Protein Digestion and Absorption: How It Works and How to Improve It

Protein Digestion and Absorption: How It Works and How to Improve It

Protein Digestion and Absorption: How It Works and How to Improve It

We all need protein for optimal health. It’s found in every cell of your body and plays a vital role in many processes that keep your body functioning. To make the protein your body needs though, you first need to consume enough protein. Then, your body must break that down through protein digestion into a form it can use.

Let’s take a closer look at protein digestion, why it’s so important, and what can get in the way of this crucial process. We’ll also look at what you can do to improve protein digestion and uncover some of the best proteins for digestibility.

Why Is Protein Digestion Important?

Protein is a critical building block of your physical body. It’s involved in the growth, maintenance, and repair of cells and tissues, including your organs, muscles, eyes, skin, hair, and nails. It also helps with the production of enzymes, hormones, and antibodies, keeping the immune system healthy. For those who are extremely physically active, it’s also an excellent source of energy, especially if you’re restricting your carbohydrate intake.

The body has a high level of protein turnover, meaning it continually breaks down proteins into amino acids and then uses different combinations of amino acids to build new proteins. You need to eat a continuous supply of protein to meet this need. However, before your body can use the dietary protein you consume, it needs to be digested and absorbed. This happens in the gastrointestinal tract (GI tract).

How Dietary Protein Is Digested

Illustration of the digestive system

The digestive system’s job is to convert the food you eat into the energy and nutrients you need for your body to function properly. 

The GI tract is part of the digestive system, and it consists of the mouth, esophagus, stomach, small intestine, large intestine, and anus. These are all joined together in what is essentially a long tube that runs through your body. Along the way, organs like the pancreas, gallbladder, and liver add various enzymes, including proteolytic enzymes (enzymes that break down proteins) to assist the process.

As the protein you eat moves through your digestive tract, it’s broken up into its constituent parts, both mechanically by movements like chewing and peristalsis, and chemically by various enzymatic reactions. The process of breaking down the complex protein molecules into smaller ones is known as hydrolysis. The protein is then absorbed and recombined through protein synthesis into new proteins that your body can use at a cellular level.

Let’s look at some key parts of the protein digestion process.

Breaking Down Protein

As you can’t swallow large chunks of food, your body immediately starts breaking it down through the process of chewing. Your saliva contains mucus, which helps keep everything moist so it can move along through the system. Interestingly, carbohydrates start digesting in the mouth with the help of an enzyme in your saliva called amylase, but proteins only really start digesting in the stomach. Proteins also take longer to break down than carbohydrates, which is why they make you feel full for longer.

From the mouth, the protein moves through your esophagus into your stomach, where secretion of hydrochloric acid takes place. The muscles in your stomach churn the food in a process called peristalsis, breaking it down further and mixing it with acidic gastric juices to create a slush called chyme.

The hydrochloric acid in your stomach has a very low pH of 1.5 – 3.5, making it extremely acidic. It contains an enzyme called pepsin, which starts the chemical breakdown of protein through denaturation. During denaturation, the proteins lose their three-dimensional structure, unfolding into polypeptide chains, which are groups of amino acids joined by peptide bonds. These amino acids are the building blocks of proteins.

The essential amino acids, which the body gets from protein, are:

  • Leucine
  • Isoleucine
  • Valine
  • Tryptophan
  • Lysine
  • Methionine
  • Histidine
  • Phenylalanine
  • Threonine

At this point, some elements of the chyme are absorbed through the walls of the stomach, but most of it passes into the small intestine within two to six hours after the food is first consumed.

Digestion and Absorption of Protein in the Small Intestine

The small intestine is a coiled tube, on average about 20 feet long (6 meters) and about an inch in diameter (3 centimeters). It is comprised of three parts: the duodenum, jejunum, and ileum. Most of the protein absorption process happens in the small intestine.

The pancreas secretes digestive enzymes, especially trypsin and chymotrypsin, and releases these into the small intestine. The trypsin activates other enzymes called proteases (or peptidases), which break the polypeptide chains down further into free amino acids or small peptides—called dipeptides or tripeptides, depending on the number of amino acids and peptide bonds in the chain.

The small intestine is lined with epithelial cells called enterocytes. These have protruding, finger-like structures called microvilli, which increase the absorptive surface area. As the liquefied protein from the stomach, now broken down into various forms of peptides and amino acids, moves through the intestinal lumen (the space inside the intestinal tube), the peptides and amino acids are absorbed by the enterocytes through a process known as active transport. Within the enterocytes, the peptides break down even further into individual amino acids and then move into the bloodstream.

Protein Delivery and Excretion

The first stop for those amino acids in the blood is the liver, which regulates the amino acid levels in the blood. If any further breakdown is needed, this also happens in the liver, releasing nitrogen-containing ammonia. This is toxic, so the liver transforms it into urea, which is then moved to the kidneys and leaves the body in the urine.

From there, the amino acids are delivered around the body by the circulatory system and recombined into new proteins as needed through protein synthesis — for example, for muscle-building — or used for fuel if the body is short on energy.

