Could look at how a given company alters its use of language over different mediums e.g. via social networking sites, to its official website, to posters, to TV adverts and how the language changes between healthy and unhealthy advertisements.

For example:

Healthy brand - Change for Life:
                           * Posters
                           * TV advert
                           * Official website

Unhealthy brand - McDonald's:
                           * Posters
                           * TV adverts
                           * Official website