Food digestion physiology varies between individuals and with the composition and size of a meal. 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), from Wikipedia, should provide some useful information.
Digestion, the hormonal process, has three phases. The cephalic phase begins when receptors in the head are stimulated by the thought, sight or smell of food, 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, as 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, permitting them to be absorbed into the blood, mostly through the wall of the small intestine. Saliva containing mucus and the enzyme amylase is secreted from salivary glands located in the mouth. 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).
In addition, papillae on the surface of the tongue secrete a small amount of the enzyme lingual lipase which begins the process of breaking down certain fats from triglycerides into diglycerides. Long chain triglycerides cannot be absorbed unless completely broken down to monoglycerides, so the process starts in the mouth and continues in the stomach. As much as 30% of fat is broken down in 1 to 20 minutes by lingual lipase, according to Wikipedia.
Food then passes through the pharynx and descends the esophagus to the stomach, a sac that stores, mixes and processes food into a milky solution called chyme. Glands lining the stomach secrete hydrochloric acid, which is necessary for protein digestion. The stomach’s high acidity (optimum pH 1.8) inhibits breakdown of carbohydrates within it. However, a small amount of the enzyme lipase is secreted 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, 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 to the gall bladder and pancreas, providing digestive enzymes and an alkaline fluid (pH 8.5) to neutralize acid emptied from the stomach.
In the duodenum, dipeptides from partially broken-down protein from the stomach are broken down to amino acids by these enzymes. Other enzymes break down the disaccharides (maltose, lactose and sucrose) into the monosaccharides glucose, fructose and galactose. The major portion of fat digestion, 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 and enter the blood directly through the small intestine wall. 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.
Passive diffusion requires no energy input from the body because it is driven by concentration. The small intestine is full of nutrients and the blood is not, so they cross over. This process is also called osmosis. Monosaccharides (“sugars”) and amino acids from protein, are transported across the membrane barrier by a process called “facilitated diffusion,” meaning they need a little help because they occur against a concentration gradient.
Finally, gastric emptying –the rate that food leaves the stomach to enter the small intestine – is tightly controlled. Liquids are emptied more quickly than solids; then carbohydrates are emptied, followed by protein, fat and fiber.