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  • Writer's pictureTerry Walker - NutriBro.

Nutrition and Food to Boost Immune System

Updated: Apr 16, 2020

This should help to explain the background to immuno-function, how microbes react with it and what we can do to boost our immune system in response to all infections as well as COVID-19.

This is a bit of a beast, so I have laid it out with a brief abstract, then the full article. I've referenced everything myself, but only included some for the readers, the other information is fairly well available on google scholar, pubmed, web of science etc. Read my page on What is Evidence if you need more information about sourcing good evidence and how to read academic journals.

Please comment, share and like to attempt to dispel myths and nonsense about this subject. Misinformation is dangerous and it's being utilised by people at the top of society to mislead and influence the public. We can help by educating and informing ourselves to move away from a society of beliefs, dogma and superstitions that lead to vulnerable people being exploited and victimised by charlatans such as homoeopaths and Donald Trump alike who can potentially harm you in their dearth of regulated, validated, monitored and inspected chemicals and water-based products.

Abstract - Immune Boosting Foods

General health is boring but appears to be the best way to have a functioning immune system. That means lots of fruit and vegetables, again boring, and as a result, this blog won't shock people into trying to buy a magic pill to cure Coronavirus, or make me famous. It is however true.

Vitamin C does not prevent catching viruses. Short term high doses have no effect on illness. Long-term sufficient vitamin C helps reduce symptoms and shorten the length of the illness. Foods high in Vitamin C are listed below. If you cook them they will lose nearly all of the vitamin C very quickly, also if left in the open air, or exposed to sunlight.

Vitamin A is super important and readily available from a healthy diet. Vitamin D is crucial and it is likely that you are or have been deficient in Vitamin D, therefore I recommend taking a supplement for that for 6 months of the year.

Zinc has a positive effect on immune function, but there is no need to supplement it to get the required levels.

Cortisol, Testosterone (man-flu), being overweight or obese and low energy from a restricted diet, all have an immuno-suppressing effect.

The Immune System

If an infectious person coughs, and one breathes in their microbes/germs/bacteria/viral host cells, that infection is passed on to that person. Immune proteins in the immediate area will create an inflammatory response. Those Y-shaped proteins have receptors that recognise foreign bodies and either kill them directly, or incredibly, transport them around until other more competent and relevantly skilled proteins can kill them. These are immunoglobulins and gammaglobulins, aka antibodies.

With COVID-19, the infection has never been seen before by any of our immune systems. It also reproduces very very quickly, outrunning the speed of the immune response. It is therefore possible that a person who is extremely exposed to the infection will develop worse symptoms than a person who catches only a few of the original microbes that then multiply.

Here's the incredible part about immune systems proteins. Their outreaching arms have 'keys' (antigen-binding sites) and if you have ever received a vaccination to say TB, there are TB shaped keys that lock together with the infectious body, recognise it, and kill it in your body. So how can this work for a microbe that has never been seen before...? Well, the genes that make immune cells shuffle around a lot and therefore don't exactly replicate, the proteins therefore have many random shaped keys to pick up chance new infections that have never been in the body. If one of these immune cells has receptors key that fits perfectly with one's own body cells, it is killed off immediately so as not to destroy the host person! (This would be an auto-immune disease.) How the host like antibodies are destroyed is too complicated to explain here, look up 'HMC or HLA' if you're more interested.

This explanation works well for bacterial infections, the kinds that can be treated with anti-biotics, I.e chlamydia, but works slightly differently with some viruses. Bacteria are cells, they eat, drink, breath and reproduce like human cells, but viruses are just segments of DNA which attach to or invade cells and change their behaviour with adverse effects. I.e. common cold, flu or Coronaviruses. Our immunoglobulins instead will bind to those virus strands and prevent them from being able to penetrate cell walls or other barriers throughout the body.

The 'complement system' can then either burst invasive bacteria cells bound to immunoglobulins or signal to phagocytes to remove cells bound with infectious DNA viruses. They are then cleared from the blood.

White blood cells, or Leukocytes, made in bone marrow can find infections via chemotaxis. They can then engulf, absorb, bleach or just kill bacteria cells and virus DNA. Lymphocyte T-cells are fairly specific to battling viruses, they sense weird virus-containing cells, kill and remove them. Interleukin-6 (the often referred to measure of inflammation) turns on T-cells to kick off the response. Some of these cells are pretty full on and go around trying to kill everything, therefore mediating cells call upon T-Suppressors chill them out a bit and turn off the immune response.

