Assume a standard Brownian motion W = (W t ) t∈ [0,1] , a Borel function f : R → R such that f (W 1 ) ∈ L 2 , and the standard Gaussian measure on the real line. We characterize that f belongs to the Besov space B 2,q ( ) := (L 2 ( ), D 1,2 ( )) ,q , obtained via the real interpolation method, by the behavior ofi=0 is a deterministic time net and P X : L 2 → L 2 the orthogonal projection onto a subspace of 'discrete'stochastic integrals x 0 + n i=1 v i−1 (X t i −X t i−1 ) with X being the Brownian motion or the geometric Brownian motion. By using Hermite polynomial expansions the problem is reduced to a deterministic one. The approximation numbers a X (f (X 1 ); ) can be used to describe the L 2 -error in discrete time simulations of the martingale generated by f (W 1 ) and (in stochastic finance) to describe the minimal quadratic hedging error of certain discretely adjusted portfolios.