When you take a bite of any food, your body immediately begins to break it down in a variety of ways. The most obvious is the mechanical breakdown of food by the teeth. Chewing breaks food into smaller pieces, which increases the surface area available for chemical interaction. And speaking of chemical interaction, a less obvious breakdown is happening in the mouth as you chew.
Saliva, which contains the aptly named enzyme salivary amylase, begins the breakdown of carbohydrates, and the lingual lipase enzyme does the same for fats. As you chew, carbohydrates are already being broken down into simple sugars in your mouth, which is why even the wholest of whole grain breads (the ones that taste like sticks) will begin to taste sweet if you chew them for a few minutes. Proteins just get wet and smashed up. Enzymatic breakdown of proteins does not begin until they reach the stomach.
When you swallow that bite, two things happen: it passes into your esophagus toward your stomach, and we start referring to the ball of mashed food and saliva as a bolus. Bolus may sound strange at first, but it sure trumps “mashed-up food ball.” The bolus is pushed toward the stomach by rhythmic muscle contractions of the esophagus called peristalsis. This mechanism works independent of gravity, so even if you stand on your head immediately after eating, the bolus will still make its way to the stomach.
After plopping into the stomach, the bolus gets further digested by the gastric juices waiting there, which are made up of water, hydrochloric acid, and more enzymes. Cells in the lining of the stomach secrete various lipase, amylase, and peptidase (also called protease) enzymes in order to break down fat, carbohydrate, and protein, respectively.
In addition to getting an acid and enzyme bath, the bolus is also being churned continuously. The stomach is literally a bag made of muscles, and those muscles are constantly constricting and relaxing. Depending on the composition of your bolus and your specific physiology, this churning and chemical breakdown continues for anywhere from two and a half to four hours. Very little absorption of nutrients occurs in the stomach, though some water, some alcohol, and many drugs can cross from the stomach into the bloodstream.
Are you comfortable with calling your eaten food a bolus yet? Great, because once the bolus is churned up in the stomach for a bit, we start calling it chyme instead. So far, that’s three names for the same bite of food. And there will be more before we’re finished. Isn’t nutrition fun?
The chyme is gradually let out into the small intestine, which is where most of the actual absorption of nutrients will take place. The small intestine is actually pretty freaking big. In the typical person, it is about 20 to 23 feet long and tightly packed with tiny finger-like projections called villi, which are covered in even smaller projections called microvilli. All of these little fingers increase the surface area of the small intestine by an amazing amount. To repeat the oft-repeated example of just how much surface area we’re talking about, if you were to stretch the small intestine smooth it would be able to cover a standard-sized tennis court. I hope that approximation was determined via math and not by physical testing!
Surface area is a big deal in digestion because it allows the nutrients more opportunity to interact with your gut wall and be absorbed. Reduced surface area is why nutrient deficiencies are more common among people suffering from celiac disease, Crohn’s disease, or any other issue that damages villi.
Almost all of the available proteins, fats, and digestible carbohydrates in the chyme are absorbed as it moves through the approximately 20 feet of the small intestine. Again, the composition of your food and your specific physiology determine how long this will take. The estimated average transit time is two and a half to three hours.
Anything that makes it to the next and final stage of digestion gets another name. When it passes from the small intestine and into the large intestine, also called the colon, it is referred to as stool or feces. Some people call it poop. That’s OK, too.
The large intestine is a little wider, but much shorter, than the small intestine. It is only about five feet long and does not have villi on its interior surface. As the stool travels the length of the large intestine, most of the remaining water is absorbed and the solids are compacted and formed into the shape and consistency that we know as stool. The food you eat and the microbes in your gut play a huge role in the consistency and moisture of your stools.
The Microbiome
Speaking of the bacteria population of the gut, let’s take a moment to discuss the microbiome. Microbiome is the term given to the collective bacteria population of your gut. It is estimated that in the average person’s colon lining, the number of individual bacteria exceeds 100 trillion. That’s up to 10 times the number of cells that make up your actual body.
Though there is still much that we do not understand about the interaction between this community of bacteria and the body, we have evidence of the microbiome playing an important part in our health. In fact, the microbiome is now being called the lost organ because of how important it is to normal bodily function. It seems almost inappropriate to call humans individuals in light of what we now know about the interactions between our bodies and the bacteria in our gut; it is more accurate to say that we are all synergistic communities.
So far, we know that the microbiome plays an important role in immunity, weight regulation, mood regulation, and possibly even longevity. Unfortunately, we do not yet know the exact nature of that role.
We do know that the human bacterial population is made up of thousands of different species, each with its own behavior and effect. All of these bacteria need food to flourish, and many species seem to prefer fiber as their food. Because all fiber comes from carbohydrates and a ketogenic diet is very low carbohydrate, one of the concerns about ketogenic eating is that it will negatively affect the composition of the microbiome. Fortunately, a ketogenic diet, particularly the Ketogenic Mediterranean Diet, does not have to be a low-fiber diet.
How We Actually Get Energy
The above overview of the digestive system helps you understand, from a systematic perspective, what happens in your body after you eat. To take it one step further, let’s talk about what your body does with the food once it is small enough to pass from your digestive system into the rest of the body, where it can be used by individual cells.
Most of what we eat is broken down into its component parts and either run through cellular pathways to extract the chemical energy (I’ll refer to energy being “burned” from now on) or used to build other things. Imagine, if you will, that you have a sandwich made from Legos. You can take the Lego-wich apart and have a pile of pieces to work with. You could assemble them back together as a rocket or a house or, as I did as a child, a multicolored nothing that you are proud of anyway. Alternately, you can eat these Legos and use them for energy. Finally, you could put the blocks in a bag and save them for later.
These are the options your body faces constantly. Most of what you give it can be torn apart and put back together differently, burned for energy, or stored for later use. Which action your body takes depends on many factors, like its current energy needs and the food available at the time.
Now that we’ve taken a journey through the digestive system, with brief detours into the microbiome and food made of Legos, we will explore the composition of food and how it is classified. Chapter 2 will be dedicated to the macronutrients: fat, protein, carbohydrates, and alcohol. Chapter 3 is about the micronutrients, vitamins, and minerals, and which are most relevant to the Ketogenic Mediterranean Diet.