I for one no I am stressed, because when this happens I am in the toilet pretty regularly.
This is the purview of the gastrointestinal (GI) tract, your esophagus, stomach, small intestines and large intestines (also known as the colon or the bowel).
When it comes to your GI tract, there’s no such thing as a free lunch.
You’ve just finished some feast, eaten a great roast turkey, somebody’s grandma’s famous mashed potatoes and gravy, a bare minimum of vegetables to give a semblance of healthiness, and oh, why not another drumstick and some corn on the cob, a slice or two of pie for dessert, ad nauseam.
You expect your gut to magically convert all that into a filtrate of nutrients in your bloodstream? It takes energy, huge amounts of it. Muscular work.
Your stomach not only breaks down food chemically, it does so mechanically as well. It undergoes systolic contractions: the muscle walls contract violently on one side of your stomach, and hunks of food are flung against the far wall, breaking them down in a cauldron of acids and enzymes.
Your small intestines do a snake dance of peristalsis (directional contraction), contracting the muscular walls at the top end in order to squeeze the food downstream in time for the next stretch of muscle to contract.
After that, your bowels do the same, and you’re destined for the bathroom soon. Circular muscles called sphincters located at the beginning and end of each organ open and close, serving as locks to make sure that things don’t move to the next level in the system until the previous stage of digestion is complete, a process no less complicated than shuttling ships through the locks of the Panama Canal.
At your mouth, stomach, and small intestines, water has to be poured into the system to keep everything in solution, to make sure that the sweet potato pie, or what’s left of it, doesn’t turn into a dry plug. By this time, the action has moved to your large intestines, which have to extract the water and return it to your bloodstream so that you don’t inadvertently excrete all that fluid and desiccate like a prune.
All this takes energy, and we haven’t even considered jaw fatigue. All told, your run-of- the-mill mammals, including us, expend 10 to 20 percent of their energy on digestion.
So back to our by-now-familiar day to day stress: You can’t waste energy on your stomach walls doing a rumba.
There isn’t time to get any nutritional benefits from digestion.
Digestion is quickly shut down during stress. The first step in that process. If you get nervous, you stop secreting saliva and your mouth gets dry.
Your stomach grinds to a halt, contractions stop, enzymes and digestive acids are no longer secreted, your small intestines stop peristalsis, nothing is absorbed.
The rest of your body even knows that the digestive tract has been shut down, as we saw two chapters ago, blood flow to your stomach and gut is decreased so that the blood-borne oxygen and glucose can be delivered elsewhere, where they’re needed.
The parasympathetic nervous system, perfect for all that calm, vegetative physiology, normally mediates the actions of digestion.
Along comes stress: turn off the parasympathetic, turn on the sympathetic, and forget about digestion. End of stress; switch gears again, and the digestive process resumes.
Most of digestion is a strategy to get your mouth, stomach, bile ducts, and so forth to work together to break your food down into its constituent parts by the time it reaches the small intestines.
The small intestines, in turn, are responsible for absorbing nutrients out of this mess and delivering them to the bloodstream.
As is apparent to most of us, not much of what we eat is actually nutritious, and a large percentage of what we consume is left over after the small intestines pick through it.
In the large intestines, the leftovers are converted to feces and eventually exit stage left.
Yet again when stress kicks in, all that stuff sitting in your large intestines, from which the nutritive potential has already been absorbed, is just dead weight.
The biology of this is quite well understood. The sympathetic nervous system is responsible. At the same time that it is sending a signal to your stomach to stop its contractions and to your small intestine to stop peristalsis, your sympathetic nervous system is actually stimulating muscular movement in your large intestine.
But why, to add insult to injury, is it so frequently diarrhea when you are truly stressed?
Relatively large amounts of water are needed for digestion, to keep your food in solution as you break it down so that it will be easy to absorb into the circulation when digestion is done. As noted, a job of the large intestine is to get that water back, and that’s why your bowels have to be so long, the leftovers slowly inch their way through the large intestine, starting as a soupy gruel and ending up, ideally, as reasonably dry stool.
Disaster strikes, stress hits, increase that large intestinal motility, and everything gets pushed through too fast for the water to be absorbed optimally.
Diarrhea, simple as that.
This information was taken from the book Sapolsky, Robert M.. Why Zebras Don't Get Ulcers