  We study the impact of the nuclear dependence of R=\sigma_L/\sigma_T on the
extraction of the F_2^A/F_2^D and F_1^A/F_1^D structure function ratios from
the data on the \sigma^A/\sigma^D cross section ratios. Guided by indications
of the nuclear dependence of R from the world data, we examine selected sets of
EMC, BCDMS, NMC and SLAC data and find that F_1^A/F_1^D < \sigma^A/\sigma^D
\leq F_2^A/F_2^D. In particular, we observe that the nuclear enhancement
(antishadowing) for F_1^A/F_1^D in the interval 0.1 < x < 0.3 becomes
significantly reduced or even disappears, which indicates that antishadowing is
dominated by the longitudinal structure function F_L. We also argue that
precise measurements of nuclear modifications of R and F_L^A have the potential
to constrain the poorly known gluon distribution in nuclei over a wide range of
x.
