Beetroot is one of the best-known food sources of dietary nitrate. The body can convert nitrate into nitric oxide, a signaling molecule involved in blood-vessel function.
Why athletes use beetroot
Nitric oxide can widen blood vessels, which supports blood flow and the delivery of oxygen and nutrients to working muscles. Many—but not all—studies using beet juice have reported improvements in aerobic endurance activities such as running, cycling, swimming, and rowing.
Juice research is not powder research
Most positive performance studies used beet juice or beet-juice concentrate with a measured nitrate content. The NIH notes that beetroot powders vary and have not been studied as extensively, so results from juice studies cannot automatically be assigned to every capsule.
Bottom line: beetroot is a stimulant-free ingredient with a plausible nitric-oxide pathway, but individual results vary. Read the NIH exercise-performance fact sheet.