Economic policies to decrease obesity (would they be effective?)
Yes, sugar tax!

This is probably as controversial as tobacco tax was back in the days. If you walk through a supermarket, you will find that half of the food section is food full of sugar. Sugar is what makes you fat, not fat itself. It has been known for a while, at least since the professional sports were invented. Yet the lobby of the enormous sugar industry keeps regulators from labeling it as hazardous and taxing its use. If you are doing a research into obesity I highly recommend devoting two hours to watch That Sugar Film (2014).

I would also recommend taking a multidisciplinary approach and bringing some arguments from biochemical and nutrition fields.