Absorption of Digested Food Products: Did you know our body does not absorb 100% of the nutrients from all the food we eat? When we eat something, it is digested, absorbed, and assimilated into our bodies. Our body can absorb about 10-90% of the nutritional content of food material. The wide range occurs because food absorption rate depends on various factors such as age, time of the day, cooking method, supplement food, and medicines are taken.  

What if our body can not absorb nutrients or a particular nutrient? The condition is called malabsorption. Lactose intolerance is one such condition in which a person can not process dairy products. The healthy food that we take can only benefit us if our body absorbs most of its nutrients. Read along to learn about the process of absorption and assimilation in the human body.

What is Absorption?

 Absorption can be defined as the process by which digested nutrients pass from the alimentary canal into the blood or lymph, and the nutrient molecules are taken into the cells of a living organism. Our food contains nutrients like proteins, carbohydrates, lipids, and vitamins. These nutrients are complex in structure and are broken into simpler forms by the process of digestion. The absorbable nutrients are amino acids, monosaccharides, fatty acids, glycerol, salts (electrolytes), vitamins, and water.

Absorption in Different Parts of Digestive System

1. Oral Cavity: Food stays for a very short span of time in the mouth; hence only some drugs and simple sugars like glucose, alcohol, etc., are absorbed in the mouth.
2. Stomach:  Food enters inside the stomach in the form of a bolus. This absorption in the stomach occurs by either osmosis, diffusion, or active transport. The stomach can absorb certain minerals and vitamins, alcohol, drugs like aspirin, and some amounts of simple sugar, etc.
3. Small Intestine: Food enters inside the small intestine in the form of chyme. The small intestine is very long and narrow, which causes the slow movement of nutrients allowing absorption.
The intestinal wall is lined with villi to increase surface area. The epithelial lining of the small intestine has microvilli which are projections of the plasma membrane to further increase the absorptive surface.  The majority of absorption of nutrients occurs through the villi of the small intestine. The apical plasma membrane housing these enzymes is composed of numerous microvilli which extend from the cell and constitute the “brush border” that helps in the absorption of nutrients from the intestinal lining into the bloodstream. Hence the small intestine is widely referred to as a site of absorption. The small intestine is divided into three regions: Duodenum, Jejunum, and Ileum.
(a) Duodenum is the first section and the shortest region of the intestine where the chyme gets mixed with bile and pancreatic juice here.
(b) The jejunum is the middle section and is the primary site of nutrient absorption.
(c) The ileum is the final section of the small intestine that absorbs the remaining nutrients. Enterocytes present on the surface of the small intestine absorbs large amounts of sodium ions from the lumen. The ileum absorbs most of the nutrients from food.

Learn About Peristalsis Here

The small intestine can absorb almost all kinds of nutrients required for the body. Amino acids and monosaccharides are absorbed into the blood capillary, whereas larger molecules like fat enter the lacteal. Products absorbed into the lacteals (lymph vessels) of the villi are fatty acids and glycerol, which then enters the lymphatic system.

4. Large Intestine:  Large intestine primarily absorbs water and products of bacterial digestion (amino acids and vitamin B complex, and vitamin K).  Most of the water is absorbed in the colon by diffusion. Whereas mineral ions are absorbed by the colon through active transport.

Absorption of Different Nutrients

The human body primarily requires carbohydrates, proteins, lipids, water vitamins, and some salts. Macronutrients like protein, carbohydrates, and lipids are digested into simpler substances like amino acids, monosaccharides, fatty acids, and glycerol, etc., respectively.   The absorption of different substances occurs in different ways. It might be by simple diffusion, osmosis,  facilitated transport, and active transport.

Absorption of Carbohydrates: Carbohydrates are the primary source of energy in humans. Carbohydrates are digested and absorbed as monosaccharides in the stomach and jejunum. Glucose and galactose are absorbed actively through sodium pump fructose are absorbed by facilitated diffusion. Glucose, galactose, and fructose are absorbed by the intestinal wall into blood capillaries.

Absorption of Protein: Proteins are the building blocks. Proteins are digested and absorbed as amino acids. Most amino acids are absorbed by active transport, but some are absorbed by facilitated transport. Absorption of amino acids occurs mainly in the duodenum and jejunum of the small intestine. Amino acid is absorbed into the blood capillaries.

Absorption of Lipid and Fat-Soluble vitamins: Lipids and fat-soluble vitamins are insoluble in water. Lipid is digested and absorbed as fatty acids and glycerol in the jejunum. Fatty acids, glycerol, and fat-soluble vitamins (A, D, E, and K) are hydrophobic. Hence, they cannot be absorbed directly into blood capillaries. These are incorporated into micelles. Micelles are small, spherical, water-soluble molecules. Fatty acids, glycerol, and fat-soluble vitamins are incorporated into micelles with the help of bile salts and phospholipids. Fatty acids, glycerol, and fat-soluble vitamins are absorbed from the micelles into the intestinal cells by simple diffusion. These absorbed nutrients are resynchronized, in the Endoplasmic reticulum, into very small fat droplets called chylomicrons. The chylomicrons are released into the lymphatic capillaries called the lacteals from the intestinal cells.

Absorption of Water-Soluble Vitamins: Most of the water-soluble vitamins (B- Complex, C, P) are absorbed into the bloodstream by simple diffusion.

