Diarrhea happens when there is an imbalance in water secretion and absorption within the body. This excess water causes the stool to pass through quicker than normal.
My fifth-grade textbook said that digestion takes more than 24 hours to complete. My 10-year-old brain was blown away by how long it took for poop to form after I’d eaten. The food is slowly pushed down through our gut, breaking it into its building blocks so that each cell can finally benefit from this tasty food.
However, when you have diarrhea (commonly referred to as ‘loose motion’ in some parts of the world), you have likely noticed that the journey of food from the mouth to the exit of the body hardly takes any time at all. Why is that?
During diarrhea, the gut cannot absorb nutrients and water
Diarrhea feels like the food takes a “fast pass” out of the gut. Instead of being systematically broken down and absorbed, which happens in normal digestion, the food gets rushed through the digestive tract and pushed out the other end.
Because of this, the gut can barely disassemble the food. Enzymes in the stomach and the small intestine dismantle larger parts of food, like starch and proteins, into smaller chunks like glucose and other such monosaccharides and amino acids. After this, cells lining the intestines must catch these nutrients, meaning that the nutrients must remain near the cells for a long enough time.
Water also needs to be absorbed, not just the water in the food, but also the water that the body secretes to aid digestion. The diarrhea fast pass prevents this water from being absorbed. (Note: This is a gross oversimplification of the digestion process; I recommend that you read this article for a more detailed description of digestion.)
This is why we experience such weird and watery poop, and why we need to go to the bathroom every few hours when we have diarrhea.
Types of Diarrhea and their causes
When lactose-intolerant people drink milk, they get diarrhea. When we eat unhygienic food, we get diarrhea. In some rare cases, when there is a more serious problem, we have prolonged bouts of diarrhea. These seem like widely different reasons for the same outcome, but as it turns out, something as commonplace as diarrhea occurs for multiple reasons.
The lactose-intolerant type of diarrhea is called osmotic diarrhea. The primary sugar in milk, lactose, associates or holds water molecules. In someone who doesn’t have lactose intolerance, the enzyme lactase will break lactose apart into glucose and galactose, which the cells can then absorb. This allows the water associated with lactose to similarly be absorbed.
However, in lactose-intolerant individuals, the lactose is never broken down. Cells in the small intestine cannot absorb lactose whole, so the lactose retains the water, which makes your poop watery.
Certain other salts and chemicals, such as antacids and salts like Epsom salt (MgSO4), are also responsible for osmotic diarrhea. The good news is that such diarrhea lasts only as long as the food remains in the system. Once the food has left the system and the person has fasted for a while, the problem usually resolves itself.
Diarrhea due to bacterial infection
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 the body.
Pathogens, like Salmonella and Giardia, can cause inflammation and can destroy the protective barriers of the gut. The gut secretes mucus, proteins and various other factors into the gut to protect it. This increases the volume of poop and, together with damaged cells not being able to absorb water and nutrients, we get a bad case of diarrhea.
The pathogen Vibrio cholera that causes cholera works in a slightly different manner. It highjacks the intestinal cell’s machinery and forces it to release large amounts of water. All that water results in large amounts of diarrhea. This is called secretory diarrhea, since the gut “secretes” water. Several laxatives also work according to this property.
Diarrhea occurs because of deranged motility of the gut
Digested food in our gut doesn’t just sprout legs and begin to move ahead in the digestive tract. Smooth muscles that line the digestive tract contract and relax to push the food forward into the next stage of digestion. This muscular propulsion is coordinated by the gut’s very own nervous system, the enteric nervous system, and the brain.
Deranged motility is when the muscle propulsion system of the gut is thrown into disarray. We still don’t know how this disarray works, nor the exact mechanisms that cause it. The enteric nervous system and its interactions with the gut, controlled through hormones and other chemical messengers, are quite complex.
The other diarrhea varieties might be a result of disturbed absorption and secretion, but there might also be a component of deranged motility. In more serious cases, such as diabetes mellitus and IBS, deranged motility seems to be central to the gut’s unwanted behavior.
Considering all this, if you try to estimate what the exact cause of your loose motion is, you might never find an answer. In some cases, there might be one simple reason that fits into a classification, but in many instances, there might be several factors contributing to the uncomfortable discomfort. Importantly, if such an ongoing condition is not treated properly and swiftly, it could become a life-threatening symptom!