Why is diet important in maintaining strong immune defenses?
The immune system works to defend the body against pathogenic organisms such as the new coronavirus SARS-CoV-2 and other insults, including environmental pollutants. However, it is also important that it does not react adversely to the harmless gut microbiota, food and body organs. To ensure the immune system functions well and provides a good defense against pathogens, first, the body needs a good supply of nutrients.
Philip Calder, Professor of Nutritional Immunology at the University of Southampton, explained to GMFH editors via email that key immune-supporting nutrients include vitamins (i.e. vitamins A, C, E, D, K and B vitamins), minerals (including zinc, copper, iron, selenium, magnesium and manganese), amino acids and essential fatty acids.
The gut microbiota affects the body’s immune response and in particular its responses to inflammation
Mounting evidence supports the theory that the gut microbiota affects the body’s immune response and in particular its responses to inflammation. As such, dietary components that promote a diverse and healthy gut microbiota are also important in supporting the body’s immune response, with probiotic-containing foods and supplements, prebiotic fibers and plant polyphenols the most widely studied immune-supporting nutrients and non-nutrients.
Beyond diet, other factors affecting the immune system and susceptibility to severe COVID-19 include aging—especially when older adults are frail—obesity, physical inactivity and stress.
Is there a particular diet that should be promoted as providing help to protect against severe COVID-19?
With an infection such as COVID-19, immune cells have greater demands for nutrients to react against the infecting pathogen. Paying attention to diet right now is also important because of the inflammatory storm triggered by severe COVID-19 infection, which has led scientists to investigate the potential of omega-3 polyunsaturated fatty acids in such a situation.
While a good nutrient supply is important for good immune function and defense against pathogens, is there a role for diet in decreasing susceptibility to and the progression of COVID-19?
The answer is yes. Indeed, focusing on the whole diet rather than on isolated nutrients is more important, mainly because the specific amounts of nutrients needed for a proper immune response have not been defined. Furthermore, scientists have noted modest effects in the association between specific nutrients or dietary supplements and reductions in COVID-19 risk.
Two recent studies emphasize the importance of diet quality for protection against severe COVID-19:
- One study in over 600,000 UK and US participant users of a smartphone-based COVID-19 symptom app that also collected dietary information found that a diet rich in healthy plant foods may reduce both the risk and severity of COVID-19, particularly in areas with higher socioeconomic deprivation.
- Similarly, a web-based survey of healthcare workers from France, Germany, Italy, Spain, the UK and the USA with substantial exposure to COVID-19 patients found that people who reported following plant-based or pescatarian diets showed a decreased risk of moderate-to-severe COVID-19.
When asked about the best diet for supporting the immune system, Professor Calder stated that, “such a diet would be plant-based with plenty of fruits, vegetables, berries, nuts, seeds and pulses but also a modest amount of animal-based foods (e.g. some lean meat, eggs, dairy and oily fish) in order to provide those nutrients that are hard to get in sufficient amounts from plants.” That diet is consistent with those regarded as generally healthy and would also promote a healthy gut microbiota, particularly if some fermented foods were included.
Is there a role for diet in supporting COVID-19 vaccine efficacy?
With the current COVID-19 vaccination program in progress, it is important to bear in mind that low intakes of several micronutrients may impair vaccine responses. That is particularly true for poor vaccine responses linked to a deficiency in micronutrients such as vitamin B6, vitamin C, vitamin D, zinc and iron, which is relevant in at-risk individuals, including frail or malnourished older adults.
Although previous studies have shown that some probiotic strains and prebiotics can help improve the effectiveness of seasonal influenza vaccines in older adults, it is not known whether they would also work in improving COVID-19 vaccine efficacy. In anticipation of upcoming clinical trials studying the impact of gut microbiota and immune system-targeted dietary strategies, a positive starting point is to ensure a good nutrition status and nutrient stores that allow for a robust defense against COVID-19 and other pathogens.
When nutrient deficiencies, especially micronutrients such as vitamin D, cannot be covered through a balanced diet alone, supplementation with micronutrients under the supervision of healthcare professionals can be considered. In that regard, to support the proper functioning of the immune system, a group of scientists have suggested recommended intakes of vitamin E, vitamin C, vitamin D and zinc, which need to be validated in well-designed clinical trials.
As Professor Calder explains: “Vaccination is an immune challenge just like infection, and a well-functioning immune system is required to achieve a good response to vaccination. Good intakes of immune-supporting nutrients and of food components that promote a diverse healthy gut microbiota are important to ensure that responses to vaccines are as good as they can be. Important nutrients to support the immune response to vaccination will include vitamins A, C, D and E, the B vitamins, iron, zinc, copper, selenium and other minerals, and most likely essential amino acids and fatty acids, although these are not well studied from the point of view of response to vaccination.”
Calder PC. Nutrition and immunity: lessons for COVID-19. Eur J Clin Nutr. 2021; 75(9):1309-1318. doi: 10.1038/s41430-021-00949-8.
Calder PC, Carr AC, Gombart AF, et al. Optimal nutritional status for a well-functioning immune system is an important factor to protect against viral infections. Nutrients. 2020; 12(4):1181. doi: 10.3390/nu12041181.
Arnardottir H, Pawelzik SC, Wistbacka UO, et al. Stimulating the resolution of inflammation through omega-3 polyunsaturated fatty acids in COVID-19: rationale for the COVID-Omega-F trial. Front Physiol. 2021; 11:624657. doi: 10.3389/fphys.2020.624657.
Louca P, Murray B, Klaser K, et al. Modest effects of dietary supplements during the COVID-19 pandemic: insights from 445 850 users of the COVID-19 symptom study app. BMJ Nutr Prev Health. 2021; 4:e000250. doi: 10.1136/bmjnph-2021-000250.
Merino J, Joshi AD, Nguyen LH, et al. Diet quality and risk and severity of COVID-19: a prospective cohort study. Gut. 2021; 70(11):2096-2104. doi: 10.1136/gutjnl-2021-325353.
Kim H, Rebholz CM, Hegde S, et al. Plant-based diets, pescatarian diets and COVID-19 severity: a population-based case-control study in six countries. BMJ Nutr Prev Health. 2021; 4:257-266. doi: 10.1136/bmjnph-2021-000272.
Rayman MP, Calder PC. Optimising COVID-19 vaccine efficacy by ensuring nutritional adequacy. Br J Nutr. 2021; 126(12):1919-1920. doi: 10.1017/S0007114521000386.
Rizzardini G, Eskesen D, Calder PC, et al. Evaluation of the immune benefits of two probiotic strains Bifidobacterium animalis ssp. lactis, BB-12® and Lactobacillus paracasei ssp. paracasei, L. casei 431® in an influenza vaccination model: a randomised, double-blind, placebo-controlled study. Br J Nutr. 2012; 107(6):876-884. doi: 10.1017/S000711451100420X.
Lomax AR, Cheung LVY, Noakes PS, et al. Inulin-type b2-1 fructans have some effect on the antibody response to seasonal influenza vaccination in healthy middle-aged humans. Front Immunol. 2015; 6:490. doi: 10.3389/fimmu.2015.00490.
Gombart AF, Pierre A, Maggini S. A review of micronutrients and the immune system-working in harmony to reduce the risk of infection. Nutrients. 2020; 12(1):236. doi: 10.3390/nu12010236.