Watermelon usually gets filed under "mostly water, not much else," and for gut health specifically, that reputation isn't entirely fair. It's one of the richest common food sources of an unusual amino acid that's only recently been studied for its direct gut effects.
An amino acid unusually concentrated in this one fruit
Watermelon is one of the richest known food sources of an amino acid called citrulline, which your body converts into another compound that supports healthy blood flow. Clinically, blood citrulline levels serve as a marker of gut lining health, since they track with how well your small intestine absorbs nutrients and drop when the gut lining is damaged.
What a recent human study found
In 2026, researchers finally looked directly at what daily watermelon does to gut bacteria in adults with overweight or obesity, one of the few times this exact question has been tested in real people instead of animals[1]. It's a promising start, though even the scientists behind it admit the human research on watermelon and gut bacteria is still pretty thin overall.
Animal studies
Diabetic mice given watermelon juice saw their gut bacteria shift and blood sugar control improve, with citrulline slowing certain carb-digesting enzymes along the way[2].The 2026 human trial
The first direct test of daily watermelon's effect on gut bacteria in adults with overweight or obesity, a promising but still early starting point.What we've learned from animal studies so far
Citrulline also seems to team up with specific beneficial bacteria to strengthen your gut lining, at least based on what's been seen so far in these animal models.
Where the rind fits in
Citrulline is especially concentrated in watermelon rind, the white part usually thrown away. Traditional cuisines that pickle or cook watermelon rind are, likely without meaning to, getting more of this compound than eating the red flesh alone gives you.
What to do with this
Watermelon's fiber content is modest compared to most other fruits in this series, so it won't be your main fiber source, but its high water content helps the hydration fiber needs to do its job properly. If hydration habits are something you struggle with generally, our guide on water and your gut covers the basics, and melon makes a similarly hydrating gut-friendly choice. For a step-by-step approach, see how to build a hydration habit you keep. The human research here is newer and thinner than for something like kiwi or avocado, and that's an honest caveat. But citrulline's blood flow benefits are already well established on their own. If you exercise regularly, that's reason enough to keep watermelon in rotation. And if you're comfortable experimenting in the kitchen, pickling the rind is an easy way to get more citrulline out of a part of the fruit most people just throw away.
Recipes using this ingredient:
- Daily watermelon intake and gut bacteria in adults with overweight or obesity
- Watermelon juice, citrulline, and gut bacteria in diabetic mice
This content is for educational purposes only and is not a substitute for professional medical advice.
