How nutrients are

absorbed in the GUT

CCSSA Congress

Durban 24. August 2018

Prof. em Rémy Meier MD

University Basel

MDB Gastro- and Nutrition Center

Basel, Switzerland

a. Oral orifice to oesophagus = about 15cm;

b. Oesophagus: 25-30 cm;

c. Length of the stomach varies between

25 and 50 cm,

d. Duodenum = roughly 30cm

e. Small bowel = 3 to 6 meters

f. Large bowel (colon) = approx. 100-120cm

g. Rectum = 15 cm long

TOTAL – approx. 8.5 meters

Anatomy of the GI tract

Digestion

Digestion occurs intraluminally and at the mucosal surface of the enterocytes, and is initiated by gastric acid as well as by a series of nutrient specific enzymes (disaccharidases, proteases, lipases, etc). Physical aspects of food break-down and mixing, which begin with chewing in the mouth, are complemented by the beneficial effects of fluid secretion that enables biochemical processes.

Absorption

Absorption is the process by which the products of

digestion and other small molecules are

transported into the epithelial cells that line the GI

tract, and from there ultimately into the blood and

lymphatic vessels which drain the tract and serve

the rest of the body. This occurs via a combination

of passive and active (energy-requiring) means.

Metabolic and digestive functions

of the intestine reside

predominantly in the proximal

part of the small bowel

(duodenum and jejunum).

The distal part of the bowel have

important functions in the

resorption of bile acids and

electrolytes.

Sites for Digestion and Absorption

Site Digestion Absorption

Mouth Carbohydrates, lipids

Stomach Lipids, proteins

Small

Bowel

Duodenum Carbohydrates, lipids,

proteins

Most nutrients in small

amounts, but

particularly calcium

Jejunum Carbohydrates (di- and

tri-saccharides),

proteins and

polypeptides, lipids

Carbohydrates,

amino-acids,

oligopeptides, lipids,

vitamins other than

B12, water, major ions

and trace elements

Ileum Any residual digestible

macromolecules

Vitamin B12 (and bile

salts), and most

nutrients in small

amounts

Colon Breakdown of

indigestible

carbohydrates (fibre)

by bacterial action

Free fatty acids

resulting from bacterial

fermentation

The digestive processes in the small intestine to take place in three phases:

• The intraluminal phase: which mainly involves hydrolysis of macronutrients (carbohydrates, proteins, lipids) by intestinal, biliary and pancreatic enzymes;

• The brush border membrane phase or mucosal phase: which concerns the further degradation of polysaccharides and long-chain peptides by the enzymes of the enterocyte microvilli; this phase incorporates absorption by the intestinal cells - predominantly of mono-and di-saccharides, amino acids and very short oligopeptides, and of the water-soluble fatty acids;

• The incorporation phase: which includes the transport of the products of digestion to the portal vein and to lymphatics.

Carbohydrates • Neither oligo- nor polysaccharides are absorbed to an appreciable

extent by the gastrointestinal tract. To be absorbed they must first be broken down to monosaccharides.

• Pancreatic and salivary amylases are necessary for starch digestion.

• Glucose, galactose and fructose are absorbed via specific transporters, and approximately 75% of all carbohydrates ingested are absorbed in the proximal 70cm of the small intestine

• The glucose is transported across the basolateral surface to the bloodstream via the glucose carrier GLUT2.

• Lactase is a β-galactosidase which hydrolyses lactose to glucose and galactose

• Galactose is absorbed by the same processes as glucose

• Fructose is taken up into the enterocytes mainly via the specific carrier GLUT5.

Lipids I

• The majority of dietary lipids are triglycerides, which amount to around 95% of the fat consumed, while the rest comprise phospholipids, cholesterol, fatty acids, and fat-soluble vitamins.

• The digestion of triglycerides begins with lingual lipases released during chewing, and continues with gastric lipases in the stomach, Digestion begins only with the impact of the pancreatic lipases stimulated by CCK. Secretin is also involved.

