The Impact of Malnutrition on the Immune System

Pathogens are microscopic organisms such as viruses, bacteria, fungi, and parasites that can cause disease. All multicellular organisms have an innate immune system that provides the first response to pathogens which can cause infection, utilising many defence mechanisms. Examples are those found in the epithelial and mucosal linings of our respiratory and gastrointestinal tracts, phagocytes that ingest pathogens, and natural killer cells that destroy infected host cells (1). Humans (along with other vertebrates) can mount a second and far more sophisticated response via the adaptive immune system. This system is more complex and takes longer to mount a defence. However, its answer is tailored to the type of invading pathogen and can bestow long-lasting protection from future infection (2).

Energy and protein malnutrition is still a significant cause of impaired immune function and increased high mortality in many developing countries (3). Malnourished individuals are more liable to fall into a vicious cycle of infection and loss of appetite. As this cycle continues, people experience reduced nutritional intake, weight loss, lowered immunity, mucosal damage and invasion by other pathogens. Malabsorption and diarrhoea contribute significantly to malnutrition, leading to further decline in immune function (4). Micronutrient malnutrition is a condition which is becoming an emerging problem in more developed countries. This is where individuals consume an excess of low-quality foods, often leading to obesity and associated health issues (5).

What nutrients are needed for a healthy immune system?

Vitamin A is needed for the development and regulation of the immune response. Studies suggest it has therapeutic effects for many infectious diseases (6). The World Health Organisation (7) estimates that vitamin A deficiency causes approximately 1.2 million deaths in children each year. Vitamin A deficiency seems to increase the risk of diarrhoea, plasmodium falciparum malaria and measles in particular. Children who are already vitamin A deficient are significantly more likely to die from measles, partly due to the difficulty of treating post-measles diarrhoea (4). Good sources of vitamin A include cheese, eggs and oily fish (8).

Vitamin C is a potent antioxidant and contributes to both innate and adaptive immune function. Increased amounts of vitamin C are needed during infection due to inflammation and increased metabolic demands. Evidence suggests that supplementation can reduce certain infections' length and severity whilst eradicating some diseases altogether (9). Intravenous administration of vitamin C in doses far beyond what can be absorbed orally may effectively treat certain viral infections (10). Good sources of vitamin C include oranges, red and green peppers and strawberries (11).

Vitamin D is essential for both the innate and adaptive immune systems. Deficiency negatively affects the body's ability to fight off infection and increases the prevalence of autoimmune issues (12). It was used to treat tuberculosis (TB) in the pre-antibiotic era by sending patients to sunshine sanatoria in the Swiss Alps (13). The effects of supplemental vitamin D was shown to be successful in a population of Muslim women living in London. Before treatment, the women were all significantly deficient in vitamin D (14). Vitamin D is one of the few vitamins individuals may not be able to get enough from food alone. The body can synthesise this vitamin from sunlight. Supplementation is recommended in the UK during the winter months (15).

Vitamin E is a fat-soluble antioxidant that helps protect membrane polyunsaturated fatty acids. It is also able to regulate the production of reactive oxygen species (ROS) (16). Supplementation has proven effective in reducing the likelihood of influenza infections in mice and was observed to reduce the risk of developing upper respiratory tract infections in nursing home residents (17). It is important to note that some studies have identified an increased risk of death in people with poor health who took high dose supplements of vitamin E (18). Most people can get enough vitamin E from a varied diet. Good sources include olive oil, nuts and seeds (19).

Folate is crucial for DNA and protein synthesis. As these are the building blocks of biological life, folate also proves to be very important for immune function, especially T lymphocytes. Deficiency appears to be linked to an increased rate of infection (20). Folate is found in small amounts in several foods. Good sources include leafy green vegetables, chickpeas and fortified breakfast cereals (21). The UK government is currently considering mandatory fortification of folate in flour and grains to reduce the incidence of neural tube defects during pregnancy.

Iron deficiency is seen in many UK population groups, including toddlers, young girls, and women of reproductive age (22). Several cells of the innate immune system can combat bacterial infections by varying iron fluxation, controlling and limiting the amount of iron available for pathogenic bacteria to grow. Deficiency has been found to alter the body's ability to mount an effective immune response (23). Good sources of iron include meat, beans and nuts (24). The specific link between iron deficiency and infectious disease risk is still unclear. In some particular cases, supplementation can worsen outcomes. For example, malaria often leads to haemolysis and anaemia. Interestingly, supplementation may actually result in disease progression, as malaria requires iron to multiply its microorganisms in the blood (4).

