The intestinal wall has a complex structure. It is lined by a single layer of rectangular cells called enterocytes. (Figure 15). These cells are bound together by proteins called "tight junctions" in their side walls and are impermeable to the passage of substances, unless the enterocytes "authorise" their opening. Thus, most of the substances we absorb pass through these cells to enter our body, rather than between them, allowing better control of what does and does not pass through. These cells also have a 'brush border' at the top (the part that faces the intestinal lumen), which are little hairs called microvilli that allow them to absorb many substances very efficiently. Also, on the surface of the microvilli are certain enzymes that help to finish digesting food, the digestion of which began in the lumen of the stomach or intestine thanks to gastric or pancreatic juices, which are liquids loaded with digestive enzymes that are released when we eat. An example of this is lactose, the sugar contained in dairy products, which is digested at the level of the intestinal microvilli thanks to an enzyme called lactase, and whose deficiency can lead to the famous lactose intolerance. When there is chronic inflammation in the intestinal wall, enterocytes can die or lose their microvilli, and this can lead to malabsorption, poor digestion or even intolerance (for example, lactose intolerance can be genetic if the gene encoding lactase is missing, or it can be acquired through excessive chronic intestinal inflammation). Also, "pores" can open up between cells due to loss or malfunction of tight junctions, so that the impermeability of these junctions is lost. There are also molecules such as gluten gliadin (the non-water-soluble part of gluten) that have the ability to bind directly to the tight junctions and open them. Thus, even in the absence of inflammation, these foods can produce intestinal porosity "per se". Intestinal porosity allows substances that should normally remain in the intestinal lumen and not enter our body, such as toxins or pieces of the wall of some intestinal bacteria called lipopolysaccharides, to pass the intestinal barrier without any control by enterocytes. Thus, these substances directly provoke a local inflammatory reaction or even pass into the bloodstream causing inflammation at a distance. For example, lipopolysaccharides can cross the blood-brain barrier, reach the brain and cause neuroinflammation (inflammation of the central nervous system). This is thought to be one of the mechanisms favouring the onset of neurodevelopmental disorders in children, such as autism for example, or neurodegenerative diseases such as Parkinson's or Alzheimer's in adults.
Pro-inflammatory foods
We know that some foods have the inherent power to inflame the intestinal wall, as the immune system recognises them as "foreign agents" and prevents them from crossing the intestinal barrier, while others only inflame the intestine if they are poorly digested.
Examples of pro-inflammatory foods are cereals containing gluten or other types of lectins, which we will discuss later, dairy products and foods containing omega-6 fatty acids (most seeds and their oils, cereals, etc.). When we speak of a food allergy, we are referring to an immune reaction produced by contact with certain substances in food, mediated directly by the cells of the immune system or by IgE-type antibodies, in which pro-inflammatory cytokines and other substances are released and a real inflammatory reaction is set in motion, which can be very dangerous. When we refer to a food intolerance, however, in principle it is usually a non-specific reaction of the digestive system, not mediated by immunity, due to deficiencies at the level of some enzymes of digestion or alterations in the microbiota that prevent a good metabolisation of some ingested substances. Although in this case there is no direct activation of the intestinal immune system, the reality is that the presence of poorly digested substances and an alteration of the local microbiota will, in practice, most likely lead to a state of inflammation at the intestinal level. Excessive activation of intestinal immune cells will cause them to massively produce molecules called pro-inflammatory cytokines, which are released into the bloodstream and serve to attract more immune cells to the inflamed area (as if the cells were "calling for reinforcements" from other units of the immune system). Thus, the cytokines will serve to increase the local inflammatory response. If exposure to the substance that has triggered the inflammation persists, for example, if we eat a food every day that we do not tolerate well, this local inflammation may become chronic, even if it is not an allergy. Thus, if the intestinal wall is inflamed, its cells, called enterocytes, which are actively involved in digestion, will not perform their function correctly and therefore, the processing and digestion of those substances that occur at the level of these enterocytes will be altered. Also, the absorption from the intestine of many beneficial substances will be less efficient, which may lead to certain nutritional deficiencies. (Figure 14). In addition, certain molecules such as gluten or casein in milk are very similar to certain molecules on the surface of our cells, such as the cells of the thyroid gland for example. Thus, if our immune system reacts to these substances, an autoimmune reaction to our own cells can occur by mistake, leading to diseases such as autoimmune thyroiditis.
