Introduction
Consumption of carbohydrates provides the body with glucose, the principal form of energy. In the Western diet, they contribute 40-60% of calories, whereas in protein-scarce diets, they comprise a higher percentage in protein-scarce diets. Four grams of carbohydrates provide one calorie. A series of chemical reactions are required for carbohydrates to be digested into monosaccharides that can be absorbed by the small intestine.
Dietary carbohydrates are broken down into three groups: simple carbs (monosaccharides), disaccharides and complex carbs (starch, fibre, glycogen). There are five types of monosaccharides: glucose, galactose, fructose, xylose and ribose. Fructose is the sweetest of the molecules. For absorption, disaccharides need hydrolysis for separation into two monosaccharides. Three disaccharides are of interest—lactose (glucose and galactose), sucrose (glucose and fructose) and maltose (glucose and glucose). Glycogen (straight and branched chains of glucose), starch (amylose, amylopectin)and glycolipids are complex carbohydrates.
Digestion, Absorption and Metabolism of Carbohydrates
The body uses carbohydrates for energy storage and nutrition. The carbohydrate content of many foods is high, including bread, potatoes, bananas and honey. The main source of carbohydrates in the human diet is polysaccharides, mostly in the form of starch. Humans must metabolise carbohydrates to use them as energy. The body absorbs glucose, which is converted from complex starches into monosaccharides. Carbohydrates begin to be digested in the mouth by salivary amylases. Pancreatic amylase further breaks down the remaining starch in the intestines. A gram of carbohydrate can provide 4 kcal of energy.
Carbohydrate metabolism pathways
- Glycolysis
- Gluconeogenesis
- Glycogenolysis
- Glycogenesis
- Pentose phosphate pathway
- Fructose metabolism
Absorption of Carbohydrates, Proteins and Lipids
Digestion of Carbohydrates
When carbohydrates are digested, disaccharides, polysaccharides and oligosaccharides are oxidised to monosaccharides.
- Enterocytes easily absorb hexoses (six-carbon sugars) such as glucose from the diet
- It is necessary to break down polysaccharides and disaccharides into smaller sugars in order to absorb them
Mouth:
- Starches are broken down by salivary amylase into maltose and polysaccharides
- In the stomach, amylases do not function because of the high pH
Stomach:
Because of its highly acidic pH, the stomach is unable to digest food properly.
Small intestine:
- Other enzymes, such as pancreatic amylase, break down carbohydrates
- Microvillus membrane on brush border digest oligosaccharides and disaccharides
Absorption of Carbohydrates
A portal circulation facilitates the absorption of carbohydrates after digestion. Transportation can either be active, passive or facilitated.
- Active transport: The transport of carbohydrates across the plasma membrane occurs through the use of transporter enzymes against the gradient of concentrations.
- Facilitated Transport: It is facilitated by transmembrane enzymes that do not require energy to move down concentration gradients.
- Passive Transport: Sugars are passively absorbed down concentration gradients without enzymatic activity or energy; this mechanism lasts the longest.
The functions of transporters vary according to their roles: passive, active or facilitated.
- SGLT1:
- Functions to transport glucose through the small intestine
- Based on an ATPase pump (Na+/K+-ATPase pump) actively generating sodium gradients
- The transport system transports to sodium, galactose or glucose and water
- Glucose transporter 2 (GLUT2)
- Located on the basolateral membranes of the small intestine and kidney, pancreas and liver
- Facilitates glucose and fructose diffusion
- By being bidirectional, the function can be altered based on cellular conditions.
- GLUT4
- Mostly found in adipose tissue
- Insulin regulates it
- Depends on cellular conditions to store glucose
- GLUT5
- Facilitates the diffusion of fructose
Passive glucose absorption is rare. The first part of the small intestine (duodenum, jejunum) absorbs most of the glucose.
Digestion of Proteins
- Consuming protein in the diet provides amino acids used to make cellular components
- A protein’s absorption requires it to be broken down into amino acids. Dipeptides, tripeptides and amino acids are digested products of protein
- A peptide-degrading enzyme called endopeptidase hydrolyzes the interior peptide bonds of proteins, while an exopeptidase enzyme hydrolyses one amino acid at a time from the C-terminus of proteins
- The small intestine and the stomach are involved in digestion
Absorption of Proteins
The absorption of proteins occurs in the small intestine, with various enzymes acting upon them.
- Free amino acids
- In the luminal membrane, cotransport of Na+- dependent amino acids take place
- Glucose and galactose cotransporters are analogous to this transporter
- Facilitated diffusion transports amino acids from the cell to the blood
- Four different carriers exist for neutral, acidic, basic and imino amino acids, respectively
- Dipeptides and tripeptides
- As compared to free amino acids, they are absorbed faster
- The luminal membrane is also capable of cotransporting dipeptides and tripeptides
- Cytoplasmic peptidases hydrolyse the dipeptides and tripeptides into amino acids after they are transported into intestinal cells
- Facilitated diffusion transports the amino acids from the cell to the blood
Digestion of Lipids
- Due to their insoluble nature in water, fats are difficult to digest and absorb
- Stomach and intestinal contents cannot mix with fat
- Among the many lipids in our bodies are triglycerides, phospholipids, cholesterol, steroids, and fat-soluble vitamins
- It is essential for lipid digestion to emulsify large lipid droplets in order to form tiny lipid droplets
- It decreases the droplet size of the lipid exposed to digestion enzymes by emulsifying it
Absorption of Lipids
- The absorptive surface of intestinal cells is in contact with the products of lipid digestion through micelles
- Over the luminal membrane, fatty acids, monoglycerides and cholesterol diffuse into the cells
- Hydrophilic glycerol does not form micelles
- Cholesterol ester, phospholipids, and triglycerides are re-esterified from lipids in intestinal cells, along with apoproteins, forming chylomicrons
- Exocytosis is the process by which chylomicrons are transported from intestinal cells
- Due to the size of the chylomicrons, they cannot enter capillaries
- This causes them to bypass lymph nodes and enter the bloodstream via the thoracic duct
Conclusion
The body uses carbohydrates for energy storage and nutrition. The main source of carbohydrates in the human diet is polysaccharides, mostly in the form of starch. Humans must metabolise carbohydrates in order to use them as energy. The body absorbs glucose, which is converted from complex starches into monosaccharides. Carbohydrates begin to be digested in the mouth by salivary amylases. Pancreatic amylase further breaks down the remaining starch in the intestines. A gram of carbohydrate can provide 4 kcal of energy.
Bread, beans, milk, popcorn, potatoes, cookies, spaghetti, soft drinks, corn and cherry pie are just a few examples of carbohydrate-rich foods. They also come in a range of shapes and sizes. Sugars, fibres, and starches are the most common and abundant types.