How does the BRAT diet help when you have the runs?

Bread (toast), rice, applesauce (but not apples), tea: standard operating procedure when you have a not-particularly-serious but one or two day bout of diarrhea.
What are the physiological mechanisms by which these foods help, singly or in combination?

First off, the BRAT diet is bananas, rice, applesauce, and toast. The one that includes tea is the BRATT diet. There’s also the BRAT(T)Y diet that includes yoghurt.

These all have one thing in common: the foods are easy to digest. Diarrhea is a natural process for eliminating contaminants and irritants in the stool. If you add more irritation, then the body will stay in “diarrhea mode.” Plus, the water concentration of diarrhea itself is a problem: if you aren’t properly hydrated, the GI tract will dry out, and thus be easier to irritate.

An analogy might be how it’s not a good idea to repeatedly pick a scab.

Actually the first question is if it does. It doesn’t.

Current guidelines actually do not promote the BRAT diet even though it is commonly still advised in real life. Certainly it can be easily misused.

While in the initial vomiting phase of gastroenteritis anything more than those foods are going to be messy and potentially slow down gastric emptying which may make vomiting worse. The priority then is getting in fluids that have some semblance of sufficient carbohydrates and electrolytes. The first solid foods should be bland boring and easy to clean in case of upchuck. After that it is actually better to segue into age appropriate real foods sooner than later as the BRAT diet for more than a day or so has inadequate nutrition to support recovery.

Here’s one review if you are interested.

IAMNA doctor, of course, so please bear (bare?) with me. I thought that the pectin in applesauce–hence Kaopectin, and the reason apple pies “set up” so easily–somehow solidified my guts. Other aspects running through my pea brain: toast = charcoal, rice = “soaking up stuff.”