The effect of linoleic acid on the whole body synthesis rates of polyunsaturated fatty acids from α-linolenic acid and linoleic acid in free-living rats.

Key Findings

Previous work performed in rats, from this laboratory found that a diet containing ALA the main precursor to DHA in human diets, was sufficient to maintain brain DHA concentrations. Rats consuming this ALA diet, synthesized DHA at a rate that was at least threefold higher than their brain DHA uptake and accretion rates, suggesting that DHA synthesis from ALA was sufficient to maintain brain DHA in these rats. The goal of this study was to feed rats’ diets with different amounts of linoleic acid (LA) and observe the effect of these diets on DHA and ARA synthesis measured in vivo. The results showed that at a LA concentration of 1.5% of the fatty acids had lower DHA synthesis–secretion rates than rats fed 53% LA. Similar to DHA synthesis, ARA synthesis was lowest in rats fed a low LA diet as compared to rats fed diets containing higher amounts of LA. In addition, plasma DHA half-life was longer in rats fed 1.5% LA compared to rats fed higher amounts of LNA, which might explain the higher tissue DHA levels. Rats fed a low level of LA had longer half-lives for EPA and DHA in the plasma than rats fed a diet containing high levels of LA. Here, rats fed diets containing high levels of LA had increased plasma ARA and decreased plasma EPA concentrations compared to rats fed diets with low levels of LA. Plasma ARA concentrations were elevated in rats fed high levels of LA, likely as a result of higher ARA synthesis rates. However, the augmented plasma n-3 PUFA concentrations appear to be due to decreased n-3 PUFA turnover in the plasma rather than increased synthesis–secretion from ALA.

Abstract

Docosahexaenoic acid (DHA) is thought to be important for brain function. The main dietary source of DHA is fish, however, DHA can also be synthesized from precursor omega-3 polyunsaturated fatty acids (n-3 PUFA), the most abundantly consumed being α-linolenic acid (ALA). The enzymes required to synthesize DHA from ALA are also used to synthesize longer chain omega-6 (n-6) PUFA from linoleic acid (LNA). The large increase in LNA consumption that has occurred over the last century has led to concern that LNA and other n-6 PUFA outcompete n-3 PUFA for enzymes involved in DHA synthesis, and therefore, decrease overall DHA synthesis. To assess this, rats were fed diets containing LNA at 53 (high LNA diet), 11 (medium LNA diet) or 1.5% (low LNA diet) of the fatty acids with ALA being constant across all diets (approximately 4% of the fatty acids). Rats were maintained on these diets from weaning for 8 weeks, at which point they were subjected to a steady-state infusion of labeled ALA and LNA to measure DHA and arachidonic acid (ARA) synthesis rates. DHA and ARA synthesis rates were generally highest in rats fed the medium and high LNA diets, while the plasma half-life of DHA was longer in rats fed the low LNA diet. Therefore, increasing dietary LNA, in rats, did not impair DHA synthesis; however, low dietary LNA led to a decrease in DHA synthesis with tissue concentrations of DHA possibly being maintained by a longer DHA half-life.