactate is considered to be less influenced by these factors. Nevertheless, few studies to date have investigated urine lactate and the overall relationship between blood lactate and urine lactate is uncertain. So, we needed to do a fundamental study of urine lactate. In this study, we investigated the change in urine lactate during a day under the circumstance where baseline blood lactate was kept at the resting level.
Purpose: The aim of this study was to investigate the circadian change of urine lactate during a day.
Methods: Eight healthy young men volunteered to participate in this study (age, 23 ± 2; height, 174.1 ± 2.3 cm; weight, 71.9 ± 6.1 kg). Subjects urinated at 08:00, and urine was again collected by a container every 2 hr thereafter until 24:00. The next morning, one final urine sample was obtained at 08:00. The subjects were prohibited from any exercise that might induce an elevated level of blood lactate production such as running. Urine lactate concentration was analyzed by HPLC. Urinary volume, urine lactate concentration, and urine collection time were used to calculate the urine lactate excretion (ULE; µg/min).
Results: Urine was collected with complete samples from 6 out of 8 subjects. Even though the ULE showed the circadian change, statistical variation was not seen. The number of subjects who showed the peak of ULE at 12:00, 14:00, 18:00, 22:00 was 1, 2, 1, 2, respectively.
Discussion: The ULE indicated circadian change, and its change was different in each subject. We previously examined the relationship between blood lactate and ULE in a step graded exercise test (unpublished). We found that ULE was not changed before and after exercise even though blood lactate increased up to 5 mmol/L in the final stage of exercise. In the current study, we assumed that a concentration of blood lactate remained at its resting level because subjects were prohibited from any vigorous exercise during the experiment days. In addition, some studies showed correlation of blood lactate and catecholamine during exercise; the peak of catecholamine was observed in the afternoon. In the current study, all subjects showed high ULE in the afternoon (12:00-18:00). However, 2 out of 6 subjects showed the highest ULE at 22:00. Our result implied that there were 2 ULE peaks in a day. We asked these 2 subjects about their behaviors during the experiment days, such as exercises, drinks, and food. Based on information we got from them, we speculated that the ingestion of oolong tea, which includes caffeine, affected ULE. Because caffeine is known to enhance the activity of the sympathetic nervous system, ULE might be affected by its activity.