How Your Diet Can Affect Your Immune System

by Matt Durkin
MSc Nutrition Specialist
22/03/2019

The human immune system is a highly sophisticated system that requires an array of nutrients to function properly. But which nutrients and where do we get these from?

In this article we will be exploring how diet affects our immune function, and outlining which foods help to protect the body from infection.

A Quick Overview of the Immune System

The immune system has two different forms of protection, known as innate and adaptive (or acquired) immunity. Innate immunity can be thought of the first line of defence, which rapidly responds to foreign cells in an attempt to eliminate them.  Examples of innate immunity include the skin, mucous membranes and cells such as macrophages which neutralise pathogens by literally engulfing them.

However, sometimes the innate immune response is not adequate enough to deal with the threat and requires the intervention of the adaptive immune system. The adaptive immune system firstly processes the pathogen to identify its specific makeup. Then, the body creates an army of cells (B cells and T cells) which are specifically designed to attack that antigen.

This aspect of immunity is highly intelligent as it has a ‘memory’, meaning that the body will respond more efficiently to the same threat in the future. This explains how vaccines work – the immune system is exposed to a tiny, harmless amount of a disease so that it can quickly recognise the pathogen if it ever enters the body again.

As you can see, the immune system is yet another example of how amazing the human body is. However for it to work properly we have to provide it with adequate nutrition.

Vitamins

Vitamins are organic compounds which are required by the human body. All vitamins are deemed as essential given that the body cannot create them. Although only needed in small amounts, hence the name micronutrients, vitamins have a range of critical roles in the body. One of these is of course the function of the immune system. Interestingly, out of the 13 recognised vitamins, 6 of them are proven to help keep illnesses at bay.

Vitamin A

Despite being widely-known for its ability to support vision, especially in dim light, vitamin A is also revered for its role in immune function.  Vitamin A is required for the development of a healthy immune system from birth, specifically the innate element of immunity.

Vitamin A helps in this regard by supporting healthy skin: the primary line of defence against pathogens. Similarly, vitamin A ensures that membranes in the respiratory tract secrete sufficient mucus to stop pathogens latching onto the body and multiplying.

There are actually two forms of vitamin A in the diet: preformed vitamin A (found in meat, fish and dairy) and the provitamin A carotenoids beta-carotene, alpha-carotene and beta-cryptoxanthin. These carotenoids, which can be found in foods such as carrots, sweet potatoes, spinach, kale and peppers, can be converted into usable vitamin A as required.

Although unfortunately common in developing countries, vitamin A deficiencies are quite rare in the western world. As vitamin A is fat-soluble, the body can store it, meaning a daily intake is not required.  

As a matter of fact, getting too much vitamin A is as much of a cause for concern in the UK as not getting enough. Too much vitamin A can occur if you consume liver more than once per week or take very high doses of cod liver oil. Pregnant women should avoid liver and supplements containing vitamin A as high amounts can harm the child.

So the take home message for vitamin A is that it is fairly straight forward to get enough through the diet without the need for supplementation and that more is definitely not better. 

Vitamin B

Vitamin B is actually made up of 8 unique nutrients which have important roles in the nervous system and transforming the food we eat into energy. Collectively known as the B vitamin complex, these nutrients are water-soluble, meaning they cannot be stored. For this reason, we have to consume them daily to maintain good health.

Alongside other key functions, vitamin B6, folate (B9) and vitamin B12 are proven in their ability to support immune function. Also known by the name of pyridoxine, vitamin B6 supports the production of antibodies. These proteins have the job of neutralising specific viruses (antigens).

As a lack of B6 delays the adaptive immune response, putting your body at greater risk of a serious illness, it is important we eat foods that contain B6 daily. Meat, fish, nuts, potatoes and bananas are just some examples of B6 sources. Fortunately B6 does not seem to be a nutrient of much concern in the British diet, but if you have poor eating habits then supplementation is a viable option.

Folate is an extremely important nutrient for the body to create DNA and RNA, the genetic coding that dictates cell function. An example of this is how pregnant women require extra folate (400µg per day) to reduce the risk of neural tube defects such as spina bifida in their babies. 

Due to its important role in DNA creation and DNA repair, it is clear to see that folate is an important element of the immune system. In humans, research has shown that low folate status leads to a reduction in circulating T cells meaning the body has a harder time to overcome viral infections.

Unlike the other B vitamins which are found in highest quantities in meat and fish, the best sources of folate are plant-based.  Legumes, green leafy vegetables and citrus fruits all provide high amounts of folate. Certain foods are also fortified with folic acid – a synthetic version of folate. The tag of ‘synthetic’ may put people off fortified foods and supplements.  However, it is accepted that folic acid is actually more bioavailable than natural folate.

Similar to vitamin B6, folate is not a nutrient of concern in the UK diet generally speaking. However, supplementation is very important for women during conception and pregnancy and can be very helpful for older adults who have impaired folate absorption. However taking high doses of folic acid can mask certain symptoms of a vitamin B12 deficiency so caution is advised. 

