Preliminary studies reported that plasma citrulline concentrations may be a reliable biochemical marker for intestinal dysfunction and absorptive enterocyte mass. The relationship between citrulline concentration and intestinal function has been supported in other studies including those examining rejection in small bowel allografts. Concentrations of citrulline are dramatically reduced in cases of mucosal damage (e.g. moderate graft rejection or viral enteritis) and strongly correlate (inversely) with severity on biopsy. Plasma citrulline concentration is lower also in patients with villous atrophy (24±13 μmol/L) than in healthy subjects (40±10 μmol/L) and patients with anorexia nervosa (39±9).