Scientific data concerning olive oil, monounsaturated fatty acids, anti-oxidant and low-density lipoprotein oxidation

Authors: Euro sciences communication in collaboration with the Atherosclerosis Research Institute, Munster University, Germany.

Introduction

Low density lipoproteins are the particles carrying the most part of cholesterol in the plasma. It is widely accepted that the increase of the low-density lipoproteins levels plays an aetiological role for the arteriosclerosis and the cardio-coronary diseases. These lipoproteins, originally harmless, are thought to be very dangerous inside the artery wall, if altered by an oxidation process. As for the lipoproteins, the possibility of low density oxidation depends both on internal factors (endogenous) and on external factors (exogenous). As for the latter, the nourishing elements are quite important over all for the various types of fatty acids and the antioxidant vitamins belonging to food. This document analyses again the low-density lipoprotein oxidation process and the nourishing elements as protagonists in the prevention of this process.

Low-density lipoprotein oxidation (atherosclerosis)

Half the cholesterol present in the blood is driven by the low-density proteins, round particles made of fats and proteins and composed by an external layer containing the so-called Apo-lipoprotein (ApoB), covering a central nucleus containing triglycerides and cholesterol( non polar fats). A particle of these lipoproteins contains about 3600 fatty acids, half of which is polyunsaturated. The low-density proteins contain also anti-oxidants; the most important is the alpha-tocopherol ( vitamin-E) the oxidation of low-density lipoproteins( per oxidation) is a chain-reaction created by free radicals over all reacting when in contact to oxygen. The polyunsaturated fatty acids are particularly exposed to the peroxidation of the lipids and to a sharing into a series of products bound to the ApoB of the low-density proteins. These last can be oxidised in the lab through the contact with the macrophages( deriving from big cells called monocytes), with smooth and endothelial muscle cells or with metal ions ( copper or iron). The low-density protein oxidation process is not known, yet and it is thought it can be inhibited by the presence of anti-oxidants, such as the ascorbic acid ( vitamin C). This oxidation is more likely to happen in the artery wall and not in the blood circle. The low density proteins containing vitamin E, instead, do not undergo this oxidation process, probably happening when the anti-oxidant” defences” are scarce and over all when there is lack of alpha- tocopherol.

The low-density lipoprotein oxidation and the atherosclerosis

The atherosclerosis process essentially starts when the lipoproteins enter the artery wall and are entrapped inside, where they have oxidative changes. The macrophages( cells forming when the monocytes, coming from the circle, penetrate the artery wall) absorb rapidly these already modified low-density proteins causing their transformation into foaming cells. The deposit of these foaming cells helps the formation of lipid streams. These do not close the artery up, but they are progressively turned into fibre plaques ( through a process similar to the scar formation) that, on their turn, gradually transform into atherosclerotic lesions.

The olive oil and the low-density oxidation

Food fatty acids can affect the low density lipoprotein oxidation in several ways. The quantity and the composition of saturated fats contribute to determine the quantity of these lipoproteins inside the artery wall. It was evidenced that the substitution of saturated fats with monounsaturated or polyunsaturated reduces the lipoproteins levels to low-density, lowering the number of these particles in the artery walls and consequently the quantity of the lipoproteins ready for oxidation. Thanks to its high content of monounsaturated fatty acids, the olive oil prevents from the oxidation process( see the section entitled ‘Effects of fatty acids on the low density lipoprotein oxidation’) and through strong anti-oxidants as the vitamin E and the phenol compounds, offers the low-density lipoproteins a further and deeper shelter.

The effects of food fatty acids on the low-lipoprotein oxidation

The real link between the use of monounsaturated / polyunsaturated fatty acids and the reduction of the low-density lipoproteins has been widely analysed. Experimental studies on rabbits show that the low-density lipoproteins rich in oleic acid( the key- fatty acids of the olive oil) are really resistant to oxidation. These data are confirmed also by studies on diets: it has been demonstrated that there is a strict link between the content of the low density proteins with linoleic acid( the main polyunsaturated fatty acids present into vegetal oils) and the oxidation process. This process is particularly relevant in a diet rich in polyunsaturated fatty acids. Scientists have tried to understand if such effect was due to the catalyzing action of the polyunsaturated fatty acids or it was due to the inhibiting action that the monounsaturated fatty acids have on the oxidation process of the low density lipoproteins. When a diet is enriched with olive oil , the content of linolenic acid of the low density lipoproteins lowers, a reduction of phagocitosis of the macrophages and a lower susceptibility to low density lipoproteins oxidation are recorded.

