How Optimal Nutrition Supports the Immune System
Vitamins and minerals — their physiological roles and where you can obtain them
Nutrition is a critical determinant of health across the human lifespan, providing the body with essential supplies for growth and development, as well as the maintenance of regular physiological functioning. While the effects of food and nutritional status have been scientifically studied throughout history, it is only in the past century, following the discovery of single-nutrient deficiencies and the advancement of laboratory techniques, that the molecular basis of nutrition has been uncovered. Likewise, as our understanding of nutrition has developed, so has our appreciation for the specific roles of micronutrients (vitamins and minerals) within the human immune system.
Put simply, optimal immune function is dependent on adequate nutrition. Conversely, when nutritional status is poor, the immune response is compromised, predisposing individuals to infection. Even though this concept is well understood, micronutrient deficiencies and the suboptimal intake of micronutrients are commonly observed around the world, greatly affecting the risk and severity of disease. This also holds true in industrialized countries, where social, economic, educational and cultural factors influence nutritional status — despite the presumed higher accessibility to nutrient-rich food.
The Immune System
In order to fully appreciate how specific nutrients influence health status, it is necessary to understand both the physiological functions and cellular composition of the immune system. The immune system comprises numerous organs and cells that have evolved to eliminate pathogens and fight off disease. In general, the immune system can be divided into two highly coordinated sub-systems, which work together to deliver an effective response: (1) the innate immune system and (2) the adaptive immune system.
The Innate Immune System
The innate immune system primarily involves fast-acting and non-specific responses to anything that the body identifies as “foreign” or “non-self.” The innate immune response is mediated by various white blood cells, which act as “first responders” to invading pathogens. These include phagocytes, macrophages, mast cells, neutrophils, eosinophils, basophils, natural killer cells and dendritic cells. In addition, the innate immune response makes use of the body’s physical barriers (such as the skin and gastrointestinal tract) and chemical barriers (such as saliva, mucus and gastric acid).
Although the various types of white blood cells that make up the innate immune system play highly specialized roles, together they account for numerous functions. These include:
- Promoting inflammation at the site of infection to recruit more immune cells
- Ingesting/engulfing foreign bodies
- Releasing anti-microbial agents that destroy foreign bodies
- Presenting foreign bodies to cells of the adaptive immune system
The Adaptive Immune System
The adaptive immune system identifies specific molecular structures on invading pathogens and delivers a targeted response over a period of days to weeks. Importantly, this form of immunity enables our body to “remember” a specific pathogen so that a faster response can be delivered when the pathogen is encountered in the future.
The adaptive immune system comprises T cells and B cells, which are capable of identifying pathogens by their specific molecular features, known as antigens. Certain T cells are responsible for directly killing damaged cells (cytotoxic T cells), while others are responsible for coordinating the immune response by releasing signalling molecules (helper T cells). B cells, on the other hand, are responsible for producing antibodies. Once antibodies are released into circulation they can bind to pathogens, labelling them for destruction. Importantly, both T cells and B cells can specialize to form memory cells, which remain in the body for extended periods of time, allowing for prolonged immunity to a specific pathogen.
Nutrition and the Immune System
During periods of infection, the immune system demands more energy, which can be derived from macronutrients (carbohydrates, fats and protein). However, optimal immune function is not simply a question of adequate energy being available; vitamins and minerals play critical roles in the delivery of both the innate and adaptive immune responses. The specific contributions of each micronutrient to the immune response, as well as some of their common dietary sources, are detailed below.
Immune cells produce reactive oxygen species and reactive nitrogen species in order to destroy pathogens. These molecules, however, are chemically unstable and must be removed by antioxidants. Vitamin C serves as one of these antioxidants and can also be used to regenerate other antioxidants in the body. Vitamin C is also known to accumulate in immune cells that ingest pathogens, where it supports these cells in destroying foreign bodies. Additionally, it reduces the formation of necrotic tissue and prevents tissue damage at sites of infection. Finally, Vitamin C stimulates the production and function of various immune cells and contributes to the differentiation of T cells and B cells.
