What is our digestive system?
Our digestive system, also referred to as our gastrointestinal system, consists of a number of tube-like organs that start at the mouth and end at the anus. There is approximately nine meters of these connected tube-like organs inside each of us. It includes our mouth, throat, oesophagus, stomach, small intestine, large intestine, rectum, anus, pancreas, liver and gallbladder. [1,2]
Our digestive system also includes approximately 100 trillion microorganisms that live in our gastrointestinal tract, known as gut microbiota or gut flora. These microbiota provide health benefits to us as their host and also support healthy digestion.
What is the role of our digestive system?
Our digestive system is responsible for converting the food and liquids we consume into energy and eliminating the indigestible components as waste products. It allows food to be broken down into nutrients, vitamins and minerals so they can be absorbed in the bloodstream and used by the body for energy, growth and repair. Carbohydrates are broken down into simple sugars glucose, fructose and sucrose, proteins are broken down into amino acids and fats are broken down into fatty acids and glycerol.[1, 2, 4]
Our microbiota also have an important role in strengthening gut integrity, energy production and immune protection.
How does digestion work?
Digestion is controlled by hormones and nerves that allow your brain and gastrointestinal tract to communicate. Digestion starts in the mouth where food is chewed, grinded and combined with saliva and enzymes to become a small round bolus for swallowing. The bolus is then transported down the throat to the oesophagus. Muscular contractions known as peristalsis then transport the bolus to the stomach. The stomach combines digestive juices with the bolus to breakdown proteins and fats. The stomach also secretes hydrochloric acid to help dissolve the bolus and eliminate any harmful bacteria. The stomach then slowly releases the end product called chime into the small intestines.
The small intestine is the primary site for the digestion and absorption of nutrients. It is responsible for transporting nutrients into the bloodstream and to the liver. Then the remaining food mass passes in to the large intestine. In the large intestine billions of harmless microorganisms feed on the remaining food mass. Dead cells are excreted, and any remaining water is reabsorbed to form a more solid mass known as the stool. Peristalsis then moves the stool from the large intestines to the rectum where it stays until it is excreted from the anus.
The liver, gallbladder and pancreas also have an important role in digestion. The pancreas produces digestive enzymes that are pumped into the small intestine and the gallbladder releases bile for dissolving fats. The liver detoxifies nutrient-rich blood transported from the gut and further processes nutrients, so they can be used by the body. [1, 2, 4]
We don’t often consider the significant amount of work performed by our digestive system as it is mostly an involuntary process. However, while we are busy getting through our day our digestive system is working hard to help us feel our best.
The role of diet on our digestive health
The old saying “we are what we eat” has never been more relevant with evidence showing that our diet plays an important role in our digestive health. What we choose to put into our mouths can impact digestive function and the approximately 100 trillion microorganisms that reside in our digestive tract. These microorganisms called gut microbiota have an important role in our body affecting digestion, inflammation and immune health. 
It is understood that poor dietary habits can impact the number and diversity of microbiota populations causing dysbiosis. Dysbiosis is where there is an overgrowth of unfavourable gut microbiota species. Dysbiosis has been associated with compromised digestive well-being. 
For those of us looking to improve our dietary intake it is reassuring to know that by making healthy dietary choices you can rapidly alter your microbiota for improved digestive health. 
Some dietary sources to include in your diet that have been shown to support healthy microbiota and digestive health include:
Consuming adequate dietary fibre is necessary for digestive health particularly the well-being of the large bowel. The National Health and Medical Research Council recommend that women consume 25g of fibre daily and men consume 30g daily. Some of the best food sources high in dietary fibre include whole grain breads and cereals, vegetables, legumes and fruit. [9,10]
The soluble fibre found in vegetables, fruits, oats and legumes helps to slow the emptying process of our stomachs. This enables us to feel fuller for longer and maintain blood sugar levels.
The insoluble fibre found in nuts, seeds and the skins of fruit and vegetables helps to ensure satiety and maintain a healthy bowel environment. Insoluble fibre supports regular bowel motions through absorbing water to help soften the large intestine contents.
