If you're eating meat, you should question how your bowels look and smell, how much meat is stuck inside there undigested and foul.
The epithelium covering the intestines is like the outer layer of skin - it's constantly being replaced, total turnover happens in less than a week. It's impossible for anything to get "stuck" for longer than that unless it's physically too large to pass (and we have teeth to deal with that, last time I checked). Even if you're eating superglue, the cells just die after a few days and anything attached to them goes along with them.
Whatever comes out when you're "cleansing" is just that - the dead cells of your own digestive tract and the bacteria living there, and there are probably a lot more of them dying than normal since you're torturing your body with starvation and external mechanical irritation in places that were never meant to be exposed to anything but half-digested food. It's no wonder it's foul, but if there is any difference based on diet, it's probably the bacteria - it makes some sense that the gut flora would be slightly different, since different bacteria help with digesting different foods.
People have been doing a LOT of dumb shit for thousands of years, and they've only recently stopped doing it not because all who practice sticking garden hoses up their ass have all been burned, but because we finally start to actually understand how our bodies work, and can throw the old misconceptions out. And you keep claiming it's "natural", but it's certainly not any kind of innate behavior a human let to grow without any outside influence would exhibit on his own, AND it requires a certain (even if small) amount of technology to do, so just how is it natural? It's something learned, like everything else we do.