By Mark Szabo
Ever wonder what happens to your food after you eat. Well keep reading and find out!
Why do we eat?
There are many answers to this question. Sometimes we eat to foster family and community. Sometimes we eat for pleasure. Sometimes we eat for pain. Sometimes we eat because we’re bored. Ultimately though, as far as our body is concerned, we eat to live; to survive and thrive.
The mechanics and the process of digestion comes from the Latin word digerere – to separate, divide, or arrange. This is a good word to describe how our bodies:
- Separate and break down food into simple molecules;
- Sort and divide those molecules based on the type of molecule and its role in the body;
- Arrange and transport those molecules through the body to feed our cells; and then
- Excrete waste products that our bodies can’t use or don’t need.
What does our food contain?
You’re probably familiar with the macronutrients: protein, carbohydrates, and fat. Then there’s the micronutrients: vitamins and minerals our bodies need. Depending on how and what you eat, there’s also phytonutrients from plants, myconutrients from fungi like mushrooms and yeast, and zoonutrients from foods that were formerly living animals. Water is also present in many foods in some quantity. And sometimes we consume other energy-containing substances like alcohol.
Fiber is another substance that we eat, but we can’t absorb as-is. Our gut microbiome likes to munch on fiber and it delivers some benefits to the body. However, a lot of it simply passes through us and is excreted by our bowels.
Whether the food we eat was formerly plant, animal, or fungi, it is chemically complex. The process of digestion takes this complexity and simplifies it into microscopically. These small pieces can then be safely transported throughout the body to feed and fuel our cells.
Typically, that means proteins are broken down into amino acids. Carbohydrates are broken down into glucose. Fats are broken down into fatty acids.
What is our gastrointestinal tract? It’s a muscular canal comprised of everything from mouth to anus which measures about 26 feet long in adults. The main function of the G.I. tract is digestion. But it does more than that. It also protects us against pathogens, detoxifies potentially harmful substances, secretes hormones and helps to process others, helps to regulate the immune system, and even contains a “second brain” more commonly knows as the enteric nervous system, or ENS. Instead of being centrally located within the cranium like our primary brain, the ENS is spread out through the entire GI tract. In addition to regulating digestion, these nerves are also involved in social engagement and emotion. Unlike the brain in our head, which gives us concrete thoughts, the signals coming from the ENS are mostly instinctive and subconscious. We tend to “feel” things through the ENS. This is where we get a “gut feeling,” and the phrase, “trust your gut.”
This is also the home of the gut microbiome, a complex collective of microorganisms, like bacteria, that live in symbiosis with us and help us to process some of the things we eat.
Where does digestion begin? Most people would probably say the stomach, a few savvy people might say the mouth, but the real answer is the brain. When the body needs food, you feel hunger. Hunger triggers your brain to get your body up to go find food. As we start thinking about food, or even when we see pictures of food, our brain tells the rest of our body to get ready to eat. Our mouth starts to salivate, our stomach starts to produce digestive enzymes, and our thoughts become ever more focused on eating. When you think about how many food ads you see during the course of a day, it’s no wonder our appetite and eating patterns can get out-of-whack. In our 21st century lives, it seems we’re constantly being told it’s time to eat.
Where do cravings come from? Remember that microbiome we mentioned earlier? There are different groups of microorganisms that help us digest different things. When we eat the things that they like to eat, the population of those colonies grows. There’s a finite amount of room in our gut, so if one population grows, it means another one starts to shrink. Over eons of evolution, these colonies have developed a working relationship with the ENS, and are able to communicate with it via chemical signals. So when you get a craving for a double-quarter pounder with cheese, that craving is actually coming from your microbiome. Once a person decides that they want to live a healthier life and starts trying to shy away from McDonalds and instead opt for home-cooked meals that nourish the body, the microbiome colony that likes to eat those cheeseburgers starts to shrink as the nutritious-loving colony starts to grow. As the cheeseburger-loving colony is shrinking, it sends signals to the ENS that says something like, “WE’RE DYING! WE NEED MORE CHEESEBURGERS!!” So the ENS sends a message to the brain that makes you want a mouth-watering cheeseburger.
