It's actually both. It keeps the digested food in your GI track longer so that you feel full almost all of the time. On top of that it affects your brain chemistry in a way that you just don't think about food. I took my first dose around 5 PM last night. I had some mild nausea and a mild headache as a side effect. I will start to take my dose before I go to bed in the future so I don't even experience those side effects but I wanted to see what the side effects, if any, would be with my first dose. It was nothing that made me uncomfortable or anything like that. A couple of Tylenol and I was fine.
I feel fine this morning though. I do feel full but it's supposed to really kick in on days 2-3 so that will be tomorrow and Sunday. I planned it like this on purpose since the weekends are the hardest and I wanted the most benefit/effect on those days.