Endurance training aiming at eliciting further increase of physical performance of competitive athletes demands serious time and intensity constraints. In addition, very high intensity training could lead to âεœover-loadingâε□ frequently associated with increased concentration of plasma lactate after maximum intensity exercise and proteinuria. We hypothesized that a newly available complex dietary (CD) supplement - by providing the necessary substances and cofactors for increased tissue metabolism - would reduce the increase in plasma lactate concentration and proteinuria after maximum intensity exercise in swimmers undergoing high intensity training and exercise (70 km/week, for 6 weeks) period. Subjects involved in the investigation were junior swimmers (n = 10). Data were collected four times during the third macrocycle of training; 1 st: before, 2 nd: after 10 days and 3 rd: 14 days after withdrawal of CD-supplement, whereas 4 th: after 10 days of placebo treatment. The study was a double-blinded random controlled investigation. In the first period, plasma lactate concentration was 8.4Ã±2.1 mmol/l, whereas protein level in the urine was 8.9Ã±5.8 mg/l. After use of CD-supplement plasma lactate concentration significantly decreased to 5.5Ã±1.9 mmol/l and proteinuria decreased to 1.3Ã±2.1 mg/l (p<0.05). Importantly, the intensity and the volume of the training did not change during the observation period. Thus, use of CD-supplement significantly reduced the increase in plasma lactate and proteinuria after maximum intensity exercise in athletes (swimmers) undergoing high endurance training despite maintained training load. We propose that the special components of CD-supplement support the mechanisms responsible for lactate elimination and reduction of protein catabolism and/or increase of protein reabsorption. These adaptations are likely to allow the athletes to undergo higher intensity training resulting in greater performance.