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Table of Contents

NUTRIDATE

Volume 32, Issue 4, 2021

NUTRIDATE

Volume 32, Issue 4, 2021

Diet and immunity: What should we eat to stay healthy?

Associate Professor Laurence Macia
Nutritional Immunometabolism Laboratory Head, University of Sydney

Introduction

Our lifestyle has dramatically changed over the last hundred years compared to the way our great grandparents lived, particularly in western countries like in North America, the United Kingdom and Australia. Diets have completely changed, physical activity and sleeping hours have declined and stress levels are higher, particularly for people living in cities. 

Figure 1: Modern lifestyles are vastly different to those of our great-grandparents

Technologies that have improved our quality of life in many ways have also indirectly had detrimental effects. Having a car or having access to public transport has eased the commute between school or work, for instance. On the downside, physical activity has significantly declined as people will take their car even for short distances instead of walking. This change of behaviour is directly paralleled with increased incidence of obesity and metabolic diseases but also of some cancers as we have discovered that muscular activity could trigger the release of factors important for our body to fight cancer. 

Similarly, mobile phones are very convenient tools for communication and to be able to get help in case of emergency. On the downside people spend hours using their phone resulting in decreased physical activity, disrupted sleep patterns and changes in the type of food consumed with increased consumption of junk food.

Junk food is also a direct product of advanced technologies. People in cities tend to be extremely busy, working for very long stressful hours with less and less time to relax and to cook. The food industry adapted its market to this demand by creating convenient food that can be quickly prepared. Warming up in a microwave a pre-made dish of lasagna for few minutes after a long day of work is an attractive alternative to spending over an hour preparing this dish from scratch. However, are those pre-made dishes healthy? 

We have unlimited access to lots of food products with a long shelf life that can be kept for months in the pantry, fridge, or freezer. Food additives, which are the key to keeping food for longer, have either anti-microbial activities, antioxidant activity or just simply make the food look nice. 

Figure 2: A fast-paced lifestyle and modern technologies have produced processed food with a long shelf life but little nutritional value.

These food additives are either natural products like citric acid found in lemon or salt but could also be made of nanoparticles like emulsifiers or titanium dioxide (these products are usually labelled E followed by numbers on food packaging). However, are these food additives safe for human consumption? 

Before discussing the effects of food on health, we need to first understand what body system keeps us healthy in the first place.

Immunity: our system of defence

Figure 3: Unlike other bodily systems, the immune system is not defined by one organ – it is diffused, with components present in almost all body sites.

Contrary to other systems in our body, such as the cardiovascular system composed of the heart and blood vessels or the nervous system constituted by the brain, spine and nerves, the immune system is not defined by one organ.

It is what we called a diffused system, having components present in almost all body sites. This property of being diffused is key to defend the body in case pathogens enter in any site. The immune system is defined as our professional system of defence, which will get activated when a danger is sensed. 

Pathogens such as bacteria, viruses and yeasts will trigger an immune response, which is the activation of the immune system to get rid of them and to repair the damaged area. Danger is also sensed when something abnormal happens like for example when a heavy object falls on your hand. The immediate immune response in this situation is called inflammation and is characterised by pain, swollen area, redness, and heat. All these features are linked to the arrival of immune cells in the damaged area.

Thus, both pathogens and dangerous situations that do not involve a pathogen can activate the immune system, which will eradicate the danger and repair the damages. 

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What is the immune system?

The immune system is a like an army with soldiers being cells and ammunition being the mediators produced by these cells. All immune cells are produced in the bone marrow. After that, some of them will keep patrolling in the blood and some will migrate and stay in tissues ready to respond. We have cells that will attack immediately within minutes of infection. These cells are called neutrophils and macrophages. They are big cells able to engulf bacteria fast and efficiently. Inside these cells, there are toxic compounds that will kill and destroy the bacteria. 

Sometimes these cells are not powerful enough to contain the infection and need extra help. To get extra help, they will attract other immune cells called lymphocytes. We have two subsets of lymphocytes: B lymphocytes and T lymphocytes. The activation of lymphocytes is complex and takes time, up to one week. However, they are highly specific to given pathogens and thus very efficient to fully eliminate them. 

Figure 4: Humans have an exquisitely sophisticated immune system designed to fight off viruses.

What are T and B lymphocytes doing specifically?

B lymphocytes produce antibodies. Antibodies are structures that will specifically attach to pathogens. As a result, these pathogens coated with antibodies cannot infect or enter cells anymore and they will be eaten by macrophages. This is an efficient way to block the entry of viruses or bacteria into cells. T lymphocytes are the other subset of lymphocytes. After being generated in the bone marrow, they need to migrate to the thymus where they will become fully functional. 

There are many different subsets of T lymphocytes which will be activated depending on the threat. For example, a subset of T cells called CD8+ T lymphocytes can directly kill an infected cell by viruses. These T lymphocytes are very important because without them a normal life is impossible. Children with a disease called DiGeorge syndrome have no thymus, they are highly susceptible to infection and have as a result a life expectancy of 2 to 3 years of age.

