This one is an assumption and does not reflect reality. I explained it in an article and linked credible source. Based on my long term experience bowel movement is not a problem at all in fact just opposite is the truth. I am glad you raised it as it might be an issue during the transition period especially when only lean meat without fat is consumed. Here is an article reflecting my experience and research on fiber https://medium.com/illumination/heres-why-plant-fiber-is-not-a-one-size-fits-all-solution-8ea19ac6d4b1