Natural Abundance Carbon Isotopic Analysis Indicates the Equal Contribution of Local Synthesis and Plasma Uptake to Palmitate Levels in the Mouse Brain

Unraveling brain palmitic acid: Origin, levels and metabolic fate

In the human brain, palmitic acid (16:0; PAM) comprises nearly half of total brain saturates and has been identified as the third most abundant fatty acid overall. Brain PAM supports the structure of membrane phospholipids, provides energy, and regulates protein stability. Sources underlying the origin of brain PAM are both diet and endogenous synthesis via de novo lipogenesis (DNL), primarily from glucose. However, studies investigating the origin of brain PAM are limited to tracer studies utilizing labelled (14C/11C/3H/2H) PAM, and results vary based on the model and tracer used. Nevertheless, there is evidence PAM is synthesized locally in the brain, in addition to obtained directly from the diet. Herein, we provide an overview of brain PAM origin, entry to the brain, metabolic fate, and factors influencing brain PAM kinetics and levels, the latter in the context of age, as well as neurological diseases and psychiatric disorders. Additionally, we briefly summarize the role of PAM in signaling at the level of the brain. We add to the literature a rudimentary summary on brain PAM metabolism.

2-Bromopalmitate treatment attenuates senescence phenotype in human adult cells - possible role of palmitoylation

Cells may undergo senescence in response to DNA damage, which is associated with cell cycle arrest, altered gene expression and altered cell morphology. Protein palmitoylation is one of the mechanisms by which the DNA damage response is regulated. Therefore, we hypothesized that protein palmitoylation played a role in regulation of the senescent phenotype. Here, we showed that treatment of senescent human vascular smooth muscle cells (VSMCs) with 2-bromopalmitate (2-BP), an inhibitor of protein acyltransferases, is associated with changes in different aspects of the senescent phenotype, including the resumption of cell proliferation, a decrease in DNA damage markers and the downregulation of senescence-associated β-galactosidase activity. The effects were dose dependent and associated with significantly decreased total protein palmitoylation level. We also showed that the senescence-modifying properties of 2-BP were at least partially mediated by the downregulation of elements of DNA damage-related molecular pathways, such as phosphorylated p53. Our data suggest that cell senescence may be regulated by palmitoylation, which provides a new perspective on the role of this posttranslational modification in age-related diseases.

Upregulated hepatic lipogenesis from dietary sugars in response to low palmitate feeding supplies brain palmitate

Palmitic acid (PAM) can be provided in the diet or synthesized via de novo lipogenesis (DNL), primarily, from glucose. Preclinical work on the origin of brain PAM during development is scarce and contrasts results in adults. In this work, we use naturally occurring carbon isotope ratios (13C/12C; δ13C) to uncover the origin of brain PAM at postnatal days 0, 10, 21 and 35, and RNA sequencing to identify the pathways involved in maintaining brain PAM, at day 35, in mice fed diets with low, medium, and high PAM from birth. Here we show that DNL from dietary sugars maintains the majority of brain PAM during development and is augmented in mice fed low PAM. Importantly, the upregulation of hepatic DNL genes, in response to low PAM at day 35, demonstrates the presence of a compensatory mechanism to maintain total brain PAM pools compared to the liver; suggesting the importance of brain PAM regulation.

Dietary triacetin, but not medium chain triacylglycerides, blunts weight gain in diet-induced rat model of obesity

Consumption of a Western diet (WD) is known to increase the risk of obesity. Short or medium chain fatty acids influence energy metabolism, and triacetin, a synthetic short chain triacylglyceride, has been shown to lower body fat under normal conditions. This study aimed to investigate if triacetin as part of a WD modifies rat weight and body fat. Male rats were fed a control diet or WD for 8 weeks. At week 8, rats in the WD group were maintained on a WD diet or switched to a WD diet containing 30% energy from medium-chain triacylglyceride (WD-MCT) or triacetin (WD-T) for another 8 weeks. At week 16, rats were euthanized and liver, adipose and blood were collected. Tissue fatty acids (FAs) were quantified by gas chromatography (GC) and hepatic FAs were measured by GC-combustion-isotope ratio mass spectrometry for δ13 C-palmitic acid (PAM)-a novel marker of de novo lipogenesis (DNL). Rats fed WD-T had a body weight not statistically different to the control group, and gained less body weight than rats fed WD alone. Furthermore, WD-T fed rats had a lower fat mass, and lower total liver and plasma FAs compared to the WD group. Rats fed WD-T did not differ from WD in blood ketone or glucose levels, however, had a significantly lower hepatic δ13 C-PAM value than WD fed rats; suggestive of lower DNL. In summary, we show that triacetin has the potential to blunt weight gain and adipose tissue accumulation in a rodent model of obesity, possibly due to a decrease in DNL.

