Citrulline is an amino acid produced in the intestine and in the liver, but the liver does not contribute significantly to circulating citrulline concentrations. The intestine is thus the only organ that normally releases significant amounts of citrulline into the blood. The investigators have designed a study looking at the value of measuring plasma citrulline concentration in patients with Crohn's disease and short bowel or normal intestinal length. Measuring the plasma citrulline concentration in short bowel patients may help to distinguish between patients who need permanent parenteral feeding from patients with just transient intestinal dysfunction. It may also help the investigators in understanding the small bowel intestinal length remaining and the absorptive integrity. In patients with normal intestinal length and Crohn's disease, it may be a reliable marker of small bowel damage and could be applied to establish therapeutic improvements. It has been demonstrated to strongly correlate (inversely) with severity on intestinal biopsies.

The investigators hypothesise that the plasma citrulline concentration is a marker for small bowel absorptive integrity and an appropriate surrogate for functional length of the small intestine.

Controlled data do not yet exist to establish the place of plasma citrulline in the assessment of small bowel function in man.

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).

A citrulline threshold of 20 µmol/L apparently permitted the classification of short bowel syndrome patients into either transient or permanent intestinal failure categories, with 92% sensitivity, 90% specificity, and 95% positive and 86% negative predictive values, respectively. Experimental studies have been carried out also in assessing the value of citrulline as a marker for severity of small bowel epithelial damage from radiation. The plasma citrulline was shown to be a simple, non-invasive and sensitive assay to monitor and quantify radiation-induced small bowel damage in mice and humans. Otherwise, the literature on citrulline as a potential marker of intestinal and nutritional integrity is young and data for specific conditions come only from single centres; there are limited data on normal ranges. More crucially, however, there has been no attempt to clarify the effect of inflammation on citrulline homeostasis. To date there is no information in respect of patients with intestinal failure in whom there has been no resection.

We hypothesise that plasma citrulline concentration reflects small bowel absorptive capacity and correlates to the functional intestinal length independently from inflammation.

Comparisons: To exclude the possibility that citrulline merely reflects inflammation, control groups (six subjects each) with short bowel syndrome without inflammation (mesenteric infarct) (negative control); and those with inflammation but no anatomical loss (active coeliac disease) (positive controls); will be studied as well as healthy volunteers.

The study is designed to utilise patients from the positive and negative control groups to permit a correlation of plasma citrulline with intestinal length and with a "gold standard" assessment of intestinal function as judged from the patients need for nutritional intervention (from normal diet to dependence on home parenteral nutrition).

Plasma citrulline will be determined by Reverse-phase High Performance Liquid Chromatography (RF-HPLC) after an overnight fast. Albumin and Routine biochemical assessment will also be performed. Gastrointestinal permeability will be determined from the double sugar test using rhamnose and lactulose, and functional absorptive capacity will be estimated by D-Xylose absorption rate.

Analysis will allow for paired comparison between patients and between groups. Differences in the clinical performance of the various parameters will be determined. The study has adequate statistical power.

Inclusion Criteria:

• Crohn's disease (CD) with massive small bowel resection at least 24 months previously (< 50cm remaining)
• Crohn's disease with small bowel resection at least 24 months previously (50-150cm remaining)
• CD with no resection
• Mesenteric infarction with massive resection > 24 months previously (< 50cm remaining)
• Mesenteric infarction with massive resection > 24 months previously (50-150cm remaining); coeliac disease.
• Healthy volunteers.
• Body mass index within the normal range

Exclusion Criteria:

• Patients with surgical resection of stomach, duodenum or pancreas; or upper gastrointestinal (UGI) bypass.
• Oral feeding > 1.0-fold the estimated basal metabolic rate as assessed using Harris and Benedict equations.
• Patients with fistulating Crohn's disease
• Patients on steroids
• Patients with other important disease, which may interfere with the study (especially diabetes and renal impairment). Alcoholism, drug abuse or any other circumstances, which may compromise the patient's ability to comply with the study requirements.
• Pregnancy.
• Corticosteroid use or octreotide during, or in, the month before the study.
• Use of glucagon-like peptide 2 (GLP2), growth hormone (GH) or glutamine or triglycerides.