# L-Citrulline: A Non-Essential Amino Acid with Important Roles in Human Health
## 8 Studies with Citrulline from Natural Sources: Watermelon
This fruit is a good source of potassium (126 mg/100 g) and contains magnesium (10 mg/100 g), calcium (5.6 mg/100 g), sodium (0.81 mg/100 g), iron (0.26 mg/100 g), copper (0.042 mg/100 g), and zinc (0.031 mg/100 g) [129]. The sugar composition in watermelon includes, in decreasing order, fructose (3.45%), followed by sucrose (2.83%) and glucose (2.45%) [130]. The main vitamin in watermelon is vitamin C (8.1 mg/100 g), providing a strong antioxidant activity against free radicals [131; 132]. This fruit is also rich in carotenoid such as lycopene and \(\beta\)-carotene. Watermelon lycopene extracts also have the capability to scavenge free radicals, acting as a natural antioxidant [133]. Many epidemiological studies have demonstrated that tomato lycopene could help to lower the risk of developing the incidence of some cancers [134]. The amounts of lycopene in watermelon fruits range from 35 to 112 mg/kg fresh weight, depending on the watermelon cultivar and ripening stages [135]. Lycopene from watermelon does not need to be heated, like tomato, for improved bioavailability, with watermelon being one of the few sources with readily available cis-isomeric lycopene [136]. Watermelon is also a rich source of \(\beta\)-carotene, which acts as an antioxidant and precursor of vitamin A. This fruit has a significant source of total phenolic and flavonoid [132]. All these functional compounds explain the health benefits of watermelon consumption. However, this review is focused on CIT benefits and we will discuss this section from this point of view but with the knowledge that other bioactive compounds present in watermelon could contribute to a specific health effect. Based on the richness of bioactive compounds, the food industry is researching different natural foods, offering healthy properties or positive effects in a particular population (such as sportspeople) as substitutes for pharmacological products [117]. Athletes frequently employ legal ergogenic aids to enhance exercise performance, with the elimination of fatigue symptoms being particularly important. Eccentric exercise produces delayed-onset muscle soreness that often persists for several days. Such muscle damage causes muscle fatigue, thus limiting performance, and negatively affecting force, peak power, and/or velocity [41]. The number of studies focusing on CIT describing enhanced exercise performance has increased recently [137]. Watermelon is a principal food source of CIT [4] and its consumption raises plasma Arg concentrations in adults [7]. Furthermore, in vitro CIT bioavailability tests have reported that a natural source of CIT (watermelon juice) could be more bioavailable than a synthetic source [138]. Recent studies have reported that diminished muscle soreness perception with just one dose of watermelon juice enriched in CIT can enhance exercise performance [40; 41]. In addition, plasma lactate levels were lower in watermelon juice enriched in CIT (3.45 g per 500 mL) than placebo, after a half-marathon race in trained runners [49]. Additionally, [PERSON] et al. [53] demonstrated the functionality of two different watermelon juices, to test a single dose of two different concentrations of CIT (0.5 and 3.3 g per 200 mL) in watermelon juice matrix and the positive effect of ellagitannins (22 mg per 200 mL) on submaximal resistance exercise performance to exhaustion in trained resistance athletes. They observed an increase in mean peak force of about 3% and a five-fold fall in the decrease in peak torque. Subjects that took the watermelon and pomegranate juice mix enriched with CIT reported the lowest muscle soreness perception values 1 h after eight repetition maximum (RM) exercises and 24 h after exercise, showing around 44-60% decrease in muscle soreness compared to placebo.