Trillions of bacteria, viruses, fungi, and other microorganisms live in the gut, forming what's known as the "gut microbiome," which is linked to digestion, immune function, and the production of certain compounds the body needs.
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Foods support this system in two different ways: some fermented products carry live microorganisms, while other foods provide the fiber and starches that bacteria already present in the gut use.
No single food can build a healthy microbiome on its own, and not all the bacteria found in fermented foods necessarily take up permanent residence in the gut. The benefit of these choices remains tied to overall dietary variety and how the products are made.
Foods that may carry live organisms
Kefir
Kefir is produced by fermenting milk or water using grains containing a mix of bacteria and yeasts. The types and amounts of microorganisms vary depending on the grains used and the fermentation and storage conditions.
Consuming kefir may allow microorganisms and fermentation byproducts to reach the digestive system. But the composition of commercial products isn't uniform, so it's best to avoid assuming all types of kefir have the same effect.
It's preferable to choose natural products with limited sugar, while following the storage instructions listed on the packaging.
Yogurt
Yogurt is one of the easiest fermented foods to add to your diet. It's made using bacteria that ferment lactose sugar and convert it into lactic acid.
Eating yogurt doesn't mean the bacteria in it will permanently colonize the gut, but it may interact with the gut environment as it passes through the digestive system.
Products vary in their live-organism content, so it's best to look for a label indicating live bacterial cultures, and choose plain varieties or those with little added sugar.
Sauerkraut
The natural fermentation of cabbage leads to the growth of lactic acid bacteria. But whether these organisms are present in the final product depends on how it's processed.
Heating and pasteurization reduce the number of live bacteria, making unpasteurized sauerkraut different from heat-treated products. Pickled products may also contain high amounts of salt, which requires attention from those who need to limit sodium.
A dietary trial published in the journal Cell showed that increasing intake of a variety of fermented foods was linked to higher microbiome diversity and reduced markers of inflammation among participants, but the findings don't mean every fermented product achieves the same benefit.
Foods that feed existing bacteria
Oats
Oats don't carry live bacteria, but they contain soluble beta-glucan fiber, which gut bacteria can ferment.
Fermenting certain fibers produces short-chain fatty acids, which play roles in colon lining health and the gut environment. Oats' effect remains part of the overall diet, not a standalone treatment for digestive disorders.
Lentils
Lentils combine plant protein with fiber, and contain resistant starch, part of which reaches the colon without being fully digested in the small intestine.
Bacteria use these compounds for fermentation, making lentils a food source for the microbiome. Rapidly increasing legume consumption may cause gas or bloating in some people, so the amount can be increased gradually.
Bananas
Bananas provide pectin and other types of fiber, while less-ripe bananas contain a larger amount of resistant starch compared to very ripe fruit.
Some gut bacteria can use these compounds, but tolerance for bananas and their effect on digestion vary between individuals.
Kiwi
Kiwi contains fiber, including pectin, alongside vitamin C and plant compounds. Clinical trials have linked consuming it to improved bowel regularity and some constipation symptoms among participants, without making it a treatment for all digestive issues.
The broader benefit comes from combining fermented foods with varied sources of fiber, such as whole grains, legumes, fruits, and vegetables. The gut microbiome needs variety more than it needs a single product labeled a "superfood."
