Eli5: what happens in the digestive track the moment you put food in your mouth?

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CW: references to eating disorders

I’m health class in high school we had a unit on eating disorders and one thing I remembered learning was that people with bulimia tend to be “normal” or “over” weight (as compared to those with anorexia) because food begins being absorbed as it’s in your mouth so even if you binge and purge you can still gain weight. I get that you can’t throw up *everything* but I don’t understand why purging food before it gets completely digested by the stomach and stuff doesn’t get rid of the calories?

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There are two matters to address here.

The first is that your body can indeed absorb a small amount of nutrition directly from your mouth. Enzymes in your saliva start breaking down starches the moment food hits your mouth, and some nutrients (mostly simple sugars and some vitamins, as well as many drugs) will dissolve in it and can be directly absorbed through the thin skin in your mouth.

The second is that your health class seems to have misrepresented bulimia, possibly dangerously. The amount of nutrition you absorb through your mouth is negligible, not nearly sufficient to maintain a healthy weight, let alone an unhealthy one. Even if you spent all day swishing sugar syrup in your mouth, it wouldn’t be enough.

Some people with bulimia are not underweight because they do not purge every meal, and because purges are not always complete. It typically takes about 30-90 minutes for food to pass through your stomach into your intestine; many meals are longer than that, so a hypothetical bulimic might get some food into their small intestine for nutrient absorption before they vomit.

**PLEASE NOTE** Even if someone is not underweight, that does not mean that they are healthy! Bulimics can have serious nutrient deficiencies, and the constant vomiting can be damaging to your throat, mouth, and especially teeth!

That’s not correct. Yes, certain things like simple sugars and alcohol can be absorbed in the mouth. However, this is at best a tiny amount. This is not the reason why (some) people with bulimia have normal or excess weight.

The causes of eating disorders are complex, and no two people are the same. A person with bulimia doesn’t necessarily binge and purge all the time. Rather, this may come in episodes, and outside those episodes the person may eat relatively normally. If the person is overweight, it may be because of what they eat in between episodes of binging and purging (simply eating “too much” is not considered binging).

In addition, it is true that purging may vary in its “effectiveness”. It all depends on the person’s specific binging-and-purging habits. If they don’t always make themselves vomit, or if the purging is delayed or “incomplete”, then the binge-eating itself may cause weight gain.

In any case, I think the main thing to note is that people with bulimia don’t typically purge all the time, after every meal. Usually it’s only after binge-eating. But again, eating disorders aren’t black-and-white, and a person may have characteristics of more than one (e.g. bulimia as well as anorexia).