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Les isomeres conjugues de l'acide linoleique (conjugated linoleic acids -CLA) sont des acides gras qui ont des proprietes biologiques potentielles interessantes pour la sante publique. Il convenait donc de completer les tables de composition des aliments afin de determiner les apports en CLA par l'alimentation et d'evaluer les taux de CLA ingeres par la population. Une etude de la composition en CLA des aliments les plus consommes par les Francais a donc ete entreprise sous l'egide du ministere de la Recherche (projet « AQS 2000 »). Dans le cadre de cette etude, nous avons analyses 54 beurres francais de fabrications locales sur 7 regions, a differentes saisons de l'annee. Les beurres analyses contiennent en moyenne 0,45g CLA/ 100 g beurre en hiver, 0,58 au printemps, et 0,80 en ete. Des variations dans les teneurs en CLA ont egalement ete constatees entre les differentes regions productrices de beurre. Les beurres issus de laits de regions de collines et de montagnes, et les beurres de Normandie, presentent les taux moyens annuels de CLA les plus eleves, et les variations de plus grandes amplitudes d'une saison a l'autre. Dans cette etude, nous avons egalement analyse la composition des beurres pour plusieurs acides gras, en observant les fluctuations saisonnieres et regionales. Les beurres d'ete contiennent plus d'acides gras insatures a longues chaines et moins d'acides gras satures a chaines moyennes que les beurres d'hiver. Les beurres de regions d'altitude et de Normandie presentent de plus grands ecarts saisonniers que le beurres de pays de plaines. Ces differences pourraient etre principalement imputables aux teneurs en acides gras polinsatures, et notamment en acide linoleique, de l'alimentation des bovins.

The metabolic fate of rumenic acid (9cis,11trans-octadecenoic acid) related to its position on the glycerol moiety has not yet been studied. In the present work, synthetic triacylglycerols (TAG) esterified with oleic and rumenic acids were prepared. Rats were force-fed synthetic dioleyl monorumenyl glycerol with (14)C labeled rumenic acid in the internal (sn-2) or in the external position (sn-1 or sn-3). Rats were then placed in metabolic cages for 16 h. At the end of the experiment, the radioactivity in tissues, carcass and expired CO(2) was measured. Rumenic acid that was esterified at the external positions on the TAG was better absorbed and oxidized to a greater extent than when esterified at the internal position. The fatty acid from the 2-TAG form was also better incorporated into the rat carcass. In the liver, rumenic acid appeared mainly in TAG (50%) and to a lesser extent in phospholipids (33%) whatever its dietary form. Moreover, analyses of lipids from Camembert cheese and butter revealed that rumenic acid was located mainly on the sn-1 or sn-3 positions (74%). Taken together, these data suggest that rumenic acid from dairy fat may be well absorbed and used extensively for energy production.

Conjugated linoleic acids (CLA) are fatty acids primarily found in dairy products. These fatty acids may have potential biological properties interesting for human health. Update of French food composition data banks was needed to determine CLA levels in French dairy products and to evaluate CLA intake by the French population. As part of this investigation, we analyzed 54 butters from local producers at different times of the year. The average CLA levels in butter were 0.45 g CLA/100 g butter in winter, 0.58 in spring, and 0.80 in summer. Regional variations of CLA levels in butters were observed from different producing areas in France. Butters from hill and mountain regions, and butters from Normandy, a prairie land, showed the highest annual average CLA levels and the largest magnitudes of variations from winter to summer. During the study, we also analyzed the total fatty acid composition of the butters, including the seasonal and regional variations. Butters produced from summer milks contained more unsaturated long chain fatty acids and less saturated middle chain fatty acids than the winter butters. Butters from mountain areas and from Normandy (prairie) showed larger differences from winter to summer than those from other prairie lands. These differences may be due to the polyunsaturated fatty acid levels, and especially to the linoleic acid levels, in the cow diets.

Deficiency in n-3 fatty acids is known to disturb the release of dopaminergic neurotransmitters in rat brain. Since isomerization reduces the bioavailability of dietary fatty acids, the effect of the conversion of alpha-linolenic acid into trans alpha-linolenic acid on the dopaminergic neurotransmission was studied. Rats were fed for 21 months with a control diet, a diet unbalanced in cis alpha-linolenic acid and containing trans alpha-linolenic acid or the same diet in which the imbalance was corrected by increasing the levels of cis alpha-linolenic acid. After 6 and 21 months of diet, the fatty acid composition and the amounts of endogenous dopaminergic neurotransmitters was assessed in the striatum, the frontal cortex and the hippocampus. The isomerization of a part of dietary alpha-linolenic acid induced some modifications of the levels of endogenous dopaminergic neurotransmitters in all brain areas but was related to a very low incorporation of trans polyunsaturated fatty acids. Increasing the dietary levels of cis alpha-linolenic acid succeeded in correcting the endogenous neurotransmitter concentrations only in the frontal cortex but not in the striatum and the hippocampus. Thus, the levels of dopamine were lowered by 95% in the hippocampus. These results suggest that in addition to the imbalance generated by their presence, trans fatty acids may directly act on the concentration of dopaminergic neurotransmitters.