It depends on length of your fast. After a short fast you can easily eat anything to break your fast. After a long fast you really have to be careful in breaking your fast.
In Russia they use Kefir to break fast. I have never done that, so can't tell you the effects of it. But it seems to me that protein and probiotics would give you diarrhea.
All fasting gurus, i.e. Dr. Shelton and alike only used citrus juice to break a longish fast.