No single food will make you ill, and no single food will protect you from every cold. But what you eat most days does shape how well your immune system works. This guide explains which foods and drinks are worth limiting, what the research actually shows, where the evidence is weaker than social media suggests, and what to eat instead.

Quick answer

The worst foods for your immune system are the ones that, eaten often, push up blood sugar, salt and alcohol intake or crowd out nutrient-rich food. The main ones are sugary drinks and added sugar, excess alcohol, very salty processed foods, deep-fried fast food, salty packaged snacks, cakes and pastries, energy drinks and a diet built mostly on ultra-processed meals. Occasional treats are not the problem. Your overall pattern, sleep and alcohol intake matter far more than any one meal.

How Does Diet Affect Your Immune System?

Your immune system is the network of cells, tissues and proteins that defends your body against infection. It needs a steady supply of energy, protein, vitamins and minerals to work properly.

Diet affects it in three main ways:

• Nutrient supply. Immune cells rely on nutrients such as protein, zinc, iron, vitamin A, vitamin C and vitamin D. A diet heavy in processed food can leave gaps.
• Inflammation. Inflammation is the body’s normal response to injury or infection. Problems start when low-level inflammation stays switched on for long periods. Diets high in sugar, salt and some fats have been linked to this kind of ongoing inflammation.
• Gut health. A large share of immune activity happens in and around the gut. Fibre-rich foods feed the gut bacteria that help keep this system balanced, while low-fibre, highly processed diets appear to do the opposite.

One important point: much of the research on specific foods and immunity comes from small human studies, laboratory tests or animal experiments. These show possible mechanisms. They do not prove that eating a particular food will make you catch more infections. The advice below reflects that uncertainty.

The 8 Worst Foods for Your Immune System

1. Sugary Drinks and Foods High in Added Sugar

Regularly eating large amounts of added sugar is one of the clearest dietary habits to cut back on, for your immune system and your health in general.

Persistently high blood sugar is known to make certain immune cells less effective. This is one reason people with poorly controlled diabetes are more prone to some infections. Neutrophils, the white blood cells that engulf and destroy bacteria, are particularly affected.

You may have seen the claim that one can of fizzy drink “shuts down” your immune system for five hours. This comes from a 1973 study in which a small group of volunteers drank 100 grams of sugar. Their neutrophils became less active at engulfing bacteria in lab tests, with the effect still measurable after five hours. That is a large dose of sugar, the study was small and old, and it did not show that the volunteers caught more infections. It is a reason to avoid frequent sugar spikes, not proof that a single sweet treat leaves you defenceless.

What to do:

• Swap sugary soft drinks, sweetened coffees and fruit juice for water, milk, unsweetened tea or sparkling water with fruit.
• Check labels for sugar under other names, such as glucose syrup, dextrose, maltose and fructose.
• Aim to keep free sugars below the limit set by the World Health Organization’s healthy diet guidance: less than 10% of daily energy, which is about 50 grams or 12 level teaspoons for someone eating 2,000 calories a day. WHO notes that going below 5% may bring extra benefits.

2. Alcohol, Especially Heavy or Binge Drinking

Heavy drinking is one of the best-documented ways diet can weaken immune defences. Doctors have long observed that people who drink heavily are more vulnerable to infections, especially pneumonia. Research reviewed by the US National Institute on Alcohol Abuse and Alcoholism shows that alcohol interferes with both the fast-acting and longer-term parts of the immune response, damages the protective lining of the gut and lungs, and that even a single episode of binge drinking can affect immune function.

Alcohol also disrupts sleep, which matters for immunity in its own right (see below).

What to do:

• Keep alcohol to a minimum. If you drink, have several alcohol-free days each week and avoid binge drinking.
• Do not drink when you are unwell or taking cold and flu medicines. Some, including paracetamol-containing products, should not be combined with alcohol.
• If you find it hard to cut down, speak to a GP or a local alcohol support service.

3. Very Salty Processed Foods

Processed meats such as bacon, ham and salami, instant noodles, stock cubes, ready meals and many restaurant dishes can contain a day’s worth of salt in one serving. Too much salt is best known for raising blood pressure, but it appears to affect immune cells too.

Two human studies stand out:

• In a long-term study linked to the Mars520 simulated space mission, a small group of healthy men had their daily salt intake lowered in stages from 12 grams to 9 grams and then 6 grams. As salt intake fell, their immune cells produced fewer inflammatory signalling proteins (IL-6 and IL-23) and more of an anti-inflammatory one (IL-10). The researchers concluded that high salt intake may push the immune system towards excessive inflammation.
• Researchers at the University of Bonn reported in 2020 that volunteers who ate an extra 6 grams of salt a day for one week had neutrophils that were noticeably worse at dealing with bacteria.

Both studies were small, so they do not prove that salty food leads to more infections. But together with the well-established blood pressure risks, they support keeping salt in check.

What to do:

• WHO advises adults to eat less than 5 grams of salt a day, which is roughly one level teaspoon in total, including salt already in food.
• Cook more meals from scratch and taste food before adding salt.
• Compare labels and choose lower-salt versions of bread, sauces and breakfast cereals.

4. Deep-Fried Foods and Fast Food

French fries, fried chicken and similar fast food are often high in calories, salt and saturated fat, and low in fibre. Some baked and fried foods may also contain industrially produced trans fats, which WHO says are not part of a healthy diet.

Research in mice published in the journal Cell in 2018 found that a high-fat, high-sugar “Western” diet put parts of the innate immune system into an inflammatory state, similar to its response to a bacterial infection, and that some of these changes persisted after the mice returned to a healthier diet. This was animal research, so it cannot tell us exactly what happens in people. Still, it fits with human evidence linking frequent fast-food diets to ongoing inflammation and poorer metabolic health.

