Written by Alana Del Sordi. Reviewed by Diana Licalzi, MS, RD, CDE

There’s a reason why Hippocrates stated, “All disease begins in the gut”. Over the past few decades, the overall health of individuals has dramatically decreased and the presence of disease is now more common than not. Changes in lifestyles, dietary patterns, increased rates of inactivity, and exposure to harmful environmental factors have led to this rapidly growing concern.

Studies have shown that gut health is linked to the development and progression of metabolic diseases, such as type 2 diabetes (T2D). Diets that are high in saturated fats, refined carbohydrates, refined sugars, and low in fiber have “been recognized as one of the major culprits of T2D with gut microbiota playing an important role in modulating effects of diet”.[12] Understanding the gut flora, why it's important, how your daily diet impacts your gut balance, and what you can do to improve and sustain functionality are all important in order to live your healthiest life.

The Gut Flora and Its Importance

The gut microbiome is composed of bacteria, archaea, and eukaryotic cells in your gastrointestinal tract.[8] Vital functions of the gut microbiome include the digestion and absorption of food, gathering energy from indigestible carbohydrates, synthesis of vitamins, aiding in the development and response of the immune system, and serving as protection against pathogens in your gastrointestinal tract.

The gut has “good” and “bad” microorganisms and an unequal balance may result in a reduction of microbial diversity. This reduction of diversity can lead to increased pathogenic bacteria, inflammation within the gut, and progress the diabetic condition.[8]

What’s more, the immune system recognizes this imbalance and tries to fight off the bad bacteria, causing irritation and further inflammation within the body. Prolonged imbalances and inflammation can cause numerous disease conditions or worsen the severity of diseases that are already present. Several studies have reported that microbiome imbalances play a role in the rapid progression of insulin resistance in individuals who have type 2 diabetes.[8] 

There are many factors that can cause disruptions to the gut microbiome, with dietary patterns being a large culprit. Westernized countries consume diets high in fats and refined carbohydrates, which contribute to increased rates of disease linked to these gut microbe imbalances.

How Poor Diet & Inflammation Shapes Your Gut 

Diet plays a vital role in shaping the health of your gut microbiota. As food goes through the process of digestion, absorption, and elimination, it interacts with your gut microbiome. The western diet is high in many inflammatory foods, including refined carbohydrates, such as white bread and pastries, fried foods, soda, and other sugar-sweetened drinks, processed meats, and packaged foods. These foods cause pro-inflammatory responses within the body and an imbalance of microorganisms.[15] 

This pro-inflammatory response combined with an imbalance of gut bacteria and excess body weight, causes the body to go into a state of low-grade inflammation. This term, known as “meta-inflammation,” is imposed by macrophages located in the colon, liver, muscles, and adipose tissues.[16]

Macrophages destroy harmful organisms and bacteria, but by doing so promote low-grade inflammation. The inflammation that occurs in adipose tissue is highly correlated to the development of insulin resistance.[15] Chronic low-grade inflammation is also associated with disturbances to the metabolic pathways that metabolize sugar in people who are obese and have T2D.[15]

Dietary Approaches for a Healthy Gut

Decreasing processed foods while increasing the intake of plant-based fibers (more on plant fibers in the next section) is highly recommended for a healthy gut microbiome.[1] Other foods that reduce inflammation and chronic disease include ones that are high in natural antioxidants and polyphenols. Antioxidants prevent or delay the damage of cells and they can aid in decreasing oxidative stress caused by carcinogens.[4]Antioxidants can be found in foods that are high in vitamin E, vitamin C, and B-carotene.[4] 

Polyphenols defend our cells from ultraviolet radiation or aggression caused by pathogens.[10]  Some foods and drinks high in polyphenols include teas, herbs, spices, red wine, onions, curly kale, leeks, broccoli, blueberries, kiwi, plums, cherries, and apples.[6]  Dried herbs that contain relatively high levels of polyphenols are oregano, thyme, rosemary, sage, parsley, dill, and coriander. Spices such as cloves (highest total phenolic content), cinnamon, coriander seed, ginger, nutmeg, and turmeric also have high polyphenol levels.[7]

Prebiotics vs. Probiotics and Their Link to Type 2 Diabetes

We always hear about probiotics and how they play an important role in balancing and maintaining a healthy gut, but we rarely hear about prebiotics and their importance. Probiotics are strains of living good bacteria while prebiotics are plant fibers that act as a food source and stimulate the growth of good bacteria.[14] Increasing your intake of prebiotic-rich foods can improve the function of your gut microbiota, strengthen intestinal permeability, suppress inflammation, and improve glucose tolerance.[8] 

Probiotics, on the other hand, help to replenish your good bacteria. Specific strains can have anti-inflammatory effects, lower glucose, and reduce LDL (bad) and total.[8] Pre- and probiotics can be found in numerous food sources. Focus on incorporating them into your diet for improved gut health.

