It has been reported that the variability of resting blood lactate concentration (BLa) is related to metabolic capacity. However, it is unclear whether the resting BLa of athletes can be utilized as a metabolic biomarker. This longitudinal case study tested the hypothesis that resting BLa levels in the morning fluctuate with a 1-year training cycle. The subject was an adult male sprinter, and BLa and blood glucose at the time of waking were measured every day for 1 year. The training cycles were divided into five phases: 1. Basic training: high-intensity and high-volume load; 2. Condition and speed training: high-intensity and low-volume load; 3. Competition training I: track race and high-intensity load; 4. Conditioning for injury; 5. Competition training II. The mean BLa levels in the basic training (1.10 ± 0.32 mmol/L and competition training I (1.06 ± 0.28 mmol/L) phases were significantly lower than in the condition and speed training (1.26 ± 0.40 mmol/L) and conditioning injury (1.37 ± 0.34 mmol/L) phases. The clarified training cycle dependence of resting BLa is suggested to be related to the ability to utilize lactate as an energy substrate with fluctuations in oxidative metabolic capacity. This case report supports the tentative hypothesis that resting BLa may be a biomarker index linked to the metabolic capacity according to the training cycle.

An increase in resting blood lactate (La−) concentration due to metabolic conditions has been reported. However, it is not clear whether resting La− changes with training cycles in athletes. The purpose of this study was to test the hypotheses that (1) the morning resting La− levels are lower in periods of high training compared to periods of low training and (2) these changes in La− concentration are related to athletes’ metabolic capacity during exercise in male college-aged rugby players. Resting La− and blood glucose concentrations were measured in the morning in eight league rugby players during the summer pre-season period (Pre-period), the training and competition season period (TC-period), and the winter post-season period (Post-period). In each period, anaerobic power, La− concentration, and respiratory responses were measured during the 40 s maximal Wingate anaerobic test (WT). The resting La− concentration in the morning was significantly lower in the TC-Period (1.9 ± 0.6 mmol/L) than in the Post-Period (2.3 ± 0.9 mmol/L). The rate of decrease in La− level immediately after the 40 s WT was significantly higher in the TC-Period than in the Post-Period. The resting La− concentration was significantly correlated with the peak oxygen uptake and the carbon dioxide output during the WT. These results support the hypothesis that an athlete’s training cycle (i.e., in season and off season) influences the resting La− levels as well as the metabolic capacity during high-intensity exercise. The monitoring of resting La− fluctuations may provide a convenient indication of the training cycle-dependent metabolic capacity in athletes.