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.
So basically, prebiotics are food for our gut microbes.
Vegetables, fruits, whole grains and legumes like peas and beans are among the best sources
of naturally occurring prebiotic fiber.
Ingredients in some packaged foods, like inulin and oligosaccharides, are also classified
as prebiotics.
In general, most people don’t eat enough fiber.
The average American eats about 16 grams of fiber per day, while the recommendation is
25 to 38 grams of fiber every day.
Now when gut microbes metabolize prebiotics, some produce short-chain fatty acids like
butyrate, acetate, and propionate.
In the gastrointestinal tract, these short-chain fatty acids nourish the cells that line the
gut and have been associated with reducing the risk for certain types of cancer, like
colorectal cancer, enhancing calcium absorption, and relieving constipation and diarrhea.
Short-chain fatty acids also enter the bloodstream and travel to other organs, where they can
act as signals to communicate with the brain as well as regulate the immune system and
inflammation.
Plant‐based or high-fiber diets promote the presence of bacteria with a higher capacity
to ferment prebiotic fiber, resulting in increased short-chain fatty acid production, which may
contribute to an overall benefit to our health.
Research on the effect of pre- and probiotics in both foods and dietary supplements is happening
as we speak, and we’re learning more all the time about their effects on health.
Even though there have been promising discoveries in certain populations, prebiotic and probiotic
supplements aren’t often given to patients in hospital settings because their health
benefits haven’t been conclusively proven.
And despite being tested for safety before going on the market, some probiotic supplements
have been associated with infections in case reports of immunocompromised patients.
Alright, here’s a quick recap.
The human gastrointestinal tract is home to trillions of microbes and their genetic makeup,
which is called the gut microbiome.
What we eat and drink can directly affect our gut microbes, particularly foods and supplements
that are categorized as probiotics and prebiotics.
Probiotics are live microorganisms that have been shown to be beneficial to health.
They’re found in foods like fermented dairy products as well as dietary supplements.
Prebiotics are food components that microbes break down and use as energy, while providing
beneficial compounds like short-chain fatty acids for our bodies to use.
The gut microbiome is an exciting and emerging area of research, and we have a lot to learn
about how it affects our health.