Digestion is the process of breaking down food to release nutrients, which the body uses for energy, growth and the repair of cells. Food and drink contain large molecules (such as protein) that must be turned into smaller molecules (such as amino acids) before they are absorbed into the bloodstream and taken to cells throughout the body, where they are needed for various biological processes.
The digestive system consists of a series of organs, each of which plays its part in the conversion of food into nutrients are absorbed into the body and anything left over unused is eliminated as waste. Our bodies depend on the daily provision of adequate amounts of nutrients, and on being able to eliminate waste products that can otherwise be toxic, so health can be compromised when the digestive system does not work properly.
Each section of the digestive system is controlled and regulated by nerves and hrmones that trigger what are known as homeostatic mechanisms, which feed back to the gut an make subtle changes in absorption rates. For example, during prgnancy, hormones signal the gut to absorb larger amounts of minerals such as iron and calcium, which are needed for the growing foetus. While these hormones stimulate the production of digestive juices and regulate appetite, nerves help control the action of the digestive system and release chemicals that speed up or delay the movement of food and the production of digestive juices.
Gut bacteria (also called gut flora or microbiome) are another crucial part of the digestive process. These healthy bacteria are necessary to properly digest food and absorb its nutrients. The digestive system is made up of the gastrointestinal (GI) tract, also known as the digestive tract, and the liver, pancreas and gallbladder. The GI tract is a series of tubes joined to form one long, convoluted tube; it comprises the mouth, oesophagus, stomach, small intestine (including colon and rectum) and anus. The entire system from mouth to anus is up to about 9 metres (30 feet) long.
The digestive process can start almost as soon as you smell or even think about food. The release of saliva in the mouth is the first stage. Saliva plays two roles: it moistens food to make it easier to form a ball, known as a bolus, and it contains an enzyme known as salivary amylase, which starts the breakdown of starch in food. Chewing and grinding food are also important for forming this soft bolus.
Swallowing pushes the bolus into the oesophagus, where it passes through the oropharynx and then the hypopharynx in the throat. At this point you cannot help but continue, as the action becomes involuntary. The oesophagus, like other parts of the gut, is made up of rings of muscle that contract and relax in a rhythmic sequence, squeezing the bolus further down towards the stomach. This process is known as peristalsis. The lower oesophageal sphincter, a ring-like muscle at the junction of the oesophagus and stomach, controls the passage of food and liquid. As food approaches the closed sphincter, the muscle gradually relaxes and lets it pass through to the stomach. The stomach then releases gastric juice, which is a mix of hydrochloric acid and the enzyme pepsin. Pepsin starts to break down proteins into peptides and amino acid, while the hydrochloric acid kills potentially harmful bacteria. Food will stay in the stomach for two to three hours and eventually form a thick paste known as chyme.
Once chyme has been formed, the muscle at the bottom of the stomach, called the pyloric sphincter, opens and the chyme enters the duodenum, the first section of the small intestine, where it mixes with more digestive enzymes released from the pancreas and also bile acids from the gallbladder. The pancreatic enzymes are responsible for the further breakdown of carbohydrates as well as fats and proteins. The bile acids are needed for the breakdown of large fat molecules into smaller molecules that can be absorbed. The chyme then passes into the ileum (the last section of the small intestine), where most of the nutrients are absorbed.
Specialized cells help absorbed materials cross the intestinal lining into the bloodstream, which carries sugars, amino acids, small fat molecules and some vitamins and minerals to the liver. The lymphatic system, which is connected to the gut (an everyday term for the gastrointestinal tract), absorbs fatty acids and other vitamins. The liver creates glycogen from sugars and carbohydrates to give the body energy, and converts dietary proteins into new proteins needed by the blood system. The liver also breaks down unwanted chemicals such as alcohol, which is detoxified and passed from the body as waste. Whatever material is left once the liver is finished goes into the large intestine, this part of which is known as the colon. The colon is approximately 5 feet (1.5 meters) long and its primary function is the storage and fermentation of indigestible matter. The colon has four parts: the ascending colon, the transverse colon, the descending colon and the sigmoid colon. In the colon, some of the water from the chyme is absorbed back into the body and faeces are formed with the remaining water, dietary fibre and other waste products, including older cells from the GI tract lining. Muscles push the faeces into the rectum and they are stored here until they are eliminated from the body through defecation.
Looking after your digestive system
A rich meal or an extra spicy curry (accompanied by one too many glasses of alcohol) is unlikely to give you anything worse than a short-lived stomach upset. But overindulging too often can cause problems such as heartburn, constipation or diarrhoea. Eating more fibre is key to preventing chronic constipation. Most people fall short of the recommended dietary intake of 25g to 30g per day. Far a healthy bowel, you need fibre from a variety of sources, such as wholemeal bread, brown rice, fruit and veg, beans and oats. Fluids are also important for digestion, so it is important to drink regularly – especially water, as it encourages the passge of waste through your digestive system and helps soften stools.
It is crucial to increase your intake of both water and fibre, because they work together in tandem: the fibre acts like a sponge, absorbing the water, without which stools will be hard and difficult to pass. Indigestion and heartburn are usually related to eating too much food. The stomach stretches after a big meal and produces more acid, which can either irritate the stomach lining or rise up into the oesophagus. This irritation can be painful and cause a burning sensation. Spicy and fatty foods and fizzy, alcoholic and caffeinated drinks can all aggravate heartburn.