Dietary sources: citrus fruits, bell peppers and brussel sprouts
Vitamin E has a diverse range of effects on immunity. Like Vitamin C, Vitamin E functions directly as an antioxidant, primarily protecting cell membranes from damage caused by free radicals (chemically unstable molecules). Additionally, Vitamin E enhances immune functions that are mediated by T cells. This is largely achieved by optimizing the helper T cell response.
Dietary sources: sunflower seeds, almonds and wheat germ
Vitamin A plays a role in both the innate and adaptive immune systems. First, it is essential for maintaining the structure and, therefore, the functional integrity of mucosal cells, which are located within several of the body’s barriers (e.g. the respiratory tract and the skin). In addition, Vitamin A contributes to the regular functioning of numerous innate immune cells, such as macrophages, neutrophils and natural killer cells. Moreover, it is necessary for the regular functioning of B cells and T cells, rendering it critical for the generation of an antibody response.
Dietary sources: dairy, fish, liver, sweet potatoes, carrots and spinach
Many cells of the innate immune system possess Vitamin D receptors. One of the key functions of Vitamin D is to stimulate the proliferation of immune cells and the production of their signalling molecules. In addition, Vitamin D has a specific role in the development of macrophages. Finally, Vitamin D is known to regulate the expression of two proteins, cathelicidin and defensin, which can directly destroy pathogens.
Dietary sources: sunlight, milk and fish oil
Vitamin B6 contributes to various facets of the immune response. Most notably, it is involved in the regulation of inflammation, the production of cell signalling molecules and the regular functioning of natural killer cells. More generally, Vitamin B6 is involved in the metabolism and synthesis of amino acids, which are the building blocks of all proteins in the body, including antibodies and cytokines (cell signalling molecules). Additionally, Vitamin B6 plays a role in the proliferation, differentiation and maturation of B cells and T cells.
Dietary sources: milk, fish, potatoes and carrots
Vitamin B9 plays a role in maintaining the innate immune response, particularly in terms of supporting natural killer cells. It also contributes to cell-mediated immune responses (i.e. by supporting the helper T cell response and contributing to the generation of a sufficient antibody response).
Dietary sources: beef, liver, black-eyed peas, spinach and asparagus
Like Vitamin B9, Vitamin B12 plays a role in the regular functioning of natural killer cells. In addition, it is known to be involved in the production of T cells.
Dietary sources: meat, fish and clams
Zinc is central to numerous immune functions. Perhaps most importantly, it is critical to the growth and differentiation of various immune cells with rapid turnover. Zinc is required for the binding of intracellular signalling molecules to T cell receptors, which results in the development and activation of T cells. Moreover, Zinc modulates the release of signalling molecules from innate immune cells, which induces the proliferation of certain T cells. Zinc also contributes to immune function by serving as an antioxidant. Finally, Zinc helps maintain the integrity of the skin and mucosal membranes, which protect against pathogen invasion.
Dietary sources: chickpeas, oysters and crabs
In addition to being essential for general cell growth and differentiation, Iron plays both a direct and indirect role in the removal of pathogens. Most notably, it forms hydroxyl radicals that are used by neutrophils to kill bacteria. Additionally, it is involved in the generation of reactive oxygen species that are used to destroy pathogens.
Dietary sources: white beans, spinach and oysters
Copper takes on a variety of functions in the immune response. It is known to accumulate at sites of infection, where it contributes to the cell signalling response (particularly the production and function of the signalling molecule interleukin-2). In addition, Copper exhibits general antimicrobial properties and serves as a free radical scavenger. Finally, Copper contributes to the proliferation of T cells and the production of antibodies.
Dietary sources: liver, crabs and cashews
Selenium is indispensable to the removal of reactive oxygen species, which is necessary for optimal immune function. There are several enzymes that utilize selenium at their active sites, enabling them to function as antioxidants and maintain cellular redox balance (through the removal of excess free radicals). In addition, selenium is involved in T cell proliferation and antibody production.
Dietary sources: brazil nuts, ham, sardines
Through the action of a highly evolved immune system, our body utilizes a wide variety of micronutrients to fight off pathogens and maintain our health status. While nutrition is certainly not the only determinant of our body’s defences, optimal nutrition helps ensure that we mount the most efficient immune response possible. With this in mind, we should look to support our immune system by obtaining adequate quantities of essential vitamins and minerals through a highly varied and nutritious diet.