Docosahexaenoic acid (DHA, 22:6n-3) and arachidonic acid (AA, 20:4n-6) are the major polyunsaturated fatty acids in the membranes of brain and retinal cells. Animals specifically deficient in dietary n-3 fatty acids have low DHA content in their membranes, reduced visual acuity and impaired learning ability. Studies on bottle-fed human infants have shown that adding DHA and AA to milk replacer-formulas can bring their concentrations in the infant blood lipids to values as high as those produced by breast-feeding and significantly improves mental development and maturation of visual function. In older subjects, diverse neuropsychiatric and neurodegenerative diseases have been associated to decreased blood levels of n-3 PUFA. Low intakes of fish or of n-3 PUFA in populations have been associated with increased risks of depression and Alzheimer disease, and n-3 PUFA, especially eicosapentaenoic acid (EPA, 20:5n-3), have shown efficacy as adjunctive treatment-and in some cases as the only treatment-in several psychiatric disorders. The mechanisms by which polyunsaturated fatty acids have an impact on neuronal functions will be reviewed: the modulation of membrane biophysical properties, regulation of neurotransmitter release, synthesis of biologically active oxygenated derivatives, and nuclear receptor-mediated transcription of genes responsive to fatty acids or to their derivatives.

The intake of individual n-6 and n-3 PUFA has been estimated in 4,884 adult subjects (2,099 men and 2,785 women), volunteers from the French SU.VI.MAX intervention trial. The food intakes of each subject were recorded in at least ten 24-h record questionnaires completed over a period of 2.5 yr, allowing the estimation of the daily intake of energy; total fat; and linoleic, alpha-linolenic, arachidonic, eicosapentaenoic (EPA), n-3 docosapentaenoic (DPA), and docosahexaenoic (DHA) acids. The mean total fat intake corresponded to 94.1 g/d (36.3% of total energy intake) in men and 73.4 g/d (38.1% of energy) in women. The intake of linoleic acid was 10.6 g/d in men and 8.1 g/d in women, representing 4.2% of energy intake; that of alpha-linolenic acid was 0.94 g/d in men and 0.74 g/d in women, representing 0.37% of energy intake, with a mean linoleic/alpha-linolenic acid ratio of 11.3. The mean intakes of long-chain PUFA were: arachidonic acid, 204 mg/d in men and 152 mg/d in women; EPA, 150 mg/d in men and 118 mg/d in women; DPA, 75 mg/d in men and 56 mg/d in women; DHA, 273 mg/d in men and 226 mg/d in women; long-chain n-3 PUFA, 497 mg/d in men and 400 mg/d in women. Ninety-five percent of the sample consumed less than 0.5% of energy as alpha-linolenic acid, which is well below the current French recommendation for adults (0.8% of energy). In contrast, the mean intakes of long-chain n-6 and n-3 PUFA appear fairly high and fit the current French recommendations (total long-chain PUFA: 500 mg/d in men and 400 mg/d in women; DHA: 120 mg/d in men and 100 mg/d in women). The intakes of alpha-linolenic acid, and to a lesser extent of linoleic acid, were highly correlated with that of lipids. Whereas the main source of linoleic acid was vegetable oils, all food types contributed to alpha-linolenic acid intake, the main ones being animal products (meat, poultry, and dairy products). The main source of EPA and DHA (and of total long-chain n-3 PUFA) was fish and seafood, but the major source of DPA was meat, poultry, and eggs. Fish and seafood consumption showed very large interindividual variations, the low consumers being at risk of insufficient n-3 PUFA intake.

Long-chain n -3 PUFA from fish are possible promising nutrients for the dietary prevention of PCa, but to-date with little epidemiological support. In contrast, studies suggest that alpha -linolenic acid intake might be a risk factor. New work, both epidemiological and experimental, is awaited to clarify these results.