The pro oxidant activity of food fatty acids

Some food fatty acids can modify the composition of the cell membrane of the monocytes, increasing the production of free radicals and causing pro-oxidant effects. A study compared the effects of a diet rich in monounsaturated and polyunsaturated fatty acids n 3( present in fish oil) or n-6( linoleic acid) on the production of super oxide anion( a free radical) in the monocytes and in the macrophages. Only the n-3 fats have determined a reduction in the production of free radicals, while the monocytes, with the introduction of n-6 monounsaturated/polyunsaturated fatty acids have presented no meaningful change and no increase. The mechanisms of this phenomenon are not known, and these results have never been reproduced. New researches will be necessary to point out the role of the different fatty acids in the pro- oxidant cellular activity; however the cells with a major quantity of monounsaturated fatty acids are less susceptible to the oxidative damage( compared to n-6 polyunsaturated fatty acids) probably as a result of the composition of the cellular membrane of such substances.

The antioxidants and the olive oil

The oxidative stress can cause several chronic diseases as the cardio-coronary diseases and cancer. The possibility that some food antioxidants, as the ones of the olive oil, can have a protective action versus the low-density lipoprotein oxidation, gave birth to a series of epidemiologic and medical studies.

Vitamin E (alphatocopherol)

Some epidemiologic studies gave evidence that high quantities of vitamin E, assumed in a period of two years at least, strongly reduce the risk of cardio coronary diseases (31-65%); these have not been confirmed by other studies, so far. The same conclusions resulted from randomized analysis providing the assumption of vitamin E. These data had not originally been planned to check cardiologic data: they were too short and the advised quantities of vitamin E were sub optimal. Now some studies are being made, which could drive us towards a final conclusion. Cambridge Hearth Antioxidant Study (CHAOS) is the only being finished so far. The results of this double blind research, controlled with a placebo, among 2000 patients with a hearth disease, have shown that the assumption of high quantities of vitamin E can strongly reduce non-final hearth-attacks but it has no influence on the global death-rate. The studies on medical interventions have been harshly criticized because a few-year treatment is supposed not to be sufficient to show the positive effects of the anti-oxidants: 20-year- long assumption could be necessary before observing positive clinic effects . It was also demonstrated that when vitamin E is added, an alpha-tocopherol increase is recorded both in the plasma and in the low density proteins. In fact there is a direct relation between the resistance of these particles and the quantity of assumed vitamin. The resistance to oxidation is higher even in individuals not assuming supplementary quantities of vitamin E, having higher tocopherol plasmatic levels.

The phenolic components

Simple phenols and phenol acids, as for example the flavonoids, can eliminate or clear up the free radicals of the low-density proteins more resistant to oxidation and to inhibit the lipid peroxidation. The phenol compounds have also an anti inflammatory and anti-haemorrhagic effect. The benefits on health of these phenol flavonoids have been recorded during the “ Seven Countries” and the “ Zutphe Elderly” studies; in fact it was observed that the medium assumption of flavonoids is not directly connected with cardio diseases. Further studies are necessary to confirm the cardio-protective qualities of such substances.

Summary and conclusions

It has been widely proved that the oxidative modifications of the low-density proteins play a key-role in the atherogenesis. The oxidation process of such particles starts with the per-oxidation of polyunsaturated fatty acids inside them. This means that the low-density lipoprotein oxidation essentially depends on their composition in fatty acids. This composition, determining the oxidation susceptibility can be influenced by food fatty acids: monounsaturated fatty acid-enriched diets, can make these lipoproteins more resistant to the oxidative modifications compared to polyunsaturated fatty acid-diets, among which the linoleic acid. The diet influences also the composition of fatty acids into cellular membranes: a monounsaturated fatty acid- enriched diet favours a high content of these substances in the cellular membranes, giving these same cells a higher resistance against the oxidative damage. A further protection against the oxidative effects is determined by some food anti-oxidants as the vitamins C and E, the flavonoids etc. Recent studies in vitro have established that not only the alpha-tocopherol, but also the phenol compounds can inhibit the low-density lipoproteins and consequently reduce the risk of atherosclerosis. Further studies will be necessary to understand fully the action of the phenolic compounds in vivo. The most studies about the Mediterranean diet mainly examined the positive effects on the cardiovascular level associated to a reduced assumption of saturated fats, and a high level of monounsaturated fatty acids, but even complex carbohydrates and fibres. The actual data suggest that a protective effect on the cardio coronary disease( probably cancer and other diseases) could be obtained even by other substances present in the Mediterranean diet, as for example the anti-oxidants present not only in fruit and vegetable but also in the olive oil. The high assumption of monounsaturated fatty acids through the olive oil consumption, can associate the advantages deriving from the reduction of cholesterol with the low-density lipoprotein lowering and the inhibition of the cellular oxidation.