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The genetic disease phenylketonuria causes devastating brain damage if the person is exposed to protein in food. Its damage can be so severe that newborns are routinely screened for the genetic defect that causes it.

The effects of PKU can be prevented with a diet that avoids all protein, but it’s incredibly challenging to follow. Even pasta has some protein, as do all meat, dairy, fish, nuts, and beans — and kids who stick to a protein-free diet and rely on a protein-substitute formula often end up with stunted growth.

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