What to do:

• Treat deep-fried food as an occasional meal rather than a routine one.
• At home, bake, grill, steam or air-fry instead of deep-frying.
• When eating out, add a salad or vegetables and skip extra salt.

5. Salty, Fried Packaged Snacks

Potato chips, corn chips, salted crackers and similar snacks combine two of the issues above: they are usually fried and heavily salted. They are also easy to overeat and tend to replace more filling, nutrient-rich snacks.

What to do:

• Keep fruit, plain nuts, yoghurt, hummus with vegetable sticks or air-popped popcorn within easy reach.
• If you buy chips, choose a small pack rather than eating from a sharing bag.

6. Cakes, Biscuits and Pastries

Cakes, cookies, doughnuts and pastries typically combine refined flour, added sugar and saturated fat, and some commercially made products contain trans fats. They are not harmful as an occasional treat, but if they become a daily habit, they add a lot of sugar and very little fibre or micronutrients.

What to do:

• Enjoy baked treats now and then, in smaller portions.
• When baking at home, try recipes that use wholemeal flour, oats, mashed banana, dates or unsweetened applesauce to replace some of the sugar and refined flour.

7. Energy Drinks

Energy drinks can contain as much sugar as regular fizzy drinks, or more, along with high doses of caffeine. Caffeine itself is not proven to harm the immune system. The bigger concern is how energy drinks are often used: to push through tiredness instead of getting enough sleep.

Sleep has a strong link with infection risk. In a 2015 study published in the journal Sleep, 164 healthy adults were exposed to a common cold virus. Those who had slept less than six hours a night in the week before were about 4.2 times more likely to develop a cold than those who slept more than seven hours.

What to do:

• If you rely on energy drinks most days, treat that as a sign you need more sleep rather than more caffeine.
• Avoid caffeine in the afternoon and evening if it affects your sleep.
• Choose sugar-free options if you do drink them occasionally, and check the caffeine content on the label.
• Energy drinks are not recommended for children, and people who are pregnant or have heart conditions should ask a doctor before drinking them.

8. A Diet Built Mainly on Ultra-Processed Meals

Ultra-processed foods are industrially made products that usually contain ingredients you would not use at home, such as flavourings, emulsifiers and modified starches. Examples include many ready meals, packaged snacks, sweetened cereals and reconstituted meat products.

WHO notes that diets containing large amounts of highly processed foods, which are often high in sodium, sugar or unhealthy fats, are associated with poorer health outcomes. The issue is less any single product and more what happens when these foods make up most of your diet: less fibre, fewer fruits and vegetables and more salt, sugar and saturated fat.

What to do:

• You do not need to avoid all processed food. Frozen vegetables, tinned beans, plain yoghurt and wholegrain bread are processed but nutritious.
• Aim to build most meals around whole or minimally processed foods.

What About Omega-6 Fats and Seed Oils?

Seed oils such as sunflower, soybean, corn and canola (rapeseed) oil are often described online as “inflammatory” because they are rich in omega-6 fats. The evidence does not support avoiding them.

The American Heart Association reviewed the research and concluded that it is incorrect to view omega-6 fats as pro-inflammatory, recommending that at least 5–10% of daily energy come from omega-6 fats. WHO’s current healthy diet guidance lists sunflower, soybean and canola oils among the unsaturated fats that are preferable to butter, ghee, lard and other saturated fats.

Where seed oils can be part of a problem is in deep-fried and ultra-processed foods, which are covered above. The oil is not the main issue there; the overall food is.

Omega-3 fats are still worth including. Good sources include oily fish such as salmon, mackerel and sardines, as well as walnuts, chia seeds and flaxseed.

What to Eat Instead to Support Your Immune System

There is no “immune-boosting” food that works on demand. A varied, balanced diet gives your immune system what it needs to function normally. WHO’s healthy diet guidance includes these practical targets for adults:

Supplements do not replace a balanced diet. Some interact with medicines, and high doses can cause harm. Speak to a doctor, pharmacist or registered dietitian before starting one, especially if you are pregnant, take regular medication or have a long-term condition.

Other Habits That Matter as Much as Food

Diet is only one part of immune health. These habits make a real difference too:

• Sleep. Most adults need at least seven hours a night.
• Physical activity. Regular moderate exercise supports overall health.
• Not smoking. Smoking damages the lungs’ defences against infection.
• Vaccination. Staying up to date with recommended vaccines is the most direct way to protect against specific infections.
• Hygiene and recovery. Handwashing and resting when unwell reduce the spread and impact of common infections. Our guide on how to get rid of winter bugs covers what to do if you are already ill.

When Should You See a Doctor?

See a GP if you get infections very often, if they take an unusually long time to clear up, if you have unexplained weight loss or ongoing tiredness, or if you need antibiotics repeatedly. These can sometimes point to an underlying condition, such as diabetes, a nutritional deficiency or an immune problem, that diet changes alone will not fix.

If you have a condition such as diabetes, kidney disease or an eating disorder, or you are pregnant, ask a doctor or registered dietitian for advice tailored to you before making major changes to your diet. You can find more practical guides in our health articles.

The Bottom Line

The foods most worth limiting for your immune system are the ones that, eaten often, flood your diet with added sugar, salt and alcohol or push out fibre and nutrients. You do not need to be perfect. Swapping sugary drinks for water, cooking more meals at home, drinking less alcohol and protecting your sleep will do more for your immune health than chasing any single “superfood”. For more everyday wellbeing advice, browse our health and wellness guides.