Twenty-four healthy young ( aged 21 1 y , body mass index 21.8 0.5 kg/m ( 2 ) ) and 25 older ( aged 75 1 y , body mass index 25.4 0.6 kg/m ( 2 ) ) men received a primed continuous L - [ ring - ( 2 ) H5 ] - phenylalanine and L - [ ring-3 ,5 - ( 2 ) H2 ] - tyrosine infusion and ingested 20 g intrinsically L - [ 1 - ( 13 ) C ] - phenylalanine-labeled protein with ( Pro + CHO ) or without ( Pro ) 60 g carbohydrate .