Table 1.

Summarized roles of citrulline in promoting intestinal immune response and gut health in human and animal models.

Subject	Design	Main Findings	References
Wistar rats; Beagle dogs; Cynomolgus monkeys	Intestinal toxicity was induced using oncological drug candidates.	In treated animals, a > 50% decrease in plasma l-citrulline levels strongly correlated with histopathological findings in the small intestine such as single-cell necrosis and mucosa atrophy, intestinal crypt necrosis, villus atrophy, enterocyte loss, and clinical signs (bloody feces and diarrhea), indicating l-citrulline as a small intestine biomarker.	[104]
Dogs (5 males/5 females per group)	Oral doses of 0.75, 1.5, and 3 mg/kg/d of MS-229 over 4 weeks to induce small intestinal toxicity.	A dose- and exposure-dependent decrease in plasma citrulline was correlated with pathological findings in the small intestine.	[105]
Preterm infants	Plasma citrulline levels were measured during the first 48 h after necrotizing enterocolitis onset.	Plasma citrulline decreased in the first 48 h suggesting ongoing intestinal injury, thus plasma citrulline measurement may provide an indication for intestinal recovery rate during the first 24 h after NEC onset.	[106]
Male Wistar rats (n = 46; 230–250 g)	Varying citrulline levels were administered as 0.5,1, 2.5, 5 g/kg/d citrulline.	The jejunum weight was significantly positively correlated with plasma citrulline, suggesting a dose-dependent intestinal adaptation in gut resected rats.	[107]
In vitro analysis using IPEC-J2 cells	Citrulline (2 mM) and Lactobacillus helveticus ASCC 511 were co-treated to IPEC-J2 cells.	Lactobacillus helveticus and citrulline exhibited synergistic effects against adhesion of pathogenic bacteria, Escherichia coli; stimulated nitric oxide; improved transepithelial electrical resistance; and stimulated tight junction proteins expression, thus, promoting intestinal health.	[98]
Female C57BL/6J mice	Mice were induced non-alcoholic steatohepatitis using fat-, fructose-, and cholesterol-rich diet followed by +/− 2.5 g l -citrulline/kg body weight.	l-citrulline alleviated non-alcoholic fatty liver disease progression via attenuation of bacterial endotoxin translocation and the loss of tight junction proteins in small intestinal tissue.	[108]
Human model	Randomized, double-blind crossover study, 10 men cycled for 60 min at 70% of their maximum workload after l-citrulline (10 g) or placebo (l-alanine) intake.	Pre-exercise l-citrulline intake prevented splanchnic hypoperfusion-induced intestinal compromise by preserving splanchnic perfusion and attenuated intestinal injury during exercise probably by enhancing arginine availability.	[109]
Mice model	Mice undergoing intestinal obstruction were divided into three groups: sham, intestinal obstruction, and citrulline group receiving a diet containing 0.6% citrulline.	Citrulline pretreatment preserved barrier integrity and modulated immune response via decreasing intestinal permeability and bacterial translocation, whereas it preserved the ileum mucosa and increased secretory IgA concentration.	[110]
Male Wistar rats (n = 15)	Ulcerative colitis was established in rats and citrulline was administered intragastrically for 7 d.	Citrulline provided protective effects by lowering the peripheral blood monocytes, the infiltration of CD68-positive monocytes, and the concentrations of MCP-1, IL-6, and IL-17A in the colon tissues of effects in ulcerative colitis rats.	[111]
Adult male Sprague–Dawley rats (180–220 g)	l-citrulline (300, 600, and 900 mg/kg body weight) was administered to rats having ethanol-induced gastric ulcer in rats.	l-citrulline elicited gastro-protective effects by attenuating gastric lesions, prevented oxidative damage, decreased nitric oxide content and increased the myeloperoxidase activity	[19]
Male Wistar rats (n = 24, 220–230 g)	Rats were assigned to either citrulline, arginine, control, or sham groups. The sham group underwent transection and other groups had an 80% resection of the small intestine.	Citrulline increased arginine levels and improved nitrogen balance after massive intestinal resection.	[112]
Adult male Sprague–Dawley rats (200–240 g)	l-citrulline (300, 600, and 900 mg/kg) was pretreated to ischemia/reperfused rats.	l-citrulline reduced the gastric mucosal lesion, prevented the production of lipid peroxidation, and inhibited the increase in myeloperoxidase activity.	[113]
Male C57/Bl6 mice (n = 65; 26–28.5 g)	Mice received intravenous infusion of endotoxin (LPS, 0.4 µg/g bodyweight per h) combined with either l-citrulline (6.25 mg/h), l-arginine (6.25 mg/h), or l-alanine (12.5 mg/h).	During endotoxemia, l-citrulline supplementation reduced intestinal microcirculatory dysfunction and increased intracellular NO production via increasing plasma and tissue concentrations of arginine and citrulline, and restored intracellular NO production in the intestine. Jejunal tissues in the l-citrulline group showed an increase in degree of phosphorylation of eNOS phosphorylation and decreased iNOS protein level.	[45]
Swiss male mice (6 weeks old)	Mice received supplemented citrulline or alanine in the drinking water for 10 d (1 g/kg/d) and on the seventh day, the animals were injected intraperitoneally with a single dose of phosphate-buffered saline (PBS) or 5-fluorouracil (200 mg/kg) for the induction of mucositis.	Citrulline administration contributed to a partial recovery of the mucosal architecture in mucositis-induced mice. There was an intermediate reduction in the histopathologic score, and functional intestinal permeability was partially rescued by citrulline treatment. Citrulline attenuated mucosal damage by reducing the size of the injured areas and decreased intestinal permeability in mucositis mice.	[114]