Rationale, Design and Participants Baseline Characteristics of a Crossover Randomized Controlled Trial of the Effect of Replacing SSBs with NSBs versus Water on Glucose Tolerance, Gut Microbiome and Cardiometabolic Risk in Overweight or Obese Adult SSB Consumer: Strategies to Oppose SUGARS with Non-Nutritive Sweeteners or Water (STOP Sugars NOW) Trial and Ectopic Fat Sub-Study

BACKGROUND

Health authorities are near universal in their recommendation to replace sugar-sweetened beverages (SSBs) with water. Non-nutritive sweetened beverages (NSBs) are not as widely recommended as a replacement strategy due to a lack of established benefits and concerns they may induce glucose intolerance through changes in the gut microbiome. The STOP Sugars NOW trial aims to assess the effect of the substitution of NSBs (the "intended substitution") versus water (the "standard of care substitution") for SSBs on glucose tolerance and microbiota diversity.

DESIGN AND METHODS

The STOP Sugars NOW trial (NCT03543644) is a pragmatic, "head-to-head", open-label, crossover, randomized controlled trial conducted in an outpatient setting. Participants were overweight or obese adults with a high waist circumference who regularly consumed ≥1 SSBs daily. Each participant completed three 4-week treatment phases (usual SSBs, matched NSBs, or water) in random order, which were separated by ≥4-week washout. Blocked randomization was performed centrally by computer with allocation concealment. Outcome assessment was blinded; however, blinding of participants and trial personnel was not possible. The two primary outcomes are oral glucose tolerance (incremental area under the curve) and gut microbiota beta-diversity (weighted UniFrac distance). Secondary outcomes include related markers of adiposity and glucose and insulin regulation. Adherence was assessed by objective biomarkers of added sugars and non-nutritive sweeteners and self-report intake. A subset of participants was included in an Ectopic Fat sub-study in which the primary outcome is intrahepatocellular lipid (IHCL) by 1H-MRS. Analyses will be according to the intention to treat principle.

BASELINE RESULTS

Recruitment began on 1 June 2018, and the last participant completed the trial on 15 October 2020. We screened 1086 participants, of whom 80 were enrolled and randomized in the main trial and 32 of these were enrolled and randomized in the Ectopic Fat sub-study. The participants were predominantly middle-aged (mean age 41.8 ± SD 13.0 y) and had obesity (BMI of 33.7 ± 6.8 kg/m2) with a near equal ratio of female: male (51%:49%). The average baseline SSB intake was 1.9 servings/day. SSBs were replaced with matched NSB brands, sweetened with either a blend of aspartame and acesulfame-potassium (95%) or sucralose (5%).

CONCLUSIONS

Baseline characteristics for both the main and Ectopic Fat sub-study meet our inclusion criteria and represent a group with overweight or obesity, with characteristics putting them at risk for type 2 diabetes. Findings will be published in peer-reviewed open-access medical journals and provide high-level evidence to inform clinical practice guidelines and public health policy for the use NSBs in sugars reduction strategies.

TRIAL REGISTRATION

ClinicalTrials.gov identifier, NCT03543644.

S-Palmitoylation of Synaptic Proteins in Neuronal Plasticity in Normal and Pathological Brains

Protein lipidation is a common post-translational modification of proteins that plays an important role in human physiology and pathology. One form of protein lipidation, S-palmitoylation, involves the addition of a 16-carbon fatty acid (palmitate) onto proteins. This reversible modification may affect the regulation of protein trafficking and stability in membranes. From multiple recent experimental studies, a picture emerges whereby protein S-palmitoylation is a ubiquitous yet discrete molecular switch enabling the expansion of protein functions and subcellular localization in minutes to hours. Neural tissue is particularly rich in proteins that are regulated by S-palmitoylation. A surge of novel methods of detection of protein lipidation at high resolution allowed us to get better insights into the roles of protein palmitoylation in brain physiology and pathophysiology. In this review, we specifically discuss experimental work devoted to understanding the impact of protein palmitoylation on functional changes in the excitatory and inhibitory synapses associated with neuronal activity and neuronal plasticity. The accumulated evidence also implies a crucial role of S-palmitoylation in learning and