Well, that's what they'd have you believe - but look it up and you'll find that calcium proprionate is a fungicide ingredient of some bread and other manufactured foods. It stops them going mouldy.
I stated to investigate what caused my IBS after consuming a couple - or was it three? - hot cross buns last year, days - maybe even weeks after they should have grown legs and walked out of the bread bin. I stuck them in the oven for a couple of minutes, lavished butter on them and consumed them eagerly and gratefully.
The sense of them passing along the lower reaches of my alimentary canal was intense and I was able to track them most of the way or so it felt. I was only free of discomfort when they had left me entirely.
The penny didn't drop until some weeks later when a fruit loaf was presented to me. It was only after eating that and having a similar experience that I looked at the label. And so began my practice of avoiding calcium proprionate. I can tell you that I'm convinced that when it comes to the chemical being "well-tolerated", that it isn't by me!
Some bread contains it, some biscuits, some other manufactured foodstuffs. A lot of bread doesn't include it (so why is it necessary at all?!) and as a general rule (useful when presented with a buffet) French sticks are a safe bet.
Of course it makes sense. You have a gut full of lovely bacteria and into that you put a fungicide. Is it any surprise there are resulting problems?