In ancient art, banqueters always recline on their left side — perhaps to aid digestion.
The élite of most advanced ancient Mediterranean societies partook of banquets lying down. We know this from iconographic records dating back to the seventh century BC. Some scholars assume that the custom was widespread in the originally nomadic tribes that finally settled in Syria or Iran, befitting their modest tent furnishings. But the social prestige that soon became associated with reclining at a banquet might owe more to the preciousness of the beds of the rich, as suggested by the outpouring of the biblical prophet Amos (around 750 BC), against those used by the Samarians: “Lying upon beds of ivory, stretched comfortably on their couches, they eat lambs taken from the flock.” (Bible, Amos 6:4–7).
So it's not surprising that one of the oldest images of a reclining banquet is a royal one: the famous bas-relief of King Assurbanipal of Assyria lying on his left side while his wife sits on the throne (pictured). This form of aristocratic banquet was widespread in the seventh century BC in Greece — the poet Archilochus wrote, “leaning on my lance I drink (wine)” — and among the Etruscans, who traded with the Greeks. It came to span the entire Mediterranean Greek and Roman civilizations.
Art historians have often noted that banqueters almost always appear to be reclining on their left sides. The usual explanation is that lying on the left leaves the right hand free to hold the dining vessels. But in funereal art there is good documentation of presumptive left-handed banqueters also reclining to the left. Jean-Marie Dentzer in his book Le motif due banquet couché dans le Proche-Orient et le monde Grec du VIIe au IVe siècle avant J.-C. (Ecole Francais, Rome, 1982) has compiled an extensive inventory of the banquet couché between the seventh and fourth centuries BC. Of the more than 700 illustrations, including at least a dozen banqueters holding pots in their left hand, not one is lying on their right side.
One explanation could lie in the anatomy of the stomach and in the digestive mechanism. The stomach has an irregular shape that curves upon itself. Its rounded base is turned to the left. There are two openings: one at the top where food enters from the oesophagus and one at the base, the pyloric orifice, from which part-digested food exits.
Eating lying down increases abdominal pressure and thus promotes gastro-oesophageal reflux, in which the acid stomach contents are forced back up the oesophagus, causing the unpleasant sensation known as heartburn. When lying on the left, the chewed food has room to expand because the curvature of the stomach is enhanced in that position. The lesser curvature on the right side of the stomach gives little space for food to resist the increased abdominal pressure and so lying on the right will soon cause reflux.
For the ancient Mediterranean civilizations, the evening meal lasted hours, and involved a lot of eating and drinking. Lying on the left would not just have reduced the risk of reflux, it would also have provided space for the large amounts of food the revellers were required to eat.