Preserved anabolic threshold and capacity as estimated by a novel stable tracer approach suggests no anabolic resistance or increased requirements in weight stable COPD patients

Abstract

Background & aims: Assessing the ability to respond anabolic to dietary protein intake during illness provides important insight in the capacity of lean body mass maintenance. We applied a newly developed stable tracer approach to assess in one session in patients with chronic obstructive pulmonary disease (COPD) and healthy older adults both the minimal amount of protein intake to obtain protein anabolism (anabolic threshold) and the efficiency of dietary protein to promote protein anabolism (anabolic capacity).

Methods: We studied 12 clinically and weight stable patients with moderate to very severe COPD (mean ± SE forced expiratory volume in 1 s: 36 ± 3% of predicted) and 10 healthy age-matched older adults. At 2-h intervals and in consecutive order, all participants consumed a mixture of 0.0, 0.04, 0.10 and 0.30 g hydrolyzed casein protein×kg ffm^-1×2 h^-1 and carbohydrates (2:1). We assessed whole body protein synthesis (PS), breakdown (PB), net PS (PS-PB) and net protein balance (phenylalanine (PHE) intake - PHE to tyrosine (TYR) hydroxylation) by IV primed and continuous infusion of L-[ring-²H₅]PHE and L-[¹³C₉,¹⁵N]-TYR. Anabolic threshold (net protein balance = 0) and capacity (slope) were determined on an individual basis from the assumed linear relationship between protein intake and net protein balance.

Results: We confirmed a linear relationship between protein intake and net protein balance for all participants (R² range: 0.9988-1.0, p ≤ 0.0006). On average, the anabolic threshold and anabolic capacity were comparable between the groups (anabolic threshold COPD vs. healthy: 3.82 ± 0.31 vs. 4.20 ± 0.36 μmol PHE × kg ffm^-1 × hr^-1; anabolic capacity COPD vs. healthy: 0.952 ± 0.007 and 0.954 ± 0.004). At protein intake around the anabolic threshold (0.04 and 0.10 g protein×kg ffm^-1×2 h^-1), the increase in net PS resulted mainly from PB reduction (p < 0.0001) whereas at a higher protein intake (0.30 g protein×kg ffm^-1×2 h^-1) PS was also stimulated (p < 0.0001).

Conclusions: The preserved anabolic threshold and capacity in clinically and weight stable COPD patients suggests no disease related anabolic resistance and/or increased protein requirements.

Trial registry: ClinicalTrials.gov; No. NCT01734473; URL: www.clinicaltrials.gov.

Keywords: COPD; Casein; Protein anabolism; Protein requirements; Protein threshold.

Figure 1 -

Study design. ffm: fat-free mass.

Figure 2 -

Depiction of the terms anabolic threshold and capacity

Figure 3 -

Plasma amino acid concentrations of PHE (A), LEU (B), and TYR (C) as a function of dietary protein intake in COPD patients (n=12) and healthy age matched controls (n=10). PHE control group: y= 155x + 44, R²=0.9987, p=0.0007. PHE COPD group: y= 156x + 41, R²=0.9864, p=0.0068. LEU control group: y= 788x + 87, R²=0.9981, p=0.0010. PHE COPD group: y= 819x + 86, R²=0.9998, p=0.0001. TYR control group: y= 303x + 43, R²=0.9928, p=0.0036. TYR COPD group: y= 329x + 43, R²=0.9966, p=0.0017. Slopes are similar between groups for all three amino acids. Statistics were done using linear regression analysis. COPD: chronic obstructive pulmonary disease. ffm: fat-free mass. LEU: leucine. PHE: phenylalanine. TYR: tyrosine.

Figure 4 -

Plasma insulin concentration as a function of dietary protein intake in COPD patients (n=12) and healthy age matched controls (n=10). Control group: y= 10.7x + 1.7, R²=0.9987, p=0.0080 (slope is significantly different from zero). COPD group: y= 15.5x + 4.0, R²=0.6318, p=0.2051 (slope is not significantly different from zero). Slopes are similar between groups. Statistics were done using linear regression analysis. COPD: chronic obstructive pulmonary disease. ffm: fat-free mass.

Figure 5 -

Mean (± SE) plasma isotope enrichments of L-[ring-²H₅]-phenylalanine (A), L-[U-¹³C₉, ¹⁵N]-tyrosine (B), L-[ring-²H₄]-tyrosine (C), L-[¹⁵N]-phenylalanine (D), L-[¹⁵N]-tyrosine (E) in COPD patients (n=12) and healthy age matched controls (n=10) during four study phases, in which we examined four levels of intake of a protein-carbohydrate mixture. Phase 0 = fasted. Phase 1 = 0.04 g protein × kg ffm^{− 1} × 2h⁻¹. Phase 2 = 0.10 g protein × kg ffm⁻¹ × 2h⁻¹. Phase 3 = 0.30 g protein × kg ffm⁻¹ × 2h⁻¹. COPD: chronic obstructive pulmonary disease.

Figure 6 -

Mean (± SE) whole body protein synthesis (A), breakdown (B), hydroxylation (C), net protein balance (D), and splanchnic PHE extraction (E) in COPD patients (n=12) and healthy age matched controls (n=10) during the four study phases, in which we examined four levels of intake of a protein-carbohydrate mixture. Phase 0 = fasted. Phase 1 = 0.04 g protein × kg ffm⁻¹ × 2h⁻¹. Phase 2 = 0.10 g protein × kg ffm⁻¹ × 2h⁻¹. Phase 3 = 0.30 g protein × kg ffm⁻¹ × 2h⁻¹. Two-factor repeated measures analysis of variance showed a significant study phase effect for all parameters (p<0.0001), except for SPE. No group effects were observed, but a study phase-by-group interaction was observed for protein breakdown (p=0.0110). Different letters (a,b,c,d) indicate statistical differences between phases, at p<0.0001, except for the difference in hydroxylation between phase 1 and 2, which was significant at p<0.05. Hydroxylation is the conversion of PHE to tyrosine by the enzyme PHE hydroxylase. Splanchnic extraction is the fraction of PHE that is extracted by the splanchnic area during its first pass after consumption. COPD: chronic obstructive pulmonary disease. ffm: fat-free mass. PHE: phenylalanine.

Figure 7.

Whole body net protein balance as a function of PHE intake (labeled and unlabeled) in healthy age matched controls (n=10) (A), and COPD patients (n=12) (B). Control group: y= 0.9536x - 4.0, R²=1, p<0.0001. COPD group: y= 0.9475x - 3.6, R²=1, p<0.0001. Both slopes are significantly different from zero. Statistics were done using linear regression analysis. Dotted lines represent the 95% confidence interval. COPD: chronic obstructive pulmonary disease. ffm = fat-free mass. Net protein balance = PHE intake - PHE hydroxylation. PHE: phenylalanine.

Figure 8.

Anabolic threshold (A) and anabolic capacity (B) in COPD patients (n=12) and healthy age matched controls (n=10). Data are depicted as a box and whiskers (min to max). Depiction of the terms anabolic threshold and anabolic capacity can be found in Figure 2. No significant differences were found between the groups for either the anabolic threshold or anabolic capacity. Statistics were done using the unpaired Student’s t-test. COPD: chronic obstructive pulmonary disease. ffm: fat-free mass.