This is somewhat, what happened to me. I found I lost all energy, and my strength was about half what it was.

We all have unique biomes in our gut, and they're tailored to us. If you lose it, eg something kills all your gut bacteria, you will be very sick. The only cure is to have a transplant from a relative. Literally fecal matter is taken from a donar, pureed, and shoved up your wahzoo.

https://en.wikipedia.org/wiki/Fecal_microbiota_transplant

My point? Your and my gut bacteria may need meat, and it could be why some cannot do the vegan thing. If your gut bacteria isn't happy, you aren't. And you can't just swap that out.