Sometimes knowing the reason WHY we are required to do something helps us be more compliant with the rules.
In a normal stomach there is a pylorus valve that keeps the food inside the stomach as it begins digestion. When food is processed enough to be released into the intestine, the pylorus opens and lets a small amount of food through. More processing, pylorus opens again, more processing... etc. In a normal size stomach the process of releasing food into the small intestine can take 1 to 2 hours depending on the size of your meal and the texture of the food you ate. Remember that the size of a normal stomach is about the size of a football or slightly larger.
After RNY, we no longer have a pylorus valve, so we need to do the work of that "trapdoor" on our own. The best way to keep food in the pouch for a while is to eat dense, solid food that is not mixed with liquid. Think of your pouch as a funnel. If you add dense food to the top bowl of a funnel, the food will very slowly be pushed through the small funnel opening -- but as soon as you add water to that food, it becomes a soupy, mushy mess that'll go through the funnel opening very quickly.
So the reason we don't drink with meals is:
1. To keep the food in our pouch as long as possible to keep us feeling full for longer and also to allow food to mix with digestive enzymes from our saliva and begin breaking down for digestion.
2. We want to avoid forcing food through that funnel opening (the Stoma) before it's ready to go. If you force food through the Stoma by drinking with meals you will eventually stretch the Stoma opening. Stretching the Stoma is much worse than stretching the pouch and something we should be most worried about. If you stretch the Stoma to the same size opening as the pouch is, you essentially have a 20-foot long intestine that is acting as your new stomach instead of a tiny pouch and stoma opening. Guess how much you can eat when you have a 20-foot stomach?? :-)
The Stoma is usually about the diameter of a lady's index finger and the pouch is about the size of a golf ball or lime.
HTH
Pam