Vitamin B12 deficiency is something that is most commonly seen in vegetarians/vegans (as B12 is only found naturally in foods of animal origin) and older adults who have an increasingly difficult time absorbing this vitamin as well.

Despite only needing 2.5µg per day – the least of any essential nutrient - B12 has a number of critical roles in health. Psychological function, energy metabolism, the nervous system, red blood cell production and immunity are all affected by a B12 deficiency.

Looking at the immune system specifically, B12 helps to activate natural killer cells and also ensures that B cells can rapidly identify and target antigens to initiate their destruction. Like folate, B12 also is required for DNA creation.

It is relatively straightforward to obtain sufficient B12 through an omnivorous diet, but those with a plant-based diet or older adults should definitely consider supplementation as well as fortified foods.

Vitamin C

Over the years vitamin C has become synonymous with a strong and healthy immune system.

This reputation is well-deserved given that vitamin C is important for both innate and adaptive immunity. Vitamin C is an important nutrient for collagen synthesis, which helps to maintain good skin condition; a primary barrier to pathogens. Vitamin C is also a powerful antioxidant which helps protect cells from free-radical damage.

It is not as clearly understood how vitamin C influences the adaptive immune system but research has shown that it enhances B and T cell function and the production of antibodies, whilst also having a role in regulating inflammatory pathways.

Research shows that adequate vitamin C status not only lowers the risk of developing an illness to begin with but also taking high amounts once illness occurs can decrease both the severity and duration of symptoms.

Good sources of vitamin C are abundant and extend far beyond oranges and orange juice. Despite citrus fruits being high in vitamin C, you may be surprised to hear that there are numerous foods that are substantially higher in this essential nutrient. Peppers, kale, broccoli, sprouts, kiwi’s, blackcurrants, rosehips and goji berries are all examples of foods that contain more than 100% of your daily vitamin C needs per 100g portion.

As you can see, it is relatively straight forward to obtain sufficient vitamin C through the diet alone. That being said, smokers, alcoholics, older adults, those fighting infections and those with inflammatory metabolic diseases (i.e. type 2 diabetes) require substantially more vitamin C and for those people supplementation is a great option. Like B vitamins, vitamin C is a water-soluble nutrient meaning we do not have the capacity to store it and subsequently require a regular intake.

Vitamin D

Vitamin D is primarily known for its ability to regulate calcium and phosphorous absorption, in turn helping us to maintain strong, healthy bones. However there are constant advancements in our understanding of vitamin D and one area which has grown quite the evidence base is how this essential nutrient impacts immunity.

Interestingly, vitamin D had unknowingly been used to treat tuberculosis before the development of antibiotics. Patients were sent to sanatoriums where part of the treatment was exposure to sunlight. Cod liver oil was also utilised – something which is renowned for being one of the world’s richest sources of vitamin D.  This was going on even before vitamin D was discovered.

Epidemiological research has consistently shown those with lower levels of vitamin D suffer from more upper respiratory tract infections (URTI’s) like the common cold. No wonder then that the winter months are often labelled ‘the cold and flu season’ as this is the time of year where people in the UK typically have the lowest levels of vitamin D due to minimal sunlight exposure.

Unlike some of the other nutrients mentioned in this article, vitamin D is very difficult to get in sufficient amounts through food alone. This statement is reinforced by the latest government statistics which show that 37% of adolescents and 29% of adults in the UK are deficient during the winter months.

Because of the sheer magnitude of the issue, the Department of Health recommend that everyone utilises supplementation through the months of September to April. ‘At risk populations’ such as older adults, those with dark skin and those who spend little time outdoors are advised to consider taking a vitamin D supplement year-round.

You may have noticed that vitamin D supplements come in two forms: D2 (ergocalciferol) and D3 (cholecalciferol). D3 is the compound our body makes in response to sunlight and is therefore the preferred source by the body. However, vitamin D2 is still effective at increasing 25-OHD levels in the blood, but just requires a relatively higher dose than if D3 is taken.

Minerals

Like vitamins, minerals are another category of micronutrients, seeing as they are only required in small amounts. Iron, zinc, copper and selenium are all classed as trace minerals and all play a role in keeping us clear of illnesses and infections.

Iron

Iron is best known for its role in the production of haemoglobin – the sticky protein within red blood cells which binds with oxygen to deliver it around the body. Iron also has an important role in both innate and adaptive immunity.

Studies have shown that a lack of iron impairs neutrophil function and reduces the number of B cells, T cells, natural killer cells and also IgA (immunoglobulin A). IgA is an antibody found mainly in the respiratory tract and digestive system and is a key element of innate immunity.

