What are Firmicutes?

The microbiome, a self-sustaining ecosystem living on and within you, may play a large role in your overall health. The two most studied phylum of bacteria are Firmicutes and Bacteroidetes. Researchers now believe that the ratio of these bacteria within the human gut might moderate whether we are obese or lean. This novel idea may revolutionize the way we treat obesity, as well as the stigma surrounding the chronic disease and related metabolic disorders. Many people suffering from obesity and its associated metabolic disorders do exercise and eat healthy diets, but still struggle to lose weight.

In a fascinating study, the weight of obese mice was drastically altered by simply shifting the communities of bacteria residing within their gut. Along with changes in weight, the lack or presence of certain bacteria changed the mice's chemistry and increased risk factors for cardiac disease, high blood pressure and diabetesAdditional studies have shown this shift in humans, too. In fact, another study recently revealed fecal transplants from the gastrointestinal (GI) tracts of obese humans has induced obesity in lean mice.

But how does bacteria cause weight gain?

The majority of the bacteria that live in our gut are anaerobes, or unable to live in oxygen. Both Bacteroidetes and Firmicutes are anaerobes. Researchers observe a higher ratio of Firmicutes to Bacteroidetes in obese humans, while in leaner humans, a higher ratio of Bacteroidetes to Firmicutes can be found.

These two types of bacteria regulate fat absorption. A good example would be two identical twins eating the same diet, but they have different ratios of Bacteroidetes and Firmicutes.

Researchers have found that Firmicutes are better at extracting energy from food as compared to Bacteroidetes. This means that if you have a large Firmicutes population in your microbiome, more of your food will be converted to energy for the body to use or store as fat. This ultimately means you are consuming more calories than someone that has a higher ratio of Bacteroidetes, even if you are consuming the same exact thing.

What does this mean to you, though? Basically, modulating the baseline of your gut microbiome population to balance your gut and encourage a higher proportion of Bacteroidetes. The modern world utilizes more antibiotics, has higher levels of stress, and are exposed to environmental pollutants, which all contribute to weight gain. Good bacteria like Bacteroidetes are reduced, allowing Firmicutes populations to grow freely.

Other than environmental factors, which are hard to avoid, you can be proactive when it comes to shifting and balancing your gut communities. We all have Firmicutes, which, when seen in higher levels are associated with obesity. Low-fat diets may decrease Firmicutes abundance; however, reducing fat intake is hard and often it does not effectively decrease gain.

However, this new research indicates that shifting Firmicutes abundance, for example with probiotics to help crowd out bad bacteria, could potentially treat obesity, or even help people that require weight gain.

Understanding how particular gut microbes encourage fat absorption may help us develop novel ways of reducing fat absorption seen in obesity and related metabolic disorders, or increase fat absorption when people are malnourished.

How can you change your Firmicutes and Bacteroidetes ratio?

Eating a high-fiber diet with good carbs helps crowd out the bad bacteria and encourages the growth of good bacteria. Firmicutes absorb fats, thus higher fat diets increase their population, which ultimately leads to weight gain.

Avoiding sugars and processed carbs is another way to allow good bacteria to grow. Firmicutes thrive on sugars, and have been known to grow rampantly in sugar factories.

Raise your intake of beans since they are among the very best foods to raise your Bacteroidetes. However, some have difficulty digesting beans. This often indicates there are too few Bacteroidetes in your digestive tract. Instead of avoiding beans, studies have shown that by adding them to your diet slowly, the symptoms may be reduced.

Start a habit of sleeping and eating on regular schedules. Data shows that our gut bacteria have a rhythm that changes throughout the day just like our sleep-wake cycle. Night work, jet lag and inconsistent eating habits upset our good bacteria just like antibiotics can.

Be proactive and try adding probiotics and prebiotics to your daily routine. A good way to increase probiotics is by regularly eating fermented foods such as yogurt, kefir, sauerkraut, and miso. When choosing a yogurt, stick with plain Greek yogurt and mix in pomegranate seeds or your favorite berries for a dose of fiber.

The outdated idea of health coming from strenuous effort and deprivation is fading. Dieting and exercise simply do not work for many people struggling with obesity and weight gain. Becoming healthy may rely on rebalancing the bacteria residing in your gut.


Ismail, Nagwa A., et al. Frequency of Firmicutes and Bacteroidetes in gut microbiota in obese and normal weight Egyptian children and adults. Arch Med Sci. 2011 Jun; 7(3): 501–507. doi:  10.5114/aoms.2011.23418

Vijay-Kumar, Matam et al. “Metabolic Syndrome and Altered Gut Microbiota in Mice Lacking Toll-Like Receptor 5.” Science (New York, N.Y.) 328.5975 (2010): 228–231. PMC.

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