Linoleic acid (LA, C18:2 (n-6)) is an essential fatty acid that is required in human nutrition for the production of eicosanoid hormones that regulate inflammation, immunity and other important cellular functions. Linoleic acid deficiency is rare because the typical Western diet contains many foods that are rich sources of LA, which following ingestion is incorporated into cell membranes. In fact, the intakes of LA in Western populations are generally too high. Linoleic acid like all polyunsaturated fatty acids is prone to oxidation because of the presence of multiple double bonds within its structure. The high levels in the typical Western diet can therefore increase the susceptibility of the cell membranes to undergo lipid peroxidation. If this peroxidation occurs in the low density lipoproteins (LDL) fraction of phospholipid membranes then it is thought that this could produce a pro-arthrogenic state within the cardiovascular system.
The intake of various fatty acids, the susceptibility of lipoproteins to oxidation and plasma antioxidant status have been measured in healthy male subjects in order to establish any associations1. Subjects with the highest intakes of LA had higher proportion of LA and α-tocopherol in LDL particles and also higher intakes of α-tocopherol and selenium, compared to those subjects with the lowest intakes of LA. The main intakes of LA for the Finnish subjects was vegetable oil and margarine, and subjects also reported cooking with the vegetable oils, which suggests that the LA was deodorised, hydrogenated and chemically altered. Using two in vitro models to determine oxidation velocity showed that LA was the first and second strongest determinant of LDL and very low density lipoprotein (VLDL) oxidation. However, monounsaturated fatty acid intake was not associated with oxidation of LDL or VLDL using these assays.
The dense LDL particles are more susceptible to lipid peroxidation than the less dense high density lipoproteins (HDL). Research shows that increasing the LA content of LDL increases the susceptibility to oxidation. This is not surprising because it is known that increasing the amount of double bonds in the structure of a fatty acid increases the risk of oxidation. The intake of LA acid in this study was 11g/d, which was equal to 4.6% of total energy, which compares to ≈6% in the United States. Research suggest that intakes above ≈7% of energy are not recommended because of increased susceptibility to cause oxidation. What is also apparent from this research and others is that many studies do not differentiate between cold pressed unprocessed virgin LA and deodorised, processed and hydrogenated LA. Therefore care should always be taken when interpreting these results.
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