New stable isotope method to measure protein digestibility and response to pancreatic enzyme intake in cystic fibrosis.
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BACKGROUND & AIMS: Adequate protein intake and digestion are necessary to prevent muscle wasting in cystic fibrosis (CF). Accurate and easy-to-use methodology to quantify protein maldigestion is lacking in CF. OBJECTIVE: To measure protein digestibility and the response to pancreatic enzyme intake in CF by using a new stable isotope methodology. DESIGN: In 19 CF and 8 healthy subjects, protein digestibility was quantified during continuous (sip) feeding for 6hby adding (15)N-labeled spirulina protein and L-[ring-(2)H5]phenylalanine (PHE) to the nutrition and measuring plasma ratio [(15)N]PHE to [(2)H5]PHE. Pancreatic enzymes were ingested after 2h in CF and the response in protein digestibility was assessed. To exclude difference in mucosal function, postabsorptive whole-body citrulline (CIT) production rate was measured by L-[5-(13)C-5,5-(2)H2]-CIT pulse and blood samples were taken to analyze tracer-tracee ratios. RESULTS: Protein digestibility was severely reduced in the CF group (47% of healthy subjects; P<0.001). Intake of pancreatic enzymes induced a slow increase in protein digestibility in CF until 90% of values obtained by healthy subjects. Maximal digestibility was reached at 100min and maintained for 80min. Stratification into CF children (n=10) and adults showed comparable values for protein digestibility and similar kinetic responses to pancreatic enzyme intake. Whole-body citrulline production was elevated in CF indicating preserved mucosal function. CONCLUSION: Protein digestibility is severely compromised in patients with CF as measured by this novel and easy-to-use stable isotope approach. Pancreatic enzymes are able to normalize protein digestibility in CF, albeit with a severe delay. Registration ClinicalTrials.gov=NCT01494909.