Protein restriction
Vegetarian diets and, for younger children, breastfeeding are common ways to limit protein intake without endangering tryptophan transport to the brain. Tryptophan
Formulas such as XLys, XTrp Analog, XLys, XTrp Maxamaid, XLys, XTrp Maxamum or Glutarex 1 are designed to provide amino acids other than lysine and tryptophan, to help prevent protein malnutrition. The entry of tryptophan into the brain is crucial in the proper synthesis of the neurotransmitter serotonin in the brain. One way to acutely cause depression, bulimia or anxiety in humans, in order to assess an individuals vulnerability to those disorders, is to supplement with a formula with all or most amino acids except tryptophan. Acute tryptophan depletion is a diagnostic procedure, not a treatment for GA1. The protein synthesis elicited by the amino acids leads circulating amino acids, including tryptophan, to be incorporated into proteins. Tryptophan is thus lowered in the brain as a result of the protein synthesis enhancement, causing circulating tryptophan to drop more than other amino acids. A relative excess of other large neutral amino acids may also compete with tryptophan for transport across the blood–brain barrier through the large neutral amino acid transporter 1. The consequence is acute tryptophan depletion in the brain and a consequent decrease in serotonin synthesis. 5-Hydroxytryptophan, a precursor of serotonin that is not metabolized to glutaryl-CoA, glutaric acid and secondary metabolites, could be used as an adjunct to selective tryptophan restriction, although it has risks.
However, the evidence in favour of selective tryptophan restriction remains insufficient and the consensus is evolving towards the restriction of lysine only. In the Amish community, where GA1 is overrepresented, patients with GA1 typically do not receive tryptophan-free formulas, either as the sole source of amino acids or as a supplement to protein restriction. Enhancement of precursor anabolic pathways
Lysine and hydroxylysine anabolic pathway enhancement
A possible way to prevent the build-up of metabolites is to limit lysine and hydroxylysine degradation, as lysine is one of the most abundant amino acids and tryptophan is one of the least abundant amino acids. Interaction of GCDH deficiency with vitamin C levels
Humans lack the enzyme L-gulonolactone oxidase, which is necessary for the synthesis of ascorbic acid (vitamin C), leaving them dependent on dietary sources of this vitamin. Vitamin C is a necessary cofactor for the utilization of lysine in collagen synthesis. Collagen, the most abundant protein in the human body, requires great amounts of lysine, the most abundant amino acid in proteins. Ascorbic acid, the main hydroxyl radical quencher, works as the cofactor providing the hydroxyl radical required for collagen cross-linking; lysine thus becomes hydroxylysine. GA1 worsens during stresses and catabolic episodes, such as fasts and infections. Endogenous catabolism of proteins could be an important route for glutaric acid production. It follows that collagen breakdown (and protein breakdown in general) should be prevented by all possible means. Ascorbic acid is used to prevent multiple organ failure and to lessen mortality and morbidity in intensive care units.