Influence of dietary nitrate supplementation on physiological and muscle metabolic adaptations to sprint interval training

We hypothesized that 4 wk of dietary nitrate supplementation would enhance exercise performance and muscle metabolic adaptations to sprint interval training (SIT). Thirty-six recreationally active subjects, matched on key variables at baseline, completed a series of exercise tests before and following a 4-wk period in which they were allocated to one of the following groups: 1) SIT and NO3--depleted beetroot juice as a placebo (SIT + PL); 2) SIT and NO3- -rich beetroot juice (similar to 13 mmol NO3-/day; SIT + BR); or 3) no training and NO3--rich beetroot juice (NT + BR). During moderate-intensity exercise, pulmonary oxygen uptake was reduced by 4% following 4 wk of SIT + BR and NT + BR (P < 0.05) but not SIT + PL. The peak work rate attained during incremental exercise increased more in SIT + BR than in SIT + PL (P < 0.05) or NT + BR (P < 0.001). The reduction in muscle and blood [lactate] and the increase in muscle pH from preintervention to postintervention were greater at 3 min of severe-intensity exercise in SIT + BR compared with SIT + PL and NT + BR (P < 0.05). However, the change in severe-intensity exercise performance was not different between SIT + BR and SIT + PL (P < 0.05). The relative proportion of type IIx muscle fibers in the vastus lateralis muscle was reduced in SIT + BR only (P < 0.05). These findings suggest that BR supplementation may enhance some aspects of the physiological adaptations to SIT.