Diet and the immune system
Professor Philip Calder
The immune system is the individual’s means of protection against pathogenic organisms including bacteria, viruses, fungi and parasites. The immune system also helps to deal with cancer cells, is involved in the response to injury and to some environmental irritants, provides tolerance to non-threatening environmental agents including proteins in foods and commensal bacteria living in the gastrointestinal tract and on the skin. Thus, a well-functioning immune system is vital to health.
The response of the immune system to challenges involves many different cell types acting together in a carefully orchestrated way to assure an appropriate reaction to the identified threat. The different cell types have their own specific roles within the response. To achieve an effective immune response, the cells move through the bloodstream between body compartments, interact closely with one another, and produce a range of chemical agents. Some of these chemicals are damaging to pathogens, while others play a role controlling the activities of the immune cells themselves.
Although it is dispersed throughout the body, the immune system has a size that is comparable to that of the liver or brain. The immune response has a high demand for energy and for building blocks. The latter are used to produce new cells and as substrates for the synthesis of important immune factors such as antibodies, cytokines (hormone-like proteins that are active within the immune response) and acute phase proteins. The increase in metabolism that accompanies an active immune response also has a demand for co-factors and regulatory molecules including several vitamins and minerals; vitamin A, zinc, copper, iron and magnesium are important in these roles. Thus, mounting an effective immune response requires significant input of energy yielding substrates, of building blocks, and of regulatory molecules.
The immune response’s demands must be met from the diet or from body stores, which are, in turn, derived from the diet. Hence good nutrition is vital to ensure an appropriate immune response. Dietary components that have been shown to be important in supporting the immune response include a number of trace elements, several water soluble and fat soluble vitamins, essential and some non-essential amino acids, and polyunsaturated fatty acids. Typically these are important as building blocks or as regulatory molecules. It may be that some other dietary components such as certain plant polyphenolic compounds are also important, but these have been less extensively studied.
Since a broad range of dietary components are necessary to support an immune response, poor nutrition is associated with a weakened immune response and with increased susceptibility to infection. This may be a problem in many population sub-groups including those with diseases that interfere with normal digestion, absorption and handling of nutrients, such as inflammatory bowel disease, short bowel syndrome or cystic fibrosis.
The immune system develops over the first years of life and its response declines in later life; the latter decline has been termed immunosenescence. Hence, there are periods of the lifecourse where individuals may manifest a sub-optimal immune response and during these periods they may be more likely to become infected and to fare poorly once infected. Another aspect of a poorer immune response at either end of the lifecourse is a poorer responsiveness to vaccination. This results in a waste of resource and creates a false sense of security in the recipients. These risks of poor immune response and increased susceptibility to infection will be exaggerated in the less well nourished. Thus, appropriate nutrition is likely to be important in ensuring good development and maturation of the immune system in infancy and early childhood and in preventing age-related immune decline. Therefore, an appreciation of the importance of good nutrition to the immune system is important across the range of health professionals.
One area that is currently receiving significant attention is the role of what is called the gut-associated immune system. This extensive and well organized structure has the role of sampling antigens within the gut lumen (arising from food or microorganisms) and then mounting an appropriate defensive or tolerogenic (i.e., mild, non-destructive) response. Recent studies suggest communication between microbes in the gut lumen and the host’s immune system, with the nature of this communication playing an important regulatory role. Such observations have given rise to ideas that modifying the microbes present in the gut lumen may provide a means by which to modify host immune responses. The aim of this would be to improve host defenses or to lower the risk of, or to treat, a disease involving an adverse immune response, such as inflammatory bowel disease. Two strategies to modify the microbes in gut have been investigated. The first is to consume live microbes (referred to as probiotics) thought to promote the required response. The second is to consume food ingredients, usually indigestible sugars, that act as substrates to promote the growth of the desired microbes; these are referred to as prebiotics. Although both of these approaches do act to modify the microbial populations residing in the gut lumen, especially the colon, the subsequent health effects are less well established. Therefore these are currently the focus of much research.
In conclusion, the immune system is important to health, enabling the host to live in a pathogen-rich environment, to cope with exposures to foreign antigens in food, and to exist peacefully with huge numbers of harmless microorganisms in the gut, on the skin and elsewhere. The immune system develops early in life and declines with ageing. A weakened immune response is associated with increased susceptibility to infections, an impaired ability to cope with being infected, and a poor response to vaccination. Poor nutrition exaggerates these states, while good nutrition better assures appropriate immune responses.
Key point: Good nutrition is important to support the immune response.