α-Linolenate reduces the dietary requirement for linoleate in the growing rat,☆☆

https://doi.org/10.1016/j.plefa.2011.08.003Get rights and content

Abstract

Background: We hypothesized that due to the absence of a dietary source of omega-3 fatty acids, the essential fatty acid (EFA) deficiency model leads to an overestimate of linoleic acid (LA) requirements. Methods: over 7 wk, young rats consumed an EFA diet containing either 0 en% linoleate (0LA) and 0 en% α-linolenate (0LNA) or a diet containing 0.5 en% LNA plus one of seven levels of added LA (0.12–4.0 en%; n=6/group).

Results: Rats consuming the 0LA–0LNA diet had the lowest final body weight, 34–68% lower LA and arachidonate in plasma and liver, 87% lower LA in epididymal fat, and an 8–20 fold higher eicosatrienoate in plasma, liver and muscle lipids. 0.5LNA completely prevented the lower growth and partly prevented the rise in eicosatrienoate seen in the 0LA–0LNA group.

Conclusion: Providing dietary LNA at 0.5 en% reduces the rat's physiological requirement for LA by an estimated factor of at least four (0.5 en% instead of 2 en%). Since LA requirements in humans are also based on the same flawed model of EFA deficiency, it is plausible that they too have been overestimated and should therefore be reinvestigated.

Introduction

Polyunsaturated fatty acids (PUFA) are key components of lipids modulating the architecture and function of cellular membranes. They are also endogenous mediators of cell signaling and gene expression, as well as precursors of several cascades of lipid mediators (eicosanoids, docosanoids, resolvins, lipoxins) [1], [2], [3], [4], [5]. The two parent PUFA—linoleate [LA; 18:2(n−6)] and α-linolenate [LNA; 18:3(n−3)]—are vitamin-like molecules needed in the diet because markedly reduced levels of either of these two fatty acids lead to well-documented deficiency symptoms (reviewed in 2 and 6). In both the rat and human, recommended adequate intakes of PUFA that prevent biochemical and physiological symptoms of deficiency and optimize tissue PUFA content correspond to approximately 2 energy (en) % of the diet for LA and 0.5 en% for LNA [6], [7], [8], [9].

Despite long acceptance of these recommended intakes for both LA and LNA, several studies suggest that the diet used to induce LA deficiency and calculate the physiological requirement for LA has been consistently flawed, thereby potentially leading to significant overestimation of the dietary requirement for LA [10], [11]. The flaw is that LA requirements have been based on correcting or preventing essential fatty acid (EFA) deficiency, i.e. a diet simultaneously deficient in both (n−6) and (n−3) PUFA. Since n−3 and n−6 PUFA are distinctly different families of nutrients with different biological functions, the absence of (n−3) PUFA in an EFA deficient diet means that EFA deficiency is not equivalent to (n−6) PUFA deficiency [6]. This is more than a theoretical objection; in the presence of 0.5 en% LNA, reproduction, early development and growth in the rat appear to be entirely normal at LA intakes as low as 0.3 en% [12], [13]. Indeed, the presence of 0.5 en% LNA in the diet prevents the growth retardation seen in classical EFA deficiency [14], [15]. Hence, the presence of 0.5 en% LNA appears to markedly reduce the symptoms of EFA deficiency, thereby apparently reducing the dietary requirement for LA to between 0.3 and 0.5 en% [12], [13] instead of the officially recommended 2 en%.

The dietary absence of (n−3) PUFA is also a problem in the few studies done to estimate dietary LA requirements in healthy humans [16], [17], [18], [19]. One adult study of experimental LA deficiency did include (n−3) PUFA in the diet but was confounded because the participants were all on enteral or parenteral nutrition for gastrointestinal disease [20]. Hence, not only has the dietary requirement for LA in healthy humans and rats always been assessed under conditions of concurrent dietary deficiency of (n−3) PUFA but, in the rat, there is also preliminary evidence that a low intake of LNA actually reduces the physiological requirement for LA; i.e. for normal growth, reproduction and development, to well below 2 en% (reviewed in [10]).

The main objective of this study was therefore to do a dose–response study in a rat model to determine whether a nutritionally adequate intake of LNA (0.5 en%) positively or negatively affects growth or other markers of LA deficiency, i.e. tissue fatty acid profiles and whole body LA balance. Using growth as the principle physiological parameter and tissue fatty acid profiles as the biochemical parameter, classical EFA deficiency, i.e. combined deficiency of (n−6) and (n−3) PUFA, was compared to specific deficiency of only (n−6) PUFA. Specific (n−6) PUFA deficiency was achieved using a diet containing no added (n−6) PUFA but an adequate intake of LNA (0.5 en%). The effect on growth and fatty acid profiles of diets containing 0.5% LNA and increasing amounts of LA was also investigated. Fatty acids in adipose tissue were measured as a marker of dietary LA and LNA intake [21].

