Digestive system

Our digestive system consists of our mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum and anus. It ingests and then breaks down and distributes the food and liquid we put into our body and then excretes the waste products. From the outset, let’s get one thing straight. The food we eat BECOMES what generates and fuels our cells. That old phrase ‘you are what you eat’ is absolutely true.

Let’s understand WHY we eat before we go any further. Like most actions in our body, it starts with hormone signaling. When you get hungry and your stomach is empty, the walls of the stomach relax, and a hormone called ‘ghrelin’, which is made in your small intestine, is released from your stomach into the bloodstream. When it circulates up into the brain, it binds to receptors that trigger specific neurons to move your body into action to find and eat food. There are other, secondary hormones that play a role in hunger regulation and satiation. These are called leptin, GLP-1s, insulin and cortisol. They are important secondary signals to your brain that other systems in your body (your fat cells, your liver and pancreas) that ‘we need to eat’.

Let’s take this piece by piece. Your stomach is empty, it’s released ghrelin into your bloodstream, your brain and nervous system pick up that signal, and start to look for food. You see an apple on your counter and the visual system in your brain lights up and sends a cascade of signals to the motor portion of your brain to get that food and put it in your mouth. As you grab the apple, your salivary glands release saliva in anticipation of that first bite, in order to help you break it down and get it into your stomach.

When you bite the apple, your mouth starts the first mechanical step of digestion, chewing & grinding so it can start to be broken down to fuel your body. Your tongue is hard at work shaping that food into a ball of sorts (called a bolus), so that when you swallow, it goes down in 1 piece. When it moves that bolus to the back of your throat, your swallow reflex kicks on and it automatically gets shuttled down your pharynx. Your epiglottis (a flap of cartilage) closes off your airway so you don’t choke. Your soft palate seals off your nasal cavity so food doesn’t come out your nose, and the muscles in your pharynx squeeze that ball of apple bits downwards towards your stomach so it can start to break it down and extract its nutrients.

Let’s pause a moment - What happens if there’s a cookie sitting next to the apple? Your brain basically turns into a toddler at a birthday party: logic goes quiet & dopamine goes wild.

Here’s what happens:

Your eyes hit the cookie and your visual reward circuitry lights up because your brain instantly remembers every delicious cookie you’ve ever eaten and how good it was. Dopamine (the ‘happiness hormone’ fires before you even touch it.

Meanwhile, the prefrontal cortex part of your brain (the “responsible adult” in your head) KNOWS it should eat the apple instead, but your brain’s limbic system (the "impulsive toddler” part) says “Nope, we want the cookie”. Since the cookie triggers a bigger predicted reward than the apple because our brains evolved to love quick calories and sugar, you reach for the cookie.

If you choose the apple anyway, congratulations. You just outsmarted 200,000 years of evolution. If you chose the cookie, also congratulations, you’re a human being with instincts you follow.