The one participant I know of was told to chug a lot of Gatorade the day before, and a little bit the day of (IIRC). The main restriction on the Gatorade (aside from quantity) was which colors were acceptable. Apparently some colors make it hard for surgeons to differentiate blood from residual bowel content.

Although he had the surgery circa 2014 and the stated purpose was to study the need to avoid opioids, not to reduce hospital stay. But like the 2006 study someone cited earlier, it was colon surgery.