The Nutrition Debate #53: On the Digestion and Absorption of Food

Food digestion physiology varies between individuals and with the composition and size of a meal. There are other factors, but a primer on what is most common and universal will be useful to understanding the various processes and their effect on our biological systems. This digest (note the pun) should provide some enlightenment.

Digestion, the hormonal process, has three phases. The cephalic phase begins when receptors in the head are stimulated by an emotional state (thought of food), sight, smell, taste and chewing. The cerebral cortex, the medulla oblongata and the hypothalamus are all involved. The vegus nerve is the messenger, triggering various enzymatic and hormonal activities. The gastric phase starts with distension, acidity, and the presence of amino acids and peptides in the stomach. The intestinal phase is initiated by distension, acidity, and osmolarity of digestive products in the intestine (we’ll explain).

Digestion, the mechanical and chemical process, begins with chewing. Chewing breaks food down to smaller units that are in turn broken down by enzymes to the smallest units to permit them to be absorbed, mostly in the small intestine, into the blood. Saliva containing mucus and the enzyme amylase is secreted from three pairs of salivary glands located in the mouth cavity. Mucus moistens the food and amylase partially digests polysaccharides (starches) into a disaccharide called maltose. About 30% of starch is broken down in the mouth (optimum pH: 6.8 or weakly acidic).

In addition, papillae on the surface of the tongue secrete the enzyme lingual lipase which begins the process of breaking down some fats from triglycerides into diglycerides. Long chain triglycerides cannot be absorbed unless completely broken down to monoglycerides, so the process starts here and continues in the stomach. As much as 30% of fat is broken down within 1 to 20 minutes of ingestion by lingual lipase alone, according to Wikipedia.

Food then passes through the pharynx and descends the esophagus to the stomach, a sac that stores and mixes and processes the food into a milky solution called chyme. Glands lining the stomach secrete hydrochloric acid, which is necessary for protein digestion by the enzyme pepsin. The stomach’s high acidity (optimum pH 1.8) inhibits the breakdown of carbohydrates within it, but it does produce a small amount of the enzyme lipase to continue the digestion of fats. So far, although the breakdown of food particles by enzymatic and mechanical action is continuing apace, virtually no absorption of nutrients into the bloodstream has occurred. The exceptions are water, some simple sugars, and other small molecules like alcohol that are absorbed in the stomach, passing through the membrane of the stomach and entering the circulatory system directly.

The final stages of digestion and most of the nutrient absorption occur in the next portion of the tract: the small intestine. The small intestine is divided into three segments – duodenum, jejunum and ileum. The duodenum in turn is connected to the hepato-pancreatic duct which connects to the liver and the gall bladder (to store and supply bile) and to the pancreas. These organs and glands provide digestive enzymes and an alkaline fluid (pH 8.5) to neutralize the acid being emptied from the stomach.

In the duodenum, dipeptides from protein in the stomach are broken down to amino acids by enzymes from the pancreas. The enzymes break down the disaccharides maltose, lactose and sucrose into the monosaccharides glucose, fructose and galactose.  The major portion of fat digestion, the breakdown of diglycerides by lipase into free fatty acids and glycerol, also happens here.

Absorption of nutrients occurs mostly in the jejunum and ileum. Amino acids (from protein) and the monosaccharides (glucose, fructose and galactose, all from carbohydrates) are water soluble nutrients and enter the blood directly through the villi and microvilli in the endothelial layer of the small intestine wall. The surface area of these many small structures (imagine a shag carpet) increases the absorption interface by 600 times, roughly equivalent to the surface of a tennis court. However, the products of fat digestion, fatty acids and glycerol, are not water soluble and therefore enter the circulation through the lymph system by a process called passive diffusion in which no energy is required.

Passive or simple diffusion requires no energy input from the body because it is driven by concentration. Fats, water and some minerals simply cross the membrane barrier because the concentration of the substance (fat, water, etc.) is lower on the other side. It is an equalization process. The small intestine is full of nutrients and the blood is not, so they cross over. This equalization process is also called osmosis, ergo ‘osmolarity of digestive products’ referred to above. 

Fructose is transported across the membrane barrier by a process called ‘facilitated diffusion,’ which means it may need a little help depending on the concentration gradient. Glucose and galactose, the other monosaccharides, as well as amino acids, also require energy for active transport, which occurs against a concentration gradient. See The Nutrition Debate #52, “The Thermic Effect of Foods,” for the energy requirements of protein digestion as compared to carbohydrates, especially of processed foods.

Finally, gastric emptying, which is the rate that food leaves the stomach to enter the small intestine, is tightly controlled. Liquids are emptied much more quickly than solids, and carbohydrates are emptied first, followed by protein, fat and fiber. Gastric emptying has attracted medical interest as “rapid gastric emptying is related to obesity and delayed gastric emptying syndrome is associated with Type 2 diabetes, aging and gastroesophageal (acid) reflux or GERD.” But that’s another subject. Next week: Paleo book reviews. 

© Dan Brown 5/20/12