Diarrhoea can result in a metabolic acidosis by causing a disproportionate loss of sodium, and thus decreasing the strong ion difference
The change in strong ion difference due to intestinal electrolyte losses
However, precisely what sort of acid-base disturbance will occur depends entirely on what is being lost, and how. Which is frequently difficult to establish. The content of the gastrointestinal fluid losses is likely to vary from patient to patient, depend strongly on the diet, and be dependent on whether there are any attempts at reclamation (i.e. if the colon is still making an effort to retain electrolytes).
Pancreatic secretions, a major source of loss for fluid with a large strong ion difference (which the bowel doesn’t get a chance to reclaim) are discussed elsewhere. Similarly, a small bowel fistula with a large output (eg. a high output ileostomy) deserves its own mention.
Broad generalizations can be made: the stool transit time is important, as is the volume. If the stool does not spend very long in the colon, reabsorption cannot occur, and electrolyte loss is inevitable.
In short, diarrhoea tends to cause a hyponatremic hypokalemic metabolic acidosis, …most of the time.
A general rule is that a metabolic acidosis due to loss of lower gastrointestinal contents will only occur if the volume of loss is massive, and the fluid being lost must have a larger than average strong ion difference.
The loss of large amounts of fluid places the kidneys at an additional disadvantage, by decreasing the glomerular filtration rate. As a result, the rate at which the acid-base disorder can be corrected decreases.
So what is the electrolyte content of diarrhoea?
I guess one might say it varies. No two stools are alike. A good article which touches on this issue reports on the the approximate electrolyte content of diarrhoea; another article produces a table which compares the electrolyte content of different kinds of gastrointestinal fluids. Again, in my hands this respectable table has become converted into a disgraceful Gamblegram.
So; the more sodium and less chloride you lose in the stool, the more severe the metabolic acidosis. Inflammatory diarrhoea (eg. Crohns disease) has the least effect, and the diarrhoea of cholera has the greatest effect. A part of this effect can be attributed to the chloride-bicarbonate exchanger in the ileum and colon.
An increase in chloride secretion (caused by a cholera toxin stimulated CTFR transporter) leads to an increase in chloride-bicarbonate exchange, which leads to a chloride-poor, bicarbonate-rich stool.
Now, this is an oversimplified diagram of the ionic movements- the reality is quite a lot more complicated.
The major point is that the movement of potassium is also important (it ends up being quite concentrated in the stool, with various authors quoting figures of 20-80mmol/L).
As a strong cation, it increases the strong ion difference of colonic contents. Therefore, the loss of vast volumes of diarrhoea is the loss of fluid with a large strong ion difference, which tends to decrease the strong ion difference of the remaining body fluids.
Interestingly, the change in the strong ion difference can be offset by an oral rehydration solution containing both sodium and glucose. A sodium-glucose co-transporter facilitates the absorption of sodium, thereby increasing the strong ion difference.