Quantitation of Human Whole-Body Synthesis-Secretion Rates of Docosahexaenoic Acid and Eicosapentaenoate Acid from Circulating Unesterified α-Linolenic Acid at Steady State

Application of Chromatographic and Spectroscopic-Based Methods for Analysis of Omega-3 (ω-3 FAs) and Omega-6 (ω-6 FAs) Fatty Acids in Marine Natural Products

Omega-3 fatty acids v(ω-3 FAs) such as EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) and omega-6 fatty acids (ω-6 FAs) such as linoleic acid and arachidonic acid are important fatty acids responsible for positive effects on human health. The main sources of ω-3 FAs and ω-6 FAs are marine-based products, especially fish oils. Some food, supplements, and pharmaceutical products would include fish oils as a source of ω-3 FAs and ω-6 FAs; therefore, the quality assurance of these products is highly required. Some analytical methods mainly based on spectroscopic and chromatographic techniques have been reported. Molecular spectroscopy such as Infrared and Raman parallel to chemometrics has been successfully applied for quantitative analysis of individual and total ω-3 FAs and ω-6 FAs. This spectroscopic technique is typically applied as the alternative method to official methods applying chromatographic methods. Due to the capability to provide the separation of ω-3 FAs and ω-6 FAs from other components in the products, gas and liquid chromatography along with sophisticated detectors such as mass spectrometers are ideal analytical methods offering sensitive and specific results that are suitable for routine quality control.

Increases in plasma n-3 tetracosapentaenoic acid and tetracosahexaenoic acid following 12 weeks of EPA, but not DHA, supplementation in women and men

Dietary feeding and stable isotope studies in rodents support that the 24-carbon omega-3 polyunsaturated fatty acids, tetracosapentaenoic acid (24:5n-3, TPAn-3) and tetracosahexaenoic acid (24:6n-3, THA), are immediate precursors to docosahexaenoic acid (DHA, 22:6n-3). In this study, we assessed for the first time, changes in TPAn-3 or THA levels following omega-3 PUFA supplementation in humans, providing insight into human omega-3 PUFA metabolism. In this secondary analysis of a double-blind randomized control trial, women and men (19 - 30 years, n = 10 - 14 per sex, per diet) were supplemented with 3 g/day EPA, DHA, or olive oil control for 12 weeks. Plasma TPAn-3 and THA concentrations were determined by gas chromatography-mass spectrometry to determine changes following supplementation in a sex-specific manner (sex x time). EPA supplementation significantly increased (p < 0.0001) plasma TPAn-3 by 215% (1.3 ± 0.1 - 4.1 ± 0.7, nmol/mL ± SEM) and THA by 112% (1.7 ± 0.2 - 3.6 ± 0.5, nmol/mL ± SEM). Furthermore, women had 111% and 99% higher plasma TPAn-3 and THA in the EPA supplemented group compared to men (p < 0.0001). There were no significant effects of time on plasma TPAn-3 or THA concentrations in the DHA supplemented or olive oil supplemented groups. In conclusion, EPA, but not DHA, supplementation in humans increased plasma TPAn-3 and THA levels, suggesting that THA accumulates prior to conversion to DHA in the n-3 PUFA synthesis pathway. Furthermore, women generally exhibit higher plasma TPAn-3 and THA concentrations compared with men, suggesting that women have a greater ability to accumulate 24-carbon n-3 PUFA in plasma via EPA and DPAn-3 elongation, which may explain the known higher DHA levels in women. Summary: In this secondary analysis of a double-blind randomized control trial, we assessed changes in omega-3 (n-3) tetracosapentaenoic acid (24:5n-3, TPAn-3) and tetracosahexaenoic acid (24:6n-3, THA) plasma levels in women and men (19 - 30 years, n = 10 - 14 per sex, per diet) following 12-weeks of n-3 PUFA supplementation (3 g/day EPA, DHA or olive oil). Women had higher plasma TPAn-3 in all supplementation groups and higher THA levels in the EPA and olive oil groups (p < 0.0001) compared to men. EPA supplementation increased (p < 0.0001) plasma TPAn-3 by 215% (1.3 ± 0.1 - 4.1 ± 0.7, nmol/mL ± SEM) and THA by 112% (1.7 ± 0.2 - 3.6 ± 0.5, nmol/mL ± SEM), but DHA supplementation had no effect. For the first time in humans, we show that plasma TPAn-3 and THA levels are higher in women and increased with EPA, but not DHA supplementation, suggesting an accumulation of THA prior to conversion to DHA in the n-3 PUFA synthesis pathway.

