Inclusion Criteria:  at critical status with APACHE II score >15 (with or without using ventilator); with the need of staying in ICU >48 hours; fasting plasma phenylalanine concentration >112 uM; and age >20 years old  Exclusion Criteria:  comorbid disorders other than the main cause for ICU admission that might also compromise survival within three months, such as cancer at terminal status; expected life span< 3 days; and undergoing hemodialysis.