Polyunsaturated fatty acids normal plasma

Approximately 60% of the dry weight of the brain is fat, a considerable proportion of which is polyunsaturated fatty acids that are present in plasma membranes. It would not be surprising if replacement of the unsaturated acids by the saturated fatly acids in membrane structure due to a dietary deficiency of polyunsaturated fatty acids played some part in development of mental illness. Indeed, it has been found that supplementation of a normal diet with polyunsaturated fatly acids can improve some mental disorder (see chapter 11). 

In the 1950s and 1960s, worldwide studies concluded that linoleate-rich dietary oils, such as corn oil and cottonseed oil, lowered the cholesterol level of human plasma, and therefore were advocated as preferred sources of polyunsaturated fatty acids. Hence, in the food industry, polyunsaturated began to mean two double bonds per molecule. More than two double bonds per molecule was associated with rapid unwanted rancidity. In the effort to minimize plasma cholesterol, food oils for humans became richer in linoleic acid (18 2o)6) and lower in linolenic acid (18 3o)3), but were also becoming deficient in essential (o3 polyunsaturates normally found in high levels in brain and nerve lipids. 

The measurements of the total amount of various essential fatty acids as co-3 fatty acids in plasma, serum, or erythrocyte membrane phospholipids have been indicated as useful markers of essential polyunsaturated fatty adds. Essential fatty acid deficiency is a clinical condition that derives from inadequate status of co-3 and co-6 fatty acids however, the symptoms are nonspecific and may not present prior to marginal essential fatty acid status. Widely used biomarkers for bicx hemi( essai-tial fatty acid deficiency are mead acid and the triene/tetraene ratio. Howcvct, the total plasma triene/tetraene ratio has been considered the gold standard for essential fatty acid deficiency. Mead acid, or 5,8,11-eicosatrienoic acid (5,8,11-20 3 co-9) is synthesized from endogenous oleic acid and is increased when there is insufficient concentrations of linoleic and a-linolenic acid to meet the needs of polyunsaturated fatty acids. Under normal conditions only trace amounts of mead acid are found in plasma. EPA and DHA inhibit mead acid synthesis. Mead acid measurement is an indicator of essential fatty acid deficiency state, while essential fatty add depletion is associated with a decrease in plasma hnoleate and arachidonate percentages. Assessment of long-term essential fatty acid intake is measured in adipose tissue, and it is considered the best indicator because of its slow tumover. - Cutoff values for the assessment of essential fatty adds and to-3 fatty acid status in erythrocytes have been reported. Proposed cutoff values for children older than 0.2 years are 0.46 mol% 20 3 co-9 (mead acid) for early suspicion of essential fatty acid defidency, 0.068 mol/mol docosapentaenoic/arachidonic acid... 

Figure 5.4 Polyunsaturated fatty acid profiles In plasma phospholipids of children with cystic fibrosis compared with those in normal children = difference not significant = p<0.05 g = p<0.01 B = p< 0.001. Reproduced with kind permission of Professor R.T. Holman and the American Oil Chemists Society, from Dietary Fats and Health, (eds E.G. Perkins and W.J. Visek), p. 255, Figure 5, 1983.

In section 3.4 we described the biosynthesis of eicosanoids from polyunsaturated fatty acids of the n-3 and n-6 families and indicated in section 3.4.8 that the balance of eicosanoids produced was important in, for example, maintaining normal vascular function. Several studies have demonstrated that altering the amounts and types of n-6 and n-3 fatty acids in the diet can influence the spectrum of eicosanoids produced. For example, substitution of fish oils in which n-3 polyunsaturated fatty acids predominate for diets in which linoleic acid (n-6) is the main polyunsaturated fatty acid (as typified by the UK diet) results in changes in plasma and platelet fatty acid profiles from arachidonic to eicoasapentaenoic acid as the predominant polyunsaturated fatty acid and a reduction in the formation by platelets of thromboxane A2, an eicosanoid that stimulates their aggregation (Table 5.7). 

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