At study entry (Day -14), blood and urine will be collected at screening to assess baseline plasma and erythrocyte membrane concentrations of docosahexaenoic acid (DHA) and eicosapentaenoic (EPA), plasma concentrations of other long chain fatty acids, fat-soluble vitamins, serum protein and urinary leukotriene concentrations. Body weight and body mass index will be obtained. During an initial 7 day observation period (Day -14 to Day -8), subjects will follow their usual enteral nutrition regimen, including use of pancreatic enzyme replacement therapy (PERT). During the 7 day run in period (Day -7 to Day -1), subjects will use Peptamen 1.5 at their normal volume of administration from 500 mL to 1,000 mL per feeding and follow their normal PERT regimen.