The Small Intestine – How Digestion Works

Things are getting serious now. The small intestine is responsible for a lot of digestion, nutrient absorption, and immune function. I am so excited to write this post. This one is bound to be a little longer than all the others because of the sheer amount of things that go on in the small intestine. When people talk about “the gut” the small intestine and large intestine are generally what they’re referring to. The small intestine is central to so many functions throughout the body. I will do my best to stay focused on the digestive function of the small intestine and not go down any lymphatic, immune, or hormone rabbit holes.

The small intestine is separated into three sections. The first section is called the duodenum, the second is called the jejunum, and the third is called the ileum. Chyme (chyme is what food is called once it’s gone through the stomach) is emptied out of the stomach little by little into the duodenum through a valve. The duodenum is the first and shortest section of your small intestine. It produces two hormones called cholecystokinin (CCK) and secretin. These hormones are triggered by the acidity of the chyme and the presence of fat and protein. They cause pancreatic juices and bile to flow into the duodenum. The duodenum continues to facilitate digestion and begins the absorption of nutrients (the stomach absorbs alcohol and some drugs, but no nutrients). The small intestine is where most nutrient absorption occurs.

Peristalsis (the contraction and relaxation of your digestive tract muscles) pushes your chyme along the small intestine into the jejunum and ileum. Inside the jejunum and ileum, your digestive enzymes and bile continue to work to digest your food. At this point, some nutrients have been broken down enough to be absorbed. Throughout both of these sections of your food will continue mixing with the digestive enzymes and bile to finish breaking down all food particles into absorbable nutrients like fatty acids and amino acids. Most absorption of nutrients is done in the jejunum and ileum. On average, it will take about 6 to 8 hours from the time you swallowed your food to the time it exits your small intestine.

The small intestine is a massive organ. If you were to stretch the small intestine all the way out, it could cover a tennis court. Your small intestine is covered in small folds and finger-like protrusions that are covered in even smaller finger-like protrusions spread all across its internal surface area. These protrusions are called villi and the smaller ones are called microvilli. Each villi is covered in cells that help absorb nutrients into the body. This is what is often referred to as the gut lining. The gut lining is only one cell thick and it’s job is to let nutrients into and keep waste, pathogens, undigested food, etc. out of the bloodstream. Inside each villi, there are blood vessels that are ready to carry the nutrients into the bloodstream.

Inside of your small intestine, there are little bugs called microbes. These microbes consist of bacteria, fungi, viruses, etc. Some of these microbes are helpful to us and others are not. All of these microbes live together in harmony when there is balance and diversity. The large intestine has an even greater number of microbes than the small intestine. The intestines are often grouped together and called the gut microbiome. These bacteria are important to digestion as they help digest foods that we cannot otherwise digest. They also create substances that we benefit from. Most of this takes place in the large intestine, but the small intestine is also involved. We will look closer at the microbiome in the large intestine post.

Microbiome research is still relatively new and there are plenty of questions that we don’t have answers to. Nevertheless, it’s important to be aware and do the best you can with the info we have. Ultimately, a wide variety of nutrient dense whole foods should be the foundation of your diet.

How Northern Digestion Affects The Small Intestine

Your brain and your digestive system are very connected. They communicate constantly. The small intestine is home to what is often called the second brain or little brain. The ENS uses some of the same chemicals and cells as the brain to control the digestive system and talk to the brain. The brain is what triggers our organs to start producing saliva, stomach acid, digestive enzymes, and other substances needed for proper digestion.

Check out the previous How Digestion Works posts to learn more about the upper digestive system:

The Brain And Mouth – How Digestion Works

The Stomach – How Digestion Works

The Liver – How Digestion Works

The Pancreas – How Digestion Works

Research has shown that a healthy gut microbiome can help you keep a stable mood. 95% of your serotonin is actually provided by the gut. The production of serotonin can be hindered by stress, disease, and/or poor diet. It’s truly a two way street between the intestines and the brain. Just another super cool example of how the body should be treated as a whole!

