7 Worst Foods for Your Immune System, According to a Dietitian

Like a cape-clad superhero, your immune system constantly monitors villainous threats, like viruses or bacteria, and jumps into action when you come down with an illness. After the COVID-19 pandemic, keeping your immune system in fighting shape for the next virus may be top of mind.

While no specific food or supplement can guarantee that you'll never get sick, your overall diet can play an important role in supporting the work your immune system does to ward off unwanted infections.

Eating a diet that is high in fiber and healthy fats and provides sufficient amounts of key nutrients, like vitamins A, E, C, D, zinc and iron, can optimally support your immune system. However, a diet lacking these nutrients and high in added sugars and saturated fat could weaken your immune system.

To give your immune system all the support it needs to keep you from getting sick, here are seven foods you should limit.

1. Refined Grains

Since the majority of your immune cells are found within the lymphoid tissue lining the gut, it is important to limit refined carbohydrates, which generally are higher in sugar and stripped of the fiber that allows a healthy gut to thrive.

According to a 2022 research review published in Allergy, dietary fiber supports the immune system in a couple of different ways.

First, fiber helps develop a healthy gut barrier, which acts as a primary defense mechanism against the bacteria and viruses in the food you eat. Second, gut bacteria produce short-chain fatty acids (SCFAs) by fermenting the fiber in your food. These SCFAs help regulate the function of your immune cells so that they perform optimally when introduced to a viral or bacterial threat.

2. Alcohol

While light to moderate alcohol intake may have some health benefits, excessive alcohol intake offers potential negative consequences—including to your gut, which plays a vital role in immunity.

A 2021 review in Toxicology Reports noted that alcohol could affect the diversity of the healthy bacteria in your gut and contribute to a less functional gut barrier—meaning pathogens can sneak past it. Excess alcohol intake can also inhibit your immune system's ability to create the antibodies you need to ward off foreign invaders, like the flu virus.

Alcohol also disrupts sleep, which is essential for the immune system to be in tip-top shape. If you like to have the occasional drink, do so in moderation—two standard drinks or less per day for men and one or less per day for women, per the Centers for Disease Control and Prevention (CDC).

3. Sugar-Sweetened Beverages

We know that too much added sugar isn't good for you. Excessive sugar intake, particularly through sugar-sweetened beverages, may negatively affect the body and contribute to a weakened immune system.

According to a 2022 review in Human Immunology, excess fructose (a type of sugar) intake alters the makeup of your gut microbiota so that the beneficial strains of bacteria have more trouble thriving. High blood sugar levels can also alter the gut's permeability, contributing to chronic inflammation. Although inflammation is a normal step in the body's innate immune response, chronic inflammation can leave your immune system too overworked and weak to ward off new infections.

4. Fried Foods

Fried foods are higher in advanced glycation end products (AGEs). These compounds naturally occur during cooking, especially at high temperatures like frying. Research, such as a 2020 article in Nutrients, found that AGEs may be associated with a less optimal gut microbiome, increased inflammation and a weaker intestinal barrier—which can all negatively affect your immune system.

5. Red Meat and Processed Meats

Although red meat can have a place in a healthy diet, excessive intake of red meat and processed meats, like bologna and hot dogs, can be bad news for your immune system. When red meat is digested, gut bacteria create a substance called trimethylamine (TMA) that is then transformed into trimethylamine N-oxide (TMAO) by the liver.

A little TMAO is nothing to worry about, but higher levels of TMAO are often associated with an unhealthy community of gut bacteria, chronic inflammation and a weakened immune system, as noted in a 2020 review in Frontiers in Immunology.

6. Caffeinated Beverages

Coffee and tea are high in antioxidants that can support a healthy immune system by reducing inflammation. But if caffeinated, they can potentially interfere with quality sleep. As noted in a 2021 review in Communications Biology, numerous research studies have linked sleep with immune function. If you enjoy caffeinated beverages, limit them to just a couple of cups daily and try to consume them at least 6-8 hours before bedtime.

7. Salty Foods

A large 2023 study published in Respiratory Research found a link between adding salt to foods and sleep apnea, a condition with which there are interruptions in breathing while sleeping. Per the CDC, sleep apnea results in a lack of quality sleep—and as we've already seen, lack of sleep is linked with a weaker immune system.

Most people generally eat a lot more salt than is recommended. According to the FDA, Americans consume about 3400 mg of sodium per day. Yet, the 2020-2025 Dietary Guidelines for Americans recommend that adults limit sodium intake to less than 2300 mg daily—or about 1 teaspoon of table salt.

To reduce your overall salt intake, consume fewer foods that are high in sodium—like fast food, salty snacks, processed meats and canned goods. And limit how often you're reaching for the salt shaker. Instead, swap the salt out for herbs and spices to ramp up the flavor in your dishes.

Frequently Asked Questions

What Are the Best Foods to Eat for the Immune System?

A 2022 review in Nutrients suggests following nutritious diets—like the Mediterranean diet, which includes foods high in fiber, omega-3 fatty acids, vitamins A, C, D and E, and zinc—can best support a healthy immune system.

We also know that staying hydrated and eating enough protein is important. Some foods to incorporate into your diet more often to support your immune system are whole grains, fruits and vegetables, nuts and seeds, legumes, lean meats and fatty fish.

What Foods Can Trigger Your Immune System?

A diet high in ultra-processed foods, added sugar, sodium and saturated fat may increase chronic inflammation and reduce your immune function. This, in turn, increases your risk of chronic health conditions such as type 2 diabetes and heart disease.

What Weakens Your Immune System the Most?

Chronic undernutrition, dehydration and excess alcohol consumption can weaken your immune system, making you more susceptible to disease. Lifestyle factors like stress and lack of sleep also impact your immune system. So when cold and flu season comes around, make sure that you aren't only eating a balanced diet but prioritizing sleep and strategies for reducing stress.

The Bottom Line

Give your immune system a fighting chance by supporting it with healthy meals packed with various nutrient-dense foods. Additionally, drinking lots of fluids, getting plenty of quality sleep and reducing stress also help bolster proper immune function. And while you want to consider limiting the foods included on this list, don't worry too much about the occasional dessert, salty snack or glass of wine. After all, you still need to enjoy life, too.