Watermelon.

Citrulline is the constituent in watermelon that carries most of the vascular rationale, and reviews of blood pressure and arterial stiffness combine citrulline supplementation and watermelon intake in the same analyses. Taking both raises total citrulline exposure without adding a separate mechanism. There is no reason to expect an additional effect from stacking them beyond the higher dose. Note that pooled analyses cannot tell you which delivery route produced a given result.

Watermelon-derived citrulline bypasses intestinal and hepatic arginase, which degrades a large share of orally ingested arginine before it reaches circulation. The result is that citrulline is a more efficient way to raise plasma arginine than arginine itself. Adding oral arginine on top does not bypass that first-pass loss. This is settled amino acid metabolism, not a claim about any outcome.

Red-fleshed watermelon carries lycopene at levels comparable with tomato, alongside its citrulline. A systematic review and meta-analysis of lycopene and male reproductive measures names watermelon among the dietary sources. The two constituents are carried in different fractions, carotenoid in the lipophilic phase and citrulline in the aqueous, so anything that separates them, such as juicing and clarifying, changes the balance.

Lycopene is a hydrocarbon carotenoid with essentially no water solubility, and its absorption depends on being incorporated into bile-salt mixed micelles. Eating watermelon with no fat at all leaves much of the lycopene fraction unabsorbed. A few grams of any dietary lipid is enough. The citrulline fraction is unaffected either way.

Lycopene and beta-carotene are absorbed through shared micellar incorporation and share the SR-BI transport route at the enterocyte. High-dose supplementation of one lowers the uptake of the other. This is a well-documented competition, which is why isolated single-carotenoid dosing behaves differently from carotenoids eaten in a mixed food. The competition applies to the supplement scenario more than to a slice of fruit.

Watermelon flesh is roughly 92 percent water and supplies potassium with it. Potassium contributes to normal blood pressure regulation through sodium handling at the kidney, which is a separate route from the citrulline and nitric oxide pathway. For anyone with reduced kidney function, added potassium load is a reason to check intake with a clinician rather than a benefit.

The two reach nitric oxide by genuinely different chemistry, one oxygen-independent through nitrite reduction and one enzymatic through nitric oxide synthase. That is the stated rationale for stacking them in sports formulations. Trials of the combination against either alone are limited, so read the pairing as mechanistically reasoned rather than demonstrated. Both also carry a modest blood-pressure-lowering signal, which is worth knowing if blood pressure is already being managed.

Ascorbate regenerates oxidised tocopherol and helps keep tetrahydrobiopterin in its reduced form, and tetrahydrobiopterin is the cofactor that keeps nitric oxide synthase coupled. When that cofactor is oxidised, the enzyme produces superoxide instead of nitric oxide. This is established enzymology, and it explains why redox status affects how much a citrulline dose actually delivers.

Ornithine and citrulline sit adjacent in the urea cycle, with ornithine transcarbamylase converting ornithine to citrulline in the mitochondrion. Supplying both raises the pool of cycle intermediates available for ammonia handling. The relationship is textbook metabolism. Whether raising the pool changes anything measurable at supplement doses is a separate question with much weaker support.