Benefits
Blood pressure with a citrulline-rich watermelon extract
In small 6-week placebo-controlled crossover studies in obese middle-aged adults with elevated blood pressure, 6 g a day of an extract its registry lists as 4 g citrulline plus 2 g arginine lowered systolic pressure, by about 15 mmHg in one study. A larger double-blind parallel trial of 6 g a day found no difference from placebo, and daily flesh or 100% juice did not change blood pressure in two controlled trials.
Blood vessel widening
In 17 healthy young adults, 500 mL of watermelon juice a day for 2 weeks kept flow-mediated dilation, a measure of how well an artery widens, higher than placebo after a glucose drink. A single 30 g dose of rind powder with 4 g of citrulline improved it in 11 young adults. In 21 postmenopausal women, 4 weeks of juice did not change it.
Fullness and body weight when replacing other snacks
In 33 overweight adults, 2 cups of watermelon a day for 4 weeks gave more fullness and less hunger than an equal-calorie cookie snack, and body weight fell instead of rising, though everyone ate watermelon first. In 17 children with overweight, a cup a day for 8 weeks lowered BMI and body fat compared with a sugary drink.
Muscle soreness and exercise performance
In 7 athletes, 500 mL of natural or citrulline-enriched watermelon juice before a hard cycling test reduced recovery heart rate and muscle soreness 24 hours later compared with placebo. In 8 active men, 16 days of a juice concentrate raised blood citrulline and arginine but did not extend time to exhaustion.
Blood arginine and lycopene levels
In controlled trials, watermelon flesh, juice concentrate and rind powder raised blood arginine, consistent with conversion of its citrulline. 100% juice raised blood lycopene in postmenopausal women but did not raise their arginine or citrulline, so results differ by product and dose.
Mechanism of action
Citrulline to arginine to nitric oxide
L-citrulline is readily absorbed and then converted to L-arginine, the substrate nitric oxide synthase uses to make nitric oxide, which relaxes blood vessels. Watermelon flesh, juice concentrate and rind powder raised plasma arginine in controlled trials, and a juice concentrate also raised plasma nitrite, a marker of nitric oxide.
Lycopene
Red watermelon flesh contains about 4.5 mg of lycopene per 100 g, a red carotenoid antioxidant. Daily 100% juice raised blood lycopene in a controlled trial, but no watermelon trial cited here linked lycopene to a health outcome.
High water content and low energy density
Raw watermelon is about 91% water with 30 kcal per 100 g. In a snack trial that found more fullness than with cookies, the authors noted it is unclear which of its components or sensory qualities caused the effect.
Clinical trials
Three small randomized placebo-controlled crossover reports from one Florida State University group, each 6 weeks per arm. The 2011 pilot gave a watermelon product supplying 2.7 g citrulline and 1.3 g arginine a day; later reports used 6 g a day, and the registry record for the 2014 report (NCT01185041) lists a watermelon extract from Milne Fruit Products with 4 g citrulline and 2 g arginine, against maltodextrin, so the registered dose was almost entirely these two amino acids and differs greatly from whole fruit; the abstracts do not say whether the amino acids were added. Funding is not stated in the abstracts (Figueroa et al. 2011, Am J Hypertens; Figueroa et al. 2012, Am J Hypertens; Figueroa et al. 2014, Am J Hypertens)
9 adults with prehypertension (2011); 14 obese adults with prehypertension or stage 1 hypertension (2012); 13 obese adults with hypertension, 10 of them women (2014). The later reports describe similar participants and may overlap.
Compared with placebo: aortic systolic pressure fell 7 mmHg in the pilot while brachial systolic and diastolic pressure did not change significantly; in 2012 brachial systolic pressure fell 15.1 mmHg and ankle systolic pressure 11.5 mmHg; in 2014 brachial and aortic systolic pressure were lower at rest and during a cold pressor test, while resting augmentation index did not change.
Randomized, double-blind, placebo-controlled parallel trial of 6 g a day of watermelon extract or placebo for 6 weeks (Massa et al. 2016, Blood Press)
40 adults with prehypertension or hypertension, about 48 years old, 20 per group.
Systolic pressure fell from 137.8 to 126.0 mmHg and diastolic from 79.2 to 72.3 mmHg within the extract group, but the change was not different from the placebo group. Sympathovagal balance did not change.
Randomized, double-blind, placebo-controlled crossover trial: 500 mL of watermelon juice a day or a calorie-matched placebo drink for 2 weeks, each followed by an oral glucose tolerance test; NIH grant support (Vincellette et al. 2021, J Nutr)
17 healthy adults (6 men, 11 women) aged 21 to 25.
