I am not sure how it works but I remember reading insulin and dopamin balance of the body were related. Dopamin is usually highly involved in brain chemistry together with serotonin and if dopamin is affected serotonin is affected as well. From what I have been reading it seems that dopamin and serotonin is working antagonistically, i.e. in a way like insulin and glucagon. So, if you are having less carbs and hence, in a way controlling your insulin usage in your body, it could be that this dopamin-serotonin balance is getting affected, albeit in a good way, as well. As far as I am aware researchers have already identified a link between depression and insulin levels or glucose intolerance I should say. They have gone further than that as well. They have looked at the mechanism involved in insulin production of the body and the necessary vitamins and minerals and their defficiencies, which I think was indirectly related to dopamin production of the body as well. If I manage to find again the medical journal that I was reading these from I will put a link here for you, if you are interested.