17 beta Estradiol Increases Liver and Serum Docosahexaenoic Acid in Mice Fed Varying Levels of alpha Linolenic Acid

Docosahexaenoic acid (DHA) is considered to be important for cardiac and brain function, and 17bestradiol (E2) appears to increase the conversion of alpha linolenic acid (ALA) into DHA. However, the effect of varying ALA intake on the positive effect of E2 on DHA synthesis is not known. Therefore, the objective of this study was to investigate the effects of E2 supplementation on tissue and serum fatty acids in mice fed a low ALA corn oil based diet (CO, providing 0.6  per cent fatty acids as ALA) or a high ALA flaxseed meal based diet (FS, providing 11.2  per cent ALA). Ovariectomized mice were implanted with a slow release E2 pellet at 3 weeks of age and half the mice had the pellet removed at 7 weeks of age. Mice were then randomized onto either the CO or FS diet. After 4 weeks, the DHA concentration was measured in serum, liver and brain. A significant main effect of E2 was found for liver and serum DHA, corresponding to 25 and 15 per cent higher DHA in livers of CO and FS rats, respectively, and 19 and 13 per cent in serum of CO and FS rats, respectively, compared to unsupplemented mice. There was no effect of E2 on brain DHA. E2 results in higher DHA in serum and liver, at both levels of dietary ALA investigated presently, suggesting that higher ALA intake may result in higher DHA in individuals with higher E2 status. (Authors abstract)

Intake of fish and blood levels of the omega 3 polyunsaturated fatty acid (n 3 PUFA) docosahexaenoic acid have been associated with health benefits to the cardiovascular system and brain in several studies.. As there is concern about the environmental sustainability of fish stocks to meet current demands for dietary DHA intake, improved understanding of the factors that increase DHA levels independently of DHA intake is required. One factor that appears to increase blood and tissue DHA is the concentration of the hormone 17b estradiol (E2). North Americans consume approximately 1,500 mg per day of ALA , while DHA intakes are approximately 70 to 90 mg per day. ALA could therefore be a potentially important source of bio available DHA through conversion to DHA; however, information on the influence of E2 on DHA levels under various levels of dietary ALA is lacking. This has implications for nutritional recommendations, especially during pregnancy, as increasing ALA intake may increase DHA supply to a fetus. In addition, previous work investigating the effect of E2 on DHA levels following ALA feeding has used ALA containing oils rather than ALA-rich foods, such as FS meal. Therefore, the goal of this study was to characterize the effect of two different levels of dietary ALA intake provided by either corn oil (CO) or FS based diets on the influence of E2 on DHA levels in liver, serum, and brain in mice. A secondary aim of this study is to determine the effects of E2 under low and high ALA intakes on other fatty acid levels.

This study shows that with elevated E2 level, DHA levels are higher in liver and serum, but not in brain, in mice fed two different levels of dietary ALA. It also demonstrates
that the previously observed positive effect of E2 on DHA levels in humans and rats is also present in mice, and that this effect of E2 is present in both levels of dietary ALA examined. This is the first study to examine the effect of varying levels of dietary ALA on the effect of E2 on DHA levels in liver and serum, the first examination of E2 effects on DHA in mice, and the first study to use FS as a source of ALA rather than oil in demonstrating the positive effect of E2 on DHA levels. These findings suggest that increasing ALA intake would result in a proportionally greater increase in DHA in an individual with higher E2 status, such as a woman compared with a man, premenopausal relative to postmenopausal woman, and a pregnant relative to a non pregnant woman. This proportionally greater increase in DHA may result in health
benefits. For example, dose response relationships have been observed between EPA and DHA intake and cardiovascular risk factors in pooled analyses of prospective and
clinical studies. A particularly steep curve exists for anti-arrhythmic effect that plateaus at approximately 750 mg per day. Thus, even small increases in DHA through enhanced conversion from ALA may have significant cardiovascular health benefits. This may explain why increasing dietary ALA is associated with lowered risk of sudden cardiac death in women but not in men.

The observed absolute differences in serum and liver DHA caused by the two factors tested in this study, the ALA rich diet and E2 supplementation, are in line with those found in the literature. Diets enriched with ALA at levels comparable to our study have been previously shown to increase liver and serum  DHA content by approximately 2.6 and 2.3 per cent , respectively, in mice. We observed absolute differences in serum and liver DHA as a percent of total fatty acids of approximately 1.6 per cent between mice fed the CO and FO diets. E2 supplementation caused absolute differences in serum and liver DHA of 0.5 and 1.1 per cent , respectively, representing 34 and 59 per cent observed diet effect. This magnitude of E2 effect is comparable to that found in the literature as differences of approximately 0.4 to 2.8 and 0.4 per cent  in liver  and erythrocyte DHA, respectively, have been observed with E2 supplementation in ovariectomized rats. The absolute difference between male and female rats is somewhat more pronounced.

The findings of this study have implications for nutritional recommendations, as it demonstrates that E2 status is associated with higher DHA status at two levels of ALA
intake. Therefore, recommendations to increase dietary ALA intake would have greater effect on DHA status in individuals with higher E2 status, for example women relative to men or in pre menopausal relative to postmenopausal women. Conversely, individuals with lower E2 status may require a greater intake of preformed DHA to receive the same health benefits. Furthermore, these results suggest that the increase in E2 that occurs in pregnancy presents a situation in which ALA is more efficiently converted to DHA.

The findings also suggest that while dietary ALA was taken up into the brain, there was no effect of E2, in increasing the level of long chain n 3 PUFA.. Using an in vitro model, it was shown that E2 significantly enhances the conversion of ALA to EPA and DPA n 3 and has a small but non significant positive effect on DHA levels in human neuroblastoma cells. This indicates that E2 has the capacity to mediate PUFA metabolism in brain cells.  In conclusion, the higher DHA in mice receiving E2 in a diet with 0.6 or 11.2  per cent ALA indicates that the positive effect of E2 is present at both of these intakes of ALA. It is therefore possible that further increases in ALA could result in greater increases in DHA status in the presence of E2, and therefore that increasing dietary ALA intake can serve as a potentially significant source of bioavailable DHA in individuals with higher E2 status. (Editors comments)