Polyunsaturated fatty acids (omega 3 and omega 6)
The current over-consumption of cereals and seeds in general in our diet leads to a problem: the presence in these foods of abundant omega-6 fatty acids. Omega-6s are a type of polyunsaturated fat which, although important in small amounts for our bodies, are pro-inflammatory if consumed in large quantities. Omega-3 fatty acids, which you have probably already heard of, and which are mainly found in fish (especially oily fish) and other marine animals, in some algae, as well as in certain seeds such as flax, chia or walnuts, are another type of polyunsaturated fat that is essential for our bodies, as we are not able to manufacture them. These fatty acids, among many other functions, such as improving our cognitive capacity by acting positively on our central nervous system, have great anti-inflammatory power and counteract the effect of omega-6 fats.
There is the so-called "omega-3 to omega-6 ratio", which is the ratio between the omega-3 and omega-6 fatty acids we ingest, and which determines whether or not inflammation occurs. Normally, the ratio should be 1:1 or slightly more, with a tolerable ratio of up to 1:5 or so (taking in five times as much omega-6 as omega-3). This is the ratio our ancestors used to have. Today, however, due to our modern diet, this ratio is usually 1:15 or even 1:20 (ingesting up to 20 times more omega-6 than omega-3). In this way, omega-3s do not have sufficient capacity to counteract the effect of omega-6s, and the "balance" of inflammation is tipped towards a pro-inflammatory state in our body.
In addition, there is the aggravating factor of the industrialisation of modern food. We eat more and more processed food, which is very rich in omega-6. In particular, the oils used to cook all these processed foods, which are usually seed oils, such as sunflower oil, for example, whose omega-6 content is very high compared to much healthier oils such as olive oil, avocado oil or coconut oil, which contain very little. The problem is difficult to solve, as it lies at several levels. On the one hand, there is the question of price, usually related to the type of pressing used to extract the oil. Seed oils tend to be cheaper, especially if they are obtained by heat extraction methods. Hot pressing yields much more oil than cold pressing, whereas virgin olive oil is usually extracted by cold pressing methods, which makes the product much more expensive. This means that the food industry does not even consider using better quality oils in their products, as it would make them much more expensive and much less competitive. The same is true in our homes, as many people use this type of oil because it is cheaper, despite the fact that sunflower oil has recently become much more expensive. On the other hand, in addition to the high content of omega-6 that these oils have in themselves, with great pro-inflammatory power, we must add the even more harmful effect on our health that the process of extraction by heat and the use of these oils at high temperatures (pastries for example) has on our health, as the polyunsaturated fatty acids (omega-3 and omega-6) are very sensitive to heat. When heated, these fats change their chemical configuration from "cis" fats to "trans" fats, as if the molecule were twisted. (Figure 17). In this way, trans fats are much more pro-inflammatory and pro-oxidant than cis fats, and promote an increase in LDL and VLDL ("bad" cholesterol). Thus, even if it is a seed with a high omega-3 content, such as flax, for example, the extraction method alters these fatty acids and makes them much less healthy if they are extracted using heat. Therefore, from a health point of view, taking ground flaxseed is not the same as taking flaxseed directly or cooking with flaxseed oil, for example. As I have already mentioned, the exposure to heat during oil extraction is often compounded by further exposure to high temperatures during cooking. If we add to this poor storage (as these oils should be protected from light and preferably kept in the refrigerator, which is rarely done), the polyunsaturated fats they contain are guaranteed to be altered, and thus inflammation.
