Factors determining blood phenylalanine in the neonatal period.

The effect of age, sex, birth weight and area of birth on blood phenylalanine has been studied in a group of 41 795 infants. The mean phenylalanine level in low birth weight infants was higher than in those of normal birth weight from six to over 43 days of age. There was a trend of increasing blood phenylalanine with age in both birth weight groups. No consistent difference was observed between the sexes. In several of the age and birth weight groups, infants living in urban areas had significantly higher blood phenylalanine levels than those in rural areas.