α-linolenic acid interconversion is sufficient as a source of longer chain ω-3 polyunsaturated fatty acids in humans: An opinion

α-linolenic acid (αLNA) conversion into the functionally important ω-3 polyunsaturated fatty acids (PUFA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), has been regarded as inadequate for meeting nutritional requirements for these PUFA. This view is based on findings of small αLNA supplementation trials and stable isotope tracer studies that have been interpreted as indicating human capacity for EPA and, in particular, DHA synthesis is limited. The purpose of this review is to re-evaluate this interpretation. Markedly differing study designs, inconsistent findings and lack of trial replication preclude robust consensus regarding the nutritional adequacy of αLNA as a source of EPC and DHA. The conclusion that αLNA conversion in humans is constrained is inaccurate because it presupposes the existence of an unspecified, higher level of metabolic activity. Since capacity for EPA and DHA synthesis is the product of evolution it may be argued that the levels of EPA and DHA it maintains are nutritionally appropriate. Dietary and supra-dietary EPA plus DHA intakes confer health benefits. Paradoxically, such health benefits are also found amongst vegetarians who do not consume EPA and DHA, and for whom αLNA conversion is the primary source of ω-3 PUFA. Since there are no reported adverse effects on health or cognitive development of diets that exclude EPA and DHA, their synthesis from αLNA appears to be nutritionally adequate. This is consistent with the dietary essentiality of αLNA and has implications for developing sustainable nutritional recommendations for ω-3 PUFA.

Influence of the nutritional status and oxidative stress in the desaturation and elongation of n-3 and n-6 polyunsaturated fatty acids: Impact on non-alcoholic fatty liver disease

Polyunsaturated fatty acids (PUFA) play essential roles in cell membrane structure and physiological processes including signal transduction, cellular metabolism and tissue homeostasis to combat diseases. PUFA are either consumed from food or synthesized by enzymatic desaturation, elongation and peroxisomal β-oxidation. The nutritionally essential precursors α-linolenic acid (C18:3n-3; ALA) and linoleic acid (C18:2n-6; LA) are subjected to desaturation by Δ6D/Δ5D desaturases and elongation by elongases 2/5, enzymes that are induced by insulin and repressed by PUFA. Maintaining an optimally low n-6/n-3 PUFA ratio is linked to prevention of the development of several diseases, including nonalcoholic fatty liver disease (NAFLD) that is characterized by depletion of PUFA promoting hepatic steatosis and inflammation. In this context, supplementation with n-3 PUFA revealed significant lowering of hepatic steatosis in obese patients, whereas prevention of fatty liver by high-fat diet in mice is observed in n-3 PUFA and hydroxytyrosol co-administration. The aim of this work is to review the role of nutritional status and nutrient availability on markers of PUFA biosynthesis. In addition, the impact of oxidative stress developed as a result of NAFLD, a redox imbalance that may alter the expression and activity of the enzymes involved, and diminished n-3 PUFA levels by free-radical dependent peroxidation processes will be discussed.

Science-based policy: targeted nutrition for all ages and the role of bioactives

Globally, there has been a marked increase in longevity, but it is also apparent that significant inequalities remain, especially the inequality related to insufficient 'health' to enjoy or at least survive those later years. The major causes include lack of access to proper nutrition and healthcare services, and often the basic information to make the personal decisions related to diet and healthcare options and opportunities. Proper nutrition can be the best predictor of a long healthy life expectancy and, conversely, when inadequate and/or improper a prognosticator of a sharply curtailed expectancy. There is a dichotomy in both developed and developing countries as their populations are experiencing the phenomenon of being 'over fed and under nourished', i.e., caloric/energy excess and lack of essential nutrients, leading to health deficiencies, skyrocketing global obesity rates, excess chronic diseases, and premature mortality. There is need for new and/or innovative approaches to promoting health as individuals' age, and for public health programs to be a proactive blessing and not an archaic status quo 'eat your vegetables' mandate. A framework for progress has been proposed and published by the World Health Organization in their Global Strategy and Action Plan on Ageing and Health (WHO (2017) Advancing the right to health: the vital role of law. https://apps.who.int/iris/bitstream/handle/10665/252815/9789241511384-eng.pdf?sequence=1&isAllowed=y . Accessed 07 Jun 2021; WHO (2020a) What is Health Promotion. www.who.int/healthpromotion/fact-sheet/en/ . Accessed 07 Jun 2021; WHO (2020b) NCD mortality and morbidity. www.who.int/gho/ncd/mortality_morbidity/en/ . Accessed 07 Jun 2021). Couple this WHO mandate with current academic research into the processes of ageing, and the ingredients or regimens that have shown benefit and/or promise of such benefits. Now is the time for public health policy to 'not let the perfect be the enemy of the good,' but to progressively make health-promoting nutrition recommendations.

