The normal anion gap metabolic acidosis associated with ureteric diversion results from the resorption of urinary chloride by the gut mucosa, and the excretion of bicarbonate in exchange.
When the ureter empties into the ileum, metabolic acidosis is seen in 80% of patients.
This is the consequence of using a piece of gut as a container and conduit for urine, a task it was never meant to perform. There are a few ways of doing this. One way is to use a piece of gut as a conduit from the shortened ureters to the skin, where it opens. A “continent” ureteric diversion is where a bladder-surrogate is formed from a piece of bowel (where the urinary sphincters are preserved; thus the urine sits in this bowel-bladder for some hours, exchanging ions, waiting to make its way into the toilet).
Urine delivered to the bowel is rich in ammonium. This ammonium substitutes for potassium in a co-transporter. Action of the co-transporter facilitates the resorption of 2 chloride ions for every resorbed ammonium and sodium ion. This results in a net gain of chloride for the body fluids, decreasing the strong ion difference. In parallel, potassium is left behind in the urine.
Meanwhile, urinary chloride is exchanged for bicarbonate, increasing the strong ion difference of the urine.
As always, the loss of a fluid with a large strong ion difference results in the decrease of total body fluid strong ion difference, resulting in a metabolic acidosis.
This used to be a major problem in the days of ureterosigmoidostomy, when the ureters emptied into a portion of the sigmoid colon. The urine had plenty of time to mingle with the colonic absorptive epithelium. An ileal conduit, by contrast, does not allow for a long urine dwell-time- and therefore does not permit excessive ion exchange to take place. The incidence of metabolic acidosis due to ileal conduit is rather low (no more than 20%) as compared with the older procedures.
A masterful thesis on this topic has been written by WS McDougal in 1992. It is present online in full text, with helpful underlined sections courtesy of a Dr Kevin Falun (or Felun), MD.