Changes in intelligence of 22 diet-treated phenylketonuric children, and of their 6 untreated and 44 unaffected siblings, were studied for an average of 27 months. On both the first and the last intelligence testings, treated children, including those treated from early infancy, had significantly lower scores than their unaffected siblings. However, those treated before 2 years were more intelligent than later-treated children. During the study treated children developed more rapidly than they had before treatment but at a significantly slower rate than that of their unaffected siblings. Age at the beginning of treatment was significantly related to initial IQ, but not to rate of development during treatment. The untreated phenylketonuric group tended to show a decline in intelligence.
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Berman et al. (1966) studied this question.