Tetracosahexaenoylethanolamide, a novel N-acylethanolamide, is elevated in ischemia and increases neuronal output

N-acylethanolamines (NAEs) are endogenous lipid-signaling molecules derived from fatty acids that regulate numerous biological functions, including in the brain. Interestingly, NAEs are elevated in the absence of fatty acid amide hydrolase (FAAH) and following CO₂-induced ischemia/hypercapnia, suggesting a neuroprotective response. Tetracosahexaenoic acid (THA) is a product and precursor to DHA; however, the NAE product, tetracosahexaenoylethanolamide (THEA), has never been reported. Presently, THEA was chemically synthesized as an authentic standard to confirm THEA presence in biological tissues. Whole brains were collected and analyzed for unesterified THA, total THA, and THEA in wild-type and FAAH-KO mice that were euthanized by either head-focused microwave fixation, CO₂ + microwave, or CO₂ only. PPAR activity by transient transfection assay and ex vivo neuronal output in medium spiny neurons (MSNs) of the nucleus accumbens by patch clamp electrophysiology were determined following THEA exposure. THEA in the wild-type mice was nearly doubled (P < 0.05) following ischemia/hypercapnia (CO₂ euthanization) and up to 12 times higher (P < 0.001) in the FAAH-KO compared with wild-type. THEA did not increase (P > 0.05) transcriptional activity of PPARs relative to control, but 100 nM of THEA increased (P < 0.001) neuronal output in MSNs of the nucleus accumbens. Here were identify a novel NAE, THEA, in the brain that is elevated upon ischemia/hypercapnia and by KO of the FAAH enzyme. While THEA did not activate PPAR, it augmented the excitability of MSNs in the nucleus accumbens. Overall, our results suggest that THEA is a novel NAE that is produced in the brain upon ischemia/hypercapnia and regulates neuronal excitation.

Brain eicosapentaenoic acid metabolism as a lead for novel therapeutics in major depression

The results of several meta-analyses suggest that eicosapentaenoic acid (EPA) supplementation is therapeutic in managing the symptoms of major depression. It was previously assumed that because EPA is extremely low in the brain it did not cross the blood-brain barrier and any therapeutic effects it exerted would be via the periphery. However, more recent studies have established that EPA does enter the brain, but is rapidly metabolised following entry. While EPA does not accumulate within the brain, it is present in microglia and homeostatic mechanisms may regulate its esterification to phospholipids that serve important roles in cell signaling. Furthermore, a variety of signaling molecules from EPA have been described in the periphery and they have the potential to exert effects within the brain. If EPA is confirmed to be therapeutic in major depression as a result of adequately powered randomized clinical trials, future research on brain EPA metabolism could lead to the discovery of novel targets for treating or preventing major depression.

Updates to the n-3 polyunsaturated fatty acid biosynthesis pathway: DHA synthesis rates, tetracosahexaenoic acid and (minimal) retroconversion

N-3 polyunsaturated fatty acids (PUFA) and the numerous families of lipid mediators derived from them collectively regulate numerous biological processes. The mechanisms by which n-3 PUFA regulate biological processes begins with an understanding of the n-3 biosynthetic pathway that starts with alpha-linolenic acid (18:3n-3) and is commonly thought to end with the production of docosahexaenoic acid (DHA, 22:6n-3). However, our understanding of this pathway is not as complete as previously believed. In the current review we provide a background of the evidence supporting the pathway as currently understood and provide updates from recent studies challenging three central dogma of n-3 PUFA metabolism. By building on nearly three decades of research primarily in cell culture and oral dosing studies, recent evidence presented focuses on in vivo kinetic modelling and compound-specific isotope abundance studies in rodents and humans that have been instrumental in expanding our knowledge of the pathway. Specifically, we highlight three main updates to the n-3 PUFA biosynthesis pathway: (1) DHA synthesis rates cannot be as low as previously believed, (2) DHA is both a product and a precursor to tetracosahexaenoic acid (24:6n-3) and (3) increases in EPA in response to DHA supplementation are not the result of increased retroconversion.

Compound-specific isotope analysis reveals no retroconversion of DHA to EPA but substantial conversion of EPA to DHA following supplementation: a randomized control trial

BACKGROUND

It has long been believed that DHA supplementation increases plasma EPA via the retroconversion pathway in mammals. However, in rodents this increase in EPA is likely due to a slower metabolism of EPA, but this has never been tested directly in humans.