Is the immune system only activated by pathogens and dangerous situations?

The answer is no. The immune system’s job is to maintain health and anything abnormal will activate this system. A common feature of non-infectious diseases is the abnormal or inadequate activation of the immune system. Obesity, for example, is characterised by a sustained low-grade inflammation. In this case, the immune system is permanently activated but at low level. An activated immune system produces products called cytokines that can damage tissues. Thus, the permanent activation of the immune system in obesity contributes to metabolic alteration such type 2 diabetes. 

While our immune system is very well trained it can still make mistakes. For example, food allergy is characterised by an abnormal response mounted against food products not dangerous in the first place. While 20 years ago food allergy was uncommon nowadays one child out of ten has food allergy in Australia. Another type of disease linked to the confusion of the immune system is called autoimmune disease. The immune system is trained to “ignore” and tolerate components of our body. However, in diseases such as type 1 diabetes, multiple sclerosis, and rheumatoid arthritis the immune system attacks our own tissues, which has dramatic consequences. 

Thus, an inappropriate activation of the immune system is the basis for many diseases.

Figure 5: Chronic inflammation is caused by lifestyle factors including poor diet, and is linked to allergies and autoimmune diseases as the immune system becomes overactivated.

Why is our immune system making this type of mistakes in the first place?

The short answer is that we do not fully know. Both genetic and environmental factors are involved but the fact that some diseases such as allergy increased over a short period of time suggests that the environment plays a great part of it.

What is in our environment that can affect our immune system?

Many behaviours typical of western lifestyle contribute to disease development. Poor diet, sleep deprivation, low physical activity and chronic stress are all key factors that can confuse the immune system and thus contribute to disease. It means that a balanced lifestyle truly makes a huge difference on health and thus easy changes can prevent disease development. 

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What in our diet can confuse our immune system?

The amount of food we eat

An inappropriate amount or type of energy available is believed to impair the function of the immune system, so it will be impacted by eating too much or not enough. Studies show that malnutrition in poor countries greatly increases susceptibility to infection. The same observation is made in overweight and obese people with for example higher risk of mortality in obese individuals during COVID-19 infection. 

The immune system needs energy to work appropriately. Humans are very good at storing fat so if we eat too much we will store lots of energy. There is a theory that this great ability to store energy contributed to our survival during famine. This energy will be stored in fat tissue, particularly in cells called adipocytes. The more fat stored the more our immune system will be activated. This activation of the immune system contributes to metabolic disease, which can potentially reduce lifespan if untreated. Similarly in cardiovascular diseases, activation of immune cells in blood vessels contributes to atherosclerosis. 

How do I know that I do not eat too much? If you eat a balanced diet as discussed later, your brain will receive information from your gastrointestinal tract (gut) that it is time to stop eating.

The type of food we eat

Food Standard Australia recommends consuming a balanced diet with a wide variety of food including fruits, vegetables and legumes, wholegrains and cereals dietary dairy and lean meat/fish. 

Figure 6: Overeating and malnutrition can impair the immune system’s function.

The Mediterranean diet is a model of healthy diet as it is characterised by the consumption of diverse types of plant food enriched in unsaturated fat (like extra virgin olive oil) which are healthier, lots of dietary fibre and limited amount of processed food and meat. 

A diverse diet will provide all the vitamins and minerals needed and will provide the energy and “building blocks” our cells need. If we have a restricted diet, we are likely to miss some of these important components and develop deficiencies. 

Should we avoid fat?

Fat is an important component of our cell membranes, so we need fat to be healthy. However, we need the right amount of fat and the right type of fat. Fat found in fish called omega-3 has lots of health benefits, particularly anti-inflammatory effects. On the other hand, polyunsaturated fat found in margarines and vegetable oils fat found in animal are pro-inflammatory. So, eating fat is a necessity to be healthy but from the right source and in the right amount. 

Are sugars (carbohydrates) bad for us?

Like fat, we need the right type and amount of carbohydrates. Sugar or carbohydrate is our main source of energy, so it is vital to have carbohydrate in our diet. However, while carbohydrates found in starch (long chain of glucose stick together present in pasta, bread or rice) is important, carbohydrates found in lollies or in soft drink is not. Healthy carbohydrates also come from fruit and vegetables.

Like the brain, the main source of energy for the immune system is glucose. When we have an infection, our immune cells will proliferate and produce lots of molecules. These processes highly rely on the transformation of glucose into energy. But if we consume too much carbohydrate they will eventually get transformed in fat and stored, so the right carbohydrate and in the right amount is needed for a healthy immunity.

Figure 7: A diverse diet rich in healthy plant-based foods supports a healthy immune system.

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Why should I eat grain and wholemeal food?

