Watermelon: the trials are really about an amino acid

Watermelon lowers blood pressure in pooled trials, by about 4 mmHg systolic across 15 studies and 415 people. Inside the same analysis, the subgroup given citrulline together with arginine got 10.4 mmHg, two and a half times as much, and the authors say plainly that the combination beats watermelon or citrulline alone. So the fruit is a delivery vehicle for a compound that works better when it arrives another way. The meta-analysis of watermelon specifically found systolic pressure, total cholesterol and LDL falling, and fasting blood sugar rising, which is the sentence nobody quotes.

The numbers, per 100 g

What a real portion looks like

• Across twenty-one trials, the watermelon amino acid changed nothing about body composition. — 21 randomised controlled trials (n = 21 trials) Evidence A source
  No substantial effect on body mass index, body weight, fat mass, waist circumference, body fat percentage or fat-free mass; subgroup analyses showed fat mass reductions in people over 40 and at doses above 6 g a day, and interventions of 3 to 8 weeks raised fat-free mass; dose-response analysis suggested non-linear relationships

What your body absorbs

• Watermelon and its amino acid lowered systolic pressure by four points, and adding arginine to it more than doubled the effect. — 15 randomised controlled trials giving 24 datasets with 415 participants (n = 415) Evidence A source
  Systolic pressure -4.02 mmHg (95% CI -6.54 to -1.50, p = 0.002) and diastolic -2.54 (-4.27 to -0.81, p = 0.004); in the subgroup given citrulline together with l-arginine, systolic -10.44 (-13.57 to -7.31) and diastolic -4.86 (-7.93 to -1.79)

What cooking and storage change

• The rind, which people throw away, raised blood citrulline and arginine and protected vessels after a blood flow blockage. — 12 volunteers (n = 12) Evidence B source
  Prolonged ischaemia caused endothelial dysfunction in the large vessels but not the microvasculature; both the rind and the rind with beetroot restored flow-mediated dilation against placebo (p<0.05) with no difference between them; plasma citrulline, arginine, nitrate and nitrite all rose after both

What people believe that the data does not support

• Europe refused the antioxidant claim for watermelon extract, as it has for every antioxidant claim on any food. — — Evidence E source
  Two register entries name watermelon and both carry the status Non-authorised; the example lists a chain of antioxidant wordings for a watermelon extract sold as ACTI-08

Who this works differently for

• Women sent home after severe pregnancy sickness kept more weight on when watermelon was added to the dietary advice. — 128 women, 64 per arm (n = 128) Evidence B source
  Weight change at the end of week 1 was -0.05 kg with watermelon against -0.5 kg in controls (p = 0.014) and at the end of week 2 +0.25 kg against -0.5 kg (p = 0.001); symptom scores, appetite, wellbeing, satisfaction and willingness to recommend the intervention were all significantly better with watermelon

What is still unknown

• Watermelon lowered blood pressure and cholesterol in pooled trials, and raised fasting blood sugar. — 9 randomised controlled trials out of 8962 studies identified (n = 9 trials) Evidence A source
  Systolic blood pressure, total cholesterol and LDL cholesterol all fell significantly; fasting blood sugar rose significantly; diastolic pressure, heart rate, BMI, body fat, arginine, insulin and C-reactive protein were unchanged

The bottom line

Citrulline does not change body composition. Twenty-one trials found no effect on weight, body mass index, fat mass, waist or fat-free mass, with the positive findings confined to subgroups the authors themselves call exploratory. The most interesting watermelon study is about the part people throw away: a microencapsulated powder made from the rind raised blood citrulline and arginine and protected blood vessels after an artificial blood flow blockage, in twelve volunteers. And there is one genuinely practical trial: after hospital discharge for severe pregnancy sickness, women given watermelon alongside dietary advice held their weight better than those given the advice alone. That was one unblinded trial of an addition to standard care, and hyperemesis is managed by clinicians.

Sources

1. wml-dr-03 · systematic review and dose-response meta-analysis of randomised controlled trials
   Overall, CIT supplementation had no substantial effects on body mass index (BMI), body weight, fat mass (FM), waist circumference (WC), body fat percentage (BFP), and fat-free mass (FFM).
   Nutrients, 2025 · checked 2026-09-22
2. wml-dr-01 · systematic review and meta-analysis of randomised controlled trials
   The results indicate that l-citrulline supplementation and watermelon intake significantly reduced SBP (-4.02 mmHg [95 % CI: -6.54 to -1.50], P = 0.002) and DBP (-2.54 mmHg [95 % CI: -4.27 to -0.81], P = 0.004) in the elderly.
   Clin Nutr ESPEN, 2025 · checked 2026-09-22
3. wml-dr-05 · randomised crossover placebo-controlled trial
   The WR and WR + B supplementation significantly restored FMD after IR injury compared to the placebo (p < 0.05).
   Food Funct, 2023 · checked 2026-09-22
4. wml-dr-06 · regulatory register entry
   POL-HC-6934 Citrullus lunatus (Watermelon) extract - ACTI-08 Good source of antioxidants Contains naturally occuring antioxidants Has antioxidant properties Acts as an antioxidants Contributes to the protection against oxidation Helps increase the antioxidative capacity of the body Helps preventing oxidation Antioxidants help protect you from free radicals Antioxidants help protect your cells and tissues from oxidation Antioxidants contribute to the total antioxidant capacity of the body Antioxidants help to protect your body by reinforcing the body's natural defence against the effects of free radicals Non-authorised
   EU Register on nutrition and health claims, заява POL-HC-6934, знімок 2026-09-21 · checked 2026-09-22
5. wml-dr-04 · randomised controlled trial
   Weight change (kg) at end of week 1, median[interquartile range] -0.05[-0.775 to + 0.50] vs. -0.5[-1.4 to + 0.1] P = 0.014 and to the end of week 2, + 0.25[-0.65 to + 0.975] vs. -0.5[-1.3 to + 0.2] P = 0.001 for watermelon and control arms respectively.
   BMC Pregnancy Childbirth, 2023 · checked 2026-09-22
6. wml-dr-02 · systematic review and meta-analysis of randomised controlled trials
   Watermelon consumption significantly decreased systolic blood pressure (SBP), totalcholesterol (TC) and low-density lipoprotein (LDL). In addition, watermelon consumption led to a significant increase in fasting blood sugar (FBS).
   Diabetes Res Clin Pract, 2023 · checked 2026-09-22