  In this letter we complete a previously introduced formalism to study the
gauge-invariant metric fluctuations from a noncompact Kaluza-Klein theory of
gravity, to study the evolution of the early universe. The evolution of both,
metric and inflaton field fluctuations are reciprocally related. We obtain that
<\delta\rho/\rho_b$ depends on the coupling of $\Phi$ with $\delta\phi$ and the
spectral index of its spectrum is: $0.9483 < n_1 < 1$.
