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In page Phenylketonuria:

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Evidence supports dietary supplementation with large neutral amino acids (LNAAs).[2] The LNAAs (e.g. leu, tyr, trp, met, his, ile, val, thr) may compete with phe for specific carrier proteins that transport LNAAs across the intestinal mucosa into the blood and across the blood–brain barrier into the brain. Its use is limited in the US due to the cost but is available in most countries as part of a low protein / PHE diet to replace missing nutrients.[citation needed]