I think you’re making a big mistake not following this diet and taking a therapeutic dose of b12 to meet your needs. There’s always stress but not eating this way adds more stress to your body at a time when you need healing.
Maybe, but maybe not.
To be honest with only a few months of the McDougall programme I was feeling great. My doctor was fair enough to say, that these few months couldn't have caused the low B12 alone. This would simply be impossible in that short time. She also didn't force me to eat meat again and didn't told me a vegan diet would be bad at all. My problem is that my body doesn't want to absorb the B12 it gets. So I have to try to bring it in all forms to my body.
A second doctor told me that going vegan from day to another was stress for my body. Now I should try to fix my levels and then slowly get back to this way of eating if I want to, so that my body can get used to it over time.
I am no doctor....I am no expert...but this sounds logical to me.
I was a big meat eater before with only a few vegetables in my diet. For me it's not unreasonable that switching my diet caused stress to my body. On the one hand I cannot say if I followed the McDougall programme right. I was feeling great but I don't know if I got everything my body needed because I usually ate the same almost daily. So no big healthy variety like some others here. On the other hand, I need to be honest to myself. Eating healthy won't solve all my body issues. I actually drink way to much coffee, I smoke, I usually don't get enough sleep, I don't exercise....
I hope you get what I am trying to say. If I would stop my vegan diet now and then die the following week, then it wouldn't be just because I was missing some veggies. There is simply so much more I need to change in my life to become healthy. And for my diet this is no Goodbye, it's just a little break.