A patient with phenylketonuria (PKU) is often placed on a diet with restricted intake of phenylalanine. The patient may be at risk for vitamin B12 deficiency if a source of vitamin B12 is not provided.
Risk factors for vitamin B12 deficiency:
(1) diet very low in animal proteins
(2) lack of vitamin B12 supplemental intake
A typical scenario is an adult on a relaxed diet, low intake of natural proteins and no vitamin supplements.
Complications:
(1) neuropsychiatric deterioration
(2) neurologic deterioration after nitrous oxide anesthesia
(3) megaloblastic anemia
Monitoring should include:
(1) serum vitamin B12 concentrations
(2) serum homocysteine (elevated in vitamin B12 deficiency)
(3) serum or urine methylmalonic acid (elevated in vitamin B12 deficiency)
Measurement of vitamin B12 concentrations alone may fail to detect a functional deficiency.
