Digestion of carbohydrates, proteins and fats

Carbohydrates, proteins and fats are the main constituents of our diet and each presents the digestive process with different problems. When we eat them they are in the form of complex structures and large molecules. Even when these are chewed and subsequently pulped in the stomach they are usually too large to be absorbed directly, which means that they have to be broken down further in the small intestine.

Dietary proteins are massive molecules made up of chains of amino acids which are connected with each other to form an intricate lattice. Protein digestion begins in the stomach where the molecules are attacked by hydrochloric acid and the powerful gastric enzyme pepsin; this breaks them down into polypeptides, which are long chains of amino acids. When chyme enters the duodenum one of the intestinal hormones, enterokinase, converts the inactive enzyme trypsinogen (produced in the pancreas) into trypsin, which together with chymotrypsin divides polypeptides into smaller chains of amino acids. A further group of intestinal enzymes, the peptidases, are responsible for the final breakdown of those chains into separate amino acids.

Starch is a polysaccharide carbohydrate, a structure made up of smaller sugar molecules. This is partly digested in the mouth with the secretion of the salivary enzyme amylase, although these, in turn, are then neutralized and digested in the stomach. Amylase is secreted in a much greater quantity in the pancreatic juice. It breaks down starch into disac-charides, which are molecules made up of two different sugars, such as maltose, lactose and sucrose. Specific enzymes maltase, lactase and sucrase are secreted which then split them into the monosaccharides, glucose, galactose and fructose; these are then easily absorbed.

Fat needs to be emulsified – mixed with water – a process which, by turning it into small particles, gives the fat-absorbing process a head start. The emulsification occurs in the small intestine with the action of the bile salts. The emulsified fats may then be broken down into fatty acids and glycerol by the action of the enzyme lipase. These, together with some undigested fat, are then absorbed.

Absorption of nutrients

The products of digestion are mostly absorbed in the small intestines. Virtually nothing is absorbed in the mouth, although some drugs such as glyceryl trinitrate (given for angina pectoris) may be absorbed directly into the circulation by being put under the tongue. The stomach has little absorptive power, allowing only small amounts of water, dissolved salts and alcohol to be absorbed. The small intestine is the most efficient organ for this process because nature has provided it with a relatively large surface area, which occurs as a result of several features.

In addition to its considerable length, the mucosa lining the small intestine lies in folds. This acts to triple the surface area. Also there are about 20 to 40 villi in every square millimetre. These are finger-like projections approximately 1mm long, each with its own blood supply and lymphatic drainage. The outermost cells of the villi are lined by the even more intricate microvilli. The effect of all these folds, fingers and micro-fingers is to increase the surface area by about 200 times. Once the products of digestion have been absorbed they are transported by the portal blood stream and by lymph in the lymphatic system. There is a substantial blood flow through the intestines and all blood drains from them into the portal vein, to pass through the liver before rejoining the general circulation. The lymph joins the circulation directly.

Water is absorbed, together with substances dissolved in it, both in the small and the large intestines. Sodium is absorbed on the external surface of the cells lining the villi in the small intestine. Potassium, another of the important electrolytes, calcium and iron are absorbed in the upper small intestine. Sugars are absorbed both as monosaccharides and disaccharides, the latter being broken down by enzymes to monosaccharides at the inner lining of the duodenal and jejunal mucosa. The sugars then diffuse into the lymphatic and capillary blood vessels, but because the blood is moving one thousand times faster than the lymph, virtually all absorbed sugars enter the portal bloodstream.

As with the absorption and digestion of carbohydrates, the final stage of protein digestion, which is the breakdown of polypeptides to amino acids, takes place at the inner surface of the intestine. The majority of the absorbed amino acids enter the portal bloodstream.

While some fat is absorbed as digested breakdown products – fatty acids and glycerol – it is now believed that more than half the fat is absorbed directly into the mucosal cells in the form of small globules or ‘micelles’ which pass between the microvilli and are ingested as small protein lined packets – the chylomicrons. If emulsification does not occur, this process is impossible. The chylomicron passes into the chyle, the lymph fluid, which eventually reaches the blood system without passing through the liver. The bile acids which have been essential for the emulsification of the fat particles are themselves absorbed in the small intestine. They enter the portal vein, and thereby return to their place of origin, the liver. This is called the entero-hepatic circulation. Finally, at the end of the digestive process there are two types of waste product, faeces and gas. Faeces are made up of indigestible food, such as fibre and bacteria from the large intestine. The gas is mostly swallowed air, and is present throughout both the small and large intestine. There is also some methane, a product of bacterial decomposition in the large bowel.