So, the immune responses in our bodies consists of: Lots (billions) of cells being produced, hormones reacting and being produced, transportation of these cells and proteins in the lymph and blood systems and enymes to function too. Therefore any nutrition strategy we want to help boost our immune system needs to provide the substrates to manufacture these cells and fluids. That is the way nutrition works. Vitamin C - ascorbic acid, doesn't necessarily fight the infections itself but works in the processes of the body as a whole to create an immune response.


It is worth noting that Vitamins A, C, and E work as antioxidants on their own and with some enzymes. That is slightly different from an pure immune response although ties in with inflammation. For viruses such as COVID-19 antioxidants appear to have little effect.

Oxygen in the body is essential, but also dangerous as it is a very reactive element. If we bond Oxygen to the very common compound Water you end up with O + H2O = H2O2 - Hydrogen Peroxide (Bleach)! This out of place can move around the body damaging cells and causing chain reactions in DNA, very bad. Other free-radicals can also simply be ionised versions of the original compound, but with a positive or negative charge. The anti-oxidant can donate an electron or hydrogen to balance the charge and neutralise the free-radical.

Immune Boosting Foods

Vitamin C - Ascorbic Acid

In 1991, the RDA from COMA of Vitamin C was 40mg per day. It has been raised to 75mg and 90mg in women and men respectively (1).

Vitamin C works in the above mentioned phagocytic cells (ones that have absorbed or killed an infectious cell) and enhances the chemotaxis effect for immune cells to find the infections. Phagocytosis is improved with Vitamin C, and it has an important role in donating electrons where oxidation can kill the virus or bacteria. It is important in the role of apoptosis (lifecycle/killing of cells) and promotes action by macrophages to clear dead cells and reduced used immune proteins from the body. With respect to viruses, Vitamin C can also promote T-Cells by acting in a gene upregulator. (1)

Vitamin C of 90mg per day is fairly easy to obtain from the diet. However, clinical studies have used higher doses to affect viruses and respiratory tract infections such as those associated with COVID-19 SARS. 90mg per day is adequate, and 200mg is considered saturating. (1)

A systematic review of the literature in 2000, found high dosage (up to 4000mg) Vitamin C to have no effect on the contraction of viruses such as common-cold, and no effect on the symptoms when a loarge dose was administered at the start of the infection. Long-term lower dosages were however found to help symptoms of cold and respiratory illness, though no prevention benefit. (2)

In 2006 another paper found a large number of randomised controlled trials that were administering about 1000mg of Vitamin C and 30mg Zinc daily had a reducing effects on colds and respiratory tract infections, both in length of illness and severity. (3)

More specifically a study in 1994 showed that elderly hospitalised patients with respiratory infections were given 100mg twice a day or placebo over 4 weeks. The treatment group had significantly higher white blood cells and improved respiratory health when compared to the placebo group. Unfortuately only 58 particpants were used, but the methods were robust. (4)

It therefore appears that daily dosages of around 100mg is useful for saturating the body with Vitamin C, so that if an infection does happen the body is ready for it. The evidence also suggests that boosting that vitamin C to 200-1000mg a day during the illness may help the symptoms and shorten the illness by a day and a half.

Foods high in Vitamin C:

All figures are per 100g. Guava 240mg, Bell Peppers 130mg, Kiwi 95mg, Broccoli 90mg, Lychees 70mg, Papaya 60mg, Oranges 55mg, Peas 50mg, Tomatoes 30mg, Kale 20mg.

Be aware that Ascorbic acid is water soluble and very very unstable. It reacts to air, ultraviolet light and heat. Cooking any of these foods very quickly removes any useful vitamin C.

Zinc in Immune Function

The recommended daily intake of Zinc is 8mg and 11mg, for women and men respectively. It is generally bound to proteins and therefore found in abundance in protein rich foods. There is not really any reason to supplement Zinc.

It is critical as a cofactor to enzymes in over 100 known bodily functions. These include breaking down nutrients to be used for energy, but also in the production of proteins and therefore cell division. Zinc is therefore important to energy levels and the processes that manufacture the cells and proteins required for an immune response (5)

Zinc is toxic in concentrations and one of the ways the immune system can fight infection is by collecting a high concentration of Zinc into one area around a cell to poisen it. Another method of immune response is to drain an infectious cell of zinc and remove its ability to function. (6)

It is worth noting that many cereals and wholgrains (containing phytates) will bind to zinc and reduce or inhibit the uptake by the gut. (7) So don't eat cereals and wholegrains with every meal.

