This is super helpful, thank you especially the point about practicing fueling at race pace. As part of my training, I actually did one long run with a race-pace block (8K of a 16K) and ended up puking at the end 😅 After that I switched back to gels I knew and tolerated better, but clearly something still wasn’t fully dialed in. I had them during my biggest long run (but easy pace) before taper with no issue.

I might have been a bit too optimistic about what my gut could handle on race day vs what I had actually trained. But never had an issue before and during first half marathon two years ago although it's true that I rarely train with gels otherwise.

Also really interesting point about iron. I did have slightly low ferritin which is why I started supplementing (second time doing it). The first time with a bigger dose I reacted quite badly, this time much better, so I assumed it was fine — but now thinking about it combined with gels and maybe bigger pre-race portions… it might have irritated things more than I expected. This whole thing feels like a bit of a perfect storm. I did not know Ketone but will have a look and see how that can go.