Foods That Increase Firmicutes in the Gut

January 4, 2026 •

3 min read

Over 100 trillion bacteria, including the prominent Firmicutes phylum, inhabit the human gut and are significantly influenced by diet. Different food groups, especially those high in refined carbohydrates and certain types of fats, can substantially increase the population of Firmicutes.

Quick Summary

Different foods can influence the abundance of the Firmicutes bacteria in the gut. Diets high in refined carbohydrates, added sugars, and saturated fats are known to increase Firmicutes, while high-fiber diets can promote a more balanced gut microbiome. Dietary choices directly impact the delicate balance of gut microbes, and understanding this relationship is key to managing overall health.

Key Points

Refined Carbohydrates: Consuming sugary drinks and processed baked goods provides ample fuel for the Firmicutes bacteria to multiply significantly.
High-Fat Western Diet: A diet high in saturated fats, combined with low fiber, is linked to an increase in Firmicutes and can disrupt the gut balance.
Prebiotic Fiber: Foods like garlic, onions, and asparagus contain prebiotic fibers that act as food for beneficial Firmicutes and support overall gut health.
Probiotic Fermented Foods: Yogurt, kefir, and kimchi introduce live bacteria, including beneficial Lactobacillus species from the Firmicutes phylum, into the gut.
Balanced Diet: A diverse diet rich in whole foods, fiber, and fermented products is key to promoting a healthy balance and preventing an overabundance of Firmicutes from a less-than-ideal diet.

The human gut microbiome is a complex ecosystem, with the phyla Firmicutes and Bacteroidetes making up a significant portion of its bacterial population. The balance between these two major groups is often studied in relation to overall health, with certain dietary patterns shown to increase the population of Firmicutes. While Firmicutes are not inherently bad, an overabundance, particularly when out of balance with other bacteria, has been linked to potential health issues.

The Impact of Refined Carbohydrates and Sugars

Refined carbohydrates and added sugars are a primary fuel source for Firmicutes, causing their population to multiply significantly. Foods rich in these ingredients include:

Sugary beverages: Soda, sweetened juices, and energy drinks.
Baked goods: White bread, pasta, pastries, cakes, and cookies.
Processed snacks: Chips, crackers, and certain breakfast cereals.

This pattern creates a vicious cycle; consuming more refined carbs feeds the Firmicutes, which in turn can increase cravings for these very foods, potentially leading to weight gain and other metabolic problems.

How High-Fat Diets Influence the Microbiome

Specific types of fats, particularly saturated fats, have also been shown to increase the Firmicutes to Bacteroidetes ratio in animal studies. This effect is often observed in what is known as a "Western diet," which is typically low in fiber and high in fat and sugar. High-fat diets can alter the composition of the gut microbiota, promoting the growth of certain Firmicutes species while potentially decreasing others.

Complex vs. Refined Carbohydrates: A Key Difference

While refined carbs increase Firmicutes in a less-than-ideal way, it is important to distinguish them from complex carbohydrates found in high-fiber foods. Dietary fiber is a complex carbohydrate that nourishes a diverse range of beneficial gut bacteria, including some species within the Firmicutes phylum that are considered beneficial, such as Faecalibacterium prausnitzii. A plant-based diet rich in whole grains, fruits, and vegetables can therefore support a healthy, balanced microbiome.

Probiotic and Fermented Foods

Some fermented foods contain live beneficial bacteria, including species of Lactobacillus, which belong to the Firmicutes phylum. These probiotic foods can help colonize the gut and boost Firmicutes levels in a healthier way compared to processed foods.

Examples of probiotic-rich fermented foods:

Yogurt with live and active cultures
Kefir
Sauerkraut
Kimchi
Miso
Tempeh

The Role of Prebiotics

Prebiotics are non-digestible fibers that serve as food for beneficial gut bacteria, including specific Firmicutes species. This helps to fuel the growth and activity of a healthy microbiome. Prebiotic-rich foods include:

Onions and garlic
Leeks
Asparagus
Bananas
Whole grains like oats and barley

Comparison of Diets and Their Effect on Firmicutes


Dietary Factor	Type of Food	Effect on Firmicutes	Overall Gut Health Impact
Refined Carbs & Sugars	White bread, soda, pastries	Significant increase	Negative; can lead to imbalance
High Saturated Fats	Red meat, processed snacks	Increase	Negative; linked to inflammation
Dietary Fiber (Prebiotics)	Onions, garlic, whole grains	Nourishes beneficial species	Positive; promotes diversity
Probiotic Fermented Foods	Yogurt, kefir, kimchi	Adds beneficial Firmicutes	Positive; introduces live cultures

Conclusion: A Balanced Approach

Understanding the link between diet and the gut microbiome is crucial for managing health. While foods high in refined carbohydrates, sugars, and saturated fats are known to increase Firmicutes, this increase is often associated with a less healthy gut profile. The key is to focus on a balanced diet rich in diverse fiber sources and fermented foods, which can promote beneficial species within the Firmicutes phylum and support overall microbiome health. Consulting with a healthcare professional can provide personalized guidance, especially when making significant dietary changes.

A note on personalized nutrition

It is important to remember that everyone's microbiome is unique, and dietary changes can affect individuals differently. For those with specific health conditions or sensitivities, gradual implementation of new foods and professional advice is recommended to monitor tolerance and effectiveness. For more detailed information on gut health and nutrition, visit the National Institutes of Health (NIH) website.

Key Takeaways for Managing Your Gut Microbiome

While diet is a powerful tool, it is one part of a larger picture. Stress management, regular exercise, and avoiding unnecessary antibiotics also play a crucial role in maintaining a balanced gut microbiome. By focusing on whole foods, fiber, and fermented products, you can help steer your gut bacteria toward a healthier composition.

Frequently Asked Questions

No, not all Firmicutes are bad. The phylum contains both beneficial and potentially harmful bacteria. The issue is often an overgrowth of certain species or an imbalanced ratio of Firmicutes to other bacteria, like Bacteroidetes.

Refined carbohydrates and sugars are easily digestible and provide a readily available energy source for many Firmicutes species. This promotes their rapid multiplication and can lead to an imbalanced gut microbiome.

Yes, diets high in saturated fats, common in Western eating patterns, have been shown in animal studies to increase the ratio of Firmicutes to Bacteroidetes and disrupt overall gut health.

Prebiotics are non-digestible fibers that serve as food for beneficial gut bacteria, helping them to thrive. Probiotics are live, beneficial microorganisms found in fermented foods or supplements that can populate the gut.

Fermented foods like yogurt, kefir, sauerkraut, and kimchi often contain Lactobacillus species, which are part of the Firmicutes phylum and can help colonize the gut with beneficial bacteria.

Significant changes to the gut microbiome can occur relatively quickly, sometimes within 24 hours of changing your diet, highlighting the direct and powerful link between what you eat and your gut bacteria.

In addition to diet, the balance can be influenced by other factors such as genetics, geography, stress levels, exercise, and the use of medications like antibiotics.