Citrulline: The Amino Acid Behind Nitric Oxide Production
Citrulline: The Amino Acid Behind Nitric Oxide Production
Ever felt your legs turn to jelly halfway through a spin class, or noticed your blood pressure reading at the GP surgery creeping up despite eating fairly "clean"? An amino acid best known for being in watermelon might be worth knowing about.
What Is Citrulline?
Citrulline is a nonessential amino acid, which just means your body can make it on its own without needing it directly from food. (cite index="1-1">L-citrulline is a nonessential amino acid produced in the body, mainly from glutamine, and obtained from the diet. On the food side, (cite index="21-14">it's found in watermelons, other melons, squash, cucumbers, pumpkins, and chickpeas — foods that turn up often enough in British kitchens, whether it's watermelon slices at a summer barbecue or cucumber in a ploughman's lunch.
Here's the interesting part: once citrulline is absorbed, (cite index="1-22">about 80% of the body's citrulline is converted in the kidneys into arginine, another amino acid. Arginine is what your body actually uses to build nitric oxide, a gas-like signal that tells blood vessel walls to relax and widen. This citrulline-to-arginine-to-nitric-oxide chain is exactly why (cite index="60-1">citrulline is among the common core ingredients found in multi-ingredient pre-workout supplements sold in gyms and supplement shops across the UK.
Why Do People Take It?
Nitric oxide and blood flow
The main reason citrulline gets attention is its role in the nitric oxide pathway. (cite index="20-1">L-citrulline is the natural precursor of L-arginine, the substrate for nitric oxide synthase in the production of nitric oxide. Better nitric oxide availability means blood vessels can relax more easily, which matters for anyone spending long hours at a desk under fluorescent office lighting with barely any outdoor movement.
Blood pressure research
Several trials have looked at citrulline specifically for blood pressure. (cite index="11-9">Some trials have studied citrulline or watermelon supplementation in adults with prehypertension or hypertension, and in normotensive individuals. Separately, (cite index="11-6">chronic supplementation has been studied for its effects on nitric oxide synthesis, blood pressure and peripheral blood flow in some study groups, though results across trials aren't always consistent.
Exercise recovery and muscle fatigue
This is where citrulline shows up most in the fitness world. (cite index="14-1">Citrulline serves as a nitric oxide precursor with the potential to increase blood flow and improve resistance exercise performance, though more research is needed to confirm its ergogenic potential. Some strength-training studies have reported modest improvements in repetitions performed, while other acute, single-dose studies have found no measurable benefit — the effect seems to depend heavily on the dose, the exercise type, and how long someone has been supplementing.
Vascular and metabolic health with age
Beyond the gym, researchers are interested in how citrulline might support blood vessel function as we get older. (cite index="72-9">Citrulline supplementation has been studied in adults with pre-hypertension or hypertension, and in relation to the blood vessel lining. This matters given how common hypertension and related cardiometabolic issues are in the UK, where diets often lean towards higher sodium and refined carbohydrates, such as chip-shop and takeaway fare.
Bioavailability & Absorption
One reason citrulline is often chosen over arginine supplements comes down to simple gut chemistry. (cite index="15-9">Citrulline has emerged as a better alternative to arginine supplementation because it is well-absorbed and elevates arginine bioavailability and nitric-oxide-dependent signalling to a greater extent compared to supplementing directly with arginine. This happens because (cite index="3-5">unlike arginine, citrulline is not quantitatively extracted from the gut or liver before reaching circulation, making it more effective at raising arginine levels and nitric oxide synthesis. In plain terms: arginine gets broken down before it can do much good, while citrulline slips past that first-pass breakdown and does its job further downstream in the kidneys.
Safety Basics
Citrulline is generally considered well tolerated. The most commonly reported issue is mild digestive discomfort — (cite index="16-3">a common side effect is an upset stomach. Because of its effect on blood vessels, people already taking medication for blood pressure or erectile dysfunction should be cautious, since (cite index="73-21">taking citrulline alongside phosphodiesterase-5 inhibitors may cause blood pressure to drop too low, leading to hypotension. Pregnant or breastfeeding women are generally advised to steer clear, as (cite index="69-26">it's recommended not to use L-citrulline if pregnant or breastfeeding due to limited safety data in these groups. Anyone with existing blood vessel conditions, low blood pressure, or who is on multiple medications should have a conversation with a doctor or pharmacist before adding it to their routine — this is especially relevant if you're also taking traditional Chinese, Malay, or Ayurvedic remedies alongside Western supplements, since interactions aren't always well studied.
Natural Food Sources
- Watermelon — by far the richest and most recognisable source, rind included
- Other melons such as honeydew and cantaloupe
- Cucumbers, commonly found in salads and sandwiches
- Pumpkin and squash, often used in curries and soups
- Chickpeas, used in hummus, curries and salads across the UK
Related Articles
Explore Related Nutrients
Final Thoughts
Citrulline sits at an interesting intersection of everyday food and modern supplement science — the same compound in a slice of summer watermelon is the one being studied for blood pressure and exercise recovery. The research has focused mostly on people with elevated blood pressure and on blood flow during and after exercise, while results for already-fit, normotensive individuals are more mixed. As with most amino acid supplements, it's not a substitute for the basics: enough sleep, regular movement, and a diet that isn't constantly tipped towards salt and refined carbs. If you're curious about adding it to your routine, especially alongside existing medication, a quick chat with a pharmacist or doctor first is the sensible move.
This article is for general informational purposes and is not a substitute for professional medical advice. Please consult a qualified healthcare professional before making any changes to your diet or supplement routine.
References
- NIH Office of Dietary Supplements
https://ods.od.nih.gov/factsheets/ExerciseAndAthleticPerformance-HealthProfessional/ - PubMed Central
https://pmc.ncbi.nlm.nih.gov/articles/PMC6073798/ - PubMed Central
https://pmc.ncbi.nlm.nih.gov/articles/PMC9592950/ - WebMD
https://www.webmd.com/diet/health-benefits-citrulline