The Polyakov loop has been used repeatedly as an order parameter in the
deconfinement phase transition in QCD. We argue that, in the confined phase,
its expectation value can be represented in terms of hadronic states, similarly
to the hadron resonance gas model for the pressure. Specifically, L(T) \approx
1/2\sum_\alpha g_\alpha \,e^(-\Delta_\alpha/T), where g_\alpha are the
degeneracies and \Delta_\alpha are the masses of hadrons with exactly one heavy
quark (the mass of the heavy quark itself being subtracted). We show that this
approximate sum rule gives a fair description of available lattice data with
N_f=2+1 for temperatures in the range 150MeV<T<190MeV with conventional meson
and baryon states from two different models. For temperatures below 150MeV
different lattice results disagree. One set of data can be described if exotic
hadrons are present in the QCD spectrum while other sets do not require such
states.Comment: 5 pages, 4 figures. Error in normalization corrected. Excited states
included. Substantially revise
