I am not aware of any previous study, but I would think phenylalanine would rise in the serum following exercise.

The physical load will not change the level of phenylalanine nor that phenylalaninmonooxygenase in phenylketonuria. An aerobic exercise that is not long lasting, can to some extent, reduce the level of some stress radicals, such as the superoxide oxygen anion. However there is always a risk of protein breakdown and increased level of phenylalanine.

see paper Article Postexercise protein metabolism in older and younger men fol...

Thanks for all your contributions, they are helpful.

Hi Ben, there's this work from Luc van Loon too... https://www.physiology.org/doi/full/10.1152/ajpendo.00543.2003?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%3dpubmed

Cheers, Javi. Very helpful.