By Dr. Gianluca Rizzo
Introduction
Research in recent decades has made great achievements in understanding the multiple possible functions of lipids.
It is easy to say polyunsaturated, but in reality when we talk about these fatty acids we refer to a family of molecules, each of which has a particular characteristic.
When we talk about polyunsaturated fatty acids (PUFA), we often stress the importance of taking a sufficient amount with the diet but we dwell little on which molecules should be taken and why. In this regard, as part of a vegetarian diet, it is often said that the required PUFA quota can be largely satisfied thanks to a diet rich in vegetable oils, nuts and seeds. To understand if this is realistic, we must take a step back and understand how our body uses these substances, the main functions but above all their metabolism.
What are polyunsaturated fats? What are their functions?
Polyunsaturated fatty acids are characterized by the presence of 2 or more double bonds, each between two adjacent carbons, along the carbon skeleton that composes them. Each double bond imposes a folding in the structure that reduces its possibility of packaging with the other molecules. This can easily be noticed through the physical state of a lipid food at room temperature. In fact, the greater the double bonds and / or molecules with double bonds, the greater the tendency of the molecules themselves to maintain a disordered disposition. This arrangement will not allow the compound to reach the solid state at room temperature, therefore, said very simply, the lipid food will be in the form of an oil. This simple information on the chemical-physical properties of lipids can tell us a lot about the foods we buy, giving us a tool to discriminate which of these can be healthy and which can represent a mere source of calories. Butter or lard are poor sources of essential fatty acids and contain mainly long and medium-long chain saturated fatty acids. It would be better to limit their use due to their marked atherogenic power, although much more harmful plant products exist. Vegetable oils are naturally found in liquid form, therefore they represent a good source of mono and polyunsaturated fats. Not all vegetable fats are healthy anyway: margarines and cocoa butter are solid at room temperature and this speaks volumes about their fatty acid composition, regardless of the healthiness of the systems used to obtain them in solid form.
The double bonds represent, however, a weak point for the aliphatic chain of lipids, therefore the greater the double bonds and the more quickly the food will be subject to deterioration and rancidity due to the oxidation processes. Olive oil is an important source of lipids due to its low saturated fatty acid content but also due to the prevalence of mono-unsaturations which limit its deterioration.
The chemical and physical properties of PUFA make them indispensable for the health of cell membranes throughout the body. The life of each cell is closely linked to the functionality of its membrane, the true heart of the cell that allows communication with the outside and the exchange of substances for metabolic purposes. This communication depends on the phospholipids that make up the bilayer and that allow the aforementioned functions; a phospholipid-rich membrane with polyunsaturated fatty acids is a more fluid and healthy membrane. Let us not forget that in the nervous system the need for PUFA is very important for the correct functionality of the various highly specialized structures.
Another important function of PUFAs concerns their role as precursors of eicosanoids, a family of cellular mediators that act in concert modulating systemic responses, with particular regard to the mechanisms of inflammation.
How many types of PUFA exist? Why are they important to health?
We can immediately make a first distinction between omega 3 (ω3) and omega 6 (ω6) which consists in the numbering of the carbon atoms along the fatty acid chain that distance the first carbon involved in a double bond from the last carbon of the chain itself . The two types of PUFA in turn can contain a variable number of double bonds and can have a longer or shorter chain.
The interesting feature from the biochemical point of view is that all animals are unable to synthesize them from scratch, but each living being has a more or less marked enzymatic capacity to lengthen the chains and increase the number of double bonds. We thus have a second distinction between short-chain polyunsaturated fatty acids, or precursors, and long-chain fatty acids (LC-PUFA). Plants have a strong propensity towards the synthesis of precursors, with a low efficiency of LC-PUFA accumulation. Conversely , animals, including humans, do not have the ability to synthesize PUFAs from scratch, so they necessarily need food sources at least for the precursors. The precursor of ω3 is called Alpha Linolenic Acid (ALA) which has three unsaturations and a carbon chain of 18 atoms (18: 3ω3). The precursor of ω6 is called Linoleic Acid (LA) which contains two unsaturations and 18 carbon atoms (18: 2ω6). Long-chain PUFAs are obtained from these precursors through a cascade of reactions that involve the action of some enzymes that perform elongation (elongase) and others that deal with adding double bonds (desaturase). Among the LC-PUFA ω3 we will mainly have Eicosapentaenoic Acid (EPA 20: 5ω3), Docosapentaenoic Acid (DPA 22: 5ω3), and Docosahexaenoic Acid (DHA 22: 6ω3). Among the LC-PUFA ω6 the most important are the Gammalinolenic Acid (GLA 18: 3ω6), the Diomogammalinolenic Acid (DGLA 20: 3ω6), and the Arachidonic Acid (AA 20: 4ω6). So far so good, but there are some problems that disturb this apparently flawless mechanism. It has been estimated that the conversion of ALA to EPA is 5-10% in healthy men and conversion to DHA is 2-5%. In women, conversion has been estimated at around 21% and around 9% respectively. In the human being the capacity of precursor maturation is not very marked and there are some phases of life such as adolescence, gestation, breastfeeding and the third age, in which the need for LC-PUFA is increased. In the child a sufficient dose of LC-PUFA allows a correct brain development (DHA can constitute up to 50% of brain tissue and retina). In the absence of this quota, the strong requests for tissue expansion could lead to visual and neuro-psychological problems of varying degrees depending on the level of deficiency. Obviously, even in the fetal and neonatal age, the expansion of the nervous tissue will require a strong dose of LC-PUFA which, in this case, becomes the mother's exclusive burden as the only food route through breast milk or placenta. In the third age it is frequent the impairment of the cognitive functions up to the dementia, and a correct dose of essential long chain fatty acids could reduce this risk and favor the improvement of mental faculties. To exacerbate these conditions of increased need, there are differences on synthetic capacity, which are reflected in different stages of life and in the gender of individuals. For example, the PUFA maturation enzyme system is still inefficient in the fetus and newborn and LC-PUFAs must be absorbed as preformed through breast milk and placenta . There is a phenomenon called "magnification" that creates a gradient across the placenta itself. We have seen that in maternal plasma the concentrations of the precursors are greater than in the placental plasma (therefore of the fetus), while the long chain polyunsaturated fatty acids are found in greater concentration in the placental plasma rather than in the maternal one. This is an elegant system that nature has devised to facilitate the potential deficiencies of the fetus, in a moment of such delicate nervous development. To ease the situation, clinical studies have shown that the ability to synthesize LC-PUFAs is greater in women than in men, supporting the needs of nurses and pregnant women, also through a mechanism in which hormonal levels of estrogen could be implicated ( as demonstrated by the 62% increase in plasma DHA in women using the contraceptive pill). Unfortunately, this leads to a rapid depletion of maternal deposits which is accentuated very much with the succession of pregnancies during the span of life. This implies that these essential fatty acids may need to be taken even in mature form.
In the third age the synthetic abilities are related to the child and therefore it is advisable to have reliable sources of LC-PUFA.
Importance of Omega-3 and Omega-6 in the Vegetarian and Vegan Diet »
