  The concept of the nuclear enhancement factor has been used since the
beginning of gamma-ray astronomy. It provides a simple and convenient way to
account for the contribution of nuclei (A>1) in cosmic rays (CRs) and in the
interstellar medium (ISM) to the diffuse gamma-ray emission. An accurate
treatment of the dominant emission process, such as hadronic interactions of
CRs with the ISM, enables one to study CR acceleration processes, CR
propagation in the ISM, and provides a reliable background model for searches
of new phenomena. The Fermi Large Area Telescope (Fermi-LAT) launched in 2008
provides excellent quality data in a wide energy range 30 MeV - 1 TeV where the
diffuse emission accounts for the majority of photons. Exploiting its data to
the fullest requires a new study of the processes of gamma-ray production in
hadronic interactions. In this paper we point out that several commonly used
studies of the nuclear enhancement factor miss to account for the spectrally
averaged energy loss fraction which ensures that the energy fraction
transferred to photons is averaged properly with the spectra of CR species. We
present a new calculation of the spectrally averaged energy loss fraction and
the nuclear enhancement factor using the QGSJET-II-04 and EPOS-LHC interaction
models.
