Tannins are one of the fundamental components of wine, and red wines in particular. They add structure to the wine giving wine that mouth puckering, drying quality. That puckering quality is the astringent action of the tannins on the lining of your mouth. It literally draws the skin tightly together.
Now imagine what happens in your stomach when you enjoy a glass of wine before dinner. Those same tannins work their magic on the lining of the stomach, drawing it a little tighter and perhaps (simply my personal experience), starting you on your way to fullness well before you’ve eaten your first bite.
Photo courtesy niseag03 vis Flickr/CC