Studies of the Schwinger-Dyson equation for the gluon propagator in the pure gauge sector have shown it to be strongly enhanced at low momenta. This enhancement in turn generates a severe suppression of the fermion propagator for massless quarks at the same low momenta, killing the pole at $p=0$. Here we compute how loops of such quarks affect the gluon propagator and find they reduce the gluon's infrared enhancement by an amount out of proportion to their relative insignificance in the perturbative short-distance regime, indicating their unquenchable importance.
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M. R. Pennington (1988) studied this question.