If you don’t know already, then let me make you aware of an interesting fact: the food that you eat isn’t just dropped from your mouth into your stomach and digested instantly; instead, the process of digestion normally takes more than 6-7 hours – or even longer.
However, when you have diarrhea (commonly referred to as ‘loose motion’ in some parts of the world), you must have noticed that the journey of food from the mouth to the exit of the body hardly takes any time at all. Why is that?
Before we can understand the mechanism of diarrhea, it helps if we take a quick look at how digestion normally takes place.
The journey of food through the body
When you first eat food, it’s broken down into smaller pieces as a result of the crushing action of the teeth. These pieces of food are then moistened by saliva, and the tongue pushes the food to the back of the mouth so that it can be swallowed. At that point, it travels through the esophagus (food pipe) and ends up in the stomach.
There, the food is processed in all sorts of ways with the help of enzymes and juices, before being passed on to the small intestine. It absorbs complex molecules, such as sugars, amino acids and fatty acids, and subsequently sends the undigested and unabsorbed food to the large intestine.
When the food finishes its travels through the digestive tract, what remains of your food heads to the rectum to await the final leg of its journey, i.e., the exit from the body. The large intestine comprises a cluster of nerves that react to pressure. Once the amount of stool in the intestine reaches critical mass, it sends a signal to the brain that you are ready to hit the restroom.
When you’re actually engaged in defecating, you consciously control the sphincter, and let the feces exit the body for good. (Note: This is a gross oversimplification of the digestion process; I recommend you read this article for a more detailed description of digestion.)
What happens during diarrhea?
All systems work perfectly under normal conditions, but when you eat something ‘bad’ (i.e., food poisoning), it ‘upsets’ (pun intended) your digestive system, and things go haywire.
When you eat something that causes a bacterial infection or inflammation of the gut, abnormal reactions occur within the stomach and a sophisticated suite of nerves (present in the gut) snap into action. The ‘threat’ signal is transmitted to the brain, which hits the ‘emergency’ button and orders the gut to flush everything out ASAP. To aid the flushing, water and other fluids mix with the ‘bad food’ and hasten the exit process of that food from within the body.
Here’s an analogy to understand the process better: suppose your bowels are like a train track and the food you ingest is like a set of cars running on it. The transit time from Station A (mouth) to Station B (anus) is normally a few hours. Everything works normally, as long as the set of trains you run on the track is good.
The moment you put a bad set of cars on the tracks, however, a string of events happen. The sensors installed along the track (nerves in the lining of the gut) detect that the set of cars is ‘alien’ or unidentified, and they send a distress signal to the supervisor (brain). The brain has two choices: it can send the cars violently back to Station A (the act of vomiting), but that would be painful and may cause damage to the track (inflammation). Or, it could send the set of bad cars barreling down to Station B.
To facilitate the swift execution of the second option (i.e., diarrhea), the brain floods the tunnels with water and other fluids. This floods the bad cars with water, making them somewhat thin, gooey and soggy, which greatly helps to chuck the bad cars out of the system in no time.
That’s why diarrhea travels so quickly through the body; sometimes, it’s even hard to distinguish between certain things that feel similar… before you’re committed.