How to strengthen your immune system

With the COVID-19 situation in full swing many people are wondering what they can do to prevent getting sick. While hygiene and social distancing are the main recommendations, it is also important to know how to strengthen your immune system to be more resilient to this or any other infection.

What is the immune system?

The immune system is the name we give to all the parts of our bodies that are involved in defending us against pathogens (things that make us sick), repairing injured tissues, and keeping abnormal growths and self-attacks in check. This includes whole organs such as the skin, cells such as B cells and T cells, and chemical compounds such as antibodies and cytokines.

Nutrients that support immune function

Energy

Insufficient energy intake can lead to nutrient deficiency and thus potentially impact immune function. Keep reading to find out which nutrients are particularly important to the immune system. Energy requirements vary depending on gender, age, weight, activity levels, etc.

Protein

The protein we eat gets broken down into amino acids that are used by to body to build new proteins, including tissues and proteins involved in immune function, such as antibodies. Protein requirements vary depending on gender, age, weight, activity levels, etc.

Micronutrients

Roles in immune function

The table below shows the roles of micronutrients involved in immune function (1).

Roles	Vitamin A	Vitamin D	Vitamin C	Vitamin E	Vitamin B6	Vitamin B12
Structural and functional integrity of mucosal barriers	X	X	X	X	X	X
Differentiation, proliferation, functioning, and movement of innate immune cells	X	X	X	X	X	X
Antimicrobial effects	X	X	X
Roles in inflammation, antioxidant effects	X	X	X	X	X
Differentiation, proliferation and functioning of T cells	X	X	X	X	X	X
Antibody production and development	X	X	X	X	X	X
Responses to antigen	X	X		X

Roles	Folate	Zinc	Iron	Copper	Selenium	Magnesium
Structural and functional integrity of mucosal barriers	X	X	X
Differentiation, proliferation, functioning, and movement of innate immune cells	X	X	X	X	X	X
Antimicrobial effects		X	X	X	X
Roles in inflammation, antioxidant effects		X	X	X	X	X
Differentiation, proliferation and functioning of T cells	X	X	X	X	X
Antibody production and development	X	X			X	X
Responses to antigen	X	X				X

Dietary sources

• Vitamin A: The preformed version of vitamin A is found only in animal-derived foods. Carotenoids, which can be converted at a low rate to vitamin A, can be found in oils, fruits and vegetables.
• Vitamin D: Mainly synthesised in our bodies by exposure of skin to the sun. Food sources include fortified margarine (not really a food, in my opinion), fatty fish such as salmon, herring and mackerel, and eggs.
• Vitamin E: Present mainly in fats and oils, followed by some vegetables, in the fats of meat, poultry and fish. There is also some vitamin E in cereals and dairy foods.
• Vitamin C: Found in fruits such as citrus, kiwi fruit, blackcurrants and guava and vegetables such as broccoli and sprouts. Vitamin C content is lost by factors such as storage time, cooking method, exposure to oxygen, etc.
• Vitamin B6: Found in organ meats, muscle meats, breakfast cereals, vegetables and fruits.
• Vitamin B12: Present in animal foods and to a lesser extent in certain algae and plants exposed to bacterial action or contaminated by soil or insects.
• Folate: The main food sources in the Australian diet are cereals, cereal products, dishes based on cereals, vegetables, legumes and fruit.
• Zinc: Found mainly in meats, fish and poultry, followed by cereals and dairy foods.
• Iron: Animal foods such as meat, fish and poultry contain haem and non-haem iron. The iron in plants such as grains and vegetables is non-haem. The absorption of non-haem iron is lower than that of haem and can be increased by including vitamin C or animal foods in the same meal. The absorption of both types of iron is decreased by the presence of calcium, zinc or phytates in the same meal.
• Copper: Found mainly in organ meats, seafood, nuts and seeds, followed by wheat bran cereals and whole grain products.
• Magnesium: Present in most green vegetables, legumes, peas, beans, nuts, some shellfish and spices. Most unrefined cereals have some magnesium as well.
• Selenium: The main dietary foods in Australia and New Zealand are seafood, poultry and eggs, followed by other muscle meats. Its content in plant foods depends on the concentration of the mineral in soil, which unfortunately is low in both countries (2).

