Our expert says:
It's a hard problem to pinpoint. It's not all that uncommon and there are a few obvious things that I can suggest. The first is what you are eating or drinking. It may be that you are drinking regularly when you run, and after a number of hours, the stomach is full of fluid, you are continuing to run, perhaps the concentration of what you are drinking is too high, these are all factors to consider. The same goes for eating, I'm not sure what you are eating or even if you are, this is all very speculative.
The other possibility is 'motion sickness'. This may sound strange, but I know that there are instances where doctors prescribe motion sickness pills for people prior to an ultramarathon, to prevent this type of nausea. That might be worth a shot for you - it either works or it doesn't but you have nothing to lose.
We also know that blood flow to the stomach is reduced during exercise and so anything that you eat before these long runs might not be digested properly causing your problems. I think that you will have to do some investigating and trial and error experimentation to see if there is anything in what you do before a long run that is different from normal - do you eat slightly more, do you drink something different, do you go at different times of the day etc? and then see what happens if you change these.
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