Zinc transports signals from external stimuli around the body, guiding both the innate and adaptive immune response (25). Low blood zinc concentrations have been found in patients with TB, Crohn's disease, diarrhoea and pneumonia. Zinc supplementation has proven effective in reducing diarrhoea's length and severity in children in developing nations, significantly reducing overall mortality (26). Good sources of zinc include meat, shellfish and dairy products (27).

The majority of the UK population can meet their nutritional requirements through a varied whole food diet. For segments of the population where deficiency has been identified, the government has made specific supplementation recommendations, as it has with vitamin D and folate. The British Association for Parenteral and Enteral Nutrition (28) suggest that at-risk groups for malnutrition include the elderly, those with long-term health conditions and anyone who abuses alcohol or drugs. In this case supplementation of specific nutrients may be necessary to prevent malnutrition. All individuals should be aware of the increasing risk of malnutrition should they consume a low-quality diet or omit whole food groups.

References

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5 - Via, M. (2012) 'The Malnutrition of Obesity: Micronutrient Deficiencies That Promote Diabetes'. ISRN Endocrinology, doi: 10.5402/2012/103472

6 - Huang, Z. et al. (2018) 'Role of Vitamin A in the Immune System'. Journal of Clinical Medicine, 7(9), doi: 10.3390/jcm7090258.

7 - Neidecker-Gonzales, O., Nestel, P. and Bouis, H. (2007) 'Estimating the Global Costs of Vitamin A Capsule Supplementation: A Review of the Literature'. Food and Nutrition Bulletin, 28(3), pp. 307–316.

8 - National Health Service (2017) Vitamin A. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-a/ (Accessed: 16 May 2020).

9 - Carr, A. and Maggini, S. (2017) 'Vitamin C and Immune Function'. Nutrients, 9(11), p. 1211.

10 - Patrick, R. (2020) Vitamin C. Available at: https://www.foundmyfitness.com/topics/vitamin-c#intravenous-vitamin-c (Accessed: 21 May 2020).

11 - National Health Service (2017) Vitamin C. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-c/ (Accessed: 16 May 2020).

12 - Aranow, C. (2011) 'Vitamin D and the Immune System'. Journal of Investigative Medicine, 59(6), pp. 881–886.

13 - Martineau, A. et al., (2007) 'A Single Dose of Vitamin D Enhances Immunity to Mycobacteria'. American Journal of Respiratory and Critical Care Medicine, 176(2), pp. 208–213.

14 - Diamond, T.H. et al., (2002) 'High bone turnover in Muslim women with vitamin D deficiency'. The Medical journal of Australia, 177(3), pp. 139–141.

15 - National Health Service (2017) Vitamin D. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/ (Accessed: 16 May 2020).

16 - Lee, G. and Han, S. (2018) 'The role of vitamin E in immunity'. Nutrients, 10(11), doi: 10.3390/nu10111614.

17 - Meydani, S., Han, S. and Wu, D. (2005) 'Vitamin E and immune response in the aged: Molecular mechanisms and clinical implications'. Immunological Reviews, 205(1), pp. 269–284.

18 - Mayo Clinic Staff (2017) Vitamin E. Available at: https://www.mayoclinic.org/drugs-supplements-vitamin-e/art-20364144 (Accessed: 16 May 2020).

19 - National Health Service (2017) Vitamin E. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-e/ (Accessed: 16 May 2020).

20 - Dhur, A., Galan, P. and Hercberg, S. (1991) 'Folate Status and Immunity'. Progress in food and nutrition science, 15(1-2), pp. 43-60.

21 - National Health Service (2017) Vitamin B. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-b/ (Accessed: 21 May 2020).

22 - Scientific Advisory Committee on Nutrition (2010) Iron and Health. Available at https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/339309/SACN_Iron_and_Health_Report.pdf (Accessed: 21 May 2020).

23 - Ward, R. et al. (2011) 'Iron and the immune system'. Journal of neural transmission, 118, pp. 315-328.

24 - National Health Service (2017) Iron. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/iron/ (Accessed: 23 May 2020).

25 - Hojyo, S. and Fukada, T. (2016) 'Roles of Zinc Signaling in the Immune System'. Journal of immunology research, doi: 10.1155/2016/6762343.

26 - Cuevas, L.E. and Koyanagi, A. (2005) 'Zinc and infection: a review'. Annals of Tropical Paediatrics, 25(3), pp. 149–160.

27 - National Health Service (2017) Others. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/others/ (Accessed: 21 May 2020).

28 - The British Association for Paraenteral and Enteral Nutrition (2018) Introduction to Malnutrition. Available at: https://www.bapen.org.uk/malnutrition-undernutrition/introduction-to-malnutrition?showall=1 (Accessed: 23 May 2020).