Digestion and nutrition absoprtion

The journey of getting back on track is a long one. And it never really ends as nothing ever stays the same, and we as humans are always changing and need to be constantly adapting to be our best. And we need to realise that through every decade of our life our body has different needs.

 

From a nutritional stand-point we need to ensure that we always get the nutrients we need from our diet.  As we age we slowly lose the ability to absorb nutrients as we produce less digestive enzymes and have lower stomach acidity. What does not help either is that most of the staple foods in modern life have the so called anti-nutrients, which are compounds which act as nutrient abosption inhibitors. 

Let’s dig deeper and first understand how nutrients are absorbed by our digestive track.

 

The Digestive system

Food moves through your GI tract by a process called peristalsis. The large, hollow organs of your GI tract contain a layer of muscle that enables their walls to move. The movement pushes food and liquid through your GI tract and mixes the contents within each organ. The muscle behind the food contracts and squeezes the food forward, while the muscle in front of the food relaxes to allow the food to move.

Mouth. Food starts to move through your GI tract when you eat. When you swallow, your tongue pushes the food into your throat. A small flap of tissue, called the epiglottis, folds over your windpipe to prevent choking and the food passes into your esophagus.

Esophagus. Once you begin swallowing, the process becomes automatic. Your brain signals the muscles of the esophagus and peristalsis begins.

Lower esophageal sphincter. When food reaches the end of your esophagus, a ringlike muscle—called the lower esophageal sphincter —relaxes and lets food pass into your stomach. This sphincter usually stays closed to keep what’s in your stomach from flowing back into your esophagus.

Stomach. After food enters your stomach, the stomach muscles mix the food and liquid with digestive juices. The stomach slowly empties its contents, called chyme, into your small intestine.

Small intestine. The muscles of the small intestine mix food with digestive juices from the pancreas, liver, and intestine, and push the mixture forward for further digestion. The walls of the small intestine absorb water and the digested nutrients into your bloodstream. As peristalsis continues, the waste products of the digestive process move into the large intestine.

Large intestine. Waste products from the digestive process include undigested parts of food, fluid, and older cells from the lining of your GI tract. The large intestine absorbs water and changes the waste from liquid into stool. Peristalsis helps move the stool into your rectum.

Rectum. The lower end of your large intestine, the rectum, stores stool until it pushes stool out of your anus during a bowel movement

Pancreas. Your pancreas makes a digestive juice that has enzymes that break down carbohydrates, fats, and proteins. The pancreas delivers the digestive juice to the small intestine through small tubes called ducts.

 

Liver. Your liver makes a digestive juice called bile that helps digest fats and some vitamins. Bile ducts carry bile from your liver to your gallbladder for storage, or to the small intestine for use.

 

Gallbladder. Your gallbladder stores bile between meals. When you eat, your gallbladder squeezes bile through the bile ducts into your small intestine.

Hormones. Cells lining your stomach and small intestine make and release hormones that control how your digestive system works. These hormones tell your body when to make digestive juices and send signals to your brain that you are hungry or full. Your pancreas also makes hormones that are important to digestion.

 

Nerves. You have nerves that connect your central nervous system—your brain and spinal cord—to your digestive system and control some digestive functions. For example, when you see or smell food, your brain sends a signal that causes your salivary glands to “make your mouth water” to prepare you to eat. You also have an enteric nervous system (ENS)—nerves within the walls of your GI tract. When food stretches the walls of your GI tract, the nerves of your ENS release many different substances that speed up or delay the movement of food and the production of digestive juices. The nerves send signals to control the actions of your gut muscles to contract and relax to push food through your intestines.

 

Nutrient absorption

The workings of the small intestine can be complex. But its role can be simply summed up in two words: nutrient absorption.

That’s because your small intestine is in charge of pulling glucose, amino acids, fatty acids, vitamins, and minerals out of food to be used by the cells. This is accomplished by tiny projections called villi. The microscopic, brush-like lining of the small intestine acts like a comb that grabs important nutrients out of the digested food that leaves your stomach.

Villi are great at absorbing nutrients because they increase the surface area of the inside of small intestine. With hundreds of thousands of villi lining your gut, that’s a lot of surface area for nutrient absorption.  Each villus (a single protrusion of the villi) is composed of a meshwork of capillaries and lymphatic vessels (called lacteals) underneath an ultra-thin layer of tissue. This special structure makes it possible to pull macro- and micronutrients out of your meals and send them to the bloodstream.  Water is also essential to this process. The small intestine uses a chemical process called diffusion to extract nutrients. Diffusion moves water and water-soluble compounds across barriers, like the villi in the small intestine. These compounds include:

Glucose (simple sugars), Amino acids (parts of proteins), Water-soluble vitamins (B vitamins and vitamin C), Minerals

Once these nutrients are diffused into the villi, it’s a straight shot to the bloodstream. That’s where these nutrients can work in cells to make proteins and create energy. 

Fats and fat-soluble vitamins (A, D, E, and K) require a few extra steps to enter the bloodstream. First, bile acids from the liver mix with fats in the small intestine. This breaks the fats down into their component fatty acids. Then, the fatty acids and other fat-soluble vitamins are absorbed by the villi into lacteals. These lymphatic vessels transport the fat-soluble compounds to the liver. That’s where they are stored and released in the body as needed. And there’s a lot of use for fatty acids and fat-soluble vitamins. Cells use the fatty acids to build cell membranes. And vitamins A, D, E, and K are useful in the body to support the health of your eyes, brain, heart, and bones.