As the remaining waste moves through the large intestine and colon, a few more nutrients are extracted. The remaining indigestible matter is dried out then finally excreted by the body in your feces.

What Can Go Wrong With Protein Digestion?

The average adult needs around 0.8 grams of protein for each kilogram of body weight per day. However, if you’re very physically active, you need at least 1.2 grams of protein for each kilogram of body weight, and as much as 2 grams if you’re an extreme athlete. Protein needs also vary based on age. Older people also need more protein — up to 1.5 grams per kilogram of body weight per day — to counteract the loss of strength and muscle mass.

That said, it’s quite possible that you’re eating the right amount of protein but your body isn’t digesting it well. When you’re not digesting and absorbing protein well, you may experience symptoms such as:

  • Indigestion
  • Stomach pain
  • Heartburn
  • Gastroesophageal reflux disease (GERD), otherwise known as acid reflux
  • Nausea
  • Bloating
  • Gas
  • Diarrhea
  • Constipation
  • Nutrient deficiencies
  • Slow muscle growth
  • Anxiety
  • Depression

The Importance of Digestive Enzymes

One of the most common reasons your body may not be digesting and absorbing protein well is a lack of the necessary enzymes — like trypsin and chymotrypsin — to break down the protein. As we age, our body produces fewer enzymes and digestion can then become more challenging.

If food isn’t digested properly, it may pass through undigested, which means you’re not getting the nutrients you need from it. Or it can stagnate in the small intestine and create a buildup of yeast and bacteria, which then creates problems like small intestinal bacterial overgrowth (SIBO).

If those undigested pieces of food manage to leak through the small intestine into the bloodstream, you may end up with a “leaky gut.” This can cause serious inflammatory and immune reactions, like food allergies, irritable bowel disease (IBS), migraines, eczema, rheumatoid arthritis, and autoimmune conditions.

How to Improve Protein Digestion and Absorption

Small bowls of food: Spread out your protein intake to improve protein digestion

If you’re experiencing any of the aforementioned symptoms, it’s worth experimenting with a few of the following options to see what works to improve your protein digestion.

Eat Enough High-Quality, Easily-Digestible Protein

Your daily protein intake should include enough high-quality protein to ensure you get all the essential amino acids (EAAs). These are the amino acids your body can’t make and must come from your nutrition. EAAs can come from a variety of different proteins, including:

  • Animal protein sources, such as meat, fish, eggs, and dairy: These are known as complete proteins as they contain all the EAAs. The most digestible animal proteins tend to be lean meats like chicken breast or fish, especially salmon.
  • Plant protein sources, including soy, beans, or legumes: These are often incomplete proteins and usually best combined with whole grains like rice or wheat to provide you with all the EAAs. Pea protein, chia seeds, and hemp hearts are some of the most easily digested plant proteins.

Chew Your Food

Chewing is the first step that breaks down protein. If you don’t chew properly, your body needs to use more enzymes to break down those big chunks of food later. When you eat, do so slowly and mindfully, chewing each mouthful well. 

Watch What and When You Drink

While drinking plenty of water is generally very good for you, if you drink it during or immediately after meals, you may dilute your stomach acid, reducing the acidity that helps to break down your food. Not surprisingly, excessive alcohol and caffeine also both interfere with the digestive process.

Spread Out Your Protein Intake

While some research varies about the upper limit of protein intake, it seems likely that there’s only so much protein your body can absorb at once. Rather than eating all your protein at once each day, eat a bit less with two or three of your meals. This way, your body can process all of the protein each time.

Stick to Protein or Carbs

Protein and carbohydrates are both macronutrients, but they’re digested in different ways. As such, when you combine them in one meal, your body has to work harder. Combine high-protein meals with green vegetables to enhance digestion.

Reduce Your Stress Levels

Stress negatively impacts your digestive system, so it’s worth doing whatever you can to reduce it. For example, practice deep breathing, go for a walk in nature, do some exercise you enjoy, spend time with loved ones, make a list of things you’re grateful for, and laugh as much as possible.

Pay Attention to Symptoms

If you regularly experience discomfort after eating certain proteins, consider cutting them out for a while to see if that helps. Once your system stabilizes, you may be able to add them back in. If your symptoms persist, consult a professional nutritionist or other health professional. 

Try a Protein Supplement

If you’re struggling to digest protein from whole foods, consider trying a protein powder supplement — either whey (a milk protein) or pea protein. Protein powders are generally very digestible. 

In particular, ioWhey Protein and ioPea Protein have been scientifically proven to cause less stomach discomfort than non-optimized whey. Furthermore, ioWhey and ioPea are also extremely bioavailable, so your body can absorb and use most of their protein content.

Improve Your Digestion and Change Your Life

Woman making the heart symbol over her belly to represent healthy protein digestion

Protein is a critically important macronutrient that your body needs daily. But before your body can use any of the important nutrients it contains, the protein must be digested and absorbed. If your body doesn’t handle these processes well, you may experience a range of mild through to severe symptoms. 

Fortunately, there are relatively easy ways to improve your digestion and it’s well worth making the effort to do so. Your body will thank you, both immediately and in the long term.