Diet and the gut microbiota: what do we know?

By Rebecca Evans

Did you know there is a third wheel in everyone’s relationship between themselves and the food they eat? This third wheel is our gut microbiota; the unique ecosystem of microorganisms residing within our intestines. While many may think their relationship with food is exclusive, emerging research is revealing that what we eat may influence our gut microbiota, which could have potential health implications⁽¹⁾. Here we have summarised the latest evidence on the relationship between our gut microbiota, health and what we know about the role of diet in this.

Health and our gut microbiota

Current research on how the gut microbiota affects our health is limited. The research is based largely on observational studies, rather than clinical trials which are considered to be the gold standard for research. Current evidence suggests a healthy gut microbiota is associated with improved immunity, reduced risk of some cancers, lower inflammation, improved bone health, vitamin synthesis, weight management and improved laxation. Comparatively an unhealthy gut microbiota caused by a microbial imbalance, termed dysbiosis, is associated with overall overweight and obesity levels, insulin resistance, leptin (an appetite hormone) resistance, high cholesterol and increased inflammation^(1-4).  While this is an emerging area of research, at this stage, it is apparent our gut microbiota is linked with our health. But what affects our gut microbiota?  

Diet and the gut microbiota

Our gut microbiota is influenced by a number of factors including age, birth mode (vaginal versus caesarean), diet, stress, antibiotic use and genetics. Furthermore, diet also influences the gut microbiota and is a modifiable factor that you can change. While our current understanding on the role of diet is also limited to observational studies, a diet that contains a variety of fibre rich plant foods appears to have a beneficial effect on the gut microbiota⁽⁵⁾.

Dietary fibres, by definition are indigestible or resist digestion, and it is fibre which appears to play a starring role when it comes to promoting beneficial gut microbiota⁽⁵⁾. In particular prebiotics, a specific type of dietary fibre, can promote the growth and/or activity of beneficial bacteria in the gut. This is known as the “prebiotic effect”.

Just like we take in food and produce waste products, our microbiota do as well. Our gut microbiota produce short chain fatty acids i.e. acetate, propionate and butyrate from the prebiotics in our diets. Short chain fatty acids have beneficial effects on our health and are associated with a reduced risk of disease, including cancer^{(3, 6)}. Eating a variety of plant foods within a high fibre diet, including whole grain and high fibre grain foods, legumes, nuts, fruit and vegetables, will ensure a varied intake of prebiotic fibres⁽⁷⁾.

Probiotics

Probiotics are live microorganisms found in supplements and fermented foods^{(2, 5)}. When foods are fermented they are broken down into a simpler form by microorganisms such as yeast and bacteria⁽⁸⁾. Common fermented foods include sauerkraut (cabbage fermented with lactic acid bacteria) and yoghurt (milk fermented with lactic acid bacteria). However, what many people don’t realise is that the indigestible components of food (i.e. prebiotic fibre) actually undergoes fermentation by the gut microbiota which means fermented foods are fermented twice, once outside the body and again in the gut⁽⁸⁾.

It is possible to ferment pre-cooked grains and legumes by adding a starter culture such as whey or yoghurt⁽⁹⁾. This is usually preceded by soaking the legumes or grains in warm water in a warm place for 12 to 24 hours. Some common fermented grains and legumes include sourdough bread, miso and tempeh, which can be bought in many shops.

How to optimise your relationship with your gut microbiota

Our gut microbiota is as unique and individual as our finger prints. It’s time we recognised the third wheel between our health and diet. For many, the good news it is that it is never too late to work on the relationship with your gut microbiota. Research suggests that our gut microbiota composition can be altered through short term dietary intervention, as well as long term habitual dietary change^{(3, 10)}.

Early research suggests an optimal diet to promote beneficial gut microbiota is high in dietary fibre (prebiotics) from plant sources. This could be achieved by increasing your intake of vegetables, fruit, legumes and nuts, as well as whole grain and high fibre grain foods⁽⁶⁾. This is consistent with Australian Dietary Guidelines and GLNC’s recommendation for all Australians to enjoy grain foods 3-4 times a day, choosing at least half as whole grain or high fibre and to enjoy legumes at least 2-3 times a week.

For a range of fibre rich recipes, cooking tips and snack ideas with grains and legumes visit the GLNC website.

References

1.Flint HJ, Scott KP, Louis P, Duncan SH. The role of the gut microbiota in nutrition and health. Nat Rev Gastroenterol Hepatol. 2012;9(10):577-89.

2.Graf D, Di Cagno R, Fak F, Flint HJ, Nyman M, Saarela M, et al. Contribution of diet to the composition of the human gut microbiota. Microb Ecol Health Dis. 2015;26:26164.

3.Scott KP, Gratz SW, Sheridan PO, Flint HJ, Duncan SH. The influence of diet on the gut microbiota. Pharmacol Res. 2013;69(1):52-60.

4.Guarner F. The Gut Microbiome: What Do We Know? Clinical Liver Disease. 2015;5(4):86-90.

5.Slavin J. Fiber and prebiotics: mechanisms and health benefits. Nutrients. 2013;5(4):1417-35.

6.Bi Y, Qin N, Yang R. Human microbiota: a neglected "organ" in precision medicine. Infectious Diseases and Translational Medicine. 2015;1(2).

7.Halmos EP, Christophersen CT, Bird AR, Shepherd SJ, Gibson PR, Muir JG. Diets that differ in their FODMAP content alter the colonic luminal microenvironment. Gut. 2015;64(1):93-100.

8.Achi OK, Ukwuru M. Cereal-Based Fermented Foods of Africa as Functional Foods. International Journal of Microbiology and Application. 2015;2(4):71-83.

9.Pinkfarm. Traditional Preparation Methods of: grains, nuts, seeds and legumes n.d. [cited 2016 18th January]. Available from: http://www.pinkfarm.com.au/real-traditions/properly-prepared-grains-nuts-seeds/.

10.David LA, Maurice CF, Carmody RN, Gootenberg DB, Button JE, Wolfe BE, et al. Diet rapidly and reproducibly alters the human gut microbiome. Nature. 2014;505(7484):559-63.