  We show that there is no uniform polynomial isoperimetric function for
finitely presented subgroups of direct products of free groups, by producing a
sequence of subgroups $G_r\leq F_2^{(1)} \times \dots \times F_2^{(r)}$ of
direct products of 2-generated free groups with Dehn functions bounded below by
$n^{r}$. The groups $G_r$ are obtained from the examples of non-coabelian
subdirect products of free groups constructed by Bridson, Howie, Miller and
Short. As a consequence we obtain that residually free groups do not admit a
uniform polynomial isoperimetric function.
