Two awkward, faintly clinical-sounding names: citrulline and L-carnitine. Athletes talk about them constantly, and increasingly so do people who simply pay attention to their own energy. But where do they actually come from? What do they really do inside the body? And what's left once the marketing slogans are stripped away?
Their origins, as it happens, couldn't be more ordinary. One is named after a piece of summer fruit: watermelon. The other after meat itself — "carnitine" comes from the Latin carnis, flesh.
Try a small experiment: type "citrulline" into Google. You'll quickly notice the top results are about erections and virility. You didn't type "watermelon" for that. Far-fetched? Not entirely. Not entirely accurate either. We'll come back to it properly, with real data, and without promising anything.
Citrulline: the amino acid that slips past the liver
The word "citrulline" comes from Citrullus, the botanical name for watermelon, where it turns up in abundance (Rimando and Perkins-Veazie, J Chromatogr A, 2005). But the body can also make its own supply, in the gut, starting from another amino acid: glutamine.
From there, it travels. Released into the bloodstream, it's converted into arginine by the kidneys. In effect, it acts as a disguised form of arginine, one that slips past the liver rather than being intercepted by it (Curis et al., Amino Acids, 2005). That's precisely why it raises blood arginine more effectively than taking arginine itself (Schwedhelm et al., Br J Clin Pharmacol, 2008).
Why does any of this matter? Because arginine is the raw material for nitric oxide, a gas that the cells lining blood vessels use to relax and let blood flow through. Its discovery earned Furchgott, Ignarro and Murad the 1998 Nobel Prize in Medicine. With age, and with certain lifestyle habits, the amount of nitric oxide available in the body tends to decline — a shift linked to a long list of age-related conditions (Allerton et al., Nutrients, 2018).
How much watermelon would that actually take?
Here's a curious fact: to get 2g of citrulline from watermelon alone, you'd need to drink roughly 1.5kg of juice a day. In one study, 780g of juice a day — the equivalent of 1g of citrulline — was enough to raise blood arginine by 12% over three weeks, while 1.56kg (2g) raised it by 22% (Collins et al., Nutrition, 2007).
Another surprise: the rind, the part almost everyone throws away, contains just as much citrulline as the flesh — sometimes more, depending on the variety — and yellow- or orange-fleshed watermelons carry more of it than red ones (Tarazona-Díaz et al., J Sci Food Agric, 2011; Rimando, 2005). A watermelon can even throw off a blood test: in one child, an unusually high citrulline reading turned out to be explained by nothing more sinister than… the sheer quantity of watermelon the child had been eating (Mandel et al., J Inherit Metab Dis, 2005).
What the studies actually show
In a double-blind trial, 22 trained men took 2.4g of citrulline a day for a week. Over a 4km cycling course, they came in 1.5% faster than on placebo and reported less muscle fatigue (Suzuki et al., J Int Soc Sports Nutr, 2016). A review pulling together the available evidence points the same way: several studies report genuine improvements in performance and recovery from exercise (Gonzalez and Trexler, J Strength Cond Res, 2020). Doses tested across the various trials range from 2.4 to 9g a day.
On blood pressure, a synthesis of 8 trials finds an average drop of 4mmHg in systolic pressure (Barkhidarian et al., Avicenna J Phytomed, 2019) — a result that holds up from one trial to the next, despite quite different study designs.
One point still needs sharpening, and it's a question of mechanism rather than outcome: measuring citrulline's immediate effect on blood flow inside the muscle, live in the lab, during exercise itself, remains genuinely difficult, and the readings collected so far are inconsistent (Gonzalez and Trexler, 2020). None of that undoes the effects seen on performance and blood pressure — it simply means the exact mechanism, second by second, is still being worked out.
And what about the erection thing?
Let's get to the question everyone's actually asking, the one Google is already asking on your behalf. Yes, citrulline has a "virile" reputation. And it didn't come from nowhere.
An erection, at bottom, is a story about blood vessels. Nitric oxide, the gas we mentioned above, is the main messenger that relaxes the tissue of the penis and lets blood flow in. When it's in short supply, that's one of the principal mechanisms behind erectile dysfunction (Burnett, J Clin Hypertens, 2006). It's also the very circuit the medications for this condition work on — including the well-known Viagra, approved in 1998, the same year as the Nobel Prize for nitric oxide. Same gas, same year (Goldstein, Sex Med Rev, 2019).
Since citrulline helps the body produce arginine, and therefore nitric oxide, researchers put the obvious question to the test. One small study gave 1.5g a day for a month to 24 men around 56 years old with mild dysfunction. After a month on placebo, 2 of them (8%) had seen their erection hardness score rise to a normal level; after a month on citrulline, 12 had (50%). No adverse effects were recorded, and the authors themselves note the result falls short of medication, at least in the short term (Cormio et al., Urology, 2011).
So there are the facts: a single study, 24 men, two months. An interesting signal, not proof. And there's a line European law simply doesn't allow anyone to cross: a food or supplement cannot be presented as preventing, treating or curing a disease (EU Regulation No 1169/2011, Article 7). Erectile dysfunction is a disease. Which means a serious brand can't promise it to you — and the fact that it doesn't is, if anything, a reassuring sign.
