New stable isotope method to measure protein digestibility and response to pancreatic enzyme intake in cystic fibrosis

Abstract

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 6 h by 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 2 h 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 100 min and maintained for 80 min. 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.

Keywords: Cystic fibrosis; Novel stable isotope method; Pancreatic enzymes; Protein digestibility; Whole-body citrulline production.

Conflict of interest statement

CONFLICT OF INTEREST

There is no Conflict of Interest to declare.

Copyright © 2013 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism. All rights reserved.

Figures

Figure 1

Overview of the study design.

Figure 1

Overview of the study design.

Figure 2

The basic principle of measuring protein digestibility with 15N-labelled protein and 2H5-labelled phenylalanine.

Figure 3

We used the compartmental model (tracee model; upper left panel) of a plasma pool (pool 1) and an intracellular pool (pool 2) of unlabeled citrulline. In this model, the amount of unlabeled citrulline in each compartment is denoted by Q (Q1 and Q2 for the pool sizes of compartments 1 and 2) and its intracellular appearance (U(2)). We used a citrulline tracer experiment (upper right panel) to calculate the rate constants and pool sizes. Thus, citrulline tracer is introduced in plasma (Pool 1 or Q1) and samples (S1) are taken from plasma. Q2 is most likely the intracellular citrulline pool. The arrows represent transfer of citrulline from one compartment to another. k(0,2) is the fractional rate constant for irreversible loss from pool 2, k(1,2) is the fractional rate of transfer of citrulline (per unit time) from pool 2 to pool 1, and k(2,1) from pool 1 to pool 2 (38). Intracellular appearance (production) of citrulline is shown entering pool 2 (U(2)=F(0,2)) and is calculated as k(0,2)*Q2. The % irreversible loss of citrulline from pool 2 is the rate of citrulline irreversible lost divided by the rate that citrulline exits pool 2 by all routes and calculated as F(0,2)/(F(0,2)+F(1,2)), and (1-% irreversible loss) is the citrulline that appears in pool 1. WbRa is therefore calculated by multiplying (1-% irreversible loss) by F(0,2). WbRa is therefore lower than the total production rate to the extent that citrulline does not appear into plasma. The obtained data for Q1, Q2, k(0,2), k(1,2) and k(0,1) for the healthy adults, CF adults and CF children are provided in the lower panel.

Figure 4

Response in protein digestibility during feeding and after intake of pancreatic enzymes (only in CF) in the whole CF group (dashed line) and healthy adult group (solid line) (left panel), and in the healthy group (solid line) and after stratification of the whole CF group into adults (dashed line) and children (dotted line) (right panel).

Figure 5

Postabsorptive whole body rate of appearance (WbRa, panel a + c) and production rate (panel b+d) of citrulline in the healthy group (solid bar) and after stratification of the whole CF group into adults (open bar) and children (cross striped bar), and after stratification of the CF group into nutritional failure (NF, vertical striped bar) and no nutritional failure (no NF, horizontal striped bar). Mean values ± SE are shown. Significance of difference as compared to the healthy group (*: P#: P<0.05).

Figure 5

Postabsorptive whole body rate of appearance (WbRa, panel a + c) and production rate (panel b+d) of citrulline in the healthy group (solid bar) and after stratification of the whole CF group into adults (open bar) and children (cross striped bar), and after stratification of the CF group into nutritional failure (NF, vertical striped bar) and no nutritional failure (no NF, horizontal striped bar). Mean values ± SE are shown. Significance of difference as compared to the healthy group (*: P#: P<0.05).