Comparative effects of well-balanced diets enriched in α-linolenic or linoleic acids on LC-PUFA metabolism in rat tissues
Introduction
The health benefits of n-3 long chain polyunsaturated fatty acids (LC-PUFA), particularly eicosapentaenoic acid (20:5 n-3, EPA) and docosahexaenoic acid (22:6 n-3, DHA), have been demonstrated in many studies. For example, EPA and DHA are precursors of eicosanoids and docosanoids involved in anti-inflammatory, anti-thrombotic and anti-aggregating signaling, and can promote vasodilatation [1]. More specifically, some studies recently underlined the involvement of the DHA-derived neuroprotectin D1 in cell survival and brain protection against neurodegenerative disorders such as Alzheimer disease [2], [3]. EPA and DHA can be obtained from the diet, especially via fish and seafood. Since marine food sources are characterized by a limited stock and a declining nutritional quality [4], [5], [6], [7], it seems useful to develop alternative sources to meet dietary n-3 PUFA requirements. As EPA and DHA are biosynthesized from α-linolenic acid (18:3 n-3, ALA), consumption of this essential precursor could be a complementary method to ensure sufficient n-3 PUFA bioavailability. However, numerous studies in humans show that dietary supplementation in ALA generally leads to increased circulating amounts of EPA and docosapentaenoic acid (22:5 n-3, DPA), but not DHA [8], [9]. This observation is consistent with the limited capacity of ALA conversion into DHA in humans, estimated to be lower than 0.05% [10]. Human studies are limited to blood measurements whereas animal studies are useful for assessing the biodistribution of n-3 PUFA in organs and tissues. DHA content in rodent tissues such as the brain or heart can thus be raised after ALA supplementation [11], [12], [13], even if this observation seems to be particularly promoted by moderate ALA supplementation levels [14].
LC-PUFA biosynthesis from ALA involves desaturases, elongases and peroxysomal β-oxidation in the liver (Fig. 1). The conversion of the n-6 linoleic acid (18:2 n-6, LA) to n-6 LC-PUFA shares a common enzymatic pathway and can thus compete with the ALA conversion, particularly for Δ6-desaturation, which is considered a rate-limiting step [15]. Because of known competition between n-6 and n-3 FA in this pathway, n-6 PUFA metabolism must be taken into consideration while studying the conversion from ALA to n-3 LC-PUFA. This concern is epidemiologically relevant because dietary LA intake has sharply risen over the past decades due to the increased consumption of vegetable oils such as soybean oil, resulting in a dietary LA/ALA ratio that now exceeds 10 [16], [17]. This imbalance is associated with the development of cardiovascular, metabolic and neuropsychiatric disorders [18], [19], and arguments in favor of a reduction in LA intake were recently raised [20], [21], but still need more experimental support. Therefore, it is important to get a better understanding of dietary ALA and LA metabolism in order to refine recommended intakes for these FA. In this regard, the liver is the main site of LA and ALA synthesis into long-chain PUFAs and their secretion into the plasma. Organs such as the brain and heart have a limited capacity to synthesize long-chain PUFAs, and therefore obtain them directly from the plasma pool. This would suggest that dietary alterations of LA or ALA, the substrates for liver desaturase and elongase enzymes, might alter the elongation-desaturation capacity of the liver and tissue fatty acid composition.
The present study provided a comprehensive assessment of erythrocyte, liver heart and brain fatty acid status in relation to liver Δ5- and Δ6-desaturase expression and activity, with the aim of examining the dependency of extra-hepatic tissue fatty acid composition on liver enzyme activity. This issue was addressed by comparing the effects of moderate ALA and LA dietary enrichment. Previous studies in rats analyzed the impact of ALA and LA supplementation on lipid tissue composition or hepatic desaturation [12], [14], [22], [23], [24], but the present work provides complementary findings for two reasons. First, we used moderately enriched diets with ALA and LA composition set to values achievable in a human diet for these FA (when expressed in energy %). In the control diet, ALA and LA composition was respectively 0.6% and 2.5% of energy, with a LA/ALA ratio of 4.2/1, which is close to nutritional recommendation determined by the French Agence Nationale de Securite Sanitaire (ANSES) [25], [26] The LA-enriched diet contained 0.6% ALA and 4.8% LA and the ALA-enriched diet 2.2% ALA and 2.5% LA (in % of energy), resulting in a LA/ALA ratio of 8/1 and 1.1/1, respectively. Second, we determined the effect of these diets on (1) the fatty acid composition in erythrocytes, liver, heart and brain, and (2) the expression and activity of the hepatic Δ5- and Δ6-desaturases.
Section snippets
Chemicals
Chemicals were purchased from Sigma-Aldrich (Saint-Quentin Fallavier, France) and solvents used in lipid analysis were from Fisher Scientific (Elancourt, France).
Animals and diets
All protocols complied with the European Union Guideline for animal care and use (2003/35/CEE). Eighteen Sprague-Dawley rats (60 g weight) were purchased from Janvier breeding center (Le Genest Saint Isle, France). They had free access to water and food. They were fed with rodent chow (Special Diet Services, Witham, UK) for one week
Body weight and food intake
After 8 weeks on the experimental diets, the mean body weight gain per animal was 359.3±47.8 g and did not differ significantly amongst the groups. Mean food consumption was 168.9±7.4 g of food/day/kg and was similar with all diets.
Red blood cells FA composition
FA profile in red blood cells (RBC) of control rats was characterized by a high PUFA proportion exceeding 40% of total FA, essentially composed of arachidonic acid (20:4 n-6, 61% of PUFA), consistent with the membrane phospholipid composition of total RBC (Table 2).
Discussion
The present study aimed at describing the effects of moderate dietary enrichment in ALA and LA on LC-PUFA biosynthesis and tissue composition in rats. The Δ6-desaturase and Δ5-desaturase expression were both activated by the ALA diet, but not by the LA diet. In a previous study, diets enriched in linseed oil (containing 61.5% ALA, 11.6% LA) or in sunflower oil (containing 13.9% ALA, 51.7% LA) led to a strong desaturase activation, compared to a pellet diet containing similar level of LA and ALA
Acknowledgments
This study received financial support from Valorex (Combourtillé, France). All authors were involved in the study design. H.B., N.B. and D.C. performed the experiments. The manuscript was written by H.B. and reviewed by P.L., F.P., V.R.. The authors thank F. Boissel, R. Marion and E. Thebault for animal care and technical assistance, and E. Reese and A. Taha for their suggestions on the article. The authors declare no conflict of interest.
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