How Your Diet Can Weaken Your Immune System

Having a strong immune system is the best way to protect yourself from infection and illness, so how can you do that?


We are used to thinking of our diet in terms of weight loss, but it has a much more important function.

It affects how you feel and how well your body functions, so your whole wellbeing is dependent on it being strong and healthy.

While a nutrient-rich and well-rounded diet supports your immune system, a diet that’s low in nutrients and high in ultra-processed foods impairs immune function.

Best foods for your immune system

As you might expect, it’s all the usual suspects and the essential nutrients that keep our immune system working well include vitamins A, B6, B12, C and D as well as copper, folate, iron, selenium and zinc.

You can get these from a healthy, varied diet including fruit and vegetables, nuts and seeds, whole grains, dairy products and meat, fish or plant proteins like pulses.

However vitamin D is now seen as key to immune health and you can’t get this from food as it is mostly produced from exposure to sunlight. Check your daily multivitamin to make sure that we’re getting enough year-round and a daily 10-microgram supplement is usually sufficient.

A healthy gut microbiome (the range of bacteria and other organisms in your intestine) has also been linked to a role in our immune system. The best way to achieve this is to have a wide variety of high-fibre plant-based foods like fruit and vegetables, whole grains, nuts, seeds, beans and lentils and fermented foods like live yoghurt or kefir can also be helpful.

What’s bad for your immune system?

On the other side of the scale, there are also foods that may weaken your immune system, so do you have many of these in your diet?

1. Added sugar

There’s no doubt that limiting how much added sugar you have definitely promotes your overall health and immune function.

Foods that significantly raise blood sugar, such as those high in added sugars, increase the production of inflammatory proteins like tumour necrosis alpha (TNF-α), C-reactive protein (CRP), and interleukin-6 (IL-6), all of which negatively affect immune function.

This is especially relevant if you have diabetes, as you can have elevated blood sugar levels for longer than people with well-regulated blood sugar levels.

What’s more, having high blood sugar levels may inhibit the response of neutrophils and phagocytes, two types of immune cells that help protect against infection.

Furthermore, it has been shown that high blood sugar levels may harm gut barrier function and drive gut bacteria imbalances, which can alter your immune response and make your body more susceptible to infection.

Several studies have linked high blood sugar levels to an impaired immune response in people with and without diabetes and diets high in added sugar may increase the susceptibility to certain autoimmune diseases, including rheumatoid arthritis.

Limiting your intake of foods and beverages high in added sugar, including ice cream, cake, sweets, and sugary beverages, can improve your overall health and promote healthy immune function.

2. Salty foods

Many foods high in salt such as crisps, ready meals and fast food may impair your body’s immune response.

Diets high in salt may trigger tissue inflammation and increase the risk of autoimmune diseases.

Salt may also inhibit normal immune function, suppress anti-inflammatory response, alter gut bacteria, and promote the generation of immune cells that are implicated in the pathogenesis of autoimmune diseases.

Researchers believe that excessive salt intake may be associated with the increase in autoimmune diseases in Western countries and that eating too much has been shown to worsen existing autoimmune diseases like ulcerative colitis, Crohn’s disease, rheumatoid arthritis, and lupus.

3. Do you have omega-6 and omega-3 fats in balance?

Western diets tend to be high in omega-6 fats and low in omega-3s. This imbalance has been associated with increased disease risk and possibly immune dysfunction.

Diets high in omega-6 fats seem to promote the expression of pro-inflammatory proteins that may weaken the immune response, while diets higher in omega-3 fats reduce the production of those proteins and enhance immune function.

What’s more, studies in people with obesity indicate that a high dietary intake of omega-6 fats may lead to immune dysfunction and increase the risk of certain conditions like asthma and allergic rhinitis

Researchers recommend that you maintain a healthy balance of omega-6 to omega-3 fats, which is considered to be around 1:1 to 4:1, to promote overall health.

This means eating more foods that are high in omega-3s — like salmon, mackerel, sardines, walnuts, and chia seeds — and fewer foods that are high in omega-6s, such as sunflower canola oil, corn oil, and soybean oil.

4. Fried foods

Once you fry food it becomes high in a group of molecules called advanced glycation end products (AGEs). These are formed when sugars react with proteins or fats during high temperature cooking, such as during frying.

