In very recent years, nutrition has explored numerous new relationships between diet and immunity. Our intestine is a crossroads of information between human metabolism and the outside world and the quality of food is a means of communication capable of influencing the ability to respond to biological stressors.
An equivocal culture on protein intake has distracted us from the true functionality of our intestine which retains some distant functions typical of ruminants, namely the ability to use vegetables. Fruits and vegetables are important in the daily diet not only as sources of sugars, mineral salts or vitamins, but above all of fibres.
The structure of the fiber is fairly well known, although it varies greatly according to the individual plant species. The most common and accredited form can be compared to sturdy cellulose strings connected by thinner strings of pectin which acts as a glue. This very robust reticular structure is in turn transversely crossed by glycoproteins.
Cellulose, pectin and proteins are compounds capable of binding and retaining large quantities of water (even 5-6 times their own weight) until they swell, increasing their volume. This reticular structure envelops the cells inside which there are the "noble" nutrients immersed in a solution rich in water. This spatial arrangement allows us to understand that all the transformations that are carried out on vegetables, such as centrifugation or squeezing, produce extracts rich in micro and macronutrients, but devoid of fiber which is eliminated. Frequently the centrifuged waste is reused for the production of sweets which in this way are more dietetic.
The human intestine is not able, like ruminants, to break down the fiber, which is eliminated through a greater production of faecal mass; this would lead to minimizing its role; actually the connection between fiber and immunity have recently emerged.
Although the intestine does not break down the fiber, it actually feeds the many species of microbes that live permanently in the colon. They feed on the remains of the human meal converting it into vitamins and new metabolites and at the end of their biological cycle they are expelled with the faeces. We have to imagine the microbiota as a boundless myriad of different microbial species living in numerical balance between them. Any event (stress, smoking, antibiotics) can modify this balance in favor of one or the other species and it is almost impossible to study these modifications in vivo, which must necessarily be examined for models in the laboratory or on guinea pigs.
It has been ascertained that the increase in the number of Streptococci, Enterobacteria and yeasts depress the populations of Akkermansia muciniphila, Bifidobacteria and Bacteroides which are the most useful species for limiting intestinal dysbiosis conditions.
The microbiota is able to only partially demolish the dietary fiber, transforming it into compounds that are apparently useless from a nutritional point of view: acetic, propionic and butyric acids and which are commonly distinguished by the acronym SCFAs (short chain fatty acids). These compounds, better known as short-chain fatty acids, play a stimulating role on the functionality of some organs. In fact, it has been ascertained in pregnant and lactating women that a diet rich in fiber stimulates the function of the thymus gland in the production of lymphocytes, the first line of attack against external aggressions (viruses, bacteria). It has been ascertained that obese subjects present an involution in the development of the thymus which above all presides over the innate immunity.
The efficiency of the immune system depends not only on the number but also on the correct ratio of all the lymphocyte species produced following an infection. Thelper lymphocytes and Treg lymphocytes, if present in the right reciprocal relationship, are able to ensure a correct immune response. In particular, the Bacteroides fragilis species is able to activate a transcription factor Foxp3+ and Interleukin 10 (IL-10). The Clostridum rhamnosus species stimulates the development of Treg cells right in the thymus gland and the Bacteroides thetaiotaomicron species in particular produces high quantities of butyric acid which, of the three fatty acids mentioned, is the one that most stimulates the production of the transcription factor Foxp3+ necessary for the development and differentiation of Tregs. Short-chain fatty acids stimulate the transformation of dietary vitamin A into new metabolites which in turn induce the development of Treg cells.
They are also able to promote the development of Th1 lymphocytes (Thelper type 1) which assist responses to invasion by viruses and bacteria (intracellular pathogens). One of the most accredited interactions between the microbiota and the immune system also lies in the action carried out by a group of bacteria called Segmented Filamentous Bacteria which, in association with Short Chain Fatty Acids, are able to enhance the development of another group of lymphocytes: Th17 , which in turn help protect against extravellular pathogens (fungi and bacteria).
The action of Treg lymphocytes has been considerably re-evaluated recently, since they preside over the regulation of the immune response at a peripheral level in the suppression of the amplification of the response itself and contextually in allergic responses. It is possible to simplify by stating that their role eliminates the effects of “friendly fire” on human body structures during a pathogen infection.
The diet
The nutritional indications of CREA in the recent dossier on the Guidelines for healthy eating (2018 revision) confirm an optimal intake of 25 g of fibers per day corresponding to 5 portions of fruit and vegetables.
Standard portions and reference sizes
Fresh fruit 150 g
Fresh vegetables 200 g
Leaf salads 80 g
Potatoes 200 g
Source: CREA Guidelines for healthy eating, 2018 revision, Rome, (pages 131-132).
In reality, the intake of fruit and vegetables alone cannot total the 25 g/day of fiber required, for this reason it is necessary to ingest suitable quantities of wholemeal pasta and legumes using dried fruit snacks for snacks. If 50 g of dried apple snacks are taken in two separate snacks (morning and afternoon), the fiber intake is equal to 7,5 g, i.e. equal to 30% of the daily requirement.
The use of dried washers, contrary to what one might imagine, was already widespread among rural populations in ancient Rome. Galen documents this when he describes the preservation of larger pears ([…]”αἱ δ'ἄπιοι καὶ μάλιστα αἱ μεγάλαι (καλοῦσι δ'αὐτὰς μενάτας οἱ παπρ'ἡμὰς μενάτας οἱ παπρ'ἡμὰσ) ι καὶ τρόϕιμον, ὥστε καὶ τέμνοντές τε αὐτὰς εἰς κυκλίσκους λεπτοὺς καὶ ξηραίνοντες ἀποτίϑενται,”[…]) […]”Pears, and especially those of greater caliber (in our parts they call them “beaten”) have some nourishment, so that after having cut them into thin rounds and dried they can be preserve”[…](Ali. Fac. VI,605). In ancient agriculture the drying of cherries and blackberries was also widespread, so that the production surpluses could cover the needs of the populations in the adverse season in the absence of crops.
Summer offers the opportunity to increase the consumption of fruits and vegetables that will boost our immune system in view of next autumn.
