Abstract

Background: Maple syrup urine disease is an inherited metabolic disorder caused by defective branched-chain amino acid catabolism. Optimal management requires maintaining plasma branched-chain amino acid concentrations within the target reference range, primarily through a strict, lifelong low-natural-protein diet supplemented with a branched-chain-free amino acid mixture, which substantially affects social functioning and is often particularly difficult to sustain during adolescence. This study aims to evaluate the feasibility of implementing a relaxed diet – a more flexible low-protein dietary regimen – in individuals with classic maple syrup urine disease.

Case Report: We performed a comparative assessment of plasma amino acid concentrations and nutritional profiles in a young adult with classic maple syrup urine disease following 7 years on a conventional diet (from 2011 to 2018) and 7 years on a relaxed dietary regimen (from 2018 to 2025). The median plasma leucine concentration during the conventional diet was 217.1 mmol/L (range 172.9-417.5), compared with 131.3 mmol/L (range 101.4-180.2) during the relaxed diet (p=0.004); isoleucine and valine values were within reference range for both diets. The patient demonstrated active engagement in dietary self-management and consistently selected appropriate portion sizes based on protein content. The overall nutritional composition of the two dietary regimens was comparable, with adequate provision of both simple and complex carbohydrates to meet daily energy requirements.

Conclusions: A simplified and relaxed diet promotes increased long-term compliance, particularly in adolescent patients transitioning from pediatric to adult care. It should be proposed only to selected patients; close monitoring of plasma amino acids is required to prevent acute metabolic decompensation.