The Micronutrient Information Center’s Health & Disease section now includes two articles on gut health. Here, we present an overview of the gut microbiota. For more information on this topic, check out these new resources online: Gut Health In Brief and Gut Health In Depth.
The gut microbiome refers to both the trillions of microorganisms that live in the gastrointestinal tract and the environment in which they live — mostly in our colon, also known as the large intestine. We provide a habitat for them; in return they provide us with helpful compounds and functions.
The organisms that live in our colon are known collectively as the gut microbiota. This includes bacteria, viruses, and yeast, but it can also refer to other microbes. However, the majority of gut microbiota research has focused on bacteria that inhabit our colon.
Functions of Gut Bacteria
The bacteria living in our gut can affect us in many ways. Their two main functions are to produce beneficial compounds from the food we eat and to provide some additional protection against disease-causing microorganisms.
Production of beneficial compounds
Gut bacteria consume some of the undigested food material that reaches the colon. The enzymes within the bacteria cells can digest some of this material further. In the process, they can produce a variety of metabolites and other small molecules.
One example is the fermentation of dietary fibers, which is described in more detail on the next page. Other examples include the transformation of bile acids, amino acids, and plant polyphenols into beneficial compounds.
These compounds can exert their influence directly inside the colon and in other parts of the body since they can be absorbed into the bloodstream and travel through the lymphatic system.
Protection against pathogens
Pathogens such as bacteria and viruses, are disease-causing microorganisms. Beneficial bacteria in the gut employ several strategies to keep the numbers of these pathogens in check.
One of the primary ways they do this is by competing with pathogens for space and nutrients. Additionally, beneficial bacteria can produce antimicrobial substances or alter the gut environment to be less inviting to pathogens.
A healthy gut microbiota
There is not one ideal composition of a healthy gut microbiota, as this varies due to people’s diets, lifestyles, and cultures. However, features of a healthy microbiota include diversity and a dominance of certain beneficial bacterial species that thrive in a low-oxygen environment.
A diverse microbiota contains many different bacterial species and is resilient to external challenges, such as infections, medications (especially antibiotics), and changes in the diet. Diversity of bacteria species also provides a diversity of bacteria functions. This allows for some bacteria to compensate for the loss of a particular strain when it does occur.
Food choices matter
Dietary choices directly impact the abundance of different bacterial species and the compounds they produce. This in turn influences the health of the colon and the entire body. An important dietary component that feeds the gut microbiota is dietary fiber, a diverse group of complex carbohydrates that cannot be digested by human enzymes in the small intestine. Fermentation of dietary fiber results in the production of beneficial short-chain fatty acids (SCFAs). These compounds directly nourish colonic epithelial cells, also called the gut barrier, and create a favorable environment for beneficial bacteria. Indirectly, SCFAs interact with cellular receptors in the colon and throughout the body, engaging a number of signaling pathways (see sidebar) that can influence inflammation, appetite, and insulin secretion, to name a few. There is a great deal of variability from person to person; what is produced depends not only on the food you consume but also which bacteria are present. This is one of several complications that make it difficult to predict how an individual will respond to specific foods.
What can I do?
When you eat, it’s not just about fueling and nourishing your body, it’s also about feeding your gut microbiota. Even though we each have a unique mixture of bacteria (and other microorganisms), the same guiding principles that benefit your overall health can also benefit the health of your gut.
- Eat a diversity of fruit, vegetables, whole grains, beans, nuts, and seeds — they all contain fiber.
- Try to consume the recommended amount of fiber each day (women: 28 grams; men: 35 grams), but if you need to increase your fiber intake, do it gradually — abrupt changes can cause discomfort and your body may need time to adapt.
- Limit highly processed foods and added sugar, as they promote the growth of less beneficial species of bacteria.
- Consume fermented foods, like kimchi, sauerkraut, yogurt, or kefir. Fermented foods can be a great source of probiotics with live bacteria, but take note that not all fermented foods contain live bacteria. Look for products with live cultures (for example, vinegar-based or pasteurized pickles do not have live cultures). To establish and maintain changes to the gut microbiota, regular consumption of a diverse, healthy diet is necessary; for example, adding fiber occasionally is not going to have the same impact. If you encounter issues making these changes, consult with a physician or dietitian for some suggestions and strategies.
GLP-1 hormone
The intestines can produce about 30 different types of peptide hormones, each with different functions. One of these gut hormones, glucagon-like peptide-1 or GLP-1, is produced by the colon after exposure to SCFAs. Thus, eating more fermentable fiber stimulates GLP-1 production, and GLP-1 helps regulate insulin secretion and appetite.
A class of drugs known as GLP-1 Receptor Agonists mimic the effect of the hormone. These drugs include semaglutide (sold under the brand names Ozempic or Wegovy) and tirzepatide (sold under the brand names Mounjaro or Zepbound).
These drugs can stimulate the GLP-1 receptor, but their effects last in the body for much longer than the naturally produced GLP-1 hormone, making them effective for the control of blood glucose levels and appetite. They are currently FDA approved for weight management or the treatment of type 2 diabetes.
| Common sources of dietary fiber* | Fiber content |
|---|---|
| 1 banana | 3 grams |
| 1 cup of broccoli | 4 grams |
| 1 cup of cooked oats | 4 grams |
| ¼ cup of almonds (a small handful) | 4 grams |
| 1 medium apple | 5 grams |
| ½ medium avocado | 7 grams |
| ½ cup of cooked lentils | 8 grams |
| 1 oz chia seeds (2 Tbsp.) | 10 grams |
