Can the bacteria living in your gut make you fat?

The importance of gut flora has become better known over the last few years, and for good reason. The balance of bacteria living in your gut is vital to health – from supporting  a strong immune system to a happy mood. “Probiotic” has become a buzzword in health conscious circles, and fermented foods like kombucha and kimchi have made their way into the mainstream. Now researchers are looking at two classes of  bacteria, Firmicutes and Bacteroidetes, and their effect on weight management and obesity. There are tens of trillions of  bacteria living in our guts, over 400 species, most all of them (in a healthy person) flourishing in our large intestine. These bacteria are hard at work, creating short-chain fatty acids like butyrate from resistant starch, and synthesizing nutrients like Vitamin K and B12.

Firmicutes and Bacteroidetes are two dominating groups of bacteria found in everyone, but research has shown that if you happen to have more firmicutes than bacteroidetes, you are more likely to put on weight and have difficulty losing it (Kotzampassi, et al. 2014). This sounds like a nightmare – a microscopic bacteria living inside of you is actually making you fatter? It sounds like a battle you are genetically destined to lose! Is that really the case? No! Do not give up, because researchers have also found that gut flora evolves to dietary change. Certain foods cause firmicutes to thrive,  while others allow bacteroidetes to outnumber firmicutes (Maslowski and Mackay,  2011). Fast food, processed foods, foods high in sugar and unhealthy fats should be avoided  if  you want to cut back on firmicutes. How to increase bacteroidetes?

1. Increase your intake of colorful veggies and other fiber rich foods like beans and lentils.

2. Include natural sources of probiotics in your diet: homemade sauerkraut, kimchi or other fermented veggies, miso, unsweetened  yogurt, kefir, or kombucha. If these are unavailable to you, it doesn’t hurt to invest in a good quality probiotic supplement.

3. Don’t forget the prebiotics! Prebiotics are food for probiotics. We get prebiotics from dietary fiber that can not be digested, and instead passes  through the digestive tract providing fuel for our beneficial bacteria. A few good prebiotic sources are jerusalem artichokes (aka sunchokes), raw  garlic and onions, green bananas, legumes, and cooked rice that has been allowed  to cool before being eaten.

4. And it can’t be stressed enough – avoid processed and sugary foods!



Gorbach SL. Microbiology of the Gastrointestinal Tract. In: Baron S, editor. Medical  Microbiology. 4th edition. Galveston (TX): University of Texas  Medical Branch at Galveston; 1996. Chapter 95. Available from:

Kotzampassi, K., Giamarellos-Bourboulis, E. J., & Stavrou, G. (2014). Obesity as a Consequence of Gut Bacteria and Diet Interactions. ISRN Obesity,  2014,  651895. doi:10.1155/2014/651895

Maslowski, K., & Mackay, C. (2011). Diet, gut microbiota, and immune responses. Nature Immunology, 12, (1), 5-­9.


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