I think it's based mostly on enzymatic production. Also, how fast you eat the food & wash it down can play a role. Chewing has an impact on this because of increased surface area of the food & the enzymes that are released in the mouth. I've heard people estimate that you could digest up to around 10,000 calories a day, but I think they pulled this # out of their ass. You could increase the amount of calories you can digest by taking digestive enzymes and probiotics. I would guess 3,000 calories on average per meal. But that's just a guess. It would vary by age, size, GI health, etc. Also, the different foods will play a role. I think people have a tendency to have an upset stomach when they gorge themselves, so that probably means something.