Watermelon: right food, wrong reason
Watermelon has a reputation among athletes that rests on citrulline, and the citrulline argument does not survive arithmetic.
The real reason to eat it is duller and considerably more useful, and it applies in exactly the week when almost everything else has been taken away.
The citrulline claim, and why it fails
L-citrulline was first isolated from watermelon, which is where the name comes from and where the reputation started.
It is a genuine compound with genuine effects. Citrulline converts to arginine, arginine is the substrate for nitric oxide, and nitric oxide affects blood flow. Supplementation has been associated with improved muscle oxygenation and fatigue resistance in some work.
The problem is the dose.
| Source | Citrulline |
|---|---|
| Watermelon flesh, per 100 g | ~100 to 350 mg |
| A 300 g serving | ~0.5 to 1 g |
| Supplement studies | 3 to 6 g |
Reaching a studied dose from watermelon requires eating a kilogram or more, daily. That is not a nutrition strategy, it is a challenge.
One study made the comparison directly, giving participants either a drink containing 6 grams of L-citrulline or 710 millilitres of watermelon juice carrying roughly 1 gram. The gap between those two is the whole point.
And a separate trial found that 2 weeks of watermelon juice supplementation improved nitric oxide bioavailability without improving endurance performance. The marker moved. The outcome did not.
And one odd detail
Citrulline concentration is not uniform through the fruit. The rind carries more than the flesh, and the central portion of the flesh carries less than the outer part.
Which produces a mildly absurd conclusion. If you were genuinely eating watermelon for citrulline, you should be eating the part everybody throws away.
Nobody is going to do that, and the figure matters because it illustrates how far the marketing has travelled from the food.
The actual reason
Watermelon is roughly 92 percent water.
| Per 100 g | Amount |
|---|---|
| Water | ~92 g |
| Calories | ~30 |
| Carbohydrate | ~7.5 g |
| Fibre | ~0.4 g |
Almost every food carrying that much water also carries meaningful fibre. Watermelon does not. It is fluid arriving in a form you can chew, with very little residue attached.
For a fighter in the final days, when the low residue list has removed most vegetables and most fruit, that combination is rare enough to matter.
And in the hours after a weigh-in, when the objective is fluid and fast carbohydrate arriving in a body with no appetite, watermelon does both at once and requires no preparation.
Where it actually earns its place
- The final days, in measured amounts. It carries less fibre than any other fruit of comparable volume. On the low residue list it is one of the small number of fruits appearing under measured amounts rather than under remove.
- The hours after the scale. Fluid, sugar, potassium and something that goes down when nothing else does. It works alongside the drinking rather than competing with it.
- Hot conditions generally. In an away camp in heat, or during a long tournament day, it is fluid you can carry in a form that does not feel like drinking more water.
A 300 gram wedge is around 90 calories, 22 grams of carbohydrate, and close to 280 millilitres of fluid.
That fluid figure is the one to hold. In the final days before a weigh-in, watermelon is fluid, and fluid is weight. It belongs in the plan rather than as a free extra, for the same reason that any drink does.
The nitrate problem
Everything above assumes the watermelon you bought is the watermelon in the nutrition tables. Frequently it is not, and the variable is nitrate.
Watermelon takes its nitrate up from the soil, and where nitrogen fertiliser has been applied heavily the fruit accumulates far more than it otherwise would.
The range is the point. In one study of melons from a producing region, nitrate concentrations ran from around 2.6 to 525 milligrams per kilogram across the samples. A 200-fold spread within a single survey, and the analysis identified excessive fertiliser use as the dominant source rather than anything about the plant.
Which means there is no such thing as the nitrate content of watermelon. There is the nitrate content of the one in front of you, and it depends on how it was grown.
I have measured this myself, in two different countries, and found readings 10 to 20 times higher than I would expect from a properly grown fruit.
The second set was in Kazakhstan in September, which is worth stating because it was mid-season fruit in a producing country rather than an early import.
That is my own testing rather than published data, and two measurements fall well short of a survey. They are consistent with the direction of the published range, and they were taken in circumstances where I would have expected the lower end.
Why it matters to a fighter specifically
Three points, and the first is the one people get wrong.
This is not an anti-doping issue. Nitrate is not a prohibited substance. Beetroot juice is used deliberately by athletes as a nitrate source and is entirely legal. Nobody has failed a test because of a watermelon.
It is a food safety issue, and the mechanism runs through nitrite. Nitrate itself is relatively inert. It converts to nitrite, and that conversion accelerates during storage, particularly at warm temperatures and in fruit that has been cut or damaged.
Which produces a specific rule. A watermelon cut in the morning and left out in a warm kitchen is a different product by the evening from the one that went into a refrigerator.
And third, in the final days this is one more uncontrolled variable in a week built entirely on controlling variables. Gastrointestinal upset from any cause in that window is a serious problem, and a food with a 200-fold range in one of its components is not a food to introduce in quantity for the first time that week.
What actually reduces the risk
Season helps and does not decide. Nitrate accumulation is highest when fruit is harvested soon after fertiliser application, and very early-season melons appearing well ahead of the local growing season are the higher-risk category.
But my own mid-season measurement in a producing country came back high, which means seasonality shifts the odds rather than removing the problem. The variable is how that particular field was fertilised, and the calendar is only a proxy for it.
