As a natural extension of this study we will perform a double-masked, randomized, crossover, placebo-controlled study for adolescent patients. Longer treatment duration will be tested: a 24 week treatment with either the 9-cis β-carotene-rich capsules or placebo followed by an additional a 24 week treatment with the other capsules and a wash-out period of 24 weeks between this two type of treatments. We predict that with this study design a maximal/increased therapeutic effect as well as efficient washout will be achieved.