We know that food gives us the energy and building blocks we need to grow, heal and thrive. To do so, it needs to be broken down into small parts that can be absorbed. This process, i.e., digestion, involves a series of tasks that take place as the food goes through various parts of our digestive system.
A healthy digestive system works effectively enough to derive nutrients from a spectrum of different diets that could be governed by geography, climate and/or access to tools. For instance, populations living near the Arctic almost exclusively consume meat-based diets, with little to no vegetables. Many populations focus on strict vegetarian foods, on the other hand. In each of these extremes, the body can still derive the nutrients it needs to thrive.
Its ability to do so depends on the collective functioning of all parts of the digestive system, each of which plays a distinct and unique role.
It may seem as though the mouth plays only a physical role in the process of digestion- but it isn’t only the point at which food enters our body and is chewed. Its job is far more complex, made possible by:
i. The tongue
Our tongue tastes whether a food is palatable and whether we should continue to eat it. From an evolutionary perspective, it was (and is) important to know if a food is helpful or potentially harmful. This is where our sense of taste plays an important role: harmful plant compounds are often bitter to taste, and the instinctive reaction to avoid bitter foods could have dramatically helped our survival. Even today, certain foods taste ‘off’ when they spoil, and recognising this helps us avoid harmful bacteria that lead to food spoilage.
Certain tastes also trigger an increase in the production of saliva from our salivary glands, which is essential for proper digestion.
ii. The salivary glands
Our mouths have three major pairs of salivary glands and hundreds of minor ones. Saliva serves to moisten food as we chew it, making it easier to swallow and move, through our oesophagus, into our stomach.
It also contains digestive enzymes that begin breaking down food in the mouth itself. Saliva contains amylase, an enzyme that breaks down starch into simple sugars, as well as a small amount of lingual lipase, an enzyme that breaks down fats.
iii. The teeth
Our teeth break food down into small pieces, for easier digestion. A sufficient amount of chewing also exposes more of the food to the digestive enzymes in saliva, further promoting their action.
From the mouth, the food makes its way to our stomach, through the oesophagus, a muscular tube that passes from the back of the mouth to the stomach.
The top of the oesophagus connects to the epiglottis, a muscle that directs food into the oesophagus and prevents it from entering the windpipe at its top end. Its bottom end connects to the stomach.
As swallowed food enters the oesophagus, it moves downward, through muscular contractions known as peristalsis, until it reaches the lower oesophageal sphincter, a muscle that separates the oesophagus from the contents of the stomach. This sphincter remains closed, except when we are swallowing food or vomiting, to prevent the acidic contents of the stomach from coming in contact with the sensitive lining of the oesophagus. Issues with the functioning of this muscle lead to acidity/heartburn.
The consumption of food triggers the cells in the stomach into producing gastric acid, which largely consists of hydrochloric acid, proteases (enzymes that break down proteins) and lipases (enzymes that break down fats).
The hydrochloric acid reduces the pH of the stomach to a highly acidic 1.5 – 3.5. This serves two purposes: it kills many microbes that may be present in the food and it activates the proteases, which require a low pH to function.
Some small molecules, such as caffeine, individual amino acids and alcohol, can be absorbed into our bloodstream through our stomach to some degree.
The adult human stomach can hold up to 4 litres of food, with 1 litre being the amount that would make us full. Food in the stomach takes around 2 to 3 hours to move into the small intestine, and it is during this time that the low pH, proteases and lipases work together to break down the ingested food.
As this liquid mass exits the stomach, it is known as chyme.
The Small Intestine
The majority of digestion and nutrient absorption takes place in the small intestine, which, contrary to its name, is normally around 20 feet long, depending on an individual’s height.
Chyme from the stomach makes its way into the duodenum, the first section of the small intestine that’s 10 to 15 inches long. In addition to chyme from the stomach, the duodenum also receives bile from the gall bladder and digestive enzymes from the pancreas.
Bile is a dark green liquid produced by the liver. Bile salts help emulsify fats, allowing them to be absorbed by the walls of our small intestine into the bloodstream.
The digestive enzymes from the pancreas break down carbohydrates, proteins and fats into their individual constituents: sugars, amino acids and fatty acids. In addition to these enzymes, the pancreas also produces bicarbonate, which helps neutralise the acidity of chyme as it moves through the small intestine.
