Root subsystems of loop extensions

We completely classify the real root subsystems of root systems of loop algebras of Kac-Moody Lie algebras. This classification involves new notions of "admissible subgroups" of the coweight lattice of a root system $\Psi$, and "scaling functions" on $\Psi$. Our results generalise and simplify earlier work on subsystems of real affine root systems.