The genuinely interesting part lies elsewhere. The artery supplying the penis is narrower than those supplying the heart, so the same early stiffening of the vessel walls shows up there first. That's why erectile dysfunction is studied as an early warning sign for vascular health more broadly (Gandaglia et al., Eur Urol, 2014). In a large study of more than 9,000 men over 55, those who went on to develop the condition carried roughly a 25% higher risk of a subsequent cardiovascular event (Thompson et al., JAMA, 2005).
So the real takeaway here isn't "take citrulline." It's this: if it happens to you, talk to a doctor, because it's sometimes the first warning sign your body gives — and it deserves to be heard. The moral of the story: citrulline is neither a love potion nor a con. It's raw material for a gas that helps your blood vessels stay supple, and long before it's a seduction talking point, that's simply a matter of everyday health.
L-carnitine: the fat shuttle
Carnitine turns up mainly in meat and dairy, though the body can manufacture its own as well. Its job, by contrast, is extremely well established: it's the shuttle that carries fatty acids into the mitochondria, where they're burned as fuel (Steiber et al., Mol Aspects Med, 2004).
Vegetarians carry less of it in the blood, though in healthy adults that gap isn't considered nutritionally meaningful (Lombard et al., Am J Clin Nutr, 1989). With age, however, the picture shifts: in frail older people, free carnitine in the blood runs lower than in those who stay robust (Malaguarnera et al., Nutrients, 2020). A simple association — it doesn't prove carnitine is the cause — but it explains why researchers keep circling back to it.
After exercise, and with age: where the evidence is strongest
After exercise: as little as 1g does the job. This is the single most interesting finding in the whole picture. In one study, 8 healthy men took 0, 1 or 2g of L-carnitine L-tartrate a day for three weeks, then performed a set of squats. The result: even at the 1g dose, blood carnitine had risen, and markers of metabolic stress and muscle damage — along with how sore participants actually felt — dropped by just as much as with 2g (Spiering et al., J Strength Cond Res, 2007). Other trials, using 2g of carnitine a day, point the same way: in 10 trained men, the area of muscle affected on MRI shrank to 41–45% of what was seen on placebo (Volek et al., Am J Physiol Endocrinol Metab, 2002), and in 18 middle-aged men and women, there was less damage and less soreness — though no gain in strength (Ho et al., Metabolism, 2010). Carnitine doesn't make you stronger. It appears to help the muscle bounce back.
With age. In 50 older adults classed as "pre-frail", 1.5g of L-carnitine a day for 10 weeks improved both a frailty index and grip strength compared with placebo, and three participants moved from "pre-frail" to "robust" altogether, against just one on placebo (Badrasawi et al., Clin Interv Aging, 2016). In a separate trial of 84 older adults who tired quickly, a higher dose — 4g a day — over 30 days was linked to a 2.1kg gain in muscle mass against 0.2kg on placebo, along with markedly lower physical and mental fatigue scores (Pistone et al., Drugs Aging, 2003).
A quieter kind of inflammation. Aging often brings with it a low-grade, silent inflammation that simmers in the background. A synthesis of 13 trials finds L-carnitine linked to a drop in CRP, a marker of inflammation, averaging −1.23mg/L, alongside modest falls in IL-6 and TNF-alpha (Haghighatdoost et al., Eur J Clin Pharmacol, 2019).
Inside the muscle itself, patience is required: in a 24-week study, high doses of carnitine taken alongside carbohydrate raised muscle carnitine by 21%, and the work produced during an exercise test by 11% (Wall et al., J Physiol, 2011).
In the interest of honesty: most of this is small-scale evidence, with doses ranging from 1 to 4g a day depending on the study. What's striking is how consistently the direction repeats itself from one trial to the next — faster recovery, less fatigue — even while the exact size of the effect still needs confirming in larger groups.
On weight: a real effect, but not a shortcut
For years, carnitine was marketed as a "fat burner" — a phrase that all but promises a shortcut. What the largest synthesis available actually shows, drawing on 37 trials and 2,292 people, is a real and measurable effect: an average of 1.2kg of weight lost, and 2.1kg less fat mass (Talenezhad et al., Clin Nutr ESPEN, 2020). It isn't a shortcut. Carnitine accompanies an active lifestyle; it doesn't replace one.
One thing worth watching
Carnitine from red meat can be converted by certain gut bacteria into a compound called TMAO, suspected of speeding up deposits inside the arteries. People who eat meat produce more of it than vegetarians do (Koeth et al., Nat Med, 2013; J Clin Invest, 2019). That's reason enough for caution, though not enough to draw firm conclusions.
As things stand, no health claim is authorized in Europe for either citrulline or L-carnitine: both remain, for now, subjects of ongoing research.
What to take from this
Citrulline, born of watermelon, helps the body produce the gas that keeps blood vessels relaxed — without being any kind of love potion. L-carnitine, born of meat, carries fat into the cell's power plants, and studies link it to faster recovery after exercise (from as little as 1g a day as L-tartrate) and, in older adults, to less fatigue and a quieter background inflammation. Two quite different jobs, neither one standing in for the other, and nothing magical about either. That might be the most useful lesson here: energy doesn't hinge on a single molecule, but on a set of well-distributed roles, each one doing its own part.
This article is published for informational and educational purposes. It does not constitute medical advice and does not replace consultation with a healthcare professional.