The internal wall of the small intestine consists of tiny, finger-like protrusions called villi that are filled with blood vessels. The purpose of villi is to increase the surface area that’s in contact with the food. Nutrients pass through the intestinal wall into the blood supply, either by passive diffusion or active transport.
Passive diffusion is the physical flow of nutrients across the gut barrier, whereas active transport involves specialised receptors on the gut barrier that transport molecules into the bloodstream.
The duodenum leads to the jejunum and subsequently the ileum, each of which is 6-10 feet long. These sections of the small intestine form a series of loops and folds that help increase the surface area for absorption of nutrients.
Different nutrients are absorbed best through different parts of the small intestine. For example, iron is absorbed in the duodenum, whereas vitamin B12 is absorbed in the ileum. Food spends between 1 and 5 hours in a healthy small intestine, with the average being about 1.5 hours.
After reaching the end of the ileum, the body has absorbed as much as it can from the food and it moves into the large intestine.
The Large Intestine
The large intestine, or colon, is the final part of the digestive system, prior to the elimination of waste. This section is approximately 5 feet in length and is mainly responsible for reabsorbing the water from digested matter as it moves towards elimination.
Chyme enters the colon as a largely liquid mass. Its water content is continually absorbed by the walls of the colon, solidifying it as it passes through.
The colon also contains the majority of our gut microbiome, a population of approximately 1 trillion bacteria that live in our digestive tract. These bacteria are generally not harmful and can in fact contribute to good health. An individual’s colon can have hundreds of different bacterial species, which have varying effects on his/her health, based on their individual characteristics and the type of food that’s eaten.
When the chyme moves from the small intestine to the colon, most of its digestible foods have been broken down and absorbed. Substances that cannot be digested remain intact at this stage and serve as fuel for colonic bacteria. Their preferred energy source is indigestible carbohydrates, collectively known as dietary fibre, especially of the soluble variety.
When these bacteria digest soluble fibre, they produce byproducts that are incredibly beneficial to human health, such as vitamins B12 and K, which are essential to our survival, as well as short chain fatty acids, which protect against colon cancer. In addition to this, gut bacteria have also been shown to activate antioxidants found in foods such as grapes, cranberry and soya, which subsequently lead to their many benefits.
It is believed that these bacteria share a symbiotic relationship with their human host. They receive a source of fermentable carbohydrates (soluble fibre) and vitamin D3, and in turn, help break down indigestible material and provide vital micronutrients to the host. Ensuring a healthy microbiome is essential for good digestive health.
The time that chyme takes to move through the large intestine (called the ‘Colonic Transit Time’, or CTT) varies considerably between individuals. It is believed to be influenced by a number of factors, including:
ii. Diet, specifically the amount of fibre, water and fat consumed
iii. The population of gut bacteria, which influence the way foods are metabolised and move through the colon
iv. Geographic location: it’s believed that individuals living in the tropics have shorter CTT than those living in temperate regions
v. Physical activity level, especially for women
vi. Food allergies and digestive diseases
As is evident, a number of factors can affect CTT, and defining a standard is extremely difficult. Studies in western populations have shown the CTT to range from 40 hours to 84 hours, whereas studies in Indian populations have shown this to be dramatically lower, at an average of 15 hours.
Even though the colon absorbs water, over 75% of healthy stool is essentially water, with the remaining 25% consisting of undigested food, metabolic waste, dead cells from the gut lining and bacteria from the colon.
Digested matter makes its way to the end of the large intestine, the rectum, where it is stored till elimination.
Tips for healthy digestion
i. Chew your food well:
The age-old advice of chewing your food thoroughly helps physically break it down into smaller pieces as well as allows salivary enzymes to begin breaking down carbohydrates and fats.
ii. Avoid antacids unless necessary:
Popping an antacid after a heavy meal has become a very common practice. However, antacids neutralise or suppress the formation of gastric acid, which may relieve immediate symptoms, but makes it difficult for our body to break down food. Consuming an acid such as lime juice or apple cider vinegar after a heavy meal can help relieve symptoms of heartburn.
iii. Add fibre rich fruits and vegetables to your diet:
Dietary fibre helps add bulk to stool as it moves through your body and, as mentioned, supports a healthy microbiome.
iv. Stay well hydrated:
Water plays an instrumental role in moving food through your digestive tract. Not being sufficiently hydrated can slow down motility and lead to constipation.