How and why does diet influence immune function?

We may take it for granted that our diet can influence the way our immune systems work. But how and why does what we eat impact the immune response? In this Honest Nutrition feature, we investigate.

The immune system is a complex network that constantly works to protect the body from antigens, which have associations with pathogens, including bacteria, toxins, parasites, and viruses.

The immune system offers two lines of defense: innate immunity and adaptive immunity.

Innate immunity is the first line of defense and consists of physical barriers, such as the skin and mucous membranes and chemical and cellular defenses. The innate immune system is nonspecific because it reacts the same way to all foreign invaders.

If the innate immune system is ineffective against a potential threat, the adaptive immune system takes over.

The adaptive immune system consists of specialized blood cells and proteins that target the specific cause of infection. The adaptive immune system has a “memory,” which is why a person’s body becomes immune to specific illnesses after initial exposure.

A person’s immune system needs to function well for them to remain healthy. Certain illnesses, medications, and lifestyle choices, such as smoking and excessive drinking, can adversely affect immune function.

Research shows that a person’s diet can impact immune health as well.

Can diet influence the immune system?

Studies suggest that a person’s diet influences their immune system, like all other aspects of health.

For example, nutrition can affect the microbiome, gut barrier function, inflammatory processes, and white blood cell function, all of which impact immune function.

Dietary patterns and individual foods have associations with increased disease risk, greater risk of allergy, and impaired immune response.

Western-type diets tend to contain high levels of saturated fat, ultra-processed foods, added sugar and salt, and overall calories. This diet is often low in foods associated with better health, such as vegetables, fruits, and fatty fish, and has strong links to an increased risk of chronic disease.

Research suggests that Western-type diets induce inflammation and alter immune system function, promoting disease development.

In contrast, diets rich in whole foods, such as vegetables, fruits, nuts, legumes, and seafood, and low in ultra-processed foods can reduce disease risk and promote healthy immune function.

Additionally, a deficiency or insufficiency of nutrients essential to immune function, including vitamin D, zinc, and vitamin C, can also affect immune response.

Nutrient deficiencies may be more common in those consuming ultra-processed diets low in whole, nutrient-dense foods.

Although it is clear that dietary choices impact overall health, including immune function, the interaction between diet and immune health is highly complex. Scientists are still learning how the foods a person consumes may help or harm immune function.

Adverse effects of unhealthy diets

Western-type diets tend to be high in refined carbohydrates, added sugars, saturated fat, and calories. This pattern of eating affects immune function in several ways.

Most of the foods in Western diets are ultra-processed and contain high levels of added sugar, which can promote inflammatory responses of the immune system.

For example, foods and beverages that significantly impact blood sugar levels, such as soda, candy, sugary cereals, and sugary baked goods, increase levels of inflammatory proteins, including tumor necrosis-alpha (TNF-alpha), C-reactive protein (CRP), and interleukin-6 (IL-6). They also interfere with the function of protective immune cells, including neutrophils and phagocytes.

A 2012 study that included 562 adults aged 85 years and older without diabetes found that the participants who had higher blood sugar levels had lower innate immune responses. They also had higher levels of CRP, which is a marker of inflammation.

Higher blood sugar levels have links to an impaired immune response in people with diabetes as well.

Also, diets high in added sugar and refined carbs may adversely alter gut bacteria, leading to dysbiosis, which involves digestive disturbances, such as bloating.

A healthy microbiome is essential to immune function because gut bacteria play a critical role in the development and function of the immune system.

Experts have also linked Western-type diets to an altered immune response due to high levels of saturated fat and added salt.

Studies indicate that diets high in saturated fat may promote inflammation, modify gut bacteria, and inhibit the functioning of white blood cells.

Diets high in added salt have links to excessive immune response, impaired inflammation regulation in the body, and an increased risk of certain autoimmune diseases, such as rheumatoid arthritis.

Western-type diets have links to an increased risk of developing several chronic diseases, including certain cancers, heart disease, and type 2 diabetes.

Researchers attribute this to the chronic low-grade inflammation and altered immune response that Western-type diets, sedentary lifestyles, and toxin exposure cause.

However, research investigating the relationship between diet and immune function is ongoing, and scientists do not entirely understand this complex relationship.

Diets for healthy immune function

While a diet high in ultra-processed foods, added sugar, and excessive calories may lead to immune dysfunction, dietary patterns rich in whole, nutrient-dense foods are beneficial for immune function.

The Mediterranean diet is rich in vegetables, legumes, nuts, fruits, whole grains, olive oil, and other healthy foods. Research has shown that it can reduce disease risk, lower markers of inflammation, and beneficially modulate gut bacteria.

Diets high in fiber, such as the Mediterranean diet, promote the production of short-chain fatty acids (SCFAs), including acetate, propionate, and butyrate. SCFAs are end products of bacterial fermentation in the gut and have health benefits.

SCFAs act locally and systemically to modulate the immune response. They maintain the health of and improve the immune defensive function of the intestinal epithelium. This is an important part of the immune system that serves as a barrier against microorganisms. It also reduces the production of inflammatory proteins from immune cells.

Diets high in fruits, vegetables, olive oil, nuts, seeds, and fatty fish contain high levels of nutrients, such as vitamin A, vitamin C, zinc, vitamin D, B6, B12, copper, folate, iron, and selenium. The immune system needs these nutrients to function optimally.

Experts know that vegetarian-based diets reduce markers of chronic inflammation, such as CRP, fibrinogen, and IL-6. This might be partly due to the array of nutrients and nonnutritive components found in fruits and vegetables strengthening the immune system response.

Foods rich in healthy fats, fiber, vitamins, protein, minerals, and beneficial plant compounds help reduce systemic inflammation, promote healthy gut bacteria balance, reduce oxidative stress and cellular damage, and improve blood sugar and insulin sensitivity. All of these activities are essential for healthy immune function

Additionally, studies show that supplementing the diet with nutrients including vitamin D, zinc, and vitamin C may help optimize immune function and reduce infection risk.

To support immune function, a person should concentrate on following a balanced dietary pattern rich in whole, nutrient-dense foods, especially plant foods, such as vegetables, fruits, legumes, nuts, and seeds. People should avoid or limit ultra-processed foods high in refined grains and added sugar.

The bottom line

It is essential to follow a healthy diet to ensure good immune function.

Studies show that while certain dietary patterns may lead to impaired immune function, other dietary patterns promote optimal immune function.

A dietary pattern low in ultra-processed foods and rich in whole, nutrient-dense foods, such as vegetables, fruits, fish, and legumes, protects against chronic disease risk and supports a healthy immune response.

Following a healthy dietary pattern and leading a lifestyle that includes stress reduction techniques, restful sleep, daily physical activity, and other healthy habits is the best way to support the immune system and reduce disease risk.

Written by Jillian Kubala, MS, RD on April 9, 2021 — Fact checked by Jessica Beake, Ph.D.

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