Gut Check: 5 Reasons Why Taking Care of Your Gut Can Help You This Flu Season

A healthy digestive system plays a key role in your overall health and immune system response.

With flu season underway — typically running from October through April, per Cleveland Clinic — you may be wondering what steps you can take to avoid getting sick, or developing a serious illness if you do get the flu.

The most obvious step is to get an annual flu shot. This vaccine is recommended in September or October for most people, according to the Centers for Disease Control and Prevention (CDC) — but you can benefit from taking this protective measure anytime during the flu season.

Your immune system, which fights outside invaders like influenza (flu) viruses, is an elaborate network in your body that is influenced by many different factors. One major determinant of how your immune system functions is the health of your gut microbiome — the bacteria and other organisms that live in your digestive tract.

“In someone who is healthy, this gut microbiome is protective against pathogens” — harmful bacteria or viruses — “that can enter the body through the food we eat or water we drink,” notes Nancee Jaffe, RDN, a gastroenterology nutritionist and the lead dietitian at the UCLA Vatche and Tamar Manoukian Division of Digestive Diseases in Los Angeles. This role can even extend outside the digestive system, potentially warding off respiratory infections like the flu.

Here are five reasons to take your gut health seriously during the flu season — and during the rest of the year, too.

1. There Are Trillions of Organisms in Your Gut, Most of Them Helpful

A huge number and variety of bacteria, fungi, parasites, and viruses live throughout your digestive system — and most of them play a helpful role in your body. “Most of them are what we call symbiotic, which means they benefit humans, and humans benefit them,” Jaffe explains. A small number of bacteria and other organisms are pathogenic, meaning they can promote disease — but they don’t necessarily cause illness unless they grow out of control.

“Our microbes are good at helping prevent the overgrowth of bacteria by basically creating competition,” says Jaffe. “They’re competing for nutrients, for sites where they can attach to the [digestive tract] lining. And that helps keep the bad bacteria at bay.”

Gut bacteria mostly live on complex carbohydrates from plant foods that the human body, by itself, can’t digest — which are collectively known as fiber. It's important to get enough fiber in your diet from a variety of healthy foods in order to promote a diverse and healthy gut microbiome.

Not only does your digestive tract host your gut microbiome, it also hosts most of the human cells that make up your immune system — about 70 percent of all these cells, according to UCLA Health. That’s no coincidence — your gut microbiome plays a key role in training your immune system to recognize which bacteria and other microbes are harmful and which ones aren’t, so that it can attack potentially harmful microbes in the food you eat.

2. It Can Be Hard to Know if Your Gut Microbiome Is Healthy

“We don’t have an agreed-upon understanding of what a healthy gut microbiome looks like,” says Jaffe. “The best thing we can say is having a lot of abundance and diversity of healthy bacteria, and making sure the balance between good and bad bacteria is right, is a good place to start.”

Unless you've taken part in certain research studies, or have had a gastrointestinal pathogen panel (used to detect bacteria, viruses, or parasites), which only happens if you have a GI issue, you probably don't know the composition of your microbiome. But even if you have this information, it doesn't necessarily help you make decisions about what to eat or whether you should take a probiotic supplement (containing beneficial bacteria).

Sometimes, of course, there are signs that your gut isn’t in great shape, which could be related to an imbalance in your gut microbiome. “The gut has many ways of letting us know when it’s not happy,” Jaffe notes, including excessive gas, bloating, abdominal discomfort, and changes in bowel movement timing or consistency.

3. Gut Bacteria Can Affect Your Immune System Outside Your Digestive Tract

While most of your immune system activity is based in your digestive tract, what happens in your gut doesn’t stay in your gut. Researchers have identified several ways that your gut bacteria may influence your immunity outside the digestive tract, one of which is by producing molecules known as short-chain fatty acids, as noted in an article published in the International Journal of Molecular Sciences.

Short-chain fatty acids are produced by some bacteria when they consume certain types of fiber, Jaffe notes. These molecules are known to stimulate immune cell activity in different areas of the body, and are also linked to your body’s maintenance of normal blood sugar and cholesterol levels.

One example of how short-chain fatty acids affect immunity can be seen in a molecule known as butyrate, as noted in an article published in the journal Nutrients. This type of short-chain fatty acid has been shown to help promote the development of different types of T cells, immune system cells that play a key role in your body’s response once it detects a pathogen that it needs to fight.

Research on how the gut microbiome could affect outcomes like flu infections is still in its infancy, according to Jaffe. But researchers already know of ways that gut microbes help the lungs control respiratory viral infections, according to an article published in the journal Mucosal Immunology. One example of this is that short-chain fatty acids and other molecules produced by gut bacteria have been shown to promote the production of certain immune system proteins (known as type 1 interferons) in the lungs.

4. What You Eat Can Influence the Diversity of Your Gut Bacteria

When it comes to feeding your gut microbes, fiber is the name of the game. Probably the best thing you can do for a healthy gut according to Jaffe is to eat a variety of fiber-rich foods — such as whole grains, beans, fruits and vegetables, and nuts and seeds. Some particularly beneficial foods include oats, barley, garlic, leeks, onions, and asparagus, since they contain certain types of fiber known to feed helpful bacteria.

“The more plant-forward the diet, probably the healthier the gut microbiome,” says Jaffe. “That means thinking about the plants that are going to show up on your plate first, and making the majority of your meals plant-based.”

Another helpful category of foods are probiotics, or those that contain high levels of helpful live bacteria — including yogurt, kefir, tempeh, kimchi, and certain pickled vegetables. “The research shows that you don’t need a ton of it. Even as little as a quarter to a half-cup per day could make a difference in the inflammatory markers in our body,” says Jaffe.

On the other side of the coin, the so-called standard American or Western diet — high in processed foods, animal protein, sugar, and alcohol, and low in fruits, vegetables, and whole grains — can disrupt your gut microbiome and drive inflammation, according to Jaffe. Inflammation refers to a type of immune system response that can have harmful effects in the body, potentially weakening your ability to fight infections. Ultra-processed foods, in particular, have been linked to changes in gut bacteria that promote chronic inflammation, according to a research review published in the journal Nutrients.

5. Factors Other Than Diet Can Affect Gut Health and Immunity

While your diet plays a key role in your gut microbiome and your body’s overall immune response, other environmental and behavioral factors may also play a role. These include psychological stress, sleep, and physical activity.

“The human gut microbiome can be influenced by the brain-gut axis,” says Jaffe, which transmits stress or emotion between the brain and the gut along what’s known as the vagus nerve. “We’ve seen that when there’s psychological stress or depression, it can promote a shift in gut bacteria composition.” Your brain and the bacteria in your gut communicate in a number of different ways, including through your immune and nervous systems, according to a research review published in the journal Physiological Reviews.

So it’s a good idea to try to reduce stress, get enough sleep, and take part in activities that you enjoy. But if you do find yourself in a stressful period, “We’ve even seen that gut bacteria can [contribute to] stress signals even more,” says Jaffe. “Which is why eating healthfully in those moments is even more important to keep the bacteria happy.”