Almost everything you eat enters your body at the level of the small intestine, located just after the stomach. Grains and wholemeal food also known as dietary fibre are the exception and will reach the colon, located at the end of the gastrointestinal tract. This is because we cannot digest dietary fibre.

To digest food, the body releases enzymes, which act as scissors cutting long food products into small entities that can enter into the blood. These products enter the blood in the small intestine. However, we do not have the enzymes to cut dietary fibre. Dietary fibre is made of long chains of carbohydrate that are thus resistant to digestion so these long chains of carbohydrate will as a result stay in the gastrointestinal tract and eventually reach the colon, where our gut microbiota is. The gut microbiota is made of trillion of micro-organisms particularly bacteria, which will use dietary fibre as a major source of energy. We have co-evolved for thousands of years with these bacteria and we need them to be healthy. 

Why do we need gut bacteria to be healthy?

Gut bacteria have many effects on our health. They help digest food and also provide vitamins. Our gut bacteria have the enzymes to digest dietary fibre; this digestion is called fermentation. The bacteria will use dietary fibre as a main source of energy but they will also produce some small molecules in this process that can enter in the blood and support our health and immune system.

Link between gut bacteria and the immune system

Figure 8: A healthy, high-fibre diet helps good gut bacteria flourish, which are important for building our immune defences.

Gut bacteria promote the development of immune cells that control the inflammatory response. Research in animals has shown that consumption of a high fibre diet protects from the development of asthma, food allergy, colorectal cancer, inflammatory bowel disease and more. Dietary fibre supports the development of beneficial bacteria and through the production of small molecules by gut bacteria. 

There is more and more evidence that this is true in humans. For example, it has been shown that giving a small gut bacterial molecule called propionic acid in humans could improve the severity of multiple sclerosis. More and more research is focusing on re-establishing a healthy gut microbiota through diet to treat diseases. However, the preventive effect of dietary fibre on disease development is more and more accepted. Thus simply eating more plant foods like vegetables, fruit, chickpeas, beans and lentils can sustain good health.

What about food additives?

There is a wide range of food additives in the food we eat. They are used for preserving the food but can have adverse impacts on health. Studies looking at emulsifiers, for instance, which can give a nice texture to mayonnaise and ice cream, have shown that they can detrimentally affect the gut microbiota and contribute to inflammation. Some countries are starting stricter regulations around food additives as they raised more and more safety concerns. Eating fresh fruits and vegetables and avoiding processed food is an easy strategy to limit the intake of food additives.

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Conclusion

Our immune system needs energy to function appropriately but needs the right source of energy and in right amount. Processed foods are not a good option for our immune system as the type and amount of energy is not appropriate and also it contains food additives, which can also compromise the gut microbiota and immune function. 

On the other hand, a healthy diet will support a healthy microbiota, which would “starve” in a diet lacking in dietary fibre. The result of this starvation is the growth of non-beneficial bacteria that will support inflammation. 

So what to eat? It is not as complicated as it seems. A diverse range of fruits, veggies, legumes and grains and moderate amount of animal products will support a strong immune system and keep you healthy. 

Student Activities

1. How have our diets and other lifestyle factors changed drastically in western societies?

2. What impacts do these changes have on people’s health?

3. What is immunity and why is it important?

4. Does the immune system just fight off pathogens? Explain your answer.

5. What are some modern diseases that result from a compromised immunity?

6. What constitutes an unhealthy diet and how can it affect immunity?

7. Explore some of the health effects of food additives on the immune system.

8. What constitutes a healthy diet and how can it support immunity?

9. Explain the importance of a high fibre diet for the immune system.

References and resources

Thompson D, 2016. Stored fat is a feat of evolution –you’re your body will fight to keep it. URL: https://theconversation.com/stored-fat-is-a-feat-of-evolution-and-your-body-will-fight-to-keep-it-52468

Australian Dietary Guidelines: https://www.eatforhealth.gov.au/guidelines

How diet can impact multiple sclerosis progression, 2020. URL: https://www.technologynetworks.com/neuroscience/news/how-diet-can-impact-multiple-sclerosis-progression-331978

Dunham W. 26 February 2015. Study links common food additives to Crohn’s disease, colitis. URL: https://www.reuters.com/article/us-science-emulsifiers-idUSKBN0LT26S20150225

Reuters staff, 2019. France to ban titanium dioxide whitener in food from 2020. URL: https://www.reuters.com/article/us-france-food-additive-idUSKCN1RT23D

Holmes A, Macia L, Simpson SJ, 2016. Poo transplants and probiotics – does anything work to improve the health of our gut? https://theconversation.com/poo-transplants-and-probiotics-does-anything-work-to-improve-the-health-of-our-gut-65480 

Probst Y, Craddock J, 2019. Eat your vegetables – studies show how plant-based diets are good for immunity. URL: https://theconversation.com/eat-your-vegetables-studies-show-plant-based-diets-are-good-for-immunity-107964 

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