Digestion is the process that breaks down food and drink into its basic chemical constituents. These basic chemicals can then be utilised within the body's own systems either as they are or rebuilt into new larger molecules. Digestion takes places in stages as the food and drink moves through the gut, enabling all usable material to be extracted efficiently as it passes through. The whole digestion process takes between a few hours and several days to mainly complete, but fibre can remain in the long intestine for weeks or even months before being removed from the body.
Digestion is a collaborative process between physical and chemical reactions controlled by the human body and microbial activity of gut microorganism that, also, facilitate the breakdown of food molecules; it is important always to keep in mind this balance between human digestive activity and symbiotic activity by non-human microorganisms within the gut as an important factor in digestive health. Another metaphor is to see the stomach as outside the body - a vast tube through the centre of the human body that takes complex molecules that are not usable by the human body and breaks these down into their smaller components for absorption into the human body, so they can be used as the building blocks to make all the complex molecules within ours bodies for their structure, energy and living.
The first stages of digestion occur in the mouth. Here, chewing breaks solid food down into smaller pieces, which opens up the inside of the food and creates a larger surface area that is exposed to the actions of digestive enzymes. These enzymes include ptyalin in saliva which begins to break down starch in food to simpler sugars.
The chewed food moves down the oesophagus into the stomach. Here, the highly acidic gastric juices - gastric juice includes hydrochloric acid - further helps the breakdown of the complex food molecules into smaller, more digestible molecules. Proteins and starch are broken down by hydrolysis from acidic actions, assisted by the physical movements of the stomach wall that churns the food around. Furthermore, pepsin, a digestive enzyme, starts to break down proteins into smaller chains of amino acids, called polypeptides.
Food remains in the stomach for several hours. The length of time the food stays in the stomach depends in part on the proportion of fat in the foods; Fat relaxes the stomach wall and slows down the whole digestion process. Fatty foods appear to be more filling because they hang around the stomach for longer than less fatty foods.
Some nutrients become simple enough to be absorbed from the stomach into the bloodstream. These nutrients move across the stomach wall into the bloodstream. These include some glucose, water-soluble minerals and vitamins B and C. Any alcohol is absorbed here, so gets into the bloodstream and circulates to all parts of the body - especially the brain - relatively quickly after being imbibed, before being broken down later in the liver to sugar.
The Small Intestine
The partly-digested food moves from the stomach into the duodenum, the first section of the small intestine; the small intestine, although small versus the large intestine, is still an impressive 6 m (20 feet) long in an adult. The duodenum contains: pancreatic juice from the (obviously) pancreas; bile from the gall bladder that emulsifies fats, breaking them down into smaller particles for digestion. In the duodenum, three main enzymes are at work: amylases like ptyalin continue to break down starches to sugars; trypsin continues to break proteins down to polypetides; lipases start to digest fats, breaking these down into fatty acids and glycerol. When food leaves the duodenum, the stomach acid has been neutralized and the rest of digestion carries on in slightly alkaline conditions.
From the duodenum, food continues down the small intestine along the jejunum and then into the ileum before leaving and entering the large intestine. By the time food leaves the small intestine, almost all the useful chemical components of your food have been broken down and absorbed into the human body for use by the body.
Within the rest of the small intestine, your food is digested by more enzymes: invertase splits sucrose from glucose and fructose; maltase breaks down maltose to glucose; lactase turns lactose into glucose and galactose (unless you don't have lactase and so are lactose intolerant); erepsin splits protein fragments down further into single amino acids; lipases finish breaking down fats into fatty acids and glycerol. The result complex molecules are broken down into their simpler building blocks - proteins to amino acids, sugars to monosaccharides and fats to fatty acids and glycerol, all of which are small enough to be absorbed across the intestinal wall into the blood system for rebuilding by other enzymes into the biochemical structures within the body. Liberated nutrients also include vitamins and minerals that were not absorbed in the stomach.
The Large Intestine
The large intestine is a wider tube that the small intestine, which runs from the base of the abdomen down to the anus. Its function is mainly to absorb water from the food remains, so that it becomes a compact mass of faeces, so these indigestible remains of your food can be eliminated through defaecation. During this stage, dietary fibre is key, because it retains water, hence the faeces is quite liquid, and it is fairly bulky so it keeps moving down from the small intestine to the anus.
In addition, further digestive processes continue to extract any last parts of your food that have not yet been utilised. The large intestine contains bacteria that help to break down undigested solids - much like bacteria aid digestion in ruminants - and help build [[Vitamin B|vitamin B}} for absorption into the body. The microorganisms digest some of the carbohydrates that the body cannot deal with, but none of these are then taken up by the body. Another well-known product of microbial action is methane, which cause flatulence and some discomfort.
Digestion does not always proceed smoothly. For example, if you are missing one of the key enzymes - e.g. lactase - then you might find it difficult to digest particular foods, or if you overload your digestive system by consuming too much rich and fatty foods. Stress - e.g. anxiety - can increase the rate of secretion of stomach acid and irritate the stomach wall, or conversely it can reduce the rate of secretion and so slow down digestion. The results are described colloquially as: indigestion, dyspepsia, acidity and heartburn. Then, there are more serious digestive complaints or illnesses: coeliac disease; Crohn's disease; ulcers etc.
Good diet is important for good digestion, and is an important part of what and how we should eat. Many modern disorders can, or may, be linked straight back to our diet and the health of our digestive systems. In general, modern diet has too much sugar, too little unprocessed food and not enough fibre.