[Zope] starting zope with su instead of -u

This feels like a huge newbie question, but I'm starting to lose hair and I was surprised not to find anything helpful in the archives. I want to try out DirectoryStorage. I gather you're not supposed to use the -u option and obviously I don't want to run as root. So I use something like su - -c START_COMMAND zope But then I get that message about how I can't run zope as root and should read the SETUID doc. But I'm not running zope as root and the SETUID doc is distinctly unhelpful with respect to using anything other than -u to drop privileges. Is there something basic I'm missing? I'm running 2.6.1 on Gentoo, if it matters. Thanks in advance, Dylan