What is it?
The gut of every human adult contains a vast amount of bacteria. This is a colony of between one and five thousand different species. Of these species some are more and less beneficial, but the ratio of the main types and the diversity of the types of bacteria is key to having a thriving colony of bacteria which is beneficial to human health.
During pregnancy, the foetal intestine is sterile, but becomes colonised during birth from the mothers skin, the environment and breastfeeding. The bacteria gradually becomes developed and diversified during the growth of the child from further dietary and environmental sources until adulthood. The gut microbiota is constantly changing and growing in response to the environment and other factors. The predominant groups of bacterial phyla are Bacteriodetes and Firmicutes.
The importance of gut microbiome:
So, why do we need bacteria in the gut? The colony of bacteria play a vital role to help humans to break down indigestible food components into absorbable molecules that otherwise we could not digest. Short chain fatty acids are also generated by bacterial activity which are a major energy source for the gut epithelium as well as proteins required for gut barrier function.
The gut microbiome is a topic currently widely researched as more and more information is coming to light about its effect on the overall health and wellbeing of humans. Some areas already well researched are its effects on gut health, immune function, cardiovascular function, neurological function and weight management [1, 2, 4, 5, 6]. The connection between the gastrointestinal tract and nervous system reveals the important and essential role of the microbiome and brain function [1]. In addition, the link between the immune system and the gut microbiome is recognised by immune system cells having the ability to distinguish harmful pathogens from other molecules such as food [2].
Gut microbiome associated health conditions:
Throughout life the gut microbiome continually changes. This can be due to diet, stress and other environmental factors such as the use of antibiotics, which can have profound effects lasting for several years. These changes can be both beneficial and harmful and this can be reflected in our overall health and disease risk. Environmental factors can cause chronic inflammation which can lead to conditions such as inflammatory bowel syndrome, cardiovascular disease, non-alcoholic fatty liver disease, obesity, poor immune function, psoriasis, acne, some cancers and have links to neurodegenerative disorders and neuropsychiatric conditions [1, 2, 3, 4, 5].
Possible environmental triggers:
There is some evidence to suggest that excessive sanitation and an over clean environment in early life may prevent the body from establishing a full range of microorganisms within the gut. The recognition of food by the immune system can then become altered causing the body to react excessively to some foods. Secondly, the use of antibiotics can cause a reduced diversity of bacteria, causing some unfavourable microorganisms to flourish whilst others struggle for competition. Another possible cause of a troublesome collection of gut microbiota are low fibre and processed diets which are poor at generating the short chain fatty acids needed to maintain a healthy gut epithelium. The type and quantity of fat in the diet could also affect the gut microbiome, saturated fats or trains fats from baked, processed foods or too high omega-6 intake can cause inflammation. Lastly, additives may also be playing a role in the promotion of gut wall inflammation.
How can we help our gut microbiota?
Then, what can we do to help? Dietary fibre appears to be the key. Diets with high intakes of dietary fibre are linked with a lower cardiovascular disease risk and lower body weights [6, 12]. Not only does it help to excrete waste products but its presence can slow down the digestion and absorption of glucose and when fermented in the large intestine they can help to reduce blood cholesterol. Dietary fibre is a type of carbohydrate. These are often illustrated in the media as bad, however we need dietary fibre and complex carbohydrates in our diet to benefit our health. Importance should be placed rather on the type of carbohydrates we consume, such as fruits, vegetables, wholegrain and legumes with less meat and alcohol. Citrus fruits in particular have been shown to reduce gut inflammation. Omega-3s which can be found in plant foods such as walnuts or algae supplements as well as vitamin D can also help to reduce inflammation [7]. Dietary fibre sources are predominantly plant-based, meaning that those who follow a plant-based diet are likely to be getting a sufficient amount.
It should be highlighted that a predominantly plant-based diet has shown multiple benefits for the gut microbiome which has great benefits for overall health. Plant-based foods digested and fermented by the gut microbiome cause secondary effects of appetite suppression which encourages overall weight loss [8]. In addition, plant-based fats and polyphenols have shown positive alterations in gut microbiota diversity [9]. Studies looking specifically at vegetarian and vegan diets in comparison to omnivorous diets have found that individuals have a more diverse and stable gut microbiome. This is likely due to higher dietary fibre content, increased short-chain fatty acid production and the anti-pathogenic and anti-inflammatory effects of polyphenols [10].
For people with diagnosed IBS, the FODMAP (Fermentable Oligo-saccharides, Disaccharides, Mono-saccharides and Polyols) diet may be beneficial to reduce symptoms [11].
NHS Low FODMAP Diet Guidance:
Prebiotics and Probiotics
The gut microbiome can be altered through the action of probiotics and prebiotics. Probiotics, such as Lactobacillus or Bifidobacterium are strains of microorganisms that can be added to food or supplemented to enable them to grow safely in the gut to benefit gut wall health. Prebiotics are types of food that when eaten they promote the growth of the most beneficial microorganisms.
Prebiotics can be found in naturally high fibre fruits and vegetables such as garlic, onions, leeks, asparagus, artichokes, tomatoes, bananas, plums and apples, grains, cereals and nuts. Probiotics contain healthy bacteria and are usually found in fermented foods such as kombucha, kimchi, sauerkraut, pickles and miso or a supplement [6].
Carbohydrates and Fibre | In More Detail
What are carbohydrates and where does fibre fit in?