Foods containing high levels of Zinc:

Per 100g. Most Seeds 7.5mg, Crab meat 7mg, Red Meat 5mg, an individual Oyster 5mg, Nuts 4-6mg, Prawns 3.5mg, Tinned lentils 1.4mg. Sprouting legumes will increase their bioavailbility to the body. Meat sourced zinc is more readily available to uptake than vegetable sourced.

Vitamin A Boosts the Immune System

Retinoic Acid or Vitamin A has a large concentration on the retina, hence the name. Carrots contain retinoids and therefore gave way to the expression about eating carrots to see in the dark. But actually to see at all, as Vit A deficiency leads to blindness.

Vitamin A is mostly stored in the liver, it is a fat soluble vitamin and therefore also resides in fatty tissues. Thus eating liver, or animal fat will increase levels of vitamin A. For veggies and vegans, it is nearly always fortified into margerine or vegetable spreads, but exists in huge quantities in certain vegetables anyway. B-Carotene is in most leafy vegetables and carrots, it is interestingly more easily absorbed from cooked food than raw, unlike vitamin C. Our bodies synthesise retinoic acid from B-Carotene.

Vitamin A induces/causes B and T-Cell responses from the immune system. It plays a strong part in the gut, where it has immunomodulatory effects by expression of chemokine receptors. (8) The retinoids also control or modulate leukocyte homing (send to correct area of inflammation or correct cell) as well as T-cell function. (9)

Together Vitamin A and D bind to protein receptors that control gene expression for the required cells of the immune system.

Foods high in Vitamin A:

The daily recommended values for vitamin A is 900micrograms.

These are foods are listed per 100g: Liver 7000-9000 mcg, Sweet Potato 1000mcg, Carrots 850mcg, Kale 700mcg, Squash 550mcg, Butter 700mcg, Spinich 450mcg, Cheese 350mcg, Mackerel 250mcg, Eggs 150mcg, Salomon 150mc.

Vitamin D deficiency and Immune System

A study measuring Vitamin D levels in September found that 13% of inner city white-caucasian and 43% of asians in the UK were deficient in Vitamin D. (10) Among athletes and healthy adults in the UK during Winter, 62% of athletes and 73% of healthy adults were found to have inadequate or deficienct levels. (11) Vitamin D supplementation worked in bringing those levels back to recommended amounts, and also significantly increased 10m sprint times and vertical jump heights.

With respect to immune functions, as mentioned above Vitamin D regulates production of cells and proteins via gene expression and the functional workings of those cells. Deficiency in Vitamin D has been shown to increase autoimmune disease problems, and an increase suseptability to faster rates of infection. (12)

How much Vitamin D

There appears to be contention in the scientific literature around the amount required. The National Institute of Health goes with 20mcg a day, so we will go with that too, I don't have time to scrutinise the evidence behind that one. Sorry.

Vitamin D is synthesised by the body under the skin when hit by UV-B rays of sunlight. Your skin type, age, location, clothing, sunscreen etc all affect this dramatically meaning that most people in the Winter high or low in the hemispheres are certainly deficient. This has neuromuscular and immuno issues.

We can however get vitamin D from a diet containing animal-products like lver and fatty dairy foods. Many cereals and breads are fortified in Vitamin D to help boost levels. Yet, out of all the vitamins and minerals in this article I would recommend supplementing Vitamin D in Winter. The blood test is also fairly easy and any GP would be able to do it if there are any suspected issues of deficiency.

Other Stuff

Also of importance to immune function are: Vitamin E, another antioxidant. Vitamin B6 (Pyridoxine), Folic Acid and Iron. Along with minerals such as Selenium and Copper.

Nothing in this article is a magic ticket. In fact, with normal or healthy levels or these micronutrients you are doing the very best you can to boost your immune system. There is limited evidence that increasing blood levels of these will actually benefit you. Maybe with the exception of Vitamin C and D when you are ill. Eating a healthy diet rich in fruits and vegetables covers the essential levels of all of these, except perhaps Vitamin D which may need supplementing to reach normal levels day-to-day in Winter.

Immune Depressing

Generally, being deficient in any of the vitamins and minerals used throughout the whole process of creating/maintaining the cells and hormones for a healthy immune system will have an obvious negative effect on the immune-function of that body.

Being overweight or Obese has negative implications for the required hormonal production of both the metabolic system and the immune system. (13) Also, having a particularly low VO2 level of fitness and therefore putting more stress on the lungs relative to the amount of body that they need to supply, appears to be having big effects in intensive care units treating people with COVID-19. (

High levels of Cortisol and stress are known to dramatically reduce the immune systems ability to fight infection. Though these are extremely hard to build randomised controlled trials for. It's not ethical to randomly give an infection to certain people. Therefore most of the trials have been done on patients receiving vaccinations, who regularly do Tai-Chi to reduce levels of cortisol for example. That's not a good experiment as those people may already have lower cortisol or may already have a stronger immune system.

