Short answer: Beetroot has more support. NIH says beetroot juice generally improved endurance in time trials, more in recreational exercisers than elite athletes, and sports nutrition groups say nitrate improves endurance in recreational athletes.1 For citrulline, NIH says research is limited and conflicting and does not strongly support it.1 We found no trial comparing them.
| Beetroot juice | Citrulline | |
|---|---|---|
| How it may work | Nitrate is turned into nitric oxide, which widens blood vessels1 | Turned into arginine, then nitric oxide1 |
| NIH summary | Generally improved performance in runners, swimmers, rowers and cyclists, not in all studies1 | Limited and conflicting; not strongly supported1 |
| Expert groups | Academy of Nutrition and Dietetics, Dietitians of Canada and ACSM: improves endurance in recreational athletes1 | Not on that list in the NIH summary |
| Study protocols | About 500 mL juice, 2.5 to 3 hours before exercise (5 to 11 mmol nitrate)1 | 8 g citrulline malate 1 hour before exercise1 |
| Blood pressure | Systolic about 4.4 mmHg lower in short trials2 | Not covered in the sources checked |
| Side notes | Pink or red urine is harmless1 | Stomach discomfort in 6 of 41 in one trial1 |
Beetroot: NIH says beetroot generally improved performance and endurance compared with placebo in time trials, with benefits more likely in recreational exercisers than elite athletes, and little study of weight training. One study suggested more nitrate helped up to a point, with no extra benefit at the highest amount. Labels usually mark juices and concentrates as foods, and studies have not tested whether beet powder supplements work like the juice.1
Citrulline: NIH describes one positive bench-press trial and one trial where citrulline without malate worsened treadmill performance, and says the research does not strongly support it.1
These are study protocols, not personalized advice; confirm any dosing with your prescriber.
A 2013 meta-analysis of 16 short crossover trials found nitrate and beetroot juice lowered systolic blood pressure by about 4.4 mmHg, with a smaller, non-significant change in diastolic; the trials lasted 2 hours to 15 days.2 If you take blood pressure medicine, tell your prescriber.
More detail: Citrulline vs arginine: which works better for nitric oxide?, Creatine vs beta-alanine: how do they differ?, and the Beetroot Juice (Dietary Nitrate) and L-Citrulline guides.