Dietary carbohydrates generally include the term ‘saccharide’ in their name. They range greatly in size from simple sugars, or monosaccharides (consisting only of single sugar units), to very large complex polysaccharides (consisting of many thousands of sugar units joined together).
As well as differences in size, carbohydrates can also differ in their chemical structure. Such differences are important because they dictate how, and where and even whether specific carbohydrates can be digested and absorbed by humans.
The three major groups of dietary carbohydrates
Simple sugars
Simple sugars or monosaccharides and disaccharides. Monosaccharides consist of just a single sugar unit. The sugars glucose and fructose are two examples of monosaccharides. Disaccharides consist of two sugar units joined together. Examples of disaccharides include; sucrose (made up of one unit of glucose plus one unit of fructose) and lactose (which is made up of one unit of glucose plus one unit of galactose).
Most monosaccharides and disaccharides in human diets are broken down very easily in the human gut and absorbed very quickly into the body.
Oligosaccharides
These occur in a limited number of plant foods such as beans and lentils, onions, garlic and artichokes. Oligosaccharides form when three to nine sugar units become tightly joined together.
This tight bonding between the component sugars is difficult for the body to break down so that these types of carbohydrates cannot be digested and absorbed in the human small intestine. Instead they continue to move on into the large intestine. Here the resident bacteria are able to break them down as bacteria contain the enzymes necessary to do this.
These indigestible carbohydrates are examples of ‘prebiotics’.
Polysaccharides
These carbohydrates have more than nine sugar units and often up to several thousands of sugar units joined together. The two major groups of polysaccharides are starch and dietary fibre.
Starch
Starch consists of long chains of the simple sugar glucose joined together. Generally, these long glucose chains are very easy for the body to break down. This means that most starches are readily digested in the small intestine, and their glucose subunits are quickly be absorbed into the bloodstream.
One particular group of starches, collectively known as resistant starch, are contained within seeds or grains, or have been changed by cooking or have a particular structure that makes them much harder to digest.
These starches behave like dietary fibre and pass down into the large intestine.
Dietary fibre
The sugars in some types of polysaccharides are bound together in unusual ways and humans do not have the enzymes to break them down. These types of polysaccharides escape digestion in the small intestine and continue to move down into the large intestine. Here they can be broken down by bacteria living in this part of the gut (that produce a wider range of enzymes).
So, while you will see dietary fibre listed separately from digestible carbohydrate on food labels, dietary fibre is in fact a type of carbohydrate. Not all dietary fibre is exactly the same. The dietary fibre in some foods can be broken down by the bacteria in the lower intestine relatively quickly. This type of fibre has been called soluble or rapidly fermentable fibre and it is digested in the upper part of the large intestine.
Other dietary fibres are very difficult indeed to break down which means that they travel much further down the large intestine before bacteria can act on them.
These fibres are known as insoluble or poorly fermentable fibre.
References
[1] Zapatera, B., Prados, A., Gómez-Martínez, S. and Marcos, A. (2015) ‘Immunonutrition: methodology and applications’, Nutrición Hospitalaria, 31 (3), pp. 145-154.
[2] Childs, C., Calder, P. and Miles, E. (2019) ‘Diet and Immune Function’, Nutrients, 11 (8), pp. E1933.
[3] Vollmer, D., West V. and Lephart, E. (2018) 'Enhancing Skin Health: By Oral Administration of Natural Compounds and Minerals with Implications to the Dermal Microbiome', International Journal of Molecular Science, 19 (10), pp. E3059.
[4] Cenit, M., Sanz, Y. and Codoñer-Franch, P. (2017) ‘Influence of gut microbiota on neuropsychiatric disorders’, World Journal of Gastroenterology, 23 (30), pp. 5486-5498.
[5] Lau, L. and Wong, S. (2018) ‘Microbiota, Obesity and NAFLD’, Advances in Experimental Medicine and Biology, 1061, pp. 111-125.
[6] Slavin, J. (2013) ‘Fiber and prebiotics: mechanisms and health benefits’, Nutrients, 5 (4), pp. 1417-1435.
[7] Bhupathiraju, S. and Tucker, K. (2011) ‘Greater variety in fruit and vegetable intake is associated with lower inflammation in Puerto Rican adults’, American Journal of Clinical Nutrition, 93 (1), pp. 37-46.
[8] Najjar, R. and Feresin, R. (2019) ‘Plant-Based Diets in the Reduction of Body Fat:Physiological Effects and Biochemical Insights’, Nutrients, 11 (11), pp. 2712.
[9] Muralidharan, J., Galiè, S., Hernández-Alonso, P., Bulló, M. and Salas-Salvadó, J. (2019) ‘Plant-Based Fat, Dietary Patterns Rich in Vegetable Fat and Gut Microbiota Modulation’, Frontiers in Nutrition, 6, pp.157.
[10] Tomova, A., Bukovsky, I., Rembert, E., Yonas, W., Alwarith, J., Barnard, N. and Kahleova, H. (2019) ‘The Effects of Vegetarian and Vegan Diets on Gut Microbiota’, Frontiers in Nutrition, 6, PP. 47.
[11] Monash University (2020) Translational Nutritional. Available Online: https://www.monash.edu/medicine/ccs/gastroenterology/research/nutrition (Accessed: 06/01/2021).
[12] Crowe, F., Roddam, A. and Key, T. (2011) ‘Fruit and vegetable intake and mortality from ischaemic heart disease: results from the European Prospective Investigation into Cancer and Nutrition (EPIC)-Heart Study’, European Heart Journal, 32 (10), pp. 1235–1243.
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