Primary outcome: flow-mediated dilation over 90 minutes after the glucose drink was higher after juice than placebo (area 838 vs 539, P = 0.03). Microvascular blood flow (P = 0.01) and resting tissue oxygen saturation (P = 0.048) were also higher; the ischemic reperfusion slope did not differ (P = 0.83).
Placebo-controlled study (NCT04781595) of a single 30 g dose of microencapsulated watermelon rind powder containing 4 g of L-citrulline, with measurements up to 120 minutes (Volino-Souza et al. 2023, Eur J Clin Nutr)
11 healthy young adults.
Flow-mediated dilation improved 60 minutes after the rind powder, and plasma citrulline and the relative change in plasma arginine rose; tissue oxygen saturation did not change.
Randomized, double-blind, placebo-controlled crossover trial (NCT03626168): two 360 mL servings a day of 100% watermelon juice or a sugar-matched isocaloric placebo for 4 weeks, 2-week washout. Funded by the American Heart Association and NIH (Ellis et al. 2021, Nutr Metab Cardiovasc Dis)
21 healthy postmenopausal women.
No change in pulse wave velocity, 24-hour blood pressure, flow-mediated dilation or blood arginine and citrulline. Serum lycopene rose (p = 0.002), and fasting glucose rose slightly (p = 0.001), a change the authors judged not clinically significant.
Randomized, controlled parallel pilot trial: 0 g, 152 g or 304 g of watermelon flesh a day for 4 weeks. Funded by the National Watermelon Promotion Board, which had no role in the study (Singh et al. 2025, Nutrients)
39 adults with elevated blood pressure, mean age 41, mean BMI 31.
Primary outcome null: mean 24-hour blood pressure after 4 weeks was 130.2 (control), 130 (1 cup) and 124.9 mmHg (2 cups), not significantly different. Glucose, insulin, lipids and nitric oxide did not change; plasma arginine and the arginine/ADMA ratio rose with watermelon compared with control (p = 0.009).
Crossover trial (NCT03380221) without randomized order: 2 cups of fresh watermelon (92 kcal) daily for 4 weeks, then, after a 2 to 4 week washout, isocaloric low-fat cookies daily for 4 weeks. Funded by the National Watermelon Promotion Board (Lum et al. 2019, Nutrients)
33 overweight or obese adults (20 men, 13 women) aged 18 to 55.
After the snack, hunger, desire to eat and prospective food consumption were lower and fullness higher with watermelon than cookies (p < 0.05). Body weight and BMI showed a snack by time interaction (p = 0.005 and 0.006), falling with watermelon and rising with cookies. Postprandial glucose and insulin did not differ. Everyone ate watermelon first, so an order effect is possible.
Lab cell study plus a 3-condition study: 500 mL of natural watermelon juice (1.17 g L-citrulline), citrulline-enriched watermelon juice (6 g citrulline in total) or placebo before a maximal effort cycling test (Tarazona-Díaz et al. 2013, J Agric Food Chem)
7 athletes.
Both watermelon juices reduced recovery heart rate and muscle soreness 24 hours later. The study was very small, and one of the two juices had added synthetic citrulline.
Randomized crossover trial: one cup a day of blenderized whole watermelon (flesh and rind) or an isocaloric sugar-sweetened beverage for 8 weeks, with a 4-week washout. From the same San Diego State University group as the cookie snack trial; funding is not stated in the abstract (Daughtry et al. 2023, Pediatr Obes)
17 children aged 10 to 17 with overweight or obesity who completed the study.
Compared with the sugary drink, watermelon lowered BMI (p = 0.032), BMI percentile (p = 0.038), body fat percentage (p = 0.036) and HbA1c (p = 0.012). Blood glucose, insulin, lipids, inflammation, liver enzymes and satiety hormones did not differ. The sugary drink raised BMI percentile from baseline (p = 0.014), so part of the difference reflects the comparator.
Randomized crossover study after a no-supplement control phase: 300 mL a day of watermelon juice concentrate (about 3.4 g L-citrulline) or an apple juice concentrate placebo for 16 days (Bailey et al. 2016, Nitric Oxide)
8 healthy recreationally active men.
Plasma citrulline, arginine and nitrite rose and muscle oxygenation during moderate exercise improved, but time to exhaustion during severe exercise did not differ between conditions. Resting systolic pressure was higher with both watermelon juice (130 mmHg) and placebo (131 mmHg) than in the control phase (124 mmHg).