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. Citrulline levels in your body are actually used clinically 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.
A very recent human study on this exact question
A 2026 study looked directly at daily watermelon and gut bacteria changes in adults with overweight or obesity, one of the few studies to look at this in real people rather than animals, though the researchers themselves noted that research on watermelon and gut bacteria specifically is still limited overall.
What earlier animal research found
A study in diabetic mice found watermelon juice shifted gut bacteria and improved blood sugar control, with citrulline meaningfully slowing down certain carb-digesting enzymes. Separate research has found citrulline working alongside specific beneficial bacteria to strengthen the gut's lining.
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.
Watermelon's fiber content is modest compared to most other fruits in this series, so it's not your main fiber source, but its high water content genuinely helps the hydration fiber needs to do its job properly. If you exercise regularly, watermelon's citrulline is worth knowing about for its established blood flow benefits, alongside whatever gut benefit the research is still figuring out. If you're comfortable experimenting in the kitchen, pickled watermelon rind is a low-effort way to use a part of the fruit most people throw away. As with several other foods in this series, the human research here is newer and thinner than for something like kiwi or avocado.
Recipes using this ingredient:
This content is for educational purposes only and is not a substitute for professional medical advice.