Iron is a nutrient of concern in the diet of numerous populations in the UK, not least women of childbearing age who require more due to the menstrual cycle. The latest National Diet and Nutrition Survey showed that 46% of adolescent girls and 29% of women between 25 and 49 have an iron intake below the lower reference intake value. 

It is certainly possible to get sufficient iron through the diet. However the negative press around red meat (the best source of haem iron) and the low bioavailability of non-haem iron (from vegetables) has resulted in millions having insufficient levels.

There are, however, ways to maximise your absorption of iron from plant-based foods. Consuming vegetables alongside a portion of meat/fish or by consuming it with a source of vitamin C can both help. As zinc and tea/coffee decrease the absorption of non-haem iron, these should be consumed at different times to iron sources or iron supplements.

Given the widespread lack of iron in the diet, supplementation is utilised by many. If you do taken an iron supplement, take with a meal (ideally one that contains vitamin C) as this will decrease the chance of suffering from stomach upset.

Zinc

Zinc has a myriad of roles in the body, from supporting brain function and vision to the maintenance of bones and testosterone levels. Despite these important functions, zinc is best known for its ability to support the immune system. Most prominently it is one of the very few nutrients which can decrease the length and severity of the common cold. 

Specifically, research has looked at how zinc lozenges can provide this benefit. Of note, one recent study has shown that zinc lozenges taken within the first 24 hours of developing a cold decreased the overall duration of the illness by a third. This benefit is even more impressive in mature adults. Due to the immune-boosting properties of zinc, as well as all of its other health benefits, it should come as no surprise that supplementation is popular.

The British Nutrition Foundation have stated that a ‘significant proportion’ of males and young females have a low zinc intake. The best sources of zinc are seafood, meat, legumes, nuts, dairy and wholegrains. Unfortunately, plant-based zinc is poorly absorbed due to also containing ‘anti-nutrients’ like phytates and oxalates which are bound to zinc. So, like iron, the best sources are animal-based.

Selenium

Selenium is a mineral that is rarely mentioned, giving the impression it does not have an important role in health. Despite only requiring around 50 micrograms per day, selenium is involved in numerous bodily processes.

One of selenium’s main roles is in the production of glutathione – a compound known to scientists as the ‘master antioxidant’. Healthy blood levels of selenium have also been shown to reduce the risk of coronary artery disease and support reproductive function.

With regards to immunity, selenium has a number of roles in the adaptive element of the immune system which are not yet fully understood.  What is known however is that a lack of selenium delays the adaptive immune response to foreign threats, meaning selenium deficient people will take significantly longer to overcome an illness.

Selenium is a nutrient of much concern in the British diet, given that a significant amount of males and females over the age of 11 have an insufficient intake. A harrowing statistic is that 51% of women between the ages of 19 and 64 are below the lower reference intake value.

This is not surprising given that the best sources of selenium (fish, seafood and nuts) are not staple foods for most. Another factor that has resulted in a decrease in selenium intake over the years is new agricultural methods that deplete the soil of selenium. This obviously does not affect the selenium content of fish but it does affect plant-based sources.

One study has shown that the selenium content in Brazil nuts (the best known selenium source) can vary massively depending on the quality of soil that the tree was grown in. While one region’s Brazil nuts provided almost 300% of the recommended amount per nut, another region provided a mere 11% per nut.

Given that many people do not like, or cannot consume, fish or nuts, and low selenium intake is such a widespread issue, supplementation is growing in popularity.

Copper

Like selenium, copper is a mineral which does not gain much attention but is important for the production of connective tissues throughout the body as well as helping the function of the immune system. Copper is needed by immune cells to produce the energy they require. Low levels of copper have also been shown to decrease the size of the thymus gland which in turn impacts the ability of T cells to defend the body against pathogens.

Copper deficiency is quite rare, given that it is easy to consume a sufficient amount by regularly eating foods such as nuts, seeds, legumes, leafy greens, and dark chocolate. However, consuming high levels of zinc (over 50mg per day) can deplete the body of copper, so it is important that if you choose to supplement with zinc you ensure it provides an amount that is beneficial and not harmful.

Summary

Hopefully this article has outlined the importance of vitamins and minerals in the function of the immune system and has given you some idea of the foods you should be eating to obtain these. Certain nutrients are difficult to obtain through food alone, while many people struggle with the challenges of maintaining a healthy, balanced diet day-in-day-out, so supplementation is a viable option.

For those looking specifically for immune system support, we have launched SimplyGo Immunity – an expertly crafted formula providing 9 essential nutrients alongside a delicious fruit blend. This powder, which is available in honey & lemon or blackcurrant flavours, is simply added to hot water to create a delicious immunity drink.

Although beyond the scope of this current article, it is also of fundamental importance to understand that even if someone has a diet that provides all essential nutrients in appropriate amounts, their immune function may still be weak if they’re stressed, their hygiene is poor or they are lacking sleep. So staying healthy year-round is much more than just what we put on our plates and this should be the take-home message for those interested in keeping illness at bay.

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