Absorption of Salts (Electrolytes): Electrolytes are an important part of nutrition. They maintain osmolarity and help in water retention—sodium, phosphate, potassium, calcium, magnesium, chloride, and bicarbonate. Most electrolytes are actively absorbed in the duodenum, while bile salts are absorbed in the ileum. Sodium absorption is coupled with glucose by active transport. Calcium absorption is enhanced by vitamin D and parathormone.

Absorption of Water: Our body is 60% water. Water, along with electrolytes, is absorbed by osmosis. About 90% of water absorption occurs in the small intestine, but some amount of absorption occurs in the stomach and large intestine as well. The absorption of water with electrolytes and digestion together maintains an osmotic balance between intestinal epithelium and blood.

Absorption of Alcohol. Alcohol is slightly lipophilic. Absorption of alcohol begins in the stomach but is mostly absorbed in the small intestine.

Mechanism of Absorption

Absorption occurs through the wall of the lumen of the gastrointestinal tract. The absorption occurs by mainly two methods: active absorption or passive absorption.

Passive absorption: Passive absorption occurs along a concentration gradient i.e., from the level of higher concentration to the level of lower concentration. This is the most direct form of membrane transport. It does not require energy. Passive absorption occurs by either osmosis or diffusion.

Diffusion: Diffusion allows the transportation of substances along concentration gradients at no expense of energy. Concentration gradients are a form of potential energy which are sufficient for diffusion. Diffusion requires a high concentration of the substance in the lumen. The molecule should be small and water-soluble to be diffusible. Diffusion may be simple or facilitated. Simple diffusion is defined as the movement of solute from the higher concentration to the lower concentration through the membrane. Few Monosaccharides like Glucose, amino acids, and ions like chloride are absorbed by simple diffusion. It is a very slow process. Facilitated transport is defined as the process of movement of solutes through the semipermeable membrane with the help of specific carrier proteins. Carrier proteins are specific molecules that bind to the nutrient molecule to form carrier metabolite complexes and transports them across the membrane. Water-insoluble nutrients like fatty acids are absorbed by this method.

Osmosis: Osmosis is defined as the process of solute or water movement through the semipermeable membrane. Water absorbed from the lumen of the GI tract is transferred to the blood as long as the solute concentration is higher in the lumen when compared to the blood.  Some nutrients can be absorbed passively.

Active absorption: Active transport is defined as the movement of solute from the lower concentration to the higher concentration at the expense of ATP. Electrolytes like sodium ions are absorbed by active transport into the blood. It is faster than passive absorption. It occurs in a particular direction. Active transportation occurs with the help of carrier proteins. Carrier molecules make the complex with a substance called carrier nutrient complex and take it across the membrane. Every single movement across the membrane requires ATP. Active transportation helps in the rapid, selective, and exhaustive intake of nutrients.  Active transport helps in maintaining osmotic pressure, ionic concentration.

Assimilation

The process of utilization of the absorbed nutrients that reaches the tissues after absorption is called assimilation. The nutrients can be stored, resynthesized, broken down, or transported further. For example, Glucose is used in respiration to generate energy. Amino acids are used for protein synthesis. The liver plays an important role in the assimilation of glucose. It converts glucose into glycogen and amino acid into a protein.  The liver also performs deamination of amino acids and releases nitrogenous waste into the blood as urea.

Study About Digestive System Here

Summary

Absorption can be defined as the process by which digested nutrients pass from the alimentary canal into the blood and lymph. The stomach can absorb alcohol, drugs like aspirin, and some amounts of simple sugar, etc. The majority of absorption of nutrients occurs through the villi of the small intestine. The large intestine primarily absorbs water. Macronutrients like protein, carbohydrates, and lipids are digested into simpler substances like amino acids, monosaccharides, fatty acids, and glycerol, etc., respectively.   

Carbohydrates are digested and absorbed as monosaccharides in the stomach and jejunum. Most amino acids are absorbed by active transport, but some are absorbed by facilitated transport. Fatty acids, glycerol, and fat-soluble vitamins (A, D, E, and K) are hydrophobic. Hence, they cannot be absorbed directly into blood capillaries. These are incorporated into micelles. These absorbed nutrients are resynchronized into chylomicrons and released into the lymphatic capillaries called the lacteals from the intestinal cells. Water, along with electrolytes, is absorbed by osmosis.  Absorption of alcohol begins in the stomach but is mostly absorbed in the small intestine. The process of utilization of the absorbed nutrients that reaches the tissues after absorption is called assimilation. The nutrients can be stored, resynthesized, broken down, or transported further.

Frequently Asked Questions (FAQs) from Absorption of Digested Food Products

Q.1. How is the assimilation of food different from absorption of food?
Ans: Absorption is defined as the process by which digested nutrients pass from the alimentary canal into the blood and lymph, whereas assimilation is the process of utilization of the absorbed nutrients that reaches the tissues after the absorption.

Q.2. What happens to the products of digestion during absorption?
Ans: During absorption, glucose and amino acid is transferred from the lumen of the small intestine to the bloodstream, whereas fatty acid and glycerine are absorbed into the lymph.

Q.3. How can vitamin C absorption be increased?
Ans: Vitamin C absorption can be increased when it is ingested along with a food rich in iron content.

Q.4. What causes poor food absorption?
Ans: Weak gut lining, food allergies, infections, certain medications can cause poor absorption of food.

Q.5. What is malabsorption?
Ans: Malabsorption is a condition in which absorption of food through the small intestine is prevented or reduced.