A person who previously lived on a steady diet of fast food that makes a decision to get healthy, must be successful at ignoring the trillions of microbiotic cells that are screaming at them. Eventually, the colonies in the gut will stabilize. The now diminished cheeseburger-loving population is crowded out by the population that likes to eat healthy meals. Instead of calling for more cheeseburgers, the gut inhabitants start asking for vegetables, lean proteins, and healthy, slow-digesting carbohydrates. So when this person decides, oh what the heck, I’ll have a cheeseburger, they often find themselves with an upset stomach afterwards. You may have experienced this yourself. Like, if I were to eat a doughnut right now, I’m sure it would taste great going down, but 20 minutes later I’ll be holding my gut going, “oh, why did I eat that?”
The process of digestion is controlled by the automatic nervous system, or ANS, which is the branch of the nervous system over which we do not have conscious control. The ANS has two sub-branches: the sympathetic nervous system (SNS), more commonly called the “fight or flight” system, and the parasympathetic nervous system (PNS), also called the “rest, digest, and repair” system.
Our GI tract reacts to both of these systems. SNS activation typically shuts down the digestive process and curbs our appetite. We stop salivating and food no longer becomes our primary thought. The GI tract itself stops moving food through, and we may experience things ranging from the “butterflies in the stomach” feeling, to nausea, vomiting, and maybe even diarrhea. PNS activation occurs when we’re resting and usually acts to regulate digestion smoothly and calmly. In cases of extreme fear, once the SNS response is exhausted, the PNS might activate causing the contents of our bowels to rapidly exit from our anus. Yes, you can actually be “scared shitless.” The takeaway from this is that stress can have a negative effect on our digestive process, so it’s good to find ways to alleviate stress in your life.
The nose also plays a role in digestion, albeit not directly. Smelling our food is an important part of digestion. It tells our GI tract that food is coming, and it also helps us decide if we should eat something or not. Once we have decided that it smells good and we should eat it, retronasal olfaction happens as we chew and swallow and the odors from which travel up the back of the throat and into the nasal passages. This is an important part of eating. It helps to tell the brain that we’re getting enough and the right kind of nutrients. It’s also why food isn’t appetizing when you have a cold and can’t smell.
The mouth is where active digestion begins. Our omnivore teeth allow us to chew a wide range of different foods. Those teeth are covered by enamel, the hardest and most chemically stable substance the body can produce.
Jaw muscles are among the strongest in the body, relative to their size, producing a biting force of about 126 pounds per square inch. In comparison, an alligator can produce nearly 3,000 PSI with its bite.
As we chew, it stimulates pleasurable neurotransmitters. This is one reason we enjoy eating (and it also helps to explain why some people chew things when they’re anxious.)
Tongues are covered in papillae, tiny little bumps that help the tongue to move food around in the mouth as we chew. Most of these papillae also contain taste buds. If you’re in your late 40’s or beyond, you might remember learning in school that you have specific taste buds in specific regions on your tongue. This isn’t actually true. All parts of your tongue can taste all flavors, as well as detect the presence of certain nutrients, like carbs and fat.
Mouths contain about 1,000 salivary glands. Most of them are very tiny, with a few larger ones in choice locations like under the tongue and along the bottom jaw line. Saliva is about 95% water with the remaining 5% comprised of mucus, enzymes, glycoproteins, and antimicrobial chemicals that help to protect us against pathogens. In addition to keeping our mouth moist, saliva also starts the digestion of starches and fats via enzymes like Amylase and Lipase, respectively.
We mentioned the gut microbiome earlier. You also have an oral microbiome living in your mouth. There are about 300 known species of beneficial bacteria living in our mouths, and researchers believe there may be as many as 700. You actually have many microbiomes in and on your body. In fact, there are more cells that belong to the microbiomes that call you home, than there are cells that make up you.
