Watermelon delivers an unusual combination of two compounds that matter for men’s health: citrulline, an amino acid that boosts nitric oxide and improves blood flow, and lycopene, an antioxidant linked to lower prostate cancer risk. These aren’t marginal trace amounts. Drinking watermelon juice measurably raises blood levels of arginine, the direct precursor to nitric oxide, and bumps plasma lycopene as effectively as tomato juice does. The downstream effects touch erectile function, blood pressure, exercise recovery, fertility, and more.
How Citrulline Powers the Benefits
Most of what makes watermelon stand out for men traces back to one amino acid: citrulline. Your body converts citrulline into arginine, which in turn serves as the raw material for producing nitric oxide. Nitric oxide relaxes the smooth muscle lining your blood vessels, widening them and improving circulation. That single mechanism ripples outward into sexual health, cardiovascular function, and physical performance.
A study in adults who drank watermelon juice confirmed that the citrulline from watermelon gets efficiently converted into arginine. Participants showed significantly higher fasting plasma arginine levels, with stable citrulline concentrations, meaning the body was actively using the citrulline rather than letting it accumulate.1Nutrition. Watermelon consumption increases plasma arginine concentrations in adults A review focused on watermelon’s cardioprotective effects described citrulline as a “recycling substrate” that increases arginine availability and, through that, promotes smooth muscle vasodilation.2PubMed Central. Watermelon Nutritional Composition with a Focus on L-Citrulline and Its Cardioprotective Health Effects-A Narrative Review
This is the engine behind most of the male-specific health claims around watermelon. When someone calls it “nature’s Viagra,” they’re pointing at citrulline. That comparison is a stretch, but it’s not baseless.
Erectile Function
The connection between citrulline and erections is straightforward: erectile function depends on nitric oxide signaling to relax blood vessels in the penis, allowing increased blood flow. Prescription drugs for erectile dysfunction work by prolonging the effects of nitric oxide. Citrulline works earlier in the chain, by helping produce more nitric oxide in the first place.
In a trial of men with mild erectile dysfunction, half of them improved from a reduced erection hardness score to a normal score after taking L-citrulline supplements, compared to about 8% on placebo. The men taking citrulline also reported roughly double the frequency of intercourse per month, and everyone whose hardness score improved said they were very satisfied with the change.3PubMed. Oral L-citrulline supplementation improves erection hardness in men with mild erectile dysfunction A separate pilot study found that a combination of L-citrulline and transresveratrol significantly improved erectile function scores in men already using phosphodiesterase-5 inhibitors, the class of drug that includes sildenafil.4PubMed Central. Oral L-citrulline and Transresveratrol Supplementation Improves Erectile Function in Men With Phosphodiesterase 5 Inhibitors
An important caveat: both of those studies used concentrated L-citrulline supplements, not watermelon itself. You’d need to eat a lot of watermelon to match the doses used in the trials, a practical problem discussed later in this article. Still, the biological pathway is real, the results in mild cases are promising, and watermelon is the richest natural food source of citrulline.
Prostate Health and Lycopene
Watermelon’s deep red color comes from lycopene, the same pigment found in tomatoes. What many people don’t realize is that watermelon can raise your blood lycopene levels just as effectively as tomato juice. A controlled study found that consuming about 20 mg of lycopene daily from either watermelon juice or tomato juice resulted in a 100 to 200% increase in plasma lycopene, with no significant difference between the two sources. Unlike tomatoes, watermelon didn’t even need to be heated for the lycopene to be well absorbed.5The Journal of Nutrition. Consumption of Watermelon Juice Increases Plasma Concentrations of Lycopene and β-Carotene in Humans
Why this matters for men: lycopene has been studied for decades in connection with prostate cancer risk. A prospective cohort study of men at high cardiovascular risk found that those in the highest quartile of lycopene intake had roughly half the risk of developing prostate cancer compared to those in the lowest quartile. A dose-response analysis from the same study identified a threshold around 5 mg per day, above which prostate cancer risk dropped by about 64%.6PubMed Central. Lycopene intake and prostate cancer risk in men at high cardiovascular risk: a prospective cohort study A broader review of watermelon lycopene’s health claims noted its effectiveness in reducing cancer and cardiovascular disorder risk.7PubMed Central. Watermelon lycopene and allied health claims
The prostate cancer evidence is observational, which means it shows association rather than definitive causation. But the relationship has held up consistently across different study designs and populations, and the biological rationale is strong: lycopene accumulates in prostate tissue and acts as a potent antioxidant there.
Fertility and Sperm Quality
Sperm cells are unusually vulnerable to oxidative damage because of their high content of polyunsaturated fatty acids and relatively limited antioxidant defenses. This is where both of watermelon’s signature compounds converge. Lycopene directly scavenges free radicals in reproductive tissue, and citrulline-derived nitric oxide supports the blood flow that sustains testicular function.
