I don't have any literature that indicates one way is better than the other. I am getting good results. Most of the time the mix is the main part of my breakfast, either by itself, or with some fresh fruit or juice. When I travel I may have some cereal as well. I have also used it as a lunch, along with some raw carrots and red grapes. In other words, empty stomach or not, both seem to be working.
Remember, this is my experience. That is all I can relate. I am not a doctor, so take this only as my opinion.