Dr. Francesco Visioli
THE ESSENTIAL FATTY ACIDS
Cell membranes, both plasma that intracellular organelles, are structures with a complex organization, in relation to the development of specialized and coordinated functions, and the establishment of different compartments.
Highly specialized, from the functional point of view, are the membranes of cells of the central and peripheral nervous system, that is, cells of which functions are based on complex processes like activation of ionic channels, receptors, secretion processes and production of seconds messengers.
These membranes require a high degree of fluidity, supported by the presence of structural lipids with a high content of long chain polyunsaturated fatty acids, that is, with more than 20 carbons and at least 4 double bonds.
Often considered only in terms of energy, fatty acids play in reality a number of key roles for the composition, the integrity and the function of the cell membrane.
It follows that a correct amount / quantity of fatty acids is essential for the optimal cell and tissue function.
It is also important to emphasize how cells and different tissues require different proportions of fatty acids, depending on their physiological roles, and that these modulations are possible through appropriate dietary strategies or pharmacological integration.
In fact, despite many fatty acids can be synthesized by mammals (including humans), those denominated essential [for example linoleic acid (18: 2n-6, LA), a-linoleic acid (ALA, 18: 3n- 3) and their elongation and desaturation] products must necessarily be ingested preformed, because the human body lacks an appropriate enzymatic kit.
In particular, the consumption of essential fatty acids of the omega 3 series [eicosapentaenoic acid (EPA, 22: 5n-3 and docosahexaenoic acid (DHA, 22: 6n-3)] is often inadequate in the Western world [1], where the consumption of marine origin foods rich in EFA is not widespread.
The above it demonstrates the indispensability of an adequate consumption of essential fatty acids (EFA) to avoid potential metabolic imbalances resulting in occurrence of diseases.
In summary, ingested fatty acids in the diet or through integration affect human health and contribute in arising EFA deficiency diseases.
OMEGA 3 FATS AND VISUAL FUNCTION
Primates bred on experimental diets deficient in EFA omega 3 (ALA series in particular, eicosapentaenoic acid EPA – 22:5n-3, docosahexaenoic acid DHA – 22:6n-3) show reduced retinal response to light, less visual acuity and reduced duration of the visual exploration time of a new stimulus [2].
Regarding the development of the human visual system, the fundamental role of the long-chain EFA is strengthened by clinical observations made on infants fed with diets devoid of EFA [3] and of populations with congenital metabolic deficiencies regarding EFA [4].
Both groups show large visual deficits that respond to drug treatment with omega 3 fatty acids.
Finally, studies of supplementation with fatty acids of the omega-3 series have provided valuable guidance links between intakes of long-chain EFA and visual function, as measured by behavioral tests, usually conducted at 2 and 4 months [5].
The biochemical basis of the activities of the health benefits omega 3 EFAs are confirmed observing that these fatty acids are especially concentrated in metabolically active neural membranes of the brain and retina [6].
In particular, docosahexaenoic acid is incorporated efficiently and selectively in the outer segments of the photoreceptors (rod outer segments, ROS), where, thanks to its high degree of unsaturation and the length of the carbon chain, intervenes in the membrane dynamic processes [ 7].
There is ample evidence that omega 3 in the diet or through supplements affect the nervous system, altering the physical properties of the membranes, the cellular enzyme activities, the structure and the number of receptors, the transport of mediators affected by carriers and the intra- and intercellular interactions.
For example, DHA plays an important role in the microenvironment of photo pigment in the outer segments of photoreceptors: phospholipids containing DHA is strongly associated with rhodopsin, the transmembrane photo pigment that plays an essential role in photo transduction [8] processes.
In addition, a subgroup of particularly rich in phospholipids DHA is actively incorporates and selectively in the discoidal membranes of ROS during the first 15 days of life; it is likely that these membrane components is actively bind to rhodopsin [9], greatly influencing the development of visual functions.
From a strategic and pharmacological point of view, you have to remember that DHA is the product of elongation and desaturation of the essential fat-linolenic acid.
The enzyme systems required for the biosynthesis of DHA from precursor are active in the first week of life, but it seems unlikely that the amounts of DHA synthesized by infants is sufficient to equalize that of infants receiving DHA preformed (through breast milk or using humanized milks).
Then the DHA is essential for optimal development of the neural system becomes a priority to ensure the diet of the infant an adequate intake of omega-3 long-chain, through both taking breast milk or using adequate amounts of omega EFA 3.
Given that the composition of cell membranes in EFA – including neuronal and retinal – is modifiable with appropriate consumer strategies (see above), the maintenance of the correct visual function in adults is also based on an adequate consumption of EFA omega-3 series, in order to provide the substrates required for the smooth application of the cellular functions and to the visual signal transduction.
MECHANISMS OF ACTION
As mentioned above, ROS contain high concentrations of DHA (about 50-70% of total fatty acids).
It is assumed that the main function performed by the DHA takes place at membrane fluidity level: phospholipids containing DHA show greater steric hindrance – and thus lower degree of caking – compared to those containing saturated fatty acids.
Increasing the proportions of DHA in such phospholipids results in an increase of the membrane fluidity and, probably, to a different degree of light refraction.
Furthermore, it has been shown that the quantities of light required to activate the rhodopsin is lower when the membranes of the rods have been enriched in DHA [8].
Among the other proposed mechanisms of action we should mention the improvement of endothelial function, widely studied in various vascular districts and demonstrated for EFA, which is expressed at the level of the retinal microvasculature.
Finally, there are many cases in which the integration with EHA omega 3 improves ocular disorders consequent to other systemic diseases, such as retinopathy associated with diabetes.
CONCLUSIONS
Essential fatty acids (not synthesized de novo by humans) are involved in the modulation of cellular processes in specialized tissues, such as the central nervous system and ocular apparatus.
It is essential to the external supply of these fatty acids, especially those of omega 3, is possible to use therapeutic strategies to modify the fatty acid composition of the membranes of these tissues.
Particular attention should therefore be paid to the EFA consumption levels, often insufficient in the Western world because their adequate intake can significantly contribute to the prevention of degenerative pathologies of neural tissue and can maintain an adequate cell functioning at ophthalmic level.
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- Neuringer M, Connor WE, Lin DS, Barstad L. Dietary omega-3 fatty acids: effects on retinal lipid composition and function in primates. In: Anderson RE, Hollyfield JG, LaVail MM, eds. Retinal degenerations. New York: CRS Press 1991:117-29.
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- SanGiovanni JP, Berkey CS, Dwyer JT et al. Dietary essential fatty acids, long-chain polyunsaturated fatty acids, and visual resolution acuity in healthy fullterm infants: a systematic review. Early Hum Dev 2000;57:165-88.
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- Mitchell DC, Litman BJ. Docosahexaenoic acid-containing phospholipids optimally promote rhodopsin activation. In: Riemersma RA, Armstrong R, Kelly RW, Wilson R, eds. Fourth International Congress on Essential Fatty Acids and Eicosanoids. Champaign: AOCS Press 1998:154-8.
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