Imagine yourself strolling along a perfectly French rue in the heart of Paris. You’ve just had lunch and you’re stuffed to the gills with French goodness. You couldn’t possibly eat another bite.
You catch the scent of something baking and suddenly you’re hungry again. You follow your nose, end up in a bakery, and the next thing you know, you’ve downed half a baguette, perhaps with a bit of cheese.
I’m sure you’ve done this before. Maybe not in Paris, but you’ve certainly experienced the feeling of being full from a meal, only to catch a scent of something baking or perhaps a lasagna fresh out of the oven and change into your stretchy pants so you can eat again.
Turns out, the sudden room that appears in your stomach post-Thanksgiving meal when the dessert pies show up is simply a routine part of what happens when you eat.
Quick quiz for you:
Where does the digestion process start?
- Stomach
- Mouth
- Brain
- Liver
Go ahead, make a guess.
You know what they say when taking a standardized test… The answer is always “C.”
That’s right. Digestion starts with your brain – it’s organized and controlled by the nervous system.
Basically, digestion starts at the scent (or sight!) of food. The brain sends signals that tell the rest of the body “food is on the way!” Those triggers tell our mouths to start salivating, the stomach to start secreting digestive enzymes… You get the idea.
Digestion is controlled by the autonomic nervous system (ANS) which regulates internal organs. It has two parts – the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). The SNS (the “flight or fight” system) tends to shut down digestion and appetite while the PNS (the “rest and relax” system) tends to regulate digestion and movement through the GI tract.
Once the food is in our mouths, you know what happens – you chew it into smaller pieces. A fun fact about chewing? It stimulates pleasurable neurotransmitters which is one reason we may love eating so much – and in my opinion, why we miss chewing when we opt for a “juice cleanse.”
As we’re chewing, the taste buds help – you guessed it – taste food and the salivary glands make saliva. This is where we find digestive enzymes like amylase which begins the digestion of starch and lipase which begins the digestion of fat.
After the food is chewed, it becomes a “bolus” which is passed to the pharynx, a fancy name for the throat. The epiglottis – think of this as a “lid” – closes when we swallow which keeps food from entering the trachea, sending it down the esophagus. While food can’t get down the trachea, liquid sometimes can – that’s why you may hear someone say water “went down the wrong pipe.”
The esophagus carries food from the mouth to the stomach. Gravity does most of the work, but wavelike muscular contractions called peristalsis handles the rest. At the end of the esophagus is the lower esophageal sphincter (LES) which regulates how quickly the food enters stomach. It also keeps food and stomach juices from splashing back up into the esophagus. When that does happen, you know it as reflux.
In the stomach food is churned and mixed it into a liquid called chyme. The pyloric sphincter at the end of the stomach regulates the passage of that chyme into the small intestine. A lot goes on in the stomach, including the secretion of gastric juice, all of which aid in the breaking down and processing of our food.
Ready for a fun fact?
When acid secretion is at its highest during digestion, the stomach has a pH nearing 1, about the same as battery acid. Whoa. This helps kill most pathogens and further breaks down foods.
When chyme begins to enter the small intestine, it happens very slowly. It takes the stomach one to four hours to empty, depending on what we’ve eaten. Carbs empty first, then protein, then fats and fibers. Not surprisingly, liquids move faster than solids. That’s why an athlete in need of a quick pre-workout boost might opt for a liquid meal high in carbs and quick-acting protein while someone who is trying to lose weight might opt for a diet higher in (healthy) fat because it will keep them feeling fuller, longer.
Seriously – how cool is the body?
Things continue to move slowly in the small intestines. It takes four to eight hours for food to work its way through. The small intestines is where most of the nutrients we eat are absorbed.
While in the part of the small intestine known as the duodenum, juices made by the pancreas are secreted that are charged with neutralizing the acidic chyme and further breaking down food. This is where the gallbladder releases bile to emulsify fat which makes it easier to absorb. Once passed through the whole small intestine, food heads to the large intestine.
Oh, but hey, don’t forget the liver!
The liver is pretty cool – it’s basically a built-in detox center. No need for those “detoxes” you read about. Most of our nutrients are screened in the liver. It will also filter toxins, drugs, and our own hormones before allowing them into general circulation. It does a whole lot more too, such as storying and releasing glucose, but that’s for another day.
The pancreas also plays a hand in digestion. It releases pancreatic juices into the duodenum at the same time that bile is released. There are a lot of digestive proteins here, as well as the hormones that balances blood glucose level.
See how it all works together?
Food spends the most time in the large intestine, hanging around for 12 to 14 more hours before it is excreted as waste. While most nutrients are absorbed in the small intestine, the large intestine can absorb some as well.
While digestion takes place in the digestive system, it is controlled in part by the endocrine system. In its simplest terms, the endocrine system is a chemical messaging system that detects changes in the body and sends out hormones (messengers) to tell the body how to respond. Hormones might tell the body to start the digestive process, signal the pancreas and bile ducts to get to work, or tell the bran “I’m full, stop eating.”
And of course, we have the aforementioned nervous system, the master controller of the entire thing. The Central Nervous System (CNS) controls energy balance, appetite and food-seeking behavior, as well as our response to sensory input such as the smell of food. The Peripheral Nervous System (PNS) controls our movements, meaning it helps us pick up and eat our food. We already discussed the ANS and how its two branches play into things, and there’s the enteric nervous system (ENS) which is the digestive system’s own localized nervous system, thought of as a second brain located in your gut.
Like the endocrine system, the nervous system uses neurotransmitters to send information from the gut to the brain and vice versa. The cool thing about neurotransmitters is that they often have different jobs depending on where they are in the body. When involved in the digestive system, they might shut down digestion, tell it to “rest and digest,” or help improve blood flow to the gut to in turn help transport and absorb nutrients.
A bit of a nerdy post?
To be sure.
But I also think it’s important.
Food is fuel. Food is what helps us perform at our best and feel our best. Food can even heal us, or at the very least set us up to fight off health issues down the road. Knowing what happens in your body when you take a bite of food – any food – is important to understanding just how imperative it is to fuel your body properly.
Got questions about digestion? Leave them in the comments or send me an email!
(And be on the lookout for more information on my nutrition coaching program, coming soon!)