As a complement to growth and the fatty acid profiles, whole-body fatty acid balance analysis [15] was performed to assess the effects of LNA and increasing amounts of LA on the rat's overall capacity to elongate and desaturate LA and LNA to their respective long chain PUFA, their accumulation in tissues, or their disappearance due to β-oxidation. At deficient LA intakes, its β-oxidation increases markedly [15], so we hypothesized that the dietary LA intake at which LA disappearance (net β-oxidation) was lowest would represent its most efficient utilization. Analogous to studies with indispensable amino acids [22], this would be equivalent to LA requirement for optimal growth.

Section snippets

Materials and methods

Animals and diets. This study was carried out in accordance with the European Community Council Directive of 24 November 1986 (86/609/EEC). Weanling (21 d old) male Wistar rats were reared in an air-conditioned room (20 °C) illuminated for 12 h (0700–1900 h). During the initial 2 wk acclimatization period, they all consumed a semi-purified diet containing 11 en% fat, and 2 en% LA and 0.5 en% LNA ([23], [24]; Table 1). Since one group continued to receive these LA and LNA levels, it was considered as

Results

Food intake, growth and skin condition. There was no significant difference in total food intake over the 7 wk study period among all the groups (mean values ranging from 991 to 1007 g/group). Fig. 1 depicts the growth curves of the two 0LA groups during the study period and also the mean growth curve (with 95% confidence interval) of all the groups receiving additional LA (0.1–4.0 LA). The growth data for the LA-supplemented groups were pooled since no significant difference by analysis of

Discussion

Our principle observation is that symptoms of classical EFA deficiency, i.e. reduction in growth and in changes in tissue ETA, ETA/AA and (n−6)DPA/DHA, were greater in the absence than in the presence of 0.5 en% LNA. Hence, as previously observed in more preliminary form [12], [13], [14], [15], a low intake of LNA protects against symptoms and fatty acids changes associated with LA deficiency in the rat. To our knowledge, this is the first dose–response study in a rat model to determine whether

Conflicts of interest statement

All authors disclose no conflicts of interest.

Acknowledgements

We thank P. Dahirel and C. Maudet for animal care and A. Linard for technical assistance. This work was partly presented at the 2006 International Society for the Study of Fatty Acids and Lipids (ISSFAL) 23–27 July, 2006, Cairns, Australia.

References (30)

  • P. Guesnet et al.

    Blood lipid concentrations of docosahexaenoic and arachidonic acids at birth determine their relative postnatal changes in term infants fed breast milk or formula

    Am. J. Clin. Nutr.

    (1999)
  • H. Mohrhauer et al.

    Effect of linolenic acid upon the metabolism of linoleic acid

    J. Nutr

    (1963)
  • A.A. Spector

    Essentiality of fatty acids

    Lipids

    (1999)
  • J.M. Alessandri

    Polyunsaturated fatty acids in the central nervous system: Evolution of concepts and nutritional implications throughout life

    Reprod. Nutr. Dev

    (2004)
  • R.S. Chapkin et al.

    Bioactive dietary long-chain fatty acids: Emerging mechanisms of action

    Br. J. Nutr.

    (2008)
  • Cited by (33)

    • Lipids for infant formulas

      2019, Cahiers de Nutrition et de Dietetique
    • Very low inadequate dietary intakes of essential n-3 polyunsaturated fatty acids (PUFA) in pregnant and lactating French women: The INCA2 survey

      2019, Prostaglandins Leukotrienes and Essential Fatty Acids
      Citation Excerpt :

      Indeed, the mean ALA intake was 0.4% EIEA, and close to 100% of the women population do not reach the RDI and adequate intake values for this PUFA (1% and 0.8% EIEA, respectively). Moreover, about 75% (pregnant) and almost 100% (lactating) could be considered at risk of n-3 PUFA deficiency since they ingest less than the minimal intake of 0.5% EIEA for optimal tissue levels of n-3 PUFA and to prevent biochemical and physiological symptoms of deficiency in the brain and retina during their active development [17,18]. Such low ALA consumption has been found again in an incomplete study conducted in 2001 on 61 pregnant women living in the South-West of France (0.4% energy) [29], but also in a very recent large prospective study conducted on 250 lactating women in the Metropolitan France (0.3% energy) [41].

    • Musings about the role dietary fats after 40 years of fatty acid research

      2018, Prostaglandins Leukotrienes and Essential Fatty Acids
      Citation Excerpt :

      It is clear that provided a minimal amount of ALA is present, the amount of DHA accumulated in plasma is regulated by the level of LA in the diet (Fig. 1). Similar results are clear from the work of Guesnet ([11], Fig. 2). Omega 3 fats as a component of a healthy diet have been extensively studied and there are now recommended intakes from a number of organisations.

    View all citing articles on Scopus

    Supported by l'Institut National de la recherche Agronomique (INRA) and the Natural Sciences and Engineering Research Council of Canada (SCC).

    ☆☆

    Contributions: PG and SCC jointly designed this project and wrote the paper. PG, MSL, JMA and MJ conducted the research and analyzed the data. PG and SCC had primary responsibility for final content. All authors read and approved the final manuscript.

    1

    Present address: LZ, 19 rue Montbauron, F-78000 Versailles, France.

    View full text