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Table 7 Low blood phenylalanine levels and action that should be taken

From: PKU dietary handbook to accompany PKU guidelines

Cause of low blood phenylalanine levels Action
Inadequate intake of natural protein • Ensure all prescribed intake of natural protein/phenylalanine is eaten.
• Check understanding of exchange system/phenylalanine content of foods.
• Re-educate as necessary.
Anabolic phase, following an intercurrent infection • Ensure all prescribed intake of natural protein is eaten.
• Repeat blood phenylalanine level, and if is still low, consider an increase of natural protein by approx. 0.5-1 g protein or 25 to 50 mg/day phenylalanine but monitor blood phenylalanine levels carefully.
Rapid growth spurt such as puberty • Increase natural protein by 0.5-1 g/day protein or phenylalanine by 25 to 50 mg/day if blood phenylalanine levels are consistently below target range.
• Increase by a further 0.5 to 1 g/day protein or phenylalanine by 25 to 50 mg/day for every 3 consecutive blood phenylalanine levels below target range.
Excess intake of infant protein substitute or overnight consumption of infant protein substitute. • Infants may take phenylalanine- free infant protein substitute overnight, which may lower morning blood concentrations. Consider reducing overnight intake if appropriate.
No obvious reason • Consider increasing natural protein by approx. 0.5-1 g protein or phenylalanine by 25 to 50 mg/day.
• Monitor blood phenylalanine levels carefully.
• It is good practice to re-check blood phenylalanine levels before any further increase in natural protein /phenylalanine intake.
  1. Table adapted from MacDonald A, White F. Amino acid disorders. In Shaw V, editor. Clinical Paediatric Dietetics: Chichester: Wiley Blackwell; 2015. p. 433–434