I flippantly summarise my version of this as “Prevent dementia, drink Coca Cola.” My own story is that I was B12 deficient because of (at the end of a chain of causation) low gut motility.
I’d drunk Coca Cola like other people drink water for my teenage years and the caffeine in it was plenty to keep my gut motility up enough to not cause issues. Eventually I went off Coca Cola for all the usual sane health reasons (but never picked up coffee or some other caffeinated drink!), my gut bacteria spilled over into my small intestine (SIBO) and increased irritable bowel syndrome (IBS) symptoms I had (yep, I am one of those people who could report that cola makes my stomach feel better—now I finally know why, and I know why it usually doesn’t replicate! Most people don’t have SIBO). I unsuccessfully tried to change my diet around several times trying to figure out what the issue was, made it worse by being flexitarian or vegetarian for a few months (very little B12!), eventually was so light- and sound-sensitive due to degraded myelin sheaths on my nerves that I could barely sleep properly and became severely depressed and increasingly mentally challenged (thankfully not to the point of dysfunctionality), until I was begging my doctors to do something.
The doctors correctly figured out the B12 deficiency (although there was some luck in that as well), but I only came across the link to my IBS by accident. I was reading The End of Alzheimers to see what I could do to fix my (mild) neural damage, decided to try a low-carb diet, noticed my IBS disappeared, and for many, many months (well over a year) did not think anything more of it, other than being happy that I had a way to manage my IBS now.
...then I looked into why low-carb would even fix IBS, came across SIBO, noticed it made the observation that I should be fine with sushi rice (indeed, I am! That was an observation I’d made a long time ago already) but not basmati rice (indeed, this is bad!), and also that it can cause B12 deficiency.
Once I realised that SIBO is caused by low gut motility, I realised I’d done this to myself by no longer drinking colas, and laughed out loud.
(To be clear, I am sticking to the low-carb diet and do not drink excessive amounts of cola, and I still don’t drink coffee, but I’m now using the SIBO self-diagnosis to carefully explore which carbs I can ingest so I can avoid some of the unfortunate side-effects of a high-protein diet (which low-carb pretty much maps to), such as kidney stones. Ow, kidney stones are bad. Do not recommend!)
Hmm, from the literature I’ve consumed so far, I had the impression that SIBO isn’t actually that common, and it’s just one possible (and not particularly wide-spread) reason for irritable bowel syndrome. That said, evidence is mounting that SIBO was absolutely [my problem](https://www.lesswrong.com/posts/fFY2HeC9i2Tx8FEnK/luck-based-medicine-my-resentful-story-of-becoming-a-medical?commentId=FXit9XoHTjevw6goD), so I have no anecdata leg to stand on. (Personally I’m gradually experimenting with the carbs my gut is okay with it, since it has such a strong dislike for most of them (especially starches, for some reason).) Thanks for the tip with the elm powder!
(Edit: Erk, sorry, I can’t seem to make that link work. This is what I get for not commenting here often!)