We perform ab initio calculations of the optical absorption and electron energy-loss spectra of (m, 0) single-walled carbon nanotubes (with m ≠ 3n for m = 7-25) in the framework of a "sum over states" (SOS) treatment of the Kohn-Sham (KS) single-particle orbitals and energies (CRYSTAL program). This approach tested on hexagonal boron nitride enables to fully assign the interband transitions in the imaginary part of the dielectric constant, in terms of atomic orbitals. As these calculations could not take into account the local field effects (depolarization effects), which take place for perpendicular polarizations in 2D and 1D periodic systems, we apply a simple method based on the Clausius-Mossotti formula, relating the SOS and coupled-perturbed KS polarizability values. This approach reproduces the main features of the spectra of boron nitride (001) surface and carbon nanotubes.