Urine Lactate after Continuous and Interval Cycling Exercise Bouts Eliciting Different Blood Lactate Concentrations

Abstract

Post-exercise urine lactate (UL) has been suggested as a novel exercise biomarker of lactate production. The few studies examining the association between post-exercise peak blood lactate (BL) and UL have reported moderate to high linear correlations. However, these studies have not considered BL concentration during exercise. Also, the range of BL values was narrow (about 10 mmol/L), thus limiting the predictive value of UL. PURPOSE: To examine the association between UL and BL concentrations during continuous and interval exercise of equal mean power. METHODS: Eleven healthy young males performed four trials in random, counterbalanced order, one week apart. All trials included 20 min of cycling with equal mean power output, performed either continuously (CON) or in the form of interval training including 48 x 10 s (HIIT10), 16 x 30 s (HIIT30), or 8 x 60 s (HIIT60) bouts at a power output corresponding to 100% of VO2max. Recovery intervals during the HIIT trials included cycling at 15% of VO2max for 150% of the exercise bout time. Capillary BL concentration was measured at rest and every 5 min during exercise, and UL concentration was measured in urine samples obtained pre- and 1 hour post-exercise, with controlled hydration. BL and UL results were analyzed using 2-way ANOVA with repeated measures (trial x time). The association between incremental area under the blood lactate curve (BL-AUC) and UL concentration was determined using linear and exponential functions. RESULTS: BL increased compared with baseline in all trials from the first 5 min of exercise (p < 0.01). BL-AUC increased from CON to HIIT10, HIIT30 and HIIT60 (61.7 ± 21.2, 83.4 ± 29.0, 97.8 ± 34.0, 147.4 ± 44.2 mmol/L x min, respectively, p < 0.01). However, post-exercise UL increased from baseline only in HIIT60 (from 1.2 ± 1.0 to 22.5 ± 23.3 mmol/L, p < 0.001). Exercise BL ranged from 3.0 to 17.7 mmol/L, while post-exercise UL ranged from 0.2 to 76.4 mmol/L. The best function describing the BL-AUC and UL relationship was exponential (r = 0.68, p < 0.05). CONCLUSIONS: The lack of increase in UL despite an increase in BL and their exponential association suggest that there may be a threshold above which BL cannot be disposed within the body and is excreted by the kidneys.