Since overweight in toddler age has become an increasing problem in the last years it is an extensive and important topic. To counteract this, new diets are constantly being launched. But how effective are they really? To answer this question we present a system modeling approach for simulating the impact of an arbitrary diet. More precisely we design a simple model for the different body weight states and their transitions, model it in COPASI and do some simulations to discuss the impact of an arbitrary diet.