Metabolomic Markers of Essential Fatty Acids, Carnitine, and Cholesterol Metabolism in Adults and Adolescents with Phenylketonuria

Abstract

Background: Individuals with phenylketonuria (PKU) have a risk of cognitive impairment and inflammation. Many follow a low-phenylalanine (low-Phe) diet devoid of animal protein in combination with medical foods (MFs).

Objective: To assess lipid metabolism in participants with PKU consuming amino acid MFs (AA-MFs) or glycomacropeptide MFs (GMP-MFs), we conducted fatty acid and metabolomics analyses.

Methods: We used subsets of fasting plasma and urine samples from our randomized crossover trial in which participants with early-treated classical and variant (milder) PKU consumed a low-Phe diet combined with AA-MFs or GMP-MFs for 3 wk each. Fatty acid profiles of red blood cell (RBC) membranes were determined for 25 adults (aged 18-49 y) with PKU and 143 control participants. Metabolomics analyses of plasma and urine samples were conducted by Metabolon for 9-10 adolescent and adult participants with PKU and for 15 control participants.

Results: RBC fatty acid profiles were not significantly different with AA-MFs or GMP-MFs. PKU participants showed higher total n-6:n-3 (ω-6:ω-3) fatty acids (mean ± SD percentages of total fatty acids: AA-MF = 5.45% ± 1.07%; controls = 4.33%; P < 0.001) and lower docosahexaenoic acid (DHA; AA-MF = 3.21% ± 0.98%; controls = 3.70% ± 1.01%; P = 0.02) and eicosapentaenoic acid (AA-MF = 0.33% ± 0.12%; controls = 0.60% ± 0.43%; P < 0.001) in RBCs than did control participants. Despite higher carnitine intake from AA-MFs than GMP-MFs (mean ± SE intake: AA-MFs = 58.6 ± 5.3 mg/d; GMP-MFs = 0.3 ± 0.01 mg/d; P < 0.001), plasma concentrations of carnitine were similar and not different from those in the control group (AA-MF compared with GMP-MF, P = 0.73). AA-MFs resulted in higher urinary excretion of trimethylamine N-oxide (TMAO), which is synthesized by bacteria from carnitine, compared with GMP-MFs (mean ± SE scaled intensity-TMAO: AA-MFs = 1.2 ± 0.1, GMP-MFs = 0.9 ± 0.1; P = 0.005). Plasma deoxycarnitine was lower in PKU participants than in control participants, suggesting reduced carnitine biosynthesis in PKU (AA-MF = 0.9 ± 0.1; GMP-MF = 1.0 ± 0.1; controls = 1.3 ± 0.1; AA-MF compared with controls, P = 0.01; GMP-MF compared with controls, P = 0.04).

Conclusions: Supplementation with DHA is needed in PKU. Carnitine supplementation of AA-MFs shows reduced bioavailability due, in part, to bacterial degradation to TMAO, whereas the bioavailability of carnitine is greater with prebiotic GMP-MFs. This trial was registered at www.clinicaltrials.gov as NCT01428258.

Figures

FIGURE 1

Metabolomics of carnitine metabolism in participants with phenylketonuria with the use of AA-MFs and GMP-MFs. Values for dietary intakes of carnitine (A) and choline (B) are means ± SEMs and are based on 3-d food records; n = 30/treatment group. Values are scaled intensity means ± SEMs for plasma deoxycarnitine (C), carnitine (D), and choline (E); PKU, n = 10/treatment group; control group, n = 15. Values are scaled intensity means ± SEMs for urinary deoxycarnitine (F), carnitine (G), and TMAO (H); n = 9/treatment group. Labeled means without a common letter differ, P < 0.05. AA-MF, amino acid medical foods; GMP-MF, glycomacropeptide medical foods; PKU, phenylketonuria; TMAO, trimethylamine N-oxide.

FIGURE 2

Carnitine synthesis and degradation pathway. TMA, trimethylamine; TMAO, Trimethylamine N-oxide.

FIGURE 3

Metabolomics of cholesterol metabolism in participants with PKU using AA-MFs and GMP-MFs. Intake of dietary cholesterol (A) and the metabolomics of cholesterol metabolism (B, C) are shown. Values are scaled intensity means ± SEMs for metabolites in plasma (classical PKU, n = 5; variant PKU, n = 5; controls, n = 15). Values for dietary intake are means ± SEMs. Total dietary cholesterol intakes are based on 3-d food records (classical PKU, n = 20; variant PKU, n = 10). Labeled means without a common letter differ, P < 0.05. AA-MF, amino acid medical foods; GMP-MF, glycomacropeptide medical foods; PKU, phenylketonuria.