After addressing the importance of strengthening our immune defenses at the table with foods that bring zinc and selenium to our body, it is now time to also see how to ensure, with a correct diet, the availability of vitamin A and D which constitute a further contribution to the activation of the immune response against bacterial and viral aggressions. Which we especially need in these times and, it seems, also for the times to come. However, it is appropriate to analyze separately the action of both nutrients of which the foods present in the Mediterranean diet are rich.
Discovered only in 1928, vitamin D has always been held responsible only for the correct formation of bones, through the regulation of the phosphorus and calcium cycle; in reality it is present chemically under two forms defined more simply as D 3 and D 2 . The D 2 form (also called ergocalciferol) is more widespread in fungi and yeast, while the D 3 form (also called colecalciferol) is more widespread in foods of animal origin, however the major source of this vitamin is human skin which underneath the direct action of the sun's rays is able to synthesize the quantities necessary for human metabolism. In figure 1 it is clearly evident how the D 3 form coming from the skin is added to that coming from the digestive system following the digestion of some foods and through the bloodstream it is transported to the liver and subsequently to the kidneys where it assumes its definitive chemical structure (carcitriol). In this mode, the vitamin is able to enter the cell nucleus through specific receptors, interacting with the DNA, conditioning its expression.
It was surprising to discover that all the cells of our immune system possess receptors for the metabolically active form of vitamin D and that they act as real sensors. The role of the vitamin is schematized in figure n. 2. First of all, it stimulates the phagocytosis activity of macrophages by increasing the production of cathelocidin, a protein with antimicrobial function, and of beta-defensin 4. Its action is then directed towards lymphocytes (CD4 in the figure) towards greater differentiation of lymphocytes T helper type 2, to the detriment of those of type 1. In turn, Treg lymphocytes also selectively increase their number with a simultaneous increase in the production of interleukins 4 and 10 (IL 4 and IL10) with a strong anti-inflammatory power .
UV rays
Fig. 1 – synthesis of vitamin D within the human body.
Fig. 2 – interaction between vitamin D and the immune systemVitamin D
Source: Author's elaboration from Baeke F., Takiishi T., Korf H., Gysemans C., Mathieu C., Vitamin D: modulator of the immune system in Current Opinion in Pharmacology 2010.
The synthesis of vitamin D excludes supplements especially if they are the result of DIY; a varied diet in which milk, cheese and oily fish are not lacking is more than sufficient to ensure the minimum required intakes. The case of problems connected with malabsorption or lack of exposure to sunlight is different; however, it should be noted that vitamin D of food origin is soluble in fats, as we will also see for vitamin A and therefore it is always correct do not eliminate the intake of dietary fats from the daily diet, especially if of vegetable origin (extra virgin olive oil).
In foods of plant origin, vitamin A is present in a chemical form which requires a preliminary conversion during the digestive process, before being able to effectively perform its function; usually in the nutritional tables the content of the vitamin is already reported in its active form (retinoic acid) in the human body. In recent years, the role of this vitamin in the development of T reg lymphocytes has been ascertained by arresting the differentiation of Th 17 lymphocytes, with a mechanism very similar to that induced by vitamin D. Th17 lymphocytes, while exercising a defense role against external aggressions to the human body determine an excess of inflammatory response which must necessarily be attenuated through the development of T reg lymphocytes. An adequate vitamin intake is therefore able to favor the development of some types of lymphocytes over others; Moreover Vitamin A maintains the integrity of intestinal tissue and its mucus lining. Some research has shown that a lack of vitamin A induces both a low presence of CD4 lymphocytes and their ability to migrate in the intestinal wall, therefore vitamin A is able to strengthen the first line of defense of our body.
Vitamin A content (μg/100 g) of some foods.
Chicken liver 36.600
Liver (sheep, beef, pork) 15.000-16.500
Sea eel 980
Dehydrated Apricots 1410
Raw carrots 1148
Butter 930
Rocket 743
Chicken egg yolk 640
Yellow Gourd 599
Source: CREA 2019 data.
Although the vitamin A is very widespread in the plant world, however the highest contents are found in the liver of animals and in eel; it too is soluble in fats therefore yellow-orange vegetables should always be consumed in combination with a fat base and if this is simpler with pumpkin, its combination with apricot is less likely. The consumption of fresh fruit in combination with fresh milk cream is undoubtedly to be preferred.
An adequate intake of vitamin A is equal to 600-700 μg/day which can be easily satisfied with 100 g of rocket or 50 g of carrots or, paradoxically, with about 60 g of eel. An egg yolk is enough to ensure a fair supply. Supplements with supplements are always to be taken on medical prescription and in any case above 3000 μg/day there is no benefit.
The summer sun and a regular consumption of fresh fruit are a valid contribution to the harmonious development of our defences; it will be appropriate to adopt this lifestyle also in the autumn that is upon us. Certainly our ancestors in ancient Rome assumed significant quantities of vitamin A thanks to the consumption of the "ficato" liver, i.e. of the liver of animals fed on figs as Galen testifies (“τὸ συκωτὸν ὀνομαζόμενόν ἐστι, τῆς προσηγορίας ταύτης τυχὸν, ἐπειδὴ πολλῶν σύκηρ ἐπειδὴ πολλῶν σύκηρ ωδῇ τοῦ μέλλοντος σϕάττεσϑαι ζώου τοιοῦτον παρασκευάζουσιν αὐτό”, “it is the one called “ficato” that takes this name because they prepare it by feeding the animal which is intended for slaughter" see De Alim. Fac. VI, 679) and it is advisable to observe the health conditions of those populations of the Po delta who have consumed eels for centuries. On the other hand, in the composition of garum, the famous fish sauce consumed by the Romans, the presence of eel was also envisaged (“Capiuntur pisces natura pingues, ut sunt salmones et anguillae”[…], […]” fish are taken for fatty as salmon and eels are"[...]), therefore this sought-after condiment did not lack vitamin A as well as D. The nutritional contents of animal liver are certainly flattering, especially for chicken liver, even if it is advisable to prefer this ingredient from animals raised with organic methods.
