My feeble understanding is some poorly understood combination of exercise intensity, heat, and osmolality will do two things.
1. Shut down the sphincter muscle to the gut (sloshy belly time)
2. Fluid will be drawn from your blood to dilute the mess in your stomach
So, I try not to eat anywhere near a climb and if I really chow down, I ride slower for a little while.
I look at body weight before and after a ride/event to assess adequacy of fluid intake. I never ever want to gain weight. If the event is like 400K or more, I am ok with up to 2% weight loss because most of that is glycogen depletion related; however, it is usually more like 1%.
The only time I had GI problems was PBP 2019 but I started the ride not well, I think it was the rabbit tartare I ate. But it was gone by the time I got to Brest