This is the sixth installment of our “Nutrition 101” video series in partnership with Osmosis, a group that focuses on health science education, highlighting the basics of several nutrition topics. For a look back at what we’ve covered so far, watch our videos on fats, carbohydrates and sugars, hydration, low-calorie sweeteners and protein.
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, along with much smaller numbers of Proteobacteria, Verrucomicrobia, Actinobacteria, and Fusobacteria. The amount and types of bacteria found in the body can vary drastically from person to person, and there’s no clear consensus as to what makes up a “healthy” human 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 our 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 Prevotella and Bacteroides 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 that we typically revert 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 must be sufficient living bacteria that survive food processing so that they’re found in the final food or beverage product, 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. Both are commonly found in foods that are labeled as containing probiotics. Probiotics are also found in dietary supplements and are added to other foods and beverages such as granola bars, protein shakes and fruit juice.
Prebiotics are food components used by host microbes, and 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. 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.
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, including colorectal cancer. Short-chain fatty acids are also associated with enhancing calcium absorption and relieving constipation and diarrhea. Because they enter the bloodstream and travel to other organs, they can act as signals to communicate with the brain as well as regulate the immune system and inflammation in the body. 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. Still, 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.
To summarize, 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, and they also provide beneficial compounds like short-chain fatty acids for our bodies to use. The study of the gut microbiome is an exciting and emerging area of research, and we have a lot to learn about how these microbes affect our health.