The rind concentrates it. Nitrate is higher near the skin than in the centre, in the same way described for citrulline. Cutting away the outer few centimetres of flesh removes a meaningful proportion, and it is the part with the least flavour anyway.
Refrigerate immediately after cutting, and cover it. The conversion to nitrite is what you are preventing, and temperature is the variable you control.
Do not keep cut melon for days. 3 to 4 days refrigerated is the general guidance and shorter is better for this reason specifically.
And if a melon tastes unusual, faintly bitter or sharp rather than sweet, do not persist with it. That is not a validated test and it is the only signal available without a meter.
The honest limit of this section
Handheld nitrate meters exist and are inexpensive, and they are the only way to know rather than guess.
Whether that is worth doing depends on where you buy fruit and how much you eat. For somebody eating watermelon occasionally, in season, from a source they trust, this is a footnote.
For somebody eating it daily in the final week of a camp, in a country where they do not know the supply chain, it is a genuine consideration and the meter costs less than one lost fight.
Published risk assessments generally conclude that melon nitrate exposure sits below the levels that would concern a regulator for the average consumer. That is a population statement. It says nothing about the individual fruit with a reading 20 times the expected value, and the range in the data is exactly why.
What else is in it
Lycopene, the carotenoid that gives it the colour, in quantities comparable to tomato. Potassium, in useful amounts, with the same correction that appears under bananas: relevant to fluid balance, not a cramp solution. Vitamins A and C.
None of those change a decision. They are reasons it is a good food rather than reasons to plan around it.
The other melons
Cantaloupe and honeydew behave similarly: high water, low fibre, low calorie density.
Melon generally is the fruit category that survives the final days best, and it helps to know it as a group rather than as one fruit. If watermelon is unavailable, the reasoning transfers.
Cantaloupe carries more vitamin A. Honeydew is slightly sweeter. Neither difference decides anything.
Fruit in the final days, as a group
Since watermelon is the exception, the rule it breaks needs setting out.
| Order | Fruit |
|---|---|
| Worst | Dried fruit, for the concentration reasons on its own page |
| Then | Berries, which carry more fibre per calorie than almost any other fruit |
| Then | Apples, pears and anything eaten with the skin |
| Middle | Citrus, which carries fibre in the membranes |
| Best tolerated | Melon of all kinds, and ripe banana in measured amounts |
That ordering surprises people who have been told berries are the superior fruit. They are, on most measures, and in the specific week where fibre is the variable being managed the ranking inverts.
Juice, which is a separate question
Juicing removes most of the fibre, which in the final days is an advantage rather than a loss, and concentrates the sugar per volume.
For a fighter that produces a narrow use. In the hours after a weigh-in, juice is fluid and fast carbohydrate arriving together with no residue at all, and that is exactly the requirement.
Outside that window it is a poorer version of the fruit: same sugar, less satiety, and none of the volume that makes whole fruit useful in a deficit.
Seeds, rind, and buying it
Seeds are edible and carry some fibre, magnesium and protein. In ordinary camp that is neutral. In the final days, seedless is the version that fits.
The rind is edible, carries more citrulline than the flesh, and is eaten pickled in several cuisines. It also carries considerably more fibre, which puts it on the wrong side of the line in the last week even though it is the part with the compound everybody wants.
On selection: weight relative to size is the most useful indicator, since a ripe melon is heavy for its dimensions because of the water. A pale or cream-coloured patch where it rested on the ground suggests it ripened in place. The hollow sound when tapped is the traditional test and is less reliable than the weight.
Once cut, keep it refrigerated and covered, and use it within 3 to 4 days.
Where it sits by phase
| When | Verdict |
|---|---|
| Ordinary camp | Fine, unremarkable, a low-calorie fruit with nothing against it |
| Final 3 to 4 days | Measured amounts, and count the fluid as fluid rather than as food |
| After the weigh-in | Genuinely useful, in the early feeds, alongside drinking |
| Fight day | Workable in the gap when the card runs late, in small amounts with fluid |
Eat it for the water
The reason watermelon appears on athlete lists is citrulline, and that reason does not hold at the quantities anybody eats.
The reason it deserves to be on a fighter's list is that it is water you can chew, with almost no fibre, arriving at the 2 points in a camp where both of those things are difficult to find.
Eat it for the water. Ignore the amino acid. And know that the fruit itself is less consistent than the table suggests.
Two complete sample documents are free to read before you buy anything.
Oleksandr Foka is a sports nutritionist with over a decade in professional sport. He wrestled freestyle from the age of 4 until he was 22, competing as a medallist and champion at Ukrainian and international level, and has worked with amateur boxers since 2017, including champions and medallists at world championships. He has worked in world title camps in boxing with Oleksandr Usyk, Oleksandr Gvozdyk, Denys Berinchyk, Vladyslav Sirenko, Murodjon Ahmadaliev, Israil Madrimov, Sergiy Bogachuk, Sabirzhan Akkalykov and Abylaikhan Zhussupov, and with the Kazakhstan national boxing team and Kazakhstan judo athletes. He currently works on the WTA tour.
Oleksandr Foka — sports nutritionist. Questions: fokaoleksandr@gmail.com
Educational content, not medical advice. For adults 18+.
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