[Boost-users] Is Boost API 32Bit only?

I try to build Boost on MSYS2-64Bit which includes 64-Bit headers of windows. Boost is calling a 32-Bit variant of windows API even when Boost is configured to build 64-Bit. There is a 64Bit pre build variant of Boost on sourceforge. That seems to run only in visual studio? Does it use a 32 Bit API as well? Is there any reason for this design? I personaly prefer any variant of GCC over visual studio due to time consuming experiences with visual studio. Kind regards Andrej