Nitrate-rich beetroot extract and citrulline, a non-essential amino acid, have been shown to be precursors of nitric oxide, which benefits athletic performance by expanding blood vessels and increasing blood flow. To determine whether combining the two produces better results, Spanish researchers gave the two supplements separately and together to 32 male triathletes for nine weeks. The dosages were 3 grams a day of citrulline and 2.1 grams a day of beetroot extract. In a study published in Biology, they reported that the combination did not improve markers of exercise-induced muscle damage, but did reduce cortisol levels, and also enabled those participants to run 5 percent farther in the standard 12-minute Cooper Test.