How to know if you’re getting enough

You can track your dietary intake for a week or two using an app that does nutrient analysis. Take the average intake of each of the nutrients mentioned above. Then go to the NHMRC’s Nutrients & Dietary Energy Calculator, enter your age, gender, pregnancy status, select “Show Nutrients” and check all the micronutrients mentioned above. You want your intake for each nutrient to be between the adequate intake (AI) or recommended daily intake (RDI) and the upper limit (UL).

Supplements

Vitamin C

Vitamin C supplements can only prevent you from getting a cold if you are under heavy physical stress. If you already have a cold, vitamin C can reduce its severity and duration when taken in doses greater than 200mg per day. A maximum dose of 2g per day is safe for healthy individuals (3)

High doses of vitamin C can stimulate the activity of phagocytes and T-cells. The antioxidant properties of vitamin C protect immune cells from oxidative stress (1).

Zinc

High doses of zinc acetate (greater than 75mg per day) can reduce the duration of a cold and its symptoms. Note that this dose is way beyond the recommended daily intake and should be only taken when sick for a short period of time. Regular daily doses of 10-15g per day can help prevent colds, at least in children (3).

Probiotics

Probiotics may help modulate the immune system to enhance the response to infection and mitigate excessive inflammation when needed. Probiotics also play an important role in gut health. They may help maintain the integrity of our intestines intact to act as a barrier against pathogens. They can stimulate the secretion of signalling molecules in the mucosa of the gut. Some strains increase secretion of antibodies that fight infections. They can also increase the efficacy of flu vaccines in elderly people (4).

The evidence that probiotics can decrease the incidence and duration of respiratory tract infections is not very clear. Likewise, there is no consensus in the optimal strain(s) or dose (3). Having said that, probiotics are generally safe for healthy individuals and have multiple potential health benefits.

Other considerations

Hot fluids

Hot fluids such as soups, teas, etc. can help to cope with symptoms of common colds. They are a good way to keep hydrate and can provide some nutrients. In addition, the steam can increase nasal airflow. Finally, warm fluids help loosen secretions making it easier to expel them from the respiratory tract (3).

Age, gender and immune function

In general, the immune system develops during childhood, achieves peak function in early adulthood (except for pregnant individuals) and declines with age (5).

While women tend to be more susceptible to autoimmune diseases, men seem to suffer more severe effects from influenza (5).

Exercise and immune function

While moderate-intensity exercise can improve immune function, there are several factors that can depress an athlete’s immune system, including:

• Repeated high-intensity training and competition
• Environmental stress, such as extreme temperature and high altitude
• Exposure to pathogens
• Insufficient food or fluid intake
• Low carbohydrate consumption
• Energy restriction leading to nutrient deficiency (6)

Sleep and stress

Poor sleep and chronic psychological stress also lower immune function (5).

Summary and recommendations

There is no magic pill or superfood you can take to prevent getting sick. However, a robust immune system should be able to fight an infection and minimise the damage. You can strengthen your immune system with:

• A healthy diet that allows you to meet recommended levels of key nutrients
• Sensible levels of physical activity
• Getting sunshine on your skin every day
• Adequate sleep
• Stress management

References

1. Gombart AF, Pierre A, Maggini S. A Review of Micronutrients and the Immune System–Working in Harmony to Reduce the Risk of Infection. Nutrients 2020;12:236.
2. National Health and Medical Research Council, Australian Government Department of Health and Ageing, New Zealand Ministry of Health. Nutrient Reference Values for Australia and New Zealand. Canberra: National Health and Medical Research Council; 2006.
3. Dietitians of Canada. Immune System – Summary of Recommendations and Evidence. In: Practice-based Evidence in Nutrition® [PEN]. 17 Mar 2020 [cited 18 Mar 2020]. Available from: https://www.pennutrition.com/KnowledgePathway.aspx?kpid=16006&trcatid=42&trid=16556. Access only by subscription. Click Sign Up on PEN login page.
4. Wu D, Lewis ED, Pae M, Meydani SN. Nutritional Modulation of Immune Function: Analysis of Evidence, Mechanisms, and Clinical Relevance. Front Immunol. 2018;9:3160.
5. Maggini S, Pierre A, Calder PC. Immune Function and Micronutrient Requirements Change over the Life Course. Nutrients. 2018;10(10).
6. Burke, Louise. Clinical Sports Nutrition, 5th Edition. McGraw-Hill Australia, 09/2015. VitalBook file.

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