Same exact thing happens to me when I am playing in long sessions. I make a point of staying away from the tables for 30 min if I have eaten anything that is really substantial or will make me feel lethargic.
I swear your body just has too much going on after eating a lot, and my play suffers.
I always will snack on stuff during a session to keep the blood sugar up, but will not eat any large meals unless I can take a serious break or are done.
Problem with this is when I am playing for a 48 hour session... means I do not eat anything substantive for two days... And then eat EVERYTHING in sight.