Resistant starch found in cooked and cooled potato and under-ripe bananas is not digested in the small intestine and is transported to the large intestine. Here it is able to support bowel health and assist the production of beneficial microbiota. 
Protein when combined with carbohydrates in the large bowel can contribute to digestive health. Protein is a dietary source of nitrogen that the microbiota in the colon need for growth, the breaking down of carbohydrates and also the production of short-chain fatty acids (SCFA). We need SCFAs as an energy source for our microbiota and tissues as well as to maintain healthy tissue integrity. 
Adequate water is essential for healthy digestive function as low water intake can reduce the water content of stools and contribute to constipation. Ensure that you consume the recommended 3.4L of water daily for men and 2.8L of water daily for women from foods and fluids. You may require greater quantities of water if you are exercising frequently, living in a warm environment or consuming caffeine. [16, 17]
Prebiotics are non-digestible food components that can alter the microbiota living in our digestive tract to offer us health benefits. Prebiotics have the potential to boost digestive health, support the immune system and improve mineral absorption
Aim to include at least 5g of prebiotics in your diet each day for digestive health. Foods that are a good dietary source of prebiotics include onion, Jerusalem artichokes, chicory root, garlic and bananas, however you would need to consume large amounts of each to reach the recommended amount. 
A probiotic is a live organism that when consumed can provide benefits to the host. Probiotics can help aid digestion, produce vitamins, assist nutrient absorption, support healthy immune function and discourage the overgrowth of harmful microorganisms.
Probiotics are a source of live bacteria that introduce and establish good bacteria in our gut, while the above-mentioned prebiotics help to nourish and support our good bacteria.
There are a number of probiotics consumed for their health benefits with some of the most common species being Lactobacillus and Bifidobacterium. Common dietary sources of probiotics include yoghurt and fermented foods such as kimchi, sauerkraut, kefir and kombucha. Probiotics are also available in supplement form and usually have a higher number of probiotics compared to food sources.
Scientific studies have shown the benefit of some particular probiotics for individual digestive conditions,. However to use probiotics for these specific digestive issues it is important to use each probiotic strain at an adequate amount scientifically shown to support the condition, as not all types of probiotics have the same effect in the body. [13, 14]
Prebiotics and Probiotics Explained
How do prebiotics and probiotics work?
The way prebiotics and probiotics function in our digestive tract is similar to the set-up in the video game Pac-Man. If we think of our digestive tract as the maze, the small ‘Pac-Dots’ that Pac-Man eats as prebiotics, the Pac-Man as a probiotic and the ghosts as bad bacteria, it may help to demonstrate their roles:
- Our digestive tract is like the Pac-Man maze where food is digested and prebiotics, probiotics and bad bacteria function.
- Prebiotics are similar to the Pac-Dots that Pac-Man eats as they are the non-digestible fibres that feed the Pac-Man who resembles the good bacteria, or probiotic.
- Probiotics are like the Pac-Man who consumes the Pac-Dots or prebiotics to grow and thrive in the digestive system.
- Bad bacteria are like the ghosts as bad bacteria are also present in our digestive tract and compete with the Pac-Man, or probiotics, to live in the gut. [20, 21]
What is a probiotic?
Probiotics are beneficial live microorganisms, found in food or supplements, that can provide us with health benefits when consumed in adequate amounts. Probiotic microorganisms, also called strains, live in our gut and support digestive and immune function as well as our overall health. [20, 22]
What do probiotics do?
Probiotics support health by breaking down indigestible foods in the large intestines, producing vitamins, supporting nutrient absorption and competing with harmful microorganisms in the gut to maintain a healthy balance of bacteria.
Consuming specific probiotics has also been associated with the management of digestive discomfort, a reduction in antibiotic-associated diarrhoea, improved ability to fight the common cold, reduced eczema and colic symptoms in infants, lactose digestion and improved immunity. Initial research is also showing a promising link between specific probiotics and weight management, brain function and blood sugar regulation. [20, 22]
Where can I find probiotics?
