Roles of unsaturated fatty acids (especially omega-3 fatty acids) in the brain at various ages and during ageing.
Among various organs, in the brain, the fatty acids most extensively studied are omega-3 fatty acids. Alpha-linolenic acid (18:3omega3) deficiency alters the structure and function of membranes and induces minor cerebral dysfunctions, as demonstrated in animal models and subsequently in human infants. Even though the brain is materially an organ like any other, that is to say elaborated from substances present in the diet (sometimes exclusively), for long it was not accepted that food can have an influence on brain structure, and thus on its function. Lipids, and especially omega-3 fatty acids, provided the first coherent experimental demonstration of the effect of diet (nutrients) on the structure and function of the brain. In fact the brain, after adipose tissue, is the organ richest in lipids, whose only role is to participate in membrane structure. First it was shown that the differentiation and functioning of cultured brain cells requires not only alpha-linolenic acid (the major component of the omega-3, omega3 family), but also the very long omega-3 and omega-6 carbon chains (1). It was then demonstrated that alpha-linolenic acid deficiency alters the course of brain development, perturbs the composition and physicochemical properties of brain cell membranes, neurones, oligodendrocytes, and astrocytes (2). This leads to physicochemical modifications, induces biochemical and physiological perturbations, and results in neurosensory and behavioural upset (3). Consequently, the nature of polyunsaturated fatty acids (in particular omega-3) present in formula milks for infants (premature and term) conditions the visual and cerebral abilities, including intellectual. Moreover, dietary omega-3 fatty acids are certainly involved in the prevention of some aspects of cardiovascular disease (including at the level of cerebral vascularization), and in some neuropsychiatric disorders, particularly depression, as well as in dementia, notably Alzheimer's disease. Recent results have shown that dietary alpha-linolenic acid deficiency induces more marked abnormalities in certain cerebral structures than in others, as the frontal cortex and pituitary gland are more severely affected. These selective lesions are accompanied by behavioural disorders more particularly affecting certain tests (habituation, adaptation to new situations). Biochemical and behavioural abnormalities are partially reversed by a dietary phospholipid supplement, especially omega-3-rich egg yolk extracts or pig brain. A dose-effect study showed that animal phospholipids are more effective than plant phospholipids to reverse the consequences of alpha-linolenic acid deficiency, partly because they provide very long preformed chains. Alpha-linolenic acid deficiency decreases the perception of pleasure, by slightly altering the efficacy of sensory organs and by affecting certain cerebral structures. Age-related impairment of hearing, vision and smell is due to both decreased efficacy of the parts of the brain concerned and disorders of sensory receptors, particularly of the inner ear or retina. For example, a given level of perception of a sweet taste requires a larger quantity of sugar in subjects with alpha-linolenic acid deficiency. In view of occidental eating habits, as omega-6 fatty acid deficiency has never been observed, its impact on the brain has not been studied. In contrast, omega-9 fatty acid deficiency, specifically oleic acid deficiency, induces a reduction of this fatty acid in many tissues, except the brain (but the sciatic nerve is affected). This fatty acid is therefore not synthesized in sufficient quantities, at least during pregnancy-lactation, implying a need for dietary intake. It must be remembered that organization of the neurons is almost complete several weeks before birth, and that these neurons remain for the subject's life time. Consequently, any disturbance of these neurons, an alteration of their connections, and impaired turnover of their constituents at any stage of life, will tend to accelerate ageing. The enzymatic activities of sytivities of synthesis of long-chain polyunsaturated fatty acids from linoleic and alpha-linolenic acids are very limited in the brain: this organ therefore depends on an exogenous supply. Consequently, fatty acids that are essential for the brain are arachidonic acid and cervonic acid, derived from the diet, unless they are synthesized by the liver from linoleic acid and alpha-linolenic acid. The age-related reduction of hepatic desaturase activities (which participate in the synthesis of long chains, together with elongases) can impair turnover of cerebral membranes. In many structures, especially in the frontal cortex, a reduction of cervonic and arachidonic acids is observed during ageing, predominantly associated with a reduction of phosphatidylethanolamines (mainly in the form of plasmalogens). Peroxisomal oxidation of polyunsaturated fatty acids decreases in the brain during ageing, participating in decreased turnover of membrane fatty acids, which are also less effectively protected against peroxidation by free radicals.