Our tongue, teeth, jaws, and mouthparts work in a coordinated dance to chew our food into a pasty ball known as a bolus. When we decide to swallow, the bolus is passed by the tongue to the back of the mouth where the epiglottis closes off the entrance to the trachea and the uvula closes off the nasopharynx to prevent food from entering our airway to the lungs and our nasal passages. Sometimes though, things go awry, usually when we’re not being mindful while we’re eating. If you’ve ever bitten your cheek, lips, or tongue, had something “go down the wrong pipe” or snorted milk out your nose when a friend told a joke at lunch, you know what I mean.
Once swallowed, the bolus travels down a mucus membrane-lined muscular tube knows as the esophagus. From here on, we have no conscious control over the process. Gravity does much of the work here, with rhythmic muscular contractions called peristalsis handling the rest. At the end of the esophagus we find the Lower Esophageal Sphincter, or LES (sometimes called the cardiac sphincter), which closes off the entrance to the stomach. The esophagus may be shorter than you think; it terminates right around your solar plexus. If you suffer from GERD (gastroesophageal reflux disease, also called heartburn) it means your LES is open more than it should be, allowing the powerful stomach acids to enter the base of your esophagus, which literally burns the esophageal tissue. If this condition becomes chronic, it can lead to damage to the esophagus which can become cancerous. (My Uncle Bill died from this.)
After passing through the LES food enters into the stomach, which sits right around the base of your breastbone and slightly to the left. The stomach secretes HCL that’s about as strong as car battery acid. People often wonder how that acid doesn’t burn away the lining of the stomach. The answer is, it does. The outermost layer of the gastric mucosa (stomach lining) is constantly being replaced after having been dissolved by the acid. This layer has no pain receptors, so we’re oblivious to the destruction. (Stomach ulcers occur when the gastric mucosa is burned down to the submucosa, which does have pain nerves.) The stomach also has stretch receptors that tell us when it’s filling up. Mindful eating helps us tune into this sense so we know when to stop.
Food that has mixed with the stomach juices becomes a mushy liquid we call chyme. At the far end of the stomach is the pyloric sphincter, which regulates entry into the small intestine. This typically happens very slowly, and it can take 1-4 hours for the stomach to completely empty, depending on what we’ve eaten. Carbs empty first, then proteins, with fats and fiber moving the slowest. Liquids empty more quickly than solids, and smaller particles empty faster than larger ones.
The small intestine is the longest portion of the GI tract and is where most nutrient absorption takes place. It is broken into three sections: the duodenum, the jejunum, and the ileum, in that order. One of the first things that happens in the duodenum is the release of pancreatic juices which contain bicarbonate to neutralize the stomach acid so it doesn’t burn the intestinal lining, along with enzymes to further break down the food particles into nutrient molecules the body can use. The gallbladder releases bile produced by the liver to emulsify fats, making them easier to absorb.
Once food has been broken down into individual molecules, these molecules are passed through the intestinal cells and enter the bloodstream via the hepatic portal vein, which leads directly to the liver. The liver filters and detoxifies things we’ve eaten or drunk so they don’t cause harm to the rest of the body.
At the end of the ileum section of the small intestine is the sphincter-like structure known as the ileocecal valve, which regulates entry to the large intestine, or colon. By this point, most absorption has already taken place, so the large intestine absorbs leftover water, salts, vitamin K, short-chain fatty acids, and some gasses. What remains is a semi-solid waste product we commonly call feces, comprised of about 1/3 dead bacteria, 1/3 inorganic material that we could not digest, and the final third of protein, sloughed-off cells, cellulose, any remaining digestive juices, and bile pigments. It takes about 12-25 hours to travel through the colon before reaching the rectum and the anus, where it is excreted outside of the body; thus concluding our trip through the G.I. tract 😊