Probiotics are available in a number of food sources and also as dietary supplements. Natural food sources that contain probiotics include yoghurt and kefir. Fermented foods that have not been treated with heat such as sauerkraut, kimchi, tempeh, miso and kombucha also contain beneficial live microorganisms. Including these food sources in your diet may be beneficial in ensuring healthy gut functioning and your gut bacteria remaining balanced.
Probiotics are also available in a variety of supplement forms including capsules, powders and drinks. [20, 22]
How many probiotics do I need?
While we understand that probiotics can benefit human health there is no current recommendation on how many or which specific sources of probiotics we should be consuming for general health.
As the microorganisms in probiotics are really small, the potency of a food source or supplement is measured in colony forming units (CFU). This CFU measure tells you how many live microorganisms you can expect to receive from each serve or dose up until the expiry date of the food or supplement.
The number of microorganisms present in foods and supplements can range from 100 million to over 10 billion. More CFUs are not always better and you should choose a dose and strain of probiotics that have been demonstrated in research to help your health condition. [20, 22, 23]
What is a prebiotic?
Prebiotics are indigestible fibres that are the food source for the beneficial microorganisms that are living in your gut. These beneficial microorganisms help to maintain our health and wellbeing. [21,30]
What do prebiotics do?
Prebiotics are indigestible by human enzymes so we are unable to break them down in our small intestine. This means prebiotics reach our large intestine still intact for our beneficial bacteria to feed on. Feeding our beneficial bacteria with prebiotics is necessary for our gut health as it allows the good bacteria to grow and contribute to the digestion of food [21,30]
Feeding these beneficial bacteria is also beneficial for our overall health as imbalances in gut bacteria have been linked with a number of digestive conditions as well as allergies, and asthma. 
Established health benefits for specific prebiotics relate to improved digestive function, increased calcium absorption and blood sugar regulation. Prebiotics can also provide energy for our cells as some of them produce short chain fatty acids when they are fermenting in the gut that are used as an energy source. [21,30]
Where can I find prebiotics?
Prebiotics are naturally found in a number of food sources, including breast milk, and are also available as supplements. Some of the best food sources that contain prebiotics include some select honeys, onion, garlic, leek, Jerusalem artichoke, chicory, banana, dandelion greens, tomato and asparagus. However, the prebiotics that are found in these fresh food sources are often present in low amounts so including prebiotic enhanced foods or supplements may be beneficial to ensure optimal gut health.
What are the types of prebiotics?
The term prebiotic includes various forms of prebiotics that you will sometimes see used on labels instead of the word prebiotic. These prebiotic forms include oligosaccharides, inulin, fructo-oligosaccharides (FOS) and galacto-oligosaccharides (GOS).
- Oligosaccharides are mostly indigestible carbohydrates found in food sources such as honey, asparagus, garlic, chicory, onion, Jerusalem artichoke, tomato, banana and wheat. 
- Fructo-oligosaccharides (FOS) are indigestible carbohydrates that are predominately sugar. They are a type of oligosaccharide. They are found in honey, leek, Brussels sprouts, artichoke, asparagus, garlic, chicory, onion, Jerusalem artichoke, tomato, banana and wheat. 
- Galacto-oligosaccharides (GOS) are indigestible galactose molecules found in the lactose of milk and milk products. They include cow’s milk and breast milk. 
- Inulin is an indigestible carbohydrate and type of dietary fibre. It is found in plant sources including chicory, onion, Jerusalem artichoke, banana and dandelion. 
- Honey Oligosaccharides (HOS) are oligosaccharides found in honey. Honey may contain as many as 24 individual oligosaccharides, including fructo –oligosaccharides and galacto-oligosaccharides.
How much prebiotic food do I need?
There are various strengths of prebiotics food sources and supplements and it is not yet understood how much nor how many prebiotics should be consumed for optimal health.
An Australian clinical study found 20g (14mL) of honey specifically selected for high prebiotic potential improved the balance of good-to-bad bacteria in the gut by nourishing beneficial bacteria Lactobacillus and Bifidobacterium while suppressing bad bacteria, Clostridium. 
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