Fiber, Resistant Starch, and Short Chain Fatty Acids: Key Nutrients in Improving Type 2 Diabetes

The human body cannot break down certain plant fibers and starches (like resistant starch) causing them to reach our large intestine where they serve as food for our gut microbiota. By resisting the normal digestion process, they also help to slow the absorption of glucose into the bloodstream. Additionally, both fiber and resistant starch increase satiety, prevent and treat constipation, decrease levels of cholesterol, and lower the risk of colon cancer.[1]

By feeding the good bacteria in the gut, fibers and resistant starch act as prebiotics.[13] Unlike us as humans, our bacteria can break down these foods through a process called fermentation. One of the by-products of fermentation are short-chain fatty acids (SCFAs). These SCFAs, in turn, serve as the major fuel source for our intestinal cells. Fueling our intestinal cells is important as they aid with the digestion of food, nutrient absorption, and protection from microbial infections.[5]

Other health-promoting effects of SCFAs include a reduced risk of inflammatory diseases, cardiovascular disease, and diabetes.[1] Short-chain fatty acids are especially important for individuals who have T2D because they increase the activity of enzymes in the “liver and muscle tissue, resulting in better blood sugar control”. [2] 

Research shows that the gut microbiota produces more short-chain fatty acids in individuals who predominantly consume a plant-based diet.[1]  Increasing the consumption of food that has fiber and food that is highly enriched with short-chain fatty acids such as fermented foods, starches and whole grains, fruits, vegetables, vinegar, and sourdough bread, will protect your body from infections caused by bad bacteria and strengthen the gut barrier function.[11]

Let's recap the relationship between the gut microbiome and type 2 diabetes

Switching your dietary habits from refined and processed foods to focusing on increasing more of a variety of fruits and vegetables into your daily life will influence your likelihood of developing disease and it can also treat existing disease(s). It is important for individuals, especially those who have type 2 diabetes to consume a diet that is high in fiber, pre- and probiotics, short-chain fatty acids, antioxidants, and polyphenols which will promote the diversity of good bacteria in the gut and reduce inflammation. Additionally, dietary fiber and resistant starch have positive metabolic health effects such as increasing the feeling of fullness, decreased weight gain, lower blood glucose and cholesterol levels, which all reduce the risk of type 2 diabetes and other diseases.[3] 

Citations

1. BMJ. (2015, September 29). High dietary fiber intake linked to health promoting short chain fatty acids: Beneficial effects not limited to vegetarian or vegan diets. ScienceDaily. Retrieved June 27, 2021 from www.sciencedaily.com/releases/2015/09/150929070122.htm
2. Brown, M. J. (2016, April 2). How Short-Chain Fatty Acids Affect Health and Weight. Healthline. https://www.healthline.com/nutrition/short-chain-fatty-acids-101#TOC_TITLE_HDR_3. 
3. Hills, R. D., Jr, Pontefract, B. A., Mishcon, H. R., Black, C. A., Sutton, S. C., & Theberge, C. R. (2019). Gut Microbiome: Profound Implications for Diet and Disease. Nutrients, 11(7), 1613. https://doi.org/10.3390/nu11071613
4. Lobo, V., Patil, A., Phatak, A., & Chandra, N. (2010). Free radicals, antioxidants and functional foods: Impact on human health. Pharmacognosy reviews, 4(8), 118–126. https://doi.org/10.4103/0973-7847.70902
5. Kong, S., Zhang, Y. H., & Zhang, W. (2018). Regulation of intestinal epithelial cells properties and functions by amino acids. BioMed research international, 2018.
6. Manach, C., Scalbert, A., Morand, C., Rémésy, C., & Jiménez, L. (2004). Polyphenols: food sources and bioavailability. The American journal of clinical nutrition, 79(5), 727-747.
7. Opara, E. I., & Chohan, M. (2014). Culinary herbs and spices: their bioactive properties, the contribution of polyphenols and the challenges in deducing their true health benefits. International journal of molecular sciences, 15(10), 19183–19202. https://doi.org/10.3390/ijms151019183
8. Sharma, S., & Tripathi, P. (2019). Gut microbiome and type 2 diabetes: where we are and where to go?. The Journal of nutritional biochemistry, 63, 101-108.
9. Antioxidants. (2021, March 03). Retrieved from https://www.hsph.harvard.edu/nutritionsource/antioxidants/
10. Pandey, K. B., & Rizvi, S. I. (2009). Plant polyphenols as dietary antioxidants in human health and disease. Oxidative medicine and cellular longevity, 2(5), 270–278. https://doi.org/10.4161/oxim.2.5.9498
11. Parada Venegas, D., De la Fuente, M. K., Landskron, G., González, M. J., Quera, R., Dijkstra, G., ... & Hermoso, M. A. (2019). Short chain fatty acids (SCFAs)-mediated gut epithelial and immune regulation and its relevance for inflammatory bowel diseases. Frontiers in immunology, 10, 277. 
12. Rodrigues, R. R., Gurung, M., Li, Z., García-Jaramillo, M., Greer, R., Gaulke, C., ... & Shulzhenko, N. Transkingdom interactions between Lactobacilli and hepatic mitochondria attenuate western diet-induced diabetes. Nature Communications, 12(1), 1-15.
13. What is Resistant Starch? The Johns Hopkins Patient Guide to Diabetes. (2020, December 2). https://hopkinsdiabetesinfo.org/what-is-resistant-starch/. 
14. What's the Difference Between Probiotics and Prebiotics? SCL Health. (n.d.). https://www.sclhealth.org/blog/2019/07/difference-between-probiotics-and-prebiotics/. 
15. Scheithauer, T., Rampanelli, E., Nieuwdorp, M., Vallance, B. A., Verchere, C. B., van Raalte, D. H., & Herrema, H. (2020). Gut Microbiota as a Trigger for Metabolic Inflammation in Obesity and Type 2 Diabetes. Frontiers in immunology, 11, 571731. https://doi.org/10.3389/fimmu.2020.571731
16. Li, C., Xu, M. M., Wang, K., Adler, A. J., Vella, A. T., & Zhou, B. (2018). Macrophage polarization and meta-inflammation. Translational research : the journal of laboratory and clinical medicine, 191, 29–44. https://doi.org/10.1016/j.trsl.2017.10.004
17. Edermaniger, L. (2020, August 19). What are short-chain fatty acids and why should you care? Retrieved from https://atlasbiomed.com/blog/what-are-short-chain-fatty-acids-and-why-should-you-care/