The mouth is where the breaking down of food begins. The amount of times you chew each bite and mix your food with your saliva (I know it’s not an appetizing picture, but it’s an important one) can either make the rest of your digestives system’s job easier or harder.

The stomach is continues the chemical breakdown process that your mouth started. It also activates other enzymes and disinfects your food to help prevent infection in your intestines.

Bile emulsifies and breaks down fats. Pancreatic juices contain bicarbonate that raises the pH of the chyme as it comes out of the stomach so it doesn’t burn the inside of your small intestine. It also contains digestive enzymes that help chemically breakdown carbohydrates, fats, and proteins.

The gut lining and villi can become inflamed and damaged. Poor diet, genetics, and dysfunction in the northern digestive system can all increase the risk of this happening. This is often referred to as leaky gut. Leaky gut can contribute to autoimmune disease, skin issues, food sensitivities, headaches, and brain issues like fatigue.

How The Small Intestine Affects The Large Intestine

The small intestine absorbs different nutrients in each section. The duodenum absorbs several minerals, glucose, vitamins B1, B2, B6, A, D, E, C, and folate, fat, etc. The jejunum absorbs lactose, maltose, water soluble vitamins, proteins, amino acids, etc. The ileum absorbs cholesterol, vitamin B12, and bile salts. The large intestine continues this process by absorbing potassium, water, and sodium chloride, vitamin K, etc.

The large intestine is your meal’s last stop before exiting the body. Everything above the large intestine paves the way for the large intestine to do it’s job. Any dysfunction above the large intestine can make its job harder. The small intestine is no exception. The autoimmune diseases we talked about above include IBD and diverticulitis. Issues like IBS, parasites, diarrhea, constipation and others can all be either helped or worsened by your small intestine.

SIBO is a common issues when it comes to small intestines. Small intestinal bacteria overgrowth is when there are strains of bacteria in the small intestine that don’t belong or when there is a sudden increase in bacteria. It is often caused by slow movement of chyme through the digestive tract. The longer food sits in your intestines, the more time it has to become a breeding ground for all sorts of bacteria. Slow moving chyme can have many causes including dehydration, recent surgery in the digestive tract, poor vagus nerve communication, stress, etc. The symptoms of SIBO include bloating, diarrhea, pain in the intestines, nausea, malnutrition, and feeling uncomfortably full after meals.

As an NTP, I do not diagnose or treat any illness or disease. However, if you are having these symptoms there are specific dietary and lifestyle recommendations I will give you to help support your small intestine and facilitate a healthy environment for the microbes that are supposed to be there.

Practical Dietary And Lifestyle Changes

• Eat a wide variety of nutrient dense, whole foods
• Focus on supporting northern digestion
• Stay hydrated
• Manage stress
• Eat foods like bone broth and healthy fats to support you gut lining
• Chew your food thoroughly

Once we start to see dysfunction in the small intestine, we should be looking very closely at northern digestion for dysfunction. Diet is so incredibly important for small intestine health. Parasites can come from unwashed raw foods, the microbiome is affected by the variety of foods we eat, and our gut lining can be supported and encouraged to repair by our consumption of healthy fats and micronutrients. Bone broth contains a ton of nutrients, collogen, and healthy fats that are needed to restore the health of the gut lining. Chewing food thoroughly reduces the work your intestines have to break down food and results in less inflammation and greater digestion and absorption of nutrients.

Summary

The small intestine is where the majority of nutrients get absorbed. Digestion continues all the way through the small intestine through the mixing of the food and digestive juices. Peristalsis moves food through the digestive tract. Each section of the small intestine absorbs different nutrients.

The microbiome begins to play a role in digestion in the small intestine. The health of your microbiome is important because overgrowths of certain bacteria can cause health issues. A diverse balance of bacteria is the goal to strive for. You can reach that goal by working to include as wide of a variety of foods as possible.

There was a lot of information in this post and your small intestine is highly involved with other systems like the immune and endocrine system in ways we didn’t even cover. The bottom line is this: You can support your small intestine by supporting northern digestion and eating a whole, nutrient dense diet.

Let me know your thoughts down below!

Have a terrific rest of your week!