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May 29, 2014

Digestion, absorption and metabolism of carbohydrates

by Helene van den Berg

Carbohydrates ingested, are mostly in the form of poly- and disaccharides and need to be broken down into simple sugars, before absorption can take place. Digestion starts in the mouth, through the salivary enzyme, ptyalin. In the intestinal tract, the pancreatic amylase, hydrolyzes the remaining starch to maltose.

No matter what we eat, whether it’s a juicy fillet steak or a crispy apple or a cheese cake, are nothing more and nothing less than proteins, carbohydrates and fats. To convert these three basic food materials into biochemical substances we can use, we need three groups of enzymes, the protein-digesting enzymes (protease), the fat-decomposing enzymes (lipases) and the carbohydrate-decomposing enzymes (amylases). The enzymatic transformation starts as soon as we take the first bite. Our bodies have a preference for carbohydrates and our digestive system acts on them first, then on the proteins and finally on the fats.

In the mouth, the well-chewed food, is mixed to a paste with the saliva; then it is transported through the esophagus into the stomach-(the more the food has been broken down and mixed with the saliva, the better for the stomach – therefore, eat slowly!). The stomach sends signals, by means of hormones, to the gallbladder and the pancreas, to make sufficient enzymes available to digest the food in the intestinal tract. The amylases in the stomach proceed digesting the carbohydrates, initiated by the saliva in the mouth. To decompose the proteins in the food paste, the stomach produces one to two liters of gastric juice every day, containing hydrochloric acid and several protein-degrading enzymes, pepsin and cathepsin. (Babies produce a special enzyme, chymosin, to break down milk protein).

Cathepsin are naturally present in the animal tissues we eat . The richest sources are liver, kidney and spleen, but they are also present in meat and fish. They are released when these foods age and are responsible for the decomposition of these foods. When we eat meat fresh, the cathepsins are initially released after the action of the hydrochloric acid in the stomach and continues to be released as the digestion progresses.

Our own internal cathepsins are important in the breakdown of damaged tissues (injuries, wounds) and in the breakdown of undesired cells , such as early cancerous cells.

There are 5 principal type of cathepsins: B, C, D, G and L:

  • Cathepsin B – hydrolyses protein with a specificity resembling papain.
  • Cathepsin C – proteolysis of prothrombin to thrombin
  • Cathepsin D – metabolism of cartilage and connective tissue. It also resembles pepsin A in it’s actions.
  • Cathepsin G – has immunological properties.
  • Cathepsin L – hydrolysis of proteins.

Hydrochloric acid has very important functions: it activates the protein-degrading enzymes; it stimulates hormone production in the stomach; destroys bacteria present in the food paste; promotes the uptake of minerals and trace elements into the bloodstream, some of which serve as coenzymes.


The monosaccharides, which have been absorbed from the intestinal tract, are carried to the liver by portal circulation. The fructose and galactose are converted into glucose. The liver cells release this glucose into the bloodstream to be distributed to the body tissues as a source of energy. The excess glucose is stored as insoluble glycogen in the liver cells. The conversion of glucose to glycogen is reversible – as the need arises, the liver mobilizes glucose from it’s glycogen stores and releases it into the bloodstream to maintain an optimal blood sugar level.

When glucose are absorbed from digested food in the small intestines, the blood sugar level rises. The liver counteracts this rise in blood sugar levels through glycogenesis (glucose converted into glycogen). Skeletal, cardiac and smooth muscles, also store glycogen from glucose, when at rest or under minor work loads; glycogen is found in small quantities in all organs of the body. The amount of reserve glycogen stored in the liver and muscle depends largely on the nature of the diet and the amount of exercise. Cardiac muscle preserves it’s reserves of glycogen tenaciously, by using blood glucose as an energy source for its muscular contraction. Brain tissue uses blood glucose(carbohydrates) almost exclusively for its energy metabolism, while skeletal muscle utilize carbohydrates or fatty acids to obtain energy for its activities.

The metabolism of carbohydrates is geared to the digestion of complex carbohydrates in the gastrointestinal tract: e.g. digestive enzymes in saliva and pancreatic secretions break down complex polysaccharides into mono- and disaccharides. These disaccharides are further digested by enzymes in the lining of the small intestines into monosaccharides, which are easily absorbed into the bloodstream, from which they pass to the liver. (refined foods bypass these digestive systems!).

Certain hormones are involved in the control of carbohydrate metabolism – insulin, adrenalin and glucagon. They work together to maintain a steady blood glucose level. The major function of glucose in the body is to produce energy, although some glucose is involved with the metabolism of cholesterol, amino acids and fat. If someone starves, the glucose stores lasts for 24-48 hours; after this glucose must be produced from the body’s fat. For glucose to be used, it needs insulin and potassium, to cross the cell membranes into the cell substance itself. Diabetics who have no insulin, or who’s body cells are unresponsive to insulin, have a high blood glucose level as result. When excess insulin is injected, the blood glucose levels falls dramatically, producing a temporary clouding of consciousness or even complete loss of consciousness. This is corrected by giving glucose rapidly by mouth or intravenously.

Therefore, insulin lower blood glucose levels. Adrenalin and glucagon raise blood glucose levels.

Carbohydrate metabolism is also affected by liver disease. Glucose, glucagon and galactose tests are abnormal with a liver dysfunctioning. In liver dysfunction glycogen storage is impaired and the administration of glucagon, leads to hypoglycemia. Galactose tolerance tests are abnormal in liver disease, since the ability to convert galactose to glucose is impaired.

The body has a limited capacity for storing glycogen, but the ability to store fat is not nearly so restricted. When carbohydrates are supplied by the diet in excess of what is needed for energy or glycogen storage, they are converted into fat and stored in the adipose tissues. Therefore, any food (includes fat and protein as well) ingested in excess of caloric expenditure, are stored as fat!. A certain minimal amount of carbohydrate must be supplied by the diet at all times to prevent the use of dietary protein as an energy source from the body’s metabolic pool of structural components (protein sparing). Protein sparing is obtained when carbohydrates and protein are ingested simultaneously.


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