OBJECTIVE

The aim of this study was to use the natural variations in 13C:12C ratio (carbon-13 isotopic abundance [δ13C]) of n-3 PUFA supplements to assess n-3 PUFA metabolism following DHA or EPA supplementation in humans.

METHODS

Participants (aged 21.6 ± 2.2 y) were randomly assigned into 1 of 3 supplement groups for 12 wk: 1) olive oil control, 2) ∼3 g/d DHA, or 3) ∼3 g/d EPA. Blood was collected before and after the supplementation period, and concentrations and δ13C of plasma n-3 PUFA were determined.

RESULTS

DHA supplementation increased (P < 0.05) plasma EPA concentrations by 130% but did not affect plasma δ13C-EPA (-31.0 ± 0.30 to -30.8 ± 0.19, milliUrey ± SEM, P > 0.05). In addition, EPA supplementation did not change plasma DHA concentrations (P > 0.05) but did increase plasma δ13C-DHA (-27.9 ± 0.2 to -25.6 ± 0.1, P < 0.05) toward δ13C-EPA of the supplement (-23.5 ± 0.22). EPA supplementation increased plasma concentrations of EPA and docosapentaenoic acid (DPAn-3) by 880% and 200%, respectively, and increased plasma δ13C-EPA (-31.5 ± 0.2 to -25.7 ± 0.2) and δ13C-DPAn-3 (-28.9 ± 0.3 to -25.0 ± 0.1) toward δ13C-EPA of the supplement.

CONCLUSIONS

In this study, we show that the increase in plasma EPA following DHA supplementation in humans does not occur via retroconversion, but instead from a slowed metabolism and/or accumulation of plasma EPA. Furthermore, substantial amounts of supplemental EPA can be converted into DHA. δ13C of n-3 PUFA in humans is a powerful and underutilized tool that can track dietary n-3 PUFA and elucidate complex metabolic questions. This trial was registered at clinicaltrials.gov as NCT03378232.

Is essential fatty acid interconversion an important source of PUFA in humans?

Humans can obtain pre-formed long-chain PUFA from the diet and are also able to convert essential fatty acids (EFA) to longer-chain PUFA. The metabolic pathway responsible for EFA interconversion involves alternating desaturation and carbon chain elongation reactions, and carbon chain shortening by peroxisomal β-oxidation. Studies using stable isotope tracers or diets supplemented with EFA show that capacity for PUFA synthesis is limited in humans, such that DHA (22 : 6n-3) synthesis in men is negligible. PUFA synthesis is higher in women of reproductive age compared with men. However, the magnitude of the contribution of hepatic PUFA synthesis to whole-body PUFA status remains unclear. A number of extra-hepatic tissues have been shown to synthesise PUFA or to express genes for enzymes involved in this pathway. The precise function of extra-hepatic PUFA synthesis is largely unknown, although in T lymphocytes PUFA synthesis is involved in the regulation of cell activation and proliferation. Local PUFA synthesis may also be important for spermatogenesis and fertility. One possible role of extra-hepatic PUFA synthesis is that it may provide PUFA in a timely manner to facilitate specific cell functions. If so, this may suggest novel insights into the effect of dietary PUFA and/or polymorphisms in genes involved in PUFA synthesis on health and tissue function.

Docosahexaenoic acid is both a product of and a precursor to tetracosahexaenoic acid in the rat

Tetracosahexaeoic acid (THA; 24:6n-3) is thought to be the immediate precursor of DHA in rodents; however, the relationship between THA and DHA metabolism has not been assessed in vivo. Here, we infused unesterified ²H₅-THA and ¹³C₂₂-DHA, at a steady state, into two groups of male Long-Evans rats and determined the synthesis-secretion kinetics, including daily synthesis-secretion rates of all 20-24 carbon n-3 PUFAs. We determined that the synthesis-secretion coefficient (a measure of the capacity to synthesize a given fatty acid) for the synthesis of DHA from plasma unesterified THA to be 134-fold higher than for THA from DHA. However, when considering the significantly higher endogenous plasma unesterified DHA pool, the daily synthesis-secretion rates were only 7-fold higher for DHA synthesis from THA (96.3 ± 31.3 nmol/d) compared with that for THA synthesis from DHA (11.4 ± 4.1 nmol/d). Furthermore, plasma unesterified THA was converted to DHA and secreted into the plasma at a 2.5-fold faster rate than remaining as THA itself (26.2 ± 6.3 nmol/d), supporting THA's primary role as a precursor to DHA. In conclusion, using a 3 h infusion model in rats, we demonstrate for the first time in vivo that DHA is both a product and a precursor to THA.