  The energy dependence of the virtual photoabsorption cross section in deep
inelastic scattering (DIS) at low $x = Q^2/W^2 < 0.1$ may be described in terms
of the saturation scale that in our approach depends on the energy,
$\Lambda^2_{sat} = \Lambda^2_{sat} (W^2)$. We briefly summarize our recent
findings that allow us to predict the exponent $C_2$ of $\Lambda^2_{sat} (W^2)
\sim (W^2)^{C_2}$ in agreement with the previous result obtained from fitting
the experimental data. The exponent $C_2$ depending on the relative magnitude
of the longitudinal and transverse contribution to the structure function,
direct measurements of the longitudinal part are urgently needed.