A review on lycopene and male infertility found that supplementation studies in both humans and animals showed decreased lipid peroxidation and DNA damage in sperm, along with increased sperm count and viability. Human trials reported improvements in sperm parameters and pregnancy rates with daily lycopene doses of 4 to 8 mg taken over three to twelve months.8PubMed Central. Lycopene and male infertility More broadly, a systematic review and meta-analysis of nutritional therapies for idiopathic male infertility found that antioxidant supplementation was associated with higher sperm concentration, improved motility, and better morphology.9PubMed Central. Pharmacologic and Nutritional Therapies in Idiopathic Male Infertility: A Systematic Review and Meta-Analysis
Watermelon isn’t typically prescribed by fertility specialists, and the evidence here is more about its constituent compounds than the whole fruit. But for men trying to conceive, regularly eating lycopene-rich foods like watermelon is a low-risk way to support sperm health alongside other lifestyle measures.
Blood Pressure and Arterial Stiffness
High blood pressure disproportionately affects men, especially before age 65, and the citrulline-to-nitric-oxide pathway directly targets the vascular mechanisms involved. Several trials have tested watermelon or watermelon extract specifically for blood pressure.
In one study of obese adults with prehypertension or hypertension, six weeks of watermelon extract supplementation reduced ankle systolic blood pressure by about 12 mm Hg and brachial systolic blood pressure by about 15 mm Hg compared to placebo.10PubMed. Watermelon extract supplementation reduces ankle blood pressure and carotid augmentation index in obese adults with prehypertension or hypertension That carotid augmentation index drop, a measure of how much pressure waves reflect back from stiff arteries, signals real improvement in vascular flexibility. A crossover trial using wild watermelon juice found a significant reduction in peripheral arterial stiffness within 30 minutes of consumption, accompanied by increased nitric oxide production.11PubMed Central. Wild Watermelon-Extracted Juice Ingestion Reduces Peripheral Arterial Stiffness with an Increase in Nitric Oxide Production
These are relatively small trials, and the blood pressure reductions seen in them wouldn’t replace medication for someone with established hypertension. But for men in the borderline range, the effect is clinically meaningful and comes without side effects.
Exercise Performance and Muscle Soreness
If you’ve ever been sore for days after a hard workout, citrulline may help. The mechanism involves both nitric oxide (which increases blood flow to working muscles, speeding nutrient delivery and waste removal) and citrulline’s role in the urea cycle, which helps clear ammonia, a byproduct of intense exercise.
A study of athletes performing maximum-effort cycling found that watermelon juice reduced muscle soreness at 24 hours and lowered recovery heart rate compared to placebo.12PubMed. Watermelon juice: potential functional drink for sore muscle relief in athletes A randomized controlled trial in non-athlete men undergoing endurance training went further, finding that those drinking watermelon juice showed greater muscle thickness gains in the chest and thigh alongside lower soreness scores the day after exercise.13PubMed Central. The Effects of Watermelon Juice on Muscle Hypertrophy, Exercise Performance, and Muscle Soreness in Non-Athlete Men Undergoing Endurance Training And a comparative study found that watermelon juice reduced delayed-onset muscle soreness just as effectively as taking pure L-citrulline supplements, with no significant difference between the two at 12 and 24 hours post-exercise.14JOSSAE (Journal of Sport Science and Education). Difference Between Watermelon Juice and Pure L-citrulline on Muscle Soreness After Eccentric Exercise: A Comparative Study
That last finding is reassuring for people who’d rather drink watermelon juice than take a supplement. For soreness reduction, the whole fruit’s citrulline appears to work comparably to isolated citrulline.
Anti-Inflammatory and Antioxidant Effects
Chronic low-grade inflammation underlies many of the conditions men worry about: heart disease, metabolic syndrome, and cancer among them. Watermelon attacks inflammation from multiple angles. Its flavonoids inhibit a key inflammatory signaling pathway and suppress the release of pro-inflammatory molecules like TNF-alpha.15Retos. Anti-oxidant and anti-inflammatory properties of watermelon (Citrullus Lanatus) have the potential to reduce oxidative stress and inflammation after exercise/physical activity: Systematic Review
In an animal model of atherosclerosis (the plaque buildup behind most heart attacks and strokes), rats fed watermelon showed significantly lower C-reactive protein, a standard marker of systemic inflammation, along with reduced oxidative stress and higher antioxidant enzyme activity compared to controls.16Nutrition Research. Watermelon consumption improves inflammation and antioxidant capacity in rats fed an atherogenic diet These animal findings don’t automatically translate to humans, but they suggest mechanisms beyond simple nitric oxide production that make watermelon broadly protective.
Blood Sugar and Body Weight
Watermelon’s high glycemic index scares some people off, but the glycemic index is measured per 50 grams of carbohydrate, and you’d have to eat a huge amount of watermelon to reach that threshold. In practice, watermelon is about 91% water, and its glycemic load per serving is low.
A four-week trial in overweight and obese adults compared daily watermelon consumption against low-fat cookies matched for calories. Watermelon didn’t spike blood sugar or insulin any more than the cookie snack did, and both glucose and insulin levels stayed stable throughout the trial.17PubMed Central. Effects of Fresh Watermelon Consumption on the Acute Satiety Response and Cardiometabolic Risk Factors in Overweight and Obese Adults A study in overweight and obese adolescents found that blended watermelon actually produced lower postprandial glucose levels than a sugar-sweetened beverage, with significantly lower blood sugar readings at 20 and 40 minutes after consumption.18PubMed Central. Effects of blenderized watermelon consumption on satiety and postprandial glucose in overweight and obese adolescents So the fear that watermelon will wreck your blood sugar is overblown. If anything, reaching for watermelon instead of a sugary drink or processed snack is a favorable swap.
