I recently answered a question on fitness.stackexchange.com asking about how the timing of drinking water would affect health or weight gain during a meal. My original answer was drawn from my earlier post here Don’t Drink And Digest, but while that has recommendations a desire was expressed to see “proof”. So here it is:
First, the amount of fluid in the stomach is positively correlated with the rate of gastric emptying. It so happens that 30 minutes is an approximate inflection point in the gastric emptying of both a solid and liquid meal, per the chart below (same source).
I’ll assert that drinking water will render the net contents of the stomach more fluid, moving the gastric emptying rate from the blue curve towards the red curve.
This seems to confirm the conclusion that avoiding liquid during the meal will help stave off the next bout of hunger because a more solid meal will take longer to empty. The 15 minutes before also seems to fit, as roughly half of liquid consumed 15 minutes beforehand will have been emptied before the meal starts.
If one further considers the dynamics, drinking water immediately before a meal will help fill up the stomach and reduce immediate hunger but will cause that meal to be digested faster, so I hypothesize that you will be hungrier sooner with this approach.