To understand how citrulline affects blood pressure, it is first necessary to understand a key molecule: nitric oxide (NO). Nitric oxide is an important signaling molecule in the body that relaxes vascular smooth muscle, leading to vasodilation, which helps lower blood pressure and improve blood flow. Citrulline itself does not directly relax blood vessels; it is an important precursor for the synthesis of another amino acid, arginine, which is the key substrate for the production of nitric oxide. In simple terms, supplementing with citrulline can increase the availability of arginine in the body, thereby supporting the continuous production of nitric oxide. This process is known as the “citrulline-arginine-NO pathway.” **Evidence-Based Analysis: How effective is citrulline in lowering blood pressure?** Multiple clinical studies have investigated the blood pressure-lowering effects of citrulline, especially in its common form, citrulline malate. The conclusions are cautious but positive. The effects are more significant in certain populations. A systematic review published in 2019 in the *Journal of Human Hypertension* indicated that citrulline supplementation is more effective in individuals with higher baseline blood pressure, such as those with prehypertension or stage 1 hypertension. For people with normal blood pressure, the blood pressure-lowering effect is usually not significant. This suggests that citrulline functions more as a regulator rather than a strong blood pressure-lowering drug. **Mild and sustained effects:** Studies have shown that taking 4–6 grams of citrulline daily for several weeks can lead to a small but statistically significant decrease in both systolic and diastolic blood pressure (usually by a few millimeters of mercury). The effect is mild and gradual, helping to improve vascular elasticity. **Potential benefits for arteriosclerosis:** In addition to lowering blood pressure, citrulline may also improve arterial endothelial function and reduce arteriosclerosis by increasing nitric oxide levels, which is an important aspect of maintaining vascular health from the root cause. **Limitations:** - **It’s not a panacea:** Citrulline is a supplement to lifestyle interventions and should not replace standard blood pressure medications. Patients with diagnosed hypertension must take medications under the guidance of a doctor and should not stop taking their medications on their own or replace them with supplements. - **Variable effects:** Individual responses to citrulline can vary due to genetic factors, intestinal absorption capacity, overall diet (such as arginine intake), and kidney function. - **Dosage and source:** To achieve the potential benefits observed in studies, several grams of citrulline per day are typically required. Relying solely on eating watermelons (which contain about 0.2–0.3 grams of citrulline per kilogram of fruit) is unlikely to provide an effective dose. Supplements are a more direct option, but high-quality products should be chosen. **Additional considerations:** - **Professional advice:** Before considering using citrulline supplements, especially if you are taking blood pressure medications (such as nitrates or angiotensin-converting enzyme inhibitors), consult a doctor or clinical nutritionist to avoid potential interactions or improper use. - **Natural foods:** Incorporating citrulline-rich natural foods (such as watermelons, cucumbers, pumpkins, and nuts) into a healthy diet is beneficial and harmless, although the amount of citrulline in these foods is limited. **References:** [1] Effect of L-citrulline supplementation on blood pressure: a systematic review and meta-analysis of randomized controlled trials. *Journal of Human Hypertension*. 2019. [2] Pharmacokinetic and pharmacodynamic properties of oral L-citrulline and L-arginine: impact on nitric oxide metabolism. *British Journal of Clinical Pharmacology*. 2008. [3] Short-term effects of L-citrulline supplementation on arterial stiffness in middle-aged men. *International Journal of Cardiology*. 2012. [4] Dose-ranging effects of citrulline administration on plasma amino acids and hormonal patterns in healthy subjects: the Citrudose pharmacokinetic study. *British Journal of Nutrition*. 2008.