The recent profusion of advice for “Ways to Boost Good Gut Health” and “Ways to Boost the Immune System” through diet and lifestyle changes, have left me very excited but wondering what does a healthy gut or a healthy immune system look like? Why does it matter? And is there a link between good gut health and the immune system?
In order to discuss what the link is so far, we need to first understand some basics about the human gut and immune system.
The Human Gut
If we take a dive into the gut that covers the stomach, small intestine and large intestine, we find the largest population of microbes, more than 38 trillion microbes (i.e. bacteria, fungi, viruses, protozoa, nematodes and other microbial species) co-existing there than anywhere in the body. This is the gut’s microbiome. In fact the gut or digestive tract is host to 90% of the body’s microbes①.
To put that into perspective, it is estimated that a typical young adult male will host:
38 trillion microbes in the large intestine;
1 trillion microbes in the dental plaque;
180 billion microbes on the skin;
100 billion microbes in the saliva;
40 billion microbes in the small intestine;
9 million microbes in the stomach
Unbelievably, the amount of microbes the human body hosts is 10 times more than the number of human cells, and therefore its shear population means it expresses at least 10 fold more unique genes than the human cells’ genome②.
Within our digestive tract, the 38 trillion microbes not only help with digestion of the food into nutrients that can be absorbed by the body, but also express chemical signals that influence the regulation of all immunity responses.
Generally, a rich and diverse gut microflora is considered to be a healthy one. A lack of diversity within the gut limits recovery from infection or antibiotics. Everyone’s gut microflora composition is unique, but a domination of one microbial specie or a lack of diversity in the gut, can be a signal for disease occurring somewhere in the body.
A well-functioning immune system is essential for survival. The immune system is a vast network of specialised cells and tissues that are spread throughout the body, constantly on alert, monitoring for signs of invaders and clearing up dead cells. Like all systems, it needs to be effective and efficient in its functions. Therefore an effective immune system deploys immune cells to fight against pathogens (harmful cells or microbes), and an efficient immune system resolves these immune cells to prevent damage to human tissue③.
Since the greatest risk of exposure to harmful invaders is in the digestive tract or on the skin, a large proportion of our immune cells reside there. Crucially, the cells of the immune system must be able to distinguish self (i.e. healthy human cells) from non-self (i.e. food and microbes), and furthermore discriminate which non-self-molecules are harmful (pathogenic) and which human cells are faulty or dead. If unable to do so, this can lead to inflammatory responses and :
Autoimmune diseases, whereby the immune system mistakenly targets healthy human cells, rather than foreign pathogens or faulty human cells; or
Hypersensitivity, whereby the immune system overreacts in a way that damages healthy tissue; or
Within the immune system, there are two subsystems known as the innate immune system and the adaptive immune system.
The innate immune system provides a general defence against harmful germs and substances. It mostly fights using immune cells such as natural killer cells and phagocytes (“eating cells”); and
The adaptive immune system has the ability to specifically recognise a pathogen and ‘remember’ it. The adaptive immune system is constantly learning and adapting, building up a library of antibodies to different pathogens, that enables it to identify previous enemies when the immune cells are exposed to it again.
Overall, the effectiveness and efficiency of the immune system varies in everyone, and can change through life. Age, obesity, alcoholism, malnutrition, genetics or acquired deficiency, are factors that can weaken the immune system.
1. Relationship Between Diet and the Gut Microbiome:
Diet has a dominant influence on the composition and metabolic capacity of the gut microflora. Diets that consist of a wide variety of fruits, vegetables, whole grains, legumes and nuts encourages a diverse gut microflora④.
Prebiotics from soybeans, wheat, onions, garlic, leeks , asparagus, provide fibre from undigested plant cell walls that will reach the microbiome in the lower digestive tract.
Antibiotic use can drastically alter the composition and diversity of the gut microflora, leading to a short-term decline in beneficial bacteria.
Avoiding sugary foods, fried foods, saturated fats and alcohol (except for moderate consumption of red wine) will help prevent serious problems not only to our body, but prevent issues caused by imbalances to the gut microflora⑥.
2. Relationship between the Gut Microbiome and Diet:
The gut microflora ferments and digests incoming foods which breaks it into essential nutrients that can be absorbed by the body⑦.
3. Relationship between Diet and the Immune System:
Diet also has a profound influence on immune development and immune responses⑧.
Eating or drinking too much sugar curbs the ability of immune cells to attack pathogens. This effect can last at least a few hours after downing a couple of sugary drinks.
The protective effects of a diet rich in fruits and vegetables, that are packed with vitamins C and E, plus beta-carotene and zinc, will reduce the risk of chronic inflammation.
Going for a wide variety of brightly coloured fruits and vegetables, including berries, citrus fruits, kiwi, apples, red grapes, raw red capsicum, cooked sweet potatoes, pumpkin and carrots, and dark green leafy vegetables such as broccoli, kale, spinach, will influence immune function. Fresh garlic is good for the immune system and is the same for some mushroom varieties such as shiitake.
While no single food can upgrade the immune system, excessive intake of micronutrients can be associated with impaired immune responses and poor nutrition can have a negative effect on the immune system. What matters is having a balanced diet that will nourish the immune cells and keep the immune system well-functioning.
4. Relationship between the Immune System and Diet:
The immune system can directly affect absorption of nutrients by the body⑨. For example, deficiency in the immune system of mice is shown to cause deficiencies in dietary absorption.
The Link between Gut Microbiome and Immune System
5. Relationship between the Gut Microbiome and the Immune System:
The gut microbes express chemical signals that influence the development, training and function of the immune system⑩.
6. Relationship between the Immune System and the Gut Microbiome:
The immune system monitors the gut microbes, exerting control over both the composition and location of the microbes. For example, immunodeficiency can alter the gut microflora’s composition and thereby the metabolic capacity of both the microflora and the body. For example, experiments with mice deficient in a certain microflora have shown links to weight gain and metabolic diseases⑪.
In summary, one of the best and easiest way of maintaining an efficient and effective immune system is to encourage a balanced gut microflora, by following a diet that has a wide variety of fruits and vegetables, legumes, grains, nuts, prebiotics and probiotics. Furthermore, undertaking some lifestyle changes, such as cutting out alcohol and smoking, exercising, reducing stress and getting adequate sleep, will boost overall well-being and help maintain a healthy immune system.