This study will determine if air-displacement plethysmography (ADP) provides accurate measurement of body composition (percent body fat and fat-free mass) in pediatric patients with intestinal failure, as compared to body composition measured by alternative standard methods, including deuterium dilution, bioimpedance analysis (BIA), dual x-ray absorptiometry (DXA) when clinically indicated, and routine anthropometric measurements.

Infants and children with intestinal failure (IF) require prolonged periods of parenteral nutrition (PN) for sustenance and growth until intestinal adaptation allows adequate enteral intake. Lean body mass (or muscle mass) preservation and accrual is an important goal during this phase of nutritional rehabilitation. However, lean body mass is not accurately measured by currently available anthropometric techniques. Thus, despite weight gain, muscle mass depletion can remain undetected.

Little data exist in the field of measuring body composition in patients with intestinal failure, where changes in body weight are commonly taken to indirectly measure gastrointestinal absorptive function. Body composition analysis is of particular importance in the management of these patients, since it can help guide medical, surgical and/or nutrition interventions, including titration of enteral or parenteral nutrition. To the investigators' knowledge there are no studies examining whether air-displacement plethysmography (ADP) is a valid measure of body composition in patients with intestinal failure.

The investigators propose a non-randomized, prospective cohort study to validate ADP as a measure of body composition in infants and children with IF.

The investigators will apply non-radioactive (stable) isotope techniques to examine the accuracy of ADP in measuring body composition in these children.

The investigators will also compare ADP with bioimpedance analysis and routine anthropometry. Additionally, if subjects have had a recent dual-energy X- ray absorptiometry (DXA) scan, the investigators will compare the ADP estimates of body composition to those obtained from DXA. The investigators hypothesize that ADP is a feasible and accurate technique for body composition measurements, and that the device can be used to serially measure lean body mass and fat mass changes in this cohort. If the feasibility and accuracy of ADP for body composition analysis can be demonstrated, the technique is likely to have wider applications in other pediatric illnesses.

Inclusion Criteria:

• Patients evaluated in the CAIR Program at Boston Children's Hospital will be eligible for participation if they meet the definition of short bowel syndrome and are under 18 years of age. Short bowel syndrome will be defined as a malabsorptive state resulting from congenital or acquired gastrointestinal disease leading to current dependence on parenteral nutrition for greater than or equal to 90 days. If the patient is on intravenous fluids, they must have a stable intravenous fluid regimen for at least 4 weeks prior to the study visit.

Exclusion Criteria:

• age < 2 years or ≥ 18 years
• presence of cardiac pacing device (BIA may theoretically interfere with pacer functioning)
• presence of any other electrical device that might interfere with the BIA such as implantable drug delivery pumps, vagal nerve stimulator and invasive cerebral perfusion monitor
• subjects will be excluded due to pregnancy
• subjects with ongoing fluid imbalance, clinically evident shifts in fluid compartments (e.g. edema, ascites) or with fluid resuscitation (defined as daily fluid intake more than 150% of maintenance or fluid boluses more than 20 ml/kg/day). Determination of fluid imbalance will be made by the physicians directly involved in the patient's care.

Patients with intestinal failure between 2-17 years of age inclusive will be recruited through the Center for Advanced Intestinal Rehabilitation (CAIR) program at Boston Children's Hospital.