In medium energy heavy ion collisions, fragments are formed in an expanding system. The isospin composition of the produced fragments may reflect the symmetry energy of such low-density nuclear matter. In fact, the simulations by antisymmetrized molecular dynamics (AMD) show that the fragment isospin composition is basically consistent with the statistical expectations even in dynamically evolving system. Isoscaling is satisfied by the AMD results. The width of the fragment isotope distribution can be explained by the ratio of the symmetry energy to the temperature if the symmetry energy at a reduced density is assumed to be relevant. This assumption is justified by studying the dependence on the density-dependent symmetry energy. The symmetry energy extracted from the AMD results is almost independent of the fragment size, which suggests that the fragment isospin composition is governed by the symmetry energy of low-density uniform matter rather than the symmetry energies for isolated nuclei.