you can as long as it is one meal replaced only and a balanced diet for the other meals, plus some kind of multivitamin. i did it while i was breastfeeding both my kids (they were a little older though and not having many feeds during the day). you cant do the tony ferguson plan as such because it is really strict with pretty much all meals replaced by shakes. in this case the body will react to the diet and produce the toxins which would transfer into your breast milk. the ingredients in the shakes are fine. i did betty baxter with my first child and they have a specific plan for breastfeeders. i even got a letter from my doctor saying it was fine, if only one meal replaced per day.