• As fat globules enter the duodenum, they are coated with bile salts. In the presence of bile salts, monoglycerides and fatty acids form “micelles.

Lipids II

• In the enterocyte, free fatty acids become associated with cytoplasmic fatty acid-binding proteins and can then be resynthesised into triglycerides.

• Newly formed triglyceride is then bound to apolipoproteins to form chylomicrons, which, after being transferred to the Golgi apparatus, leave the cell by exocytosis at the basolateral membrane of the cell.

• Chylomicrons are thus delivered first to the lymphatics and from there to the systemic circulation.

Proteins I

• The digestion of protein is based on a series of hydrolytic reactions which split the α-peptide bonds between the adjacent amino acid residues which form the primary sequence of the polypeptide chain.

• The gastric phase of digestion is dependend of pepsin where they begin the cleaving of proteins and polypeptides into shorter peptide chains.

• In the small intestine pancreatic proteases take over as the principal proteolytic enzymes. Trypsinogen is activated to trypsin by the brush-border enzyme enterokinase. Chymotrypsin, elastase and carboxypeptidases are also involved.

Proteins II

• About 30% of the hydrolysis takes place in the duodenal lumen, the jejunum being the site of hydrolysis of the remaining long-chain polypeptides.

• All protein is degraded to a mixture of tripeptides, dipeptides and single amino-acids as it reaches the small intestine’s mucosal surface.

• The absorption of protein products is predominantly in the upper jejunum.

• Transport of amino acids into the enterocytes is facilitated by electrochemical gradients involving sodium and, in some cases, chloride ions.

Proteins III

• Dipeptide and tripeptide transport is driven by a hydrogen ion concentration gradient that is actively generated by Na+/H+ exchangers.

• Generally, the absorption of oligopeptides is more efficient than that of amino-acids.

Medical Physiology Boron, Boulpaep Saunders

Vitamins I

• Water-soluble vitamins

- Most free vitamins are absorbed by the cells of the

intestinal epithelium mainly via simple diffusion according

to concentration gradients, aided by specific sodium-

dependent transporter proteins.

- Vitamin B12 is depended from the intrinsic factor of the

stomach. The intrinsic factor–B12 complex passes down

the intestine in is taken up int the terminal ileum.

Vitamins II

• Fat-soluble vitamins

- Fat-soluble vitamins are absorbed along with the

digestion products of lipids, segregating into micelles

and passing into the lymph as for lipid in general.

Jejunum Ileum

Iron + +

Calcium + +

Magnesium +/- +

Water/Na/K + ++ (++ )

Bile Salts - ++

B12 - ++

Water, electrolytes and trace

elements

Medical Physiology Boron, Boulpaep Saunders

Net absorption of water and

electrolytes in the small and large

bowel is largely driven by active ion-

pumping of the Na+-K+ pump, which

induces transmembrane

concentration differences and electro-

chemical gradients which drive Na+

linked co-transport of important

luminal components including amino

acids, glucose, bile acids and others.

Medical Physiology Boron, Boulpaep Saunders

Water-, electrolyte absorption

Colon

• The colon serves mainly to reabsorb fluid, and as the source of short-chain fatty acids which are made available by bacterial fermentation of non-absorbed carbohydrate (fibre).

• Typically 80-90% of the ileal fluid delivered to the colon will be reabsorbed from the gut lumen. Sodium ions are transported actively from the lumen to the blood, while chloride ions are absorbed in a process linked to the secretion of bicarbonate by colonocytes.

Summary

• The digestion and absorption of macronutrients (carbohydrates, lipids, proteins) and micronutrients (vitamins and minerals), essential for the requirements of the human body, take place predominantly in the small intestine.

• In most cases digestion occurs both in the gastrointestinal lumen and at the mucosal surface.

• Substrate-specific enzyme processes exist for polysaccharide carbohydrates and polypeptides.

• Lipid handling is more complex, requiring the creation of intraluminal emulsions and the micelles which present lipid degradation products to the enterocytes.

• The intestine (small bowel and colon) has additional mechanisms for absorption and re-absorption of water and key electrolytes.