Biochemical and Behavioral Effects of a Commercial “Low Phenylalanine” Diet in Growing Rats

Summary Liquid diets made from LoPhe powder and water and containing additional L-Phe in amounts of 0.2, 0.4, 0.8, and 5.0% were given to mother rats immediately after delivery for the following 3 weeks and then to the offspring for the next 5 weeks. The result was a faulty development of liver PheH and brain GS. An overall impairment of growth and behavior tests done at age 45 and 52 days suggested a lag in maturation of approximately 10 days. LoPhe without additional L-Phe could be fed to rats only after day 21. When given to mother rats from the time of delivery, offspring did not survive. Although highly deficient in L-Phe and producing a complete arrest of growth, this diet resulted in normal brain GS of offspring; however, liver PheH was greatly decreased after a temporary rise. The 5.0% L-Phe produced neither phenylalaninemia nor an adaptation of PheH, but decreased brain GS slowly and consistently over the entire time of the study. The Phe and Tyr serum levels at no time correlated with the dietary intake of these two amino acids. Our data show, at least for the rat, that serum Phe levels in growing animals do not reflect adequacy of intake of Phe.