How Much Watermelon You Actually Need
Here’s the practical problem: clinical trials of citrulline for erectile function or exercise recovery typically use doses of 1.5 to 6 grams of L-citrulline per day. Fresh watermelon flesh contains roughly 1 to 2 grams of citrulline per kilogram, depending on variety and ripeness. That means you’d need to eat somewhere around 1 to 3 kg of watermelon flesh daily to match the lower end of supplement doses used in research. A review on watermelon and vascular health noted that getting a sufficient dose can be impaired by the sheer volume of fruit required, sometimes more than 700 mL of juice.19PubMed Central. Current Evidence of Watermelon (Citrullus lanatus) Ingestion on Vascular Health: A Food Science and Technology Perspective
There’s a workaround most people overlook: the rind. An analysis of watermelon parts found that the rind contains significantly more citrulline and arginine than the flesh, about 1.5 times more of each.20PubMed Central. Free Amino Acids and Volatile Aroma Compounds in Watermelon Rind, Flesh, and Three Rind-Flesh Juices The flesh still had the highest citrulline content per serving in another assessment, while the outer skin topped the charts for total phenolic content and antioxidant activity.21Applied Sciences. Bioavailability of Citrulline in Watermelon Flesh, Rind, and Skin Using a Human Intestinal Epithelial Caco-2 Cell Model Blending rind into smoothies or pickling it (common in some culinary traditions) is a practical way to increase your citrulline intake without having to eat an absurd quantity of flesh.
For lycopene, the threshold is more achievable. The prostate cancer study mentioned earlier identified benefits starting around 5 mg per day, and about two cups of diced watermelon provides roughly 9 to 13 mg of lycopene. Regular watermelon consumption during the season it’s available can realistically keep your lycopene levels in a favorable range.
Gut Health and Brain Circulation
Emerging research suggests watermelon affects the gut microbiome in ways that may compound its other benefits. In a trial of overweight and obese adults, daily watermelon consumption increased populations of bacterial species positively associated with glucose metabolism and immune function.22Human Nutrition & Metabolism. Dietary watermelon and gut microbiome modulation in adults with overweight or obesity This is early-stage science, but the direction is interesting: watermelon may improve metabolic health partly by feeding beneficial gut bacteria, not just by delivering citrulline and lycopene directly.
The nitric oxide boost from citrulline also reaches the brain. Enhanced cerebral blood flow supports cognitive function, and citrulline’s antioxidant properties may help protect neurons from oxidative damage. A review on citrulline’s effects on brain health highlighted its role in supporting endothelial function and vasodilation in cerebral circulation, with potential to mitigate age-related cognitive decline. The review noted that dietary sources like watermelon juice combine citrulline’s bioactivity with additional antioxidants like lycopene, potentially amplifying neuroprotective effects.23Quality in Sport. Citrulline – The Powerhouse Nutrient for Cardiometabolic and Brain Health While this research is largely theoretical at present, it suggests the circulatory benefits of watermelon extend above the neck.
Skin and UV Protection
Lycopene accumulates in your skin, where it acts as a mild internal sunscreen against ultraviolet damage. An experimental study found that topical lycopene application inhibited markers of UV-B-induced photodamage in a dose-dependent manner, including reduced skin thickening and protection of cell proliferation markers that UV normally suppresses.24PubMed. Protective effects of lycopene against ultraviolet B-induced photodamage This won’t replace sunscreen, but consistent dietary lycopene intake adds a layer of antioxidant defense in the skin. For men who are less likely than women to use daily sun protection products, this incidental benefit from eating watermelon is worth knowing about.
Who Should Be Careful
Watermelon is a high-potassium fruit, which is fine for most people but can be dangerous for men with chronic kidney disease. When kidney function is impaired, the body can’t excrete potassium efficiently, and eating large amounts of potassium-rich foods can lead to dangerously elevated blood potassium levels. A clinical perspective from nephrologists highlighted cases where excessive watermelon consumption by kidney disease patients led to hyperkalemia, with risk of cardiac complications.25PubMed Central. Watermelon-induced hyperkalemia in chronic kidney disease patients: perspective from Pakistan If you have kidney disease or are on dialysis, talk to your doctor about how much watermelon is safe for you.
For everyone else, the main “risk” of watermelon is gastrointestinal discomfort from eating too much at once. The fruit is high in fructose and water, and large servings can cause bloating or loose stools in sensitive individuals. If you’re taking blood pressure medication, be aware that watermelon’s blood-pressure-lowering effect could theoretically add to your drug’s effect, though this hasn’t been shown to cause problems in studies. And men on nitrate-based medications for angina should be cautious with anything that boosts nitric oxide, though watermelon’s effect is far gentler than prescription drugs.