Glycomacropeptide Safety and Its Effect on Gut Microbiota in Patients with Phenylketonuria: A Pilot Study

Abstract

Glycomacropeptide (GMP) represents a good alternative protein source in Phenylketonuria (PKU). In a mouse model, it has been suggested to exert a prebiotic role on beneficial gut bacteria. In this study, we performed the 16S rRNA sequencing to evaluate the effect of 6 months of GMP supplementation on the gut microbiota of nine PKU patients, comparing their bacterial composition and clinical parameters before and after the intervention. GMP seems to be safe from both the microbiological and the clinical point of view. Indeed, we did not observe dramatic changes in the gut microbiota but a specific prebiotic effect on the butyrate-producer Agathobacter spp. and, to a lesser extent, of Subdoligranulum. Clinically, GMP intake did not show a significant impact on both metabolic control, as phenylalanine values were kept below the age target and nutritional parameters. On the other hand, an amelioration of calcium phosphate homeostasis was observed, with an increase in plasmatic vitamin D and a decrease in alkaline phosphatase. Our results suggest GMP as a safe alternative in the PKU diet and its possible prebiotic role on specific taxa without causing dramatic changes in the commensal microbiota.

Keywords: Agathobacter; GMP; Subdoligranulum; calcium homeostasis; diet; glycomacropeptide; gut microbiota; nutritional therapy; phenylketonuria; vitamin D.

Figure 1

Alpha- and Beta-diversity between timepoints. PKU patients were sampled at baseline (T0) and after 6 months of GMP intake (T1). (A) Boxplots showing the metrics Chao1, Observed Species, Shannon index, and PD whole tree. Black dots represent outlier samples. (B,C). PCoA of the Unweighted and Weighted Unifrac distances with a boxplot of samples’ distribution for, respectively, the first and second coordinates and the first and third coordinates. T0 samples are depicted in orange; T1 in green. Solid dots represent single samples, empty circles represent the group average. For each group, confidence ellipse are reported. No statistically significant differences were observed among groups for both biodiversity indices.

Figure 2

Taxonomy analysis. (A) Histogram chart showing the family level distribution of PKU patients at T0 and T1. (B) Bar plot of the main bacterial genera. For both panels, bacteria were selected according to their relative abundance (at least 1%); the rest of the abundances were grouped in the “Other genera” group. The asterisk indicates significant correlation (* p < 0.05).

Figure 3

Correlation of bacterial genera and SCFA. Heatmap showing the Euclidean correlation R-values occurring between the most abundant bacterial genera and fecal SCFAs. Clustering was performed through a complete agglomeration method. Asterisk indicates significant (p < 0.05) correlation (asymptotic p-value).

Figure 4

Microbiota, biochemical values, and SCFA interactions. PCA shows correlations between the most abundant genera and significantly changed ones (as reported in Supplementary Table S3), the key biochemical parameters, and butyrate. Samples are divided by timepoints: T0 (baseline) and T1 (after 6 months of GMP intervention). Color gradients and transparency depend on the variable loading contribution; the arrow length and distance from the middle indicate the magnitude of the separation. Positively correlated variables point to the same quadrant of the plot; negatively correlated variables point to opposite sides. The first and the third principal component (dimensions) are shown, along with their eigenvalues (percentage of variances).