5 Worst Foods for Your Immune System, Say Experts

Updated: Jun. 12, 2024

Research says these treats can dampen your body's defenses at the exact time of year you need your immunity to be fiercest. Avoid the worst foods for your immune system this season.

sugar in black coffee
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Your immune system is an impressive force of cells, tissues, and organs that hustles to shield you from microscopic threats like bacteria and viruses. Academic institutions like Harvard Health highlight that the health of your gut microbiome—trillions of beneficial microbes living in your intestines—is central to your body’s ability to produce disease-fighting proteins. The state of these microbes is greatly influenced by what you eat. Among the biggest offenders for your wellness is eating the worst foods for your immune system.

Nutrition experts agree that consuming a diet plentiful in fiber, fruits, vegetables, whole grains and legumes is critical for maintaining a healthy microbiome and controlling inflammation, which is key to a responsive immune system. Conversely, unhealthy foods for your immune system can upset this balance, leading to sustained inflammation and a weakened immune defense. This increases your risk of illness.

With the holidays and virus season coinciding with each other—causing you to find yourself traveling in tight quarters or crowding indoors with a crew—you can avoid a few indulgences that often surface this time of year. Take care of your body and avoiding what experts say are the worst foods for your immunity, so your body can take care of you, too.

Unhealthy food and fast food with donuts, chocolate, burgers and sweets top view
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1. Sugar

Reaching for that holiday cookie, punch, or even wine might seem like innocent relief for your sweet tooth, but what you’re doing is catapulting your blood glucose levels into hazardous elevation. This can temporarily weaken your immune system, including your white blood cells, which lessens their ability to fend off germs.

Nutrition guidelines generally advise limiting “added sugars”—those extra spoonfuls put into foods during manufacturing—to less than 10% of your daily caloric intake. On a diet of 2,000 calories daily, that’s no more than 200 calories from added sugars, which equates to a pretty excessive 12 teaspoons. The American Heart Association is even stricter, suggesting no more than nine teaspoons for men and six for women.

To sidestep sugar, opt for the natural sweetness of fruits like berries or apples, which have a milder effect on blood glucose. When you’re thirsty, choose water with a twist of fruit, herbal teas (which can warm up your whole body in the chilly months!), or seltzer water. These beverages hydrate and nourish you without impeding your immune system.

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Strips of fragrant fried bacon. Macro background.
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2. Processed meats

Processed meats—think bacon, sausages, and deli slices—might make a mouth-watering holiday breakfast or charcuterie board, but they’re also among the worst foods for your immune system. These meats come packed with preservatives and saturated fat (the unhealthy type of fat you want to keep to a minimum). Regularly eating these can lead to chronic inflammation, which keeps your immune system in a constant state of alert—and, over time, can wear it down.

Then there’s the issue of AGEs—advanced glycation end-products found in processed meats. Alexis Supan, RD, MPH, registered dietitian at the Cleveland Clinic Center for Human Nutrition, states that AGEs “cause inflammation throughout the body, deplete our antioxidant levels, negatively impact gut health, and damage our cells.”

Swap out processed meats for leaner proteins that are minimally processed, such as grilled chicken breast, fish, or plant-based options like lentils and chickpeas. These protein sources are lower in saturated fats and free from the preservatives in processed meats, helping to reduce inflammation and support your immune system.

If you like the convenience of deli meats, look for preservative-free options, often labeled as “nitrate-free” or “uncured”…and, consume these at a minimum.

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3. Refined grains

Baked goods, pie, breads and even crackers can be among the worst foods for your immune system. Not only are they nutritionally sparse, but they also lead to abrupt blood sugar fluctuations—suppressing your immune system and making it more challenging for your body to combat infections.

Instead of reaching for foods made with refined grains, try incorporating whole-grain alternatives like brown rice, quinoa, barley, and whole-grain versions of bread and pasta into your diet. These whole grains don’t spike your blood sugar as dramatically, thanks to their lower glycemic index. They’ll also provide additional nutrients and fiber.

I Ate Bread Every Day for a Week—Here’s What Happened

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4. Alcohol

Excessive alcohol consumption can break down the gut barrier, creating a passage for harmful bacteria to enter your bloodstream. This intrusion triggers inflammation and can weaken the cells that keep a vigilant watch for infections.

Supan comments on the serious repercussions of sustained, excessive alcohol use: “Heavy, chronic alcohol consumption can also lower our bodies’ ability to create T cells and B cells, the antibodies we use to fight infection.”

For a gut health boost that can bolster your immunity, kombucha is a great alternative to alcohol. Not only is it fizzy and fun; it’s also rich in probiotics that strengthen the gut barrier.

Cup of coffee
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5. Caffeine

The quick energy boost from caffeine, particularly in energy drinks, has its drawbacks—a big one being its impact on sleep quality and duration. Poor sleep, in turn, is a well-documented factor that can impair immunity, as outlined in a 2021 paper published in Communications Biology. Especially in the evening, beware of caffeinated beauty products (which can seep into your bloodstream) and treats at the cafe counter, since a chocolatey indulgence or even a little caffeinated breath mint can pique your energy, making them among the worst foods for your immune system.

For an energy lift that’s gentler on your body, consider green tea or a modest amount of black coffee. Both provide caffeine, but with additional health benefits and without the high sugar content. (Opt for more than three to five cups per day—think of one cup as one tea bag or one scoop of ground or instant coffee poured in a standard-sized mug.) Also consider a smoothie made with a blend of fruits, vegetables, and a source of protein, such as Greek yogurt or protein powder. These alternatives offer a gradual release of energy, helping to avoid the disruptive effects of caffeine on sleep.

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