Dietary nitrate, best known from beetroot juice, is one of the better-supported supplements for endurance. It can lower the oxygen cost of exercise, but the size of the effect depends on who you are.
What the research broadly shows:
A common dose is around 6-8 mmol of nitrate (roughly 400-500 mg), taken about 2-3 hours before exercise
Several days of loading may work better than a single dose for some people
Recreational and moderately trained athletes tend to respond more than highly trained ones, where effects are often small or absent
Antibacterial mouthwash can blunt the effect, because mouth bacteria help convert nitrate to nitrite