Content Reviewers:Rishi Desai, MD, MPH
The gastrointestinal tract is home to trillions of microbes, collectively called the gut microbiome.
It was previously thought that there were about ten times as many microbial cells in our bodies as there are human cells, but more recent estimates have it at closer to a one-to-one ratio, with the balance tipped just slightly toward the microbes.
In other words, it looks like we’re slightly more microbe than human!
The gut microbiome is dominated by two main groups of bacteria: Bacteroidetes and Firmicutes, with much smaller numbers of Proteobacteria, Verrucomicrobia, Actinobacteria, and Fusobacteria.
The amount and types of bacteria can vary drastically from person to person, and there’s no clear consensus on what makes up a “healthy” microbiome.
Microbes are found throughout the gastrointestinal tract, but most are in the large intestine, or colon. And since what we eat and drink passes through the gastrointestinal tract every day - it’s no surprise that our diet affects our gut microbiome.
For example, people who eat a high-fiber diet tend to have higher levels of Prevotella, and those with a diet higher in protein and fat have more Bacteroides, both of which are members of the Bacteroidetes group.
In fact, studies have shown that even a single day of a strict animal-based diet or plant-based diet can alter the microbiome composition, but we often revert back to our regular microbiome once our diets go back to normal.
Two parts of our diet that are uniquely able to affect the microbiome are probiotics and prebiotics.
Probiotics are live microorganisms that offer a health benefit - for example, by helping to enhance or restore health to our gut microbiome.
Many of the microorganisms that naturally live in our bodies are similar to microorganisms found in probiotic foods, drinks, and dietary supplements.
Probiotic bacteria are found in fermented dairy products like yogurt and kefir as well as foods like kimchi and sauerkraut, though not all types of fermented foods necessarily qualify as a “probiotic.”
For a food or drink to be considered “probiotic”, there have to be sufficient living bacteria that survive food processing so that they’re in the food or beverage, and the bacteria that survive have to be ones that are known to benefit human health - based on research studies.
Two well studied groups of bacteria are Lactobacillus, which is in the Firmicutes group, and Bifidobacterium, a type of Actinobacteria, and both are commonly found in foods that contain probiotics.
Probiotics are also found in dietary supplements and are added to other foods and beverages, like granola bars, protein shakes, and fruit juice.
Prebiotics are food components used by host microbes, and therefore they offer a health benefit too.
Many prebiotics are found in high-fiber foods that aren’t broken down by human digestive enzymes, and make it to the large intestine where they’re fermented by gut bacteria.