  Gamma-ray Bursts (GRBs) prompt emission spectra are often fitted with the
empirical ''Band" function, namely two power laws smoothly connected. The
typical slope of the low energy (sub-MeV) power law is $\alpha_{B}\simeq -1$.
In a small fraction of long GRBs this power law splits into two components such
that the spectrum presents, in addition to the typical $\sim$ MeV $\nu F_{\nu}$
peak, a break at the order of a few keV or hundreds keV. The typical power law
slopes below and above the break are -0.6 and -1.5 respectively. If the break
is a common feature, the value of $\alpha_{B}$ could be an ''average'' of the
spectral slopes below and above the break in GRBs fitted with Band function. We
analyze the spectra of 27 (9) bright long (short) GRBs detected by the Fermi
satellite finding a low energy break between 80 keV and 280 keV in 12 long
GRBs, but in none of the short events. Through spectral simulations we show
that if the break is moved closer (farther) to the peak energy a relatively
harder (softer) $\alpha_{B}$ is found by fitting the simulated spectra with the
Band function. The hard average slope $\alpha_{B}\simeq-0.38$ found in short
GRBs suggests that the break is close to the peak energy. We show that for 15
long GRBs best fitted by the Band function only, the break could be present,
but it is not identifiable in the Fermi/GBM spectrum, because either at low
energies, close to the detector limit for relatively soft
$\alpha_{B}\lesssim-1$, or in the proximity of the energy peak for relatively
hard $\alpha_{B}\gtrsim-1$. A spectrum with two breaks could be typical of GRB
prompt emission, though hard to identify with current detectors. Instrumental
design such that conceived for the THESEUS space mission, extending from 0.3
keV to several MeV and featuring a larger effective area with respect to
Fermi/GBM, can reveal a larger fraction of GRBs with a spectral energy break.
