  The behavior of the structure function at high energies (high densities) is
directly associated to the gluon distribution. In this paper we analyze the
asymptotic behavior of the structure function at very high densities
considering a leading and a higher twist relation between these two quantities.
We verify that (a) if the leading twist relation between the structure function
and the gluon distribution is used, we recover the black disc limit, and (b) a
softer behavior is predicted if a higher twist relation between $F_2$ and $xG$
is considered. While the first behavior is well-established by the current high
density approaches, the higher twist behavior is a new result. In both cases,
the $F_2$ structure function unitarizes and the Froissart boundary is not
violated in the asymptotic regime of high density QCD.
