Abstract:
Optimal nutritional status is of high importance for the proper functioning of the immune system and can be achieved by adequate intake of macronutrients and micronutrients. In order for the immune response to be adequate in eliminating pathogens, but also in preventing and fighting allergies, autoimmune and chronic diseases, it is necessary for the diet to be balanced in intake of all necessary macronutrients and micronutrients.
Vegetarianism is becoming an increasingly widespread type of diet around the world. In relation to the omnivorous diet, the vegetarian diet (especially vegan) is significantly different in terms of intake of certain foods, and therefore may be different in terms of intake of certain macro- and micronutrients, which can affect the immune system.
Many studies have shown that in vegetarians, and especially vegans, some hidden nutritional deficiencies in minerals (Fe, Se, Zn, Ca, Cu), vitamins (B12, D, A, B2, B3 and B6), omega 3 fatty acids (as well as the less favorable ratio of omega 3 / omega 6 fatty acids) and, less frequently, high-value amino acids rich in sulphur, can be found. Indeed, it has been shown that there are many differences between a vegetarian and an omnivorous type of diet in immune status and immune response.
Studies conducted in recent years show that the level of immune markers can be altered in vegetarians. For example, vegetarian diets have been connected with decreased levels of many inflammatory markers - cytokines / chemokines, acute phase proteins and prostaglandins, including: CRP (C reactive protein), IL (Interleukin) -1β, IL-6, TNF (tumor necrosis factor) -α, decreased ratio of TNF-α to IL-10, decreased levels of SAA (serum amyloid A), ferritin, fibrin, prostaglandin PgE2, antibodies (IgG, IgM and IgE); decreased count of leukocytes, neutrophils, lymphocytes, monocytes, eosinophils, basophils, platelets, and some of their fractions (Natural killer cells - NK cells, T lymphocytes, B lymphocytes, CD4 +, CD8 +), as well as an altered activity (decreased proliferation of T lymphocytes after mitogen stimulation, decreased phagocytic activity of neutrophils and monocytes and production of reactive oxygen species, as well as decreased cytotoxic activity of NK cells), and reversal of proinflammatory immune response to antiinflammatory response. Also, the altered ratio of omega 3 to omega 6 fatty acids has been shown, as well as the status of markers of antioxidant protection, which may affect endogenous production of proinflammatory and antiinflammatory eicosanoids, degree of inflammation and level of tissue damage during inflammation. For example, in vegetarians, some studies have shown the reduced levels of anti-inflammatory and antioxidant omega 3 fatty acids, as well as the reduced antioxidant protection (decreased levels of glutathione, uric acid and increased levels of homocysteine), connected with vitamin B12, selenium and sulphur-rich amino acids deficiency.
However, studies on vegetarianism and the immune response are quite rare, very often contradictory in the results, and there is no definitive conclusion. In addition, many studies have been done on individuals with autoimmune diseases and already altered immune status (ulcerative colitis, atopic dermatitis, etc.) by translating from omnivorous to vegetarian diets, while other studies have been done comparing vegetarians and omnivorous subjects without dietary intervention, but often included special groups of subjects (e.g., pregnant women, the elderly, people with polycystic ovary syndrome, diabetes, etc.), which all contributed to the conflicting findings.
Examining what kind of diet is the most effective for a satisfactory immune response is especially important during the present COVID-19 pandemic, as well as long-standing pandemics of chronic diseases related to nutrition and altered immune status (e.g., obesity, diabetes, cardiovascular diseases, autoimmune diseases, cancer). Given the increase in vegetarianism and the great heterogeneity of the vegetarian diets in the world, and due to the conflicting results of the existing studies on the impact of vegetarian vs. omnivorous diet to immune status and response, more detailed research is needed, which will exclude the effects of all potential confounding factors, such as: gender, age, smoking status, physical activity, the presence of obesity and metabolic syndrome, the presence of certain diseases and conditions that may affect immune status, as well as the use of therapy and supplementation which may affect immune status, the type of vegetarianism (veganism or lacto-ovo-vegetarianism) and length of vegetarianism, as almost all previous studies have not been well controlled for such factors. There is a particular lack of studies examining the specific characteristics of a vegetarian diet (e.g., intake of certain macro- and micronutrients, intake of certain foods), as well as the status of macro- and micronutrients in the body (e.g., omega-3/6 fatty acids, minerals and vitamins) and examined their association with the immune response and markers of antioxidant protection and oxidative stress.
