  In this work, we show that a class of metric-affine gravities can be reduced
to a Riemann-Cartan one. The reduction is based on the cancelation of the
nonmetricity against the symmetric components of the spin connection. A
heuristic proof, in the Einstein-Cartan formalism, is performed in the special
case of diagonal unitary tangent metric tensor. The result is that the
nonmetric degrees of freedom decouple from the geometry. Thus, from the point
of view of isometries on the tangent manifold, the equivalence might be viewed
as an isometry transition from the affine group to the Lorentz group,
$A(d,\mathbb{R})\longmapsto SO(d)$. Furthermore, in this transition, depending
on the form of the starting action, the nonmetricity degrees might present a
dynamical matter field character, with no geometric interpretation in the
Riemann-Cartan geometry.
