Nice to know this is a known problem; there doesn't seem to be a definitive answer, but the experience of others is really helpful. It's a problem that seems to be getting worse for me. Stage 1 is no longer fancying any sort of food (although liquids, often milk shakes may be acceptable for longer). Stage 2 is throwing up
My stomach contents at this stage may extend back a few meals, or may be negligible. I then find I can ride a long time in comfort on a completely empty stomach (I don't seem to be carbohydrate-dependent), although on a 600 last June I experienced Stage 3 (?) feeling really cold (possibly a consequence of having no food to burn?) and shivery on a not particularly cool evening.
I'd say causes of the syndrome are multi-factorial. For me sleep deprivation may have a role ... when all I really want to do is sleep, my digestive system seems to switch off.
The kind of food I eat doesn't seem to have a role, but I'm normally careful about my diet.
Pushing even
slightly harder than usual is another negative factor. I've experienced stage 1 after spending 100 km or so chatting along with somebody who is riding maybe 0.5 kph faster than my normal pace. The difference is so slight it's barely perceptible, but does tell over a distance.
In discussion with my GP, we considered that blood flowing to feed leg muscles may deprive the digestive system of the blood it requires and switching over can take time.
On some rides none of the above happens; I eat what I like mostly at controls, have loads of energy to spare, and can happily share with the pace of others. The trouble is I can't predict how things are going to turn out. On the recent Fenlands 600 I got to stage 1 (not fancying food) at Sleaford, tried to fix the problem by riding easily, but that didn't work so, for safety's sake, I abandoned at Gainsborough rather than risk stage 2 (throwing up) striking me down on a damp night in the remote reaches of the Fens with no escape easily available. I caught the train from Gainsborough, and felt completely fine again after an hours rest and a pannier-conditioned cheese sandwich.
To deal with the problem I'm trying to ride at my own pace (even at the expense of being anti-social), not worry too much about taking on food as fuel, but eat regularly nevertheless, and try to start events well-rested.