Prevent emerge from downgrading packages

I have a lot of packages (>10) installed with ~amd64 keyword and I want to keep my base system stable. But emerge -uD world will downgrade all unstable package.
I want solution for emerging only upgrades not masking old versions every time.

There was an -U key in old emerge, I wonder why they don't keep it.

From serverfault Andrew

• You didn't say what you want to see when time passes and new versions appear. Suppose that a new version of the ~amd64 package appears - do you want to upgrade it (i.e. follow the new development) or just sit on what you already have (i.e., work around a bug in stable)?

  If you want to upgrade your ~amd64 packages automatically to the latest unstable version, please add a line to your package.keywords file:

  foo-bar/baz ~amd64
  

  Otherwise, just mask all old versions using package.mask

  <foo-bar/baz-1.2.3
  

  The second approach will create warnings about unavailable packages, but will not attempt to downgrade anything.

  From AEP