The aim of the present work was to determine whether changes in blood lactate concentration [La-] between 2 different points of a training season would allow to predict changes in fat oxidation (FatOx) or carbohydrate oxidation (CHOx) in men and women. Fourteen men and 10 women performed a step incremental exercise test on a cycle ergometer at 2 points of the training season (spring and fall). Blood lactate concentration was measured during the final 30 seconds of each step and immediately after task failure, while mean FatOx and CHOx during the last minute of each stage was estimated using indirect calorimetry. Subsequently, the difference between the 2 points of the training season at each common power output during the incremental test was calculated for [La-] (Delta[La-]), FatOx (DeltaFatOx), and CHOx (DeltaCHOx). The significant level was set at p < 0.05. The results showed [La-] was strongly and inversely associated with FatOx (men: -0.821 ± 0.112; women: -0.914 ± 0.038) and strongly and positively associated with CHOx (men: 0.937 ± 0.027; women: 0.945 ± 0.032) at each point of the training season. Contrary, a broad range of correlations were determined for the relationship between Delta[La-] and DeltaFatOx (men: -0.207 ± 0.531; women: -0.384 ± 0.502) or DeltaCHOx (men: 0.292 ± 0.427; women: 0.324 ± 0.475). In conclusion, assessing [La-] alone might be considered as an effective way to indirectly assess substrate oxidations as long as [La-], FatOx, and CHOx during the incremental test remain stable throughout the training season. Contrary, the novel and most important finding is changes in [La-] do not predict changes in substrate oxidations as the training season progresses.
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