“…The diet is undoubtedly difficult, particularly since phenylalanine is an essential amino acid. Extremely worrying side effects have been observed including anaemia (Royston and Parry, 1962), retardation of growth (Fisch, Gravem, and Feinberg, 1966), severe protein deficiency (Pitt, 1967), and even death (Dodge et al, 1959). There is evidence too that early infantile undernutrition can be detrimental to intellectual development (Davison and Dobbing, 1966).…”