Twice-a-day exercise increases acute MCT1 gene expression in skeletal muscle but does not change the lactate curve after 3 weeks of training in adult men

We have previously demonstrated that different modalities of endurance exercise combined with lower muscle glycogen content elicit several physiological and molecular benefits in men. In this study, we hypothesized that these exercise strategies modulate monocarboxylate transporters (MCTs) and plasma lactate. We investigated MCT1 and MCT4 gene expression after two forms of exercise (i.e., once daily and twice-a-day) under low carbohydrate (CHO) availability (Acute - Study 1) and whether three weeks under once daily or twice-a-day training differentially affected plasma lactate during exercise (Chronic - Study 2). In Study 1, five participants performed a high-intensity interval exercise (HIIE) 2 h (twice-a-day) or 15 h (once daily) after exercise and diet manipulations to reduce endogenous CHO stores or without previous CHO manipulation (Control). Muscle biopsies were collected before, right after, and 3 h after HIIE. In Study 2, plasma lactate was measured during a graded exercise test before and after three weeks of once-daily (n=7) or twice-a-day training (n=7). MCT1 gene expression increased from before to after and 3-h post-HIIE only in the twice-a-day exercise (P<0.05). MCT4 gene expression was unaltered in all conditions (P>0.05). The plasma lactate curve shifted to the right in both training approaches, without differences in lactate slope reduction between once-daily (-0.49±0.58 mmol·L-1·min-1) and twice-a-day (-0.46±0.73 mmol·L-1·min-1) exercise. In conclusion, twice-a-day training increased acute MCT1 gene expression but did not result in chronic changes in plasma lactate response during exercise.