Steriods and Testosterone levels also depress the immune system. (14) Man-Flu is real. Although most of the experiments have been on Salomon and Rats.

Sugars and simple carbohydrates

I could find no particular evidence for high or low carbohydrate diets with respect to immune function and common colds and Flu. Just that it is better to have a higher carbohydrate diet when fighting infections as it leads to lower rates of pneumonia. (15) This would likely have been in the form of glucose as it was administered enterally.

There is an extremely strong relationship between the gut and immune health, as many cells are either produced there or are triggered there. I would guess that the gut microbiome is therefore important to be balanced with bacteria and yeasts that reside there. The evidence for ProBiotics is pretty weak for boosting immune function. Though one study showed that probiotics do strengthen and prolong the effects of a vaccinations, which is good news for not catching it again. (16)

I would also guess, further to this, that an over reliance on sugary foods promotes an imbalance of gut bacteria, possibly yeast based, and therefore dampens the immune system. That is another Doctoral research project.


1. Carr, A. C., & Maggini, S. (2017). Vitamin C and immune function. Nutrients, 9(11), 1211.

2. Douglas, R. M., Chalker, E. B., & Treacy, B. (2000). Vitamin C for preventing and treating the common cold. The Cochrane database of systematic reviews, (2), CD000980-CD000980.

3. Wintergerst, E. S., Maggini, S., & Hornig, D. H. (2006). Immune-enhancing role of vitamin C and zinc and effect on clinical conditions. Annals of Nutrition and Metabolism, 50(2), 85-94.

4. Hunt, C., Chakravorty, N. K., Annan, G., Habibzadeh, N., & Schorah, C. J. (1994). The clinical effects of vitamin C supplementation in elderly hospitalised patients with acute respiratory infections. International journal for vitamin and nutrition research, 64(3), 212-219.

5. Dardenne, M. (2002). Zinc and immune function. European journal of clinical nutrition, 56(3), S20-S23.

6. Haase, H., & Rink, L. (2014). Multiple impacts of zinc on immune function. Metallomics, 6(7), 1175-1180.

7. Miller, L. V., Krebs, N. F., & Hambidge, K. M. (2007). A mathematical model of zinc absorption in humans as a function of dietary zinc and phytate. The Journal of nutrition, 137(1), 135-141.

8. Mora, J. R., Iwata, M., & Von Andrian, U. H. (2008). Vitamin effects on the immune system: vitamins A and D take centre stage. Nature Reviews Immunology, 8(9), 685-698.

9. Pino-Lagos, K., Benson, M. J., & Noelle, R. J. (2008). Retinoic acid in the immune system. Annals of the New York Academy of Sciences, 1143.

10. Ford, L., Graham, V., Wall, A., & Berg, J. (2006). Vitamin D concentrations in an UK inner-city multicultural outpatient population. Annals of clinical biochemistry, 43(6), 468-473.

11. Close, G. L., Russell, J., Cobley, J. N., Owens, D. J., Wilson, G., Gregson, W., ... & Morton, J. P. (2013). Assessment of vitamin D concentration in non-supplemented professional athletes and healthy adults during the winter months in the UK: implications for skeletal muscle function. Journal of sports sciences, 31(4), 344-353.

12. Aranow, C. (2011). Vitamin D and the immune system. Journal of investigative medicine, 59(6), 881-886.

13. Samartı́n, S., & Chandra, R. K. (2001). Obesity, overnutrition and the immune system. Nutrition Research, 21(1-2), 243-262.

14. Schroderus, E., Jokinen, I., Koivula, M., Koskela, E., Mappes, T., Mills, S. C., ... & Poikonen, T. (2010). Intra-and intersexual trade-offs between testosterone and immune system: implications for sexual and sexually antagonistic selection. The American Naturalist, 176(4), E90-E97.

15. Masters, B., Aarabi, S., Sidhwa, F., & Wood, F. (2012). High‐carbohydrate, high‐protein, low‐fat versus low‐carbohydrate, high‐protein, high‐fat enteral feeds for burns. Cochrane Database of Systematic Reviews, (1).

16. Zimmermann, P., & Curtis, N. (2018). The influence of probiotics on vaccine responses–A systematic review. Vaccine, 36(2), 207-213.

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