Systematic reviews and meta-analyses have found that supplementation with nitrate tends to work best when taken approximately 2 to 3 hours before exercise.[1] Meta-analyses have also found that the effects of nitrate might be influenced by training status. For example, some meta-analyses have found that effects tend to be diminished in endurance athletes with a high performance level (e.g., elite athletes) and that nitrate tends to work best in recreational athletes or nonathletes during longer-duration time-to-exhaustion tests (i.e., tests measuring how long a person can run/cycle at a low to moderate intensity).[2][3][4] That said, some analyses have shown nitrate to work best during shorter exercise bouts (lasting between 2 and 10 min).[1] Nitrate might also be more effective in men than women; however, very few studies in women have been published, so more research is needed to understand whether a sex difference exists.[3][4][1]
The major problem with such analyses is that very few randomized controlled trials have been specifically designed to examine dose-response effects, time course effects, sex differences, etc. Plus, the existing studies include a small number of participants. Further high-quality research is needed to understand when and in whom nitrate works best.