If levels become too high in your body, they can contribute to inflammation and cellular damage and thought to weaken the immune system in several ways. These include promoting inflammation, depleting your body’s antioxidant mechanisms, inducing cellular dysfunction, and negatively affecting gut bacteria.

Researchers believe that a diet high in AGEs may increase susceptibility to diseases like malaria and increase the risk of medical conditions like metabolic syndrome, certain cancers, and heart disease .

Cutting back on foods like French fries, fried meat and fish will reduce your intake of AGE and they aren’t good for overall health and may cause immune dysfunction.

5. Processed and grilled meats

Like fried foods these are high in AGEs and the highest content is found in  fried bacon, roasted skin-on chicken, and grilled steak.

Processed meats are also high in saturated fat and some research suggests that diets high in saturated fats and low in unsaturated fats may contribute to immune system dysfunction and may contribute to systemic inflammation .

Additionally, a high intake of these foods has been linked to various diseases, including colon cancer, increased disease risk and may harm your immune system.

6. Fast food

An occasional treat is not usually a problem. but eating this too frequently has been linked to many negative health outcomes.

As well as taking a toll on your immune system, diets high in fast food and highly processed foods may drive inflammation, increase gut permeability, and cause bacteria imbalance in the gut, all of which can negatively affect your immune health.,

7. Foods that contain certain additives

Many food items, especially ultra-processed foods, contain additives to improve shelf life, texture, and taste. Some of these may negatively affect your immune response.

Carboxymethylcellulose (CMC) and polysorbate-80 (P80) are commonly used emulsifiers that have been linked to immune dysfunction in rodent studies. Although findings confirm they are not toxic, they can lead to important effects on lipid metabolism by causing changes in the expression of some genes associated with obesity.

For example, some emulsifiers, which are added to processed foods to improve texture and shelf life, can alter gut bacteria, harm your gut lining, and induce inflammation, all of which can cause immune dysfunction.

Also, corn syrup, salt, artificial sweeteners, and the natural food additive citrate may also negatively affect your immune system.

8. Highly refined carbohydrates

Eating highly refined carbs like white bread and sugary baked goods too often may harm your immune system.

These are types of high glycaemic foods that cause a spike in your blood sugar and insulin levels, potentially leading to the increased production of free radicals and inflammatory proteins like CRP.

Such a diet rich may alter gut bacteria, which can negatively affect your immune systems, so choose nutritious, high fibre carbohydrate sources like starchy vegetables, oats, fruit, and legumes over refined carbohydrates to support immune health.

9. Saturated fats

A diet high in saturated fats and low in unsaturated fats has been associated with immune dysfunction.

High saturated fat intake can activate certain signaling pathways that induce inflammation, thus inhibiting immune function. High fat diets may also increase your susceptibility to infection by suppressing your immune system and white blood cell function.

Studies in rodents have suggested that high fat diets can cause gut bacteria changes and damage the intestinal lining, potentially increasing infection and disease risk.

Eating a well-balanced diet high in fibre and healthy fat sources is a good way to support immune health.

10. Artificial sweeteners in food and drink

I have written on this topic often, more in relation to the fact that artificial sweeteners pose health risks and make no real contribution to weight loss, so it is something to be aware of.

Certain artificial sweeteners have been linked to altered gut bacteria composition, increased inflammation and imbalance in the gut, and blunted immune response.

Some researchers think that overusing artificial sweeteners may be detrimental to immune health and unfortunately manufacturers keep changing the name of their products so the ones consistently listed as higher risk are aspartame, sucralose and saccharin. Always read the label!

Although not all studies agree, and some have shown that moderate daily intake of those sweeteners does not cause changes in gut bacteria or immune function. However, rather than risk them if you are a dedicated sweetener user, you might want to shift to a more natural sweetener such as small amounts of honey or maple syrup or stevia-based ones as they are herbal as opposed to artificial.  

 Helpful information:

So what does this mean in real terms?  A good place to start is by limiting food and drink that are high in added sugars and salt, processed meats, and fried foods, all of which may have adverse effects on your body’s immune function.

If you can also limit your intake of ultra-processed foods that will help too as much as possible.

If sugar is a problem for you – and I hold my hand up for that one – then this article by nutritionist Patrick Holford can help.