Powerade contains high fructose syrup, and fructose is not easy on the stomach WHILE running, akin to eating an apple. A 4.5% glucose solution, with some flavor, should work just fine. 400ml should absorb in about 15 min., the flavor and the glucose just make it more appealing to drink, helping the runner to get the entire dose down - crucial early in a marathon and in a hot race like Berlin.
I think J.T. Daniels would back me up on this one.