Do rocket and beetroot raise nitric oxide levels in your body?
Rocket and beetroot demonstrably increase the availability of nitric oxide in your body, with measurable effects on blood pressure and arterial flexibility, especially if you are somewhat older or your blood pressure tends to run high. If you use antibacterial mouthwash shortly afterwards, it largely switches off that mechanism.
Dietary nitrate from beetroot and rocket raises the amount of nitric oxide (NO) in your body. The process starts in the mouth: bacteria on your tongue convert nitrate into nitrite, which is then converted into NO in your blood vessels. After twelve weeks of daily beetroot extract, nitrate levels in the blood rise five to six times, and nitrite levels (the direct forerunner of NO) rise one and a half to two times.
What this does for your blood vessels depends on who you are. In older people who already had slightly elevated blood pressure, average blood pressure fell by around 4 mmHg after two weeks of beetroot juice. In young, healthy people there was no effect. Twelve weeks of beetroot extract made the arteries more flexible in postmenopausal women, even without any drop in blood pressure. In people with completely normal blood pressure, four days of beetroot juice changed nothing about how their blood pressure responded to exercise or cold.
Four weeks of beetroot juice also changes the mix of bacteria in your mouth: the bacteria that convert nitrate increase, which in theory supports NO production even further. Antibacterial mouthwash does the opposite: it kills those same bacteria, undermining the effect of dietary nitrate. If you regularly use antibacterial mouthwash, you get less benefit from the nitrate in beetroot and rocket.
For athletes the picture is more modest. In well-trained athletes, beetroot juice had no measurable effect on performance. The explanation is that fit people already produce a lot of NO, so extra nitrate makes little difference. In young healthy women there was also no effect on muscle strength, regardless of the phase of their cycle. The effect of dietary nitrate is therefore most pronounced in people who have more room for improvement in their blood vessel function, such as older people or those with the beginnings of raised blood pressure.
This answer draws on many studies combined, including six studies in people and one smaller trial involving fourteen participants. The findings on NO increases and blood pressure reduction in older people come from several independent studies that all point in the same direction. The findings on arterial flexibility and mouth bacteria each come from a single study. The findings in athletes and young women are also based on individual small studies; the evidence for "no effect" in those groups is therefore still limited.