In a parallel-group design , 32 healthy older men ( aged 71 1 y ) received a primed continuous infusion of L - [ ring - ( 2 ) H5 ] - phenylalanine , L - [ ring-3 ,5 - ( 2 ) H2 ] - tyrosine , and L - [ 1 - ( 13 ) C ] - leucine , and ingested 25 g intrinsically L - [ 1 - ( 13 ) C ] - phenylalanine and L - [ 1 - ( 13 ) C ] - leucine labeled casein dissolved in bovine milk serum ( Cas + Serum ) or water ( Cas ) .