Anti-nutrients and nutrient inhibitors

The term “anti-nutrients” suggests what they are. Whereas nutrients are substances that nourish plants and animals to grow and live, anti-nutrients earn their title because they can block the absorption of nutrients. Anti-nutrients are naturally found in animals and many plant-based foods. In plants, they are compounds designed to protect from bacterial infections and being eaten by insects. People who are at high risk for diseases related to mineral deficiencies, such as osteoporosis with calcium deficiency or anemia with iron deficiency, may wish to monitor their food choices for anti-nutrient content.

Protease inhibitors are substances that inhibit the actions of trypsin, pepsin and other proteases in the gut, preventing the digestion and subsequent absorption of protein. For example, Bowman–Birk trypsin inhibitor is found in soybeans.

Lipase inhibitors interfere with enzymes, such as human pancreatic lipase, that catalyze the hydrolysis of some lipids, including fats. For example, the anti-obesity drug orlistat causes a percentage of fat to pass through the digestive tract undigested.[7]

Amylase inhibitors in beans, which prevent the action of enzymes that break the glycosidic bonds of starches and other complex carbohydrates, preventing the release of simple sugars and absorption by the body.

There are several compounds in the foods we eat classified as anti-nutrients. Examples include:

Glucosinolates in cruciferous vegetables (broccoli, Brussels sprouts, cabbage)—can prevent the absorption of iodine, which may then interfere with thyroid function and cause goiter. Those already with an iodine deficiency or a condition called hypothyroidism are most susceptible. Although widely recognized for their putative health benefits, also interfere with the uptake of iodine and flavonoids, and chelate metals (e.g., iron and zinc) thus reducing their absorption.

Lectins in legumes (beans, peanuts, soybeans), whole grains—can interfere with the absorption of calcium, iron, phosphorus, and zinc. 

Trypsin Some proteins can also be antinutrients, such as the trypsin inhibitors. These enzyme inhibitors interfere with digestion.

Avidin is an antinutrient found in active form in raw egg whites. It binds very tightly to biotin (vitamin B7) and can cause deficiency of B7 (in extreme cases).

Oxalates in green leafy vegetables, tea (particularly in rhubarb, tea, spinach, parsley and purslane). Oxalates bind to calcium and prevent its absorption in the human body.

Phytates (phytic acid) can be found in whole grains, seeds, legumes, some nuts. It has a strong binding affinity to minerals such as calcium, magnesium, iron, copper, and zinc. This results in precipitation, making the minerals unavailable for absorption in the intestines. Phytic acids are common in the hulls of nuts, seeds and grains and of great importance in agriculture animal nutrition due to the mineral chelation and bound phosphates released into the environment. Without the need to use milling to reduce phytate (including nutrient), the amount of phytic acid is commonly reduced in animal feeds by adding histidine acid phosphate type of phytases to them.

Saponins in legumes, whole grains—can interfere with normal nutrient absorption.

Flavonoids are a widespread form of antinutrients (in, tea, coffe, legums)  which are a group of polyphenolic compounds that include tannins. These compounds chelate (bind) metals such as iron and zinc and reduce the absorption of these nutrients, but they also inhibit digestive enzymes and may also precipitate proteins.

Considerations

  • It is not known how much nutrient loss occurs in our diets because of anti-nutrients, and the effects vary among individuals based on their metabolism and how the food is cooked and prepared. Many anti-nutrients like phytates, lectins, and glucosinolates can be removed or deactivated by soaking, sprouting, or boiling the food before eating.
  • Studies on vegetarians who eat diets high in plant foods containing anti-nutrients do not generally show deficiencies in iron and zinc, so the body may be adapting to the presence of anti-nutrients by increasing the absorption of these minerals in the gut.
  • Anti-nutrients may also exert health benefits. Phytates, for example, have been found to lower cholesterol, slow digestion, and prevent sharp rises in blood sugar. Many anti-nutrients have antioxidant and anticancer actions, so avoiding them entirely is not recommended.
  • Excessive intake of dietary fiber can reduce the transit time through the intestines to such a degree that other nutrients cannot be absorbed. However, this effect is often not seen in practice and reduction of absorbed minerals can be attributed mainly to the phytic acids in fibrous food.

Some strategies

  1. Keep a journal of how you feel. If you are sensitive to some of the anti-nutrients you will probably experience some unconfort like bloating or gas.
  2. Avoid eating nuts with meals.
  3. Always soak/boil/ferment veggies and nuts.
  4. Cook one-pot meals where you add veggies and cook them for while.
  5. If you choose to have legumes have them as a meal alone. Anti-nutrients affect the absorption of nutrients eaten at the same meal. Therefore to lower this risk, it is recommended to avoid eating large quantities of foods containing anti-nutrients at one meal. A strategy could be to alter the timing of eating foods with anti-nutrients. Another example is to drink tea between meals instead of with a meal to reduce the chances of iron being poorly absorbed, or taking a calcium supplement a few hours after eating a high-fiber wheat bran cereal that contains phytates.
  6. By concurrently eating mineral absorption enhancers, such as garlic and onions, one can get the best of both worlds by improving the bioavailability of iron and zinc in plant foods.

Further reading

https://askthescientists.com/nutrient-absorption/

https://www.niddk.nih.gov/health-information/digestive-diseases/digestive-system-how-it-works

https://med.libretexts.org/Courses/American_Public_University/APU%3A_Basic_Foundation_of_Nutrition_for_Sports_Performance_(Byerley)/03%3A_Digestion_and_Absorption/3.3%3A_The_Digestion_and_Absorption_Process

https://www.visiblebody.com/learn/digestive/digestive-absorption-and-elimination

https://en.wikipedia.org/wiki/Antinutrient#:~:text=Widespread%20form%20of%20antinutrients%20are,and%20may%20also%20precipitate%20proteins.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7600777/pdf/nutrients-12-02929.pdf