The Gut & Gut Bacteria

When most people talk about digestion of the food we eat, they talk about the chemical and mechanical breakdown of carbohydrates, proteins and fats.

School Of Nutrition Posted Oct 26, 2016 Future Fit Training

The Gut & Gut Bacteria

But there is another key part to our digestive system that is not as well known - digestion that involves our gut bacteria.

The human digestive system is home to trillions of bacteria. The average adult has a gut population that weighs more than 1kg, has 10 times more cells than the human body and our bacteria’s waste accounts for around half the mass of our feces. These microorganisms are diverse but live in highly structured colonies, the organization of which is completely individual. Although similarities exist in members of the same family, household or even work colleagues, everyone’s balance of bacteria is unique and will depend on their individual lifestyle, home and diet.

What do they do?

Gut bacteria are important in basic digestion. Without gut microflora we would not be able to utilize all the carbohydrate we eat, as we lack enzymes to break down some complex polysaccharides, fiber, oligosaccharides and sugar alcohols. The microorganisms in our guts do this for us, turning these carbohydrates into short-chain fatty acids (SCFAs), such as acetic acid, propionic acid and butyric acid. These SCFAs can then be absorbed from the large intestine and used by the body. Bacterial fermentation of complex carbohydrates can be responsible for as much as 10% of total energy gained from our food (1).

Microorganisms also break down proteins such as enzymes, bile salts, collagen and elastin allowing for the amino acids to be reused but also preventing the production of toxins and increase the lower gut’s absorption of water, lessening the risk of dehydration.

What else do they do?

Our gut bacteria’s role in digestion has been known for some time but just how important these organisms are to our health and wellbeing is only just being realized.

As well as providing an energy source for the rest of the body, SCFAs have been shown to provide a vital local energy source for muscle and gut cells, improving mechanical digestion, supporting the intestinal barrier and inhibiting the growth of harmful bacteria. A lack of SCFAs causes poor gut mobility and increases the risk of the sort of gut damage linked to IBS. The correct balance supports the immune system and poor populations have been linked with increased risks of allergy.

Gut bacteria are responsible for a large proportion of our Vitamin K production and are important in the manufacture of all B Vitamins. The right balance of favourable gut microorganisms has also been shown to improve nutrient absorption, particularly of Calcium (maybe by as much as 50%) and Magnesium (2).

Intestinal microorganisms also produce many of the neurochemicals used by the brain to regulate mental processes such as learning, memory and mood. Animal studies have shown that early exposure to normal gut bacteria is important for brain development and the development of behavior (3). Although much more research is needed, studies have suggested poor diversity of gut bacteria maybe associated with a number of mental health problems, including eating disorders, anxiety and depression. Boosting diversity certainly results in lower perceived stress and anxiety and measurably lowers levels of the stress hormone cortisol. Mice bred to have no bacteria in their gut are found to be excessively active and show greater risk-taking behavior (4).

There are also measurable differences in how individuals digest food depending on which microbes they have in the gut and it is thought that these differences may have a role in weight gain & obesity. Short-chain fatty acids (SCFAs) affect fat, glucose, and cholesterol metabolism. SCFAs have been shown to beneficially affect glucose metabolism, normalising blood glucose levels and increasing the efficiency of glucose handling. They also increase fatty acid oxidation in tissue, decreasing fat storage in white adipose tissue and increasing loss of heat energy through the brown adipose tissue (5). These differences may be linked to the gut organism’s role in phytonutrient breakdown. Lots of research has shown that eating a wide variety of colourful fruits and vegetables, that contain a diverse mix of phytonutrients reduces disease risk and leads to better health and improved academic and sports performance. However, eating lots of phytonutrients doesn’t work for everyone and its now thought that the it may be possible that micro-organisms living within the digestive tract are responsible for breaking down the phytonutrients into their bio-active forms (6). Supplementing with probiotics has been found to boost the performance benefits of many natural sports supplements (7). 

How to boost beneficial gut microorganisms

There are a number of factors that contribute to the health of your gut microbiome, including your environment, the amount of exercise and sleep you get, and the use of antibacterial cleaners or antibiotics but the number one factor that determines what microbes live in your gut is diet. High sugar, high simple or refined carbohydrate diets have been found to promote the growth of unfavourable gut bacteria at the cost of the beneficial ones and cause lower diversity.

Probiotic, Prebiotic, and Symbiotic foods all improve the quality and diversity of the microorganisms in your digestive system. Probiotic foods or supplements are those that contain a quantity of beneficial microorganisms that aim to improve the balance the intestinal microflora. Prebiotics are foods high in lower gut digested carbohydrates that stimulate and feed the growth of the beneficial bacterial species and symbiotic foods contain both pre- and pro-biotics. Fermented foods such as sauerkraut, pickles, kimchi, kefir, kombucha and unprocessed yogurt are symbiotic and can support favourable bacteria growth and improve diversity. Getting a good proportion of your carbohydrates from vegetables, particularly vegetables high in resistant starch that act as prebiotics boost both the quality and diversity of your microbiome. High intakes of polyphenols found in brightly coloured fruits and vegetables may also support gut bacteria.

Further reading

Julia Enders (2015). Gut: the inside story of our body’s most under-rated organ. Scribe Publications.

Kellman (2015). The microbiome diet: The scientifically proven way to restore your gut health. De Capo Lifelong books.


(1)  Besten et al (2013) The role of short-chain fatty acids in the interplay between diet, gut microbiota and host energy metabolism, J Lipid Res 54(9). 2325-2340

(2)  Scholz-Ahrens et al  (2007) Prebiotics, probiotics and symbiotic affect mineral absorption, bone mineral density and bone structure. J. Nutr. March 2007 vol. 137 no. 3 838S-846S

(3)  Rea et al (2016). The microbiome: A key regulator of stress and neuroinflammation.

(4)  Diaz Heijtz et al. (2011). Normal gut microbiota modulates brain development and behavior. Proc Natl Acad Sci USA 2011;108:3047–3052

(5)  Hartstra et al (2015) Insights Into the Role of the Microbiome in Obesity and Type 2 Diabetes. Diabetes Care 2015 Jan; 38(1): 159-165.

(6)  Lampe JW, Chang JL. Inter-individual differences in phytochemical metabolism and disposition. Semin Cancer Biol 2007;17:347–53.

(7)  VIDEO by Nutrition facts 2012.’t+Use+Antiseptic+Mouthwash&fwp_content_type=video

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