Finally, I would like to clarify the difference between omega-3 fatty acids of plant origin (ALA) and those of animal origin (EPA and DHA). The latter are the ones that are really useful for our bodies, as they are the ones that really have an immunoregulatory, mainly anti-inflammatory, effect. Once ingested, ALA must be converted in the body into EPA and then into DHA. The conversion rate varies from person to person, but generally does not exceed 10%. As a result, a large proportion of the plant-based omega-3s we ingest are never converted into active molecules in our bodies. Thus, a person who only consumes products of plant origin, even if they have a high intake of foods rich in omega-3 ALA, will probably not reach the minimum amounts of DHA and EPA that our body requires. It is for this reason that EPA and DHA supplementation is highly recommended for people following this type of diet.
Lectins
Humans have long been omnivorous animals. Throughout evolution, our digestive system has specialised in digesting and assimilating many green plants, wild fruits (especially berries and nuts) and animal products (meat, eggs and fish), as these foods were what we could mainly gather or hunt. For millions of years our digestive system has had very little contact with cereals or leguminous plants. Although we sometimes found "wild" cereals or legumes that we ate, they were far from being our main source of food. However, with the arrival of the Neolithic revolution, where man learned to cultivate the land and domesticate animals, our eating style changed completely. This was about 8,000 to 10,000 years ago. From that time onwards, the consumption of cereals and cereal flours increased dramatically, and to a lesser extent that of pulses, because they were foods that could be easily preserved. And since then, this has only increased. Our digestive system, despite its adaptability and flexibility, has not been able to adapt to such a drastic change in such a short time (yes, yes, 10 000 years is a very short time in the history of evolution). As a result, over-consumption of these foods has put "stress" on our gut and its guardians, the cells of the immune system.
One of the reasons why grains and legumes promote inflammation is the presence of substances called lectins. Lectins are proteins found in many plants. Most cereals except rice, millet, teff and little else contain them. In fact, gluten is one type of lectin, although other non-gluten cereals, such as maize or oats, contain similar lectins. All legumes also contain lectins, as do solanaceous plants such as tomatoes, potatoes and aubergines. Lectins are chemical substances that allow plants to defend themselves against predators, as they do not have claws or teeth like animals, nor can they flee when attacked. On the one hand, they produce a certain toxicity and cause intestinal inflammation, so many animals and insects avoid eating them. On the other hand, lectins allow plants to survive, favouring the expansion of their seeds, as the animal gut often has no mechanism to digest them. Thus, the seed ingested by an animal will pass through its digestive tract and come out intact in the faeces a few hours later, while the animal will probably have moved on. In this way the seed will be "planted" elsewhere, surrounded by the wonderful fertiliser that is faeces. Lectins are therefore a very clever and effective form of plant defence. In small doses, they are not a problem for our organism, as our digestive tract can tolerate them. But the problem comes when we ingest too much. And this is what has been happening more and more. From the invention of agriculture to the present day, the consumption of lectin-laden foods has risen sharply, especially since the mid-20th century. It should not be forgotten that, back in the 1960s-70s, health authorities began to recommend the consumption of cereals, milk, cheese, margarine and potatoes as the basis of our diet. These recommendations were later transformed into the famous "nutritional pyramid", which first appeared in Sweden in 1974 and was later exported to the rest of the world, recommending the percentage of each type of food that we should eat for good health. According to this nutritional pyramid, which varied little from country to country, products such as cereals, bread, rice, pasta and dairy products should be at the base of our diet. Today, these recommendations have been more than contested by many scientists, and it has even been demonstrated that the high consumption of carbohydrates has a harmful effect on health and is linked to many chronic diseases such as diabetes and cardiovascular disease. However, this way of eating has become established in Western societies and most people today still think that healthy eating is just a matter of eating this way. This is not to say that these foods should not be eaten, but they should be eaten in moderation and, preferably, with the knowledge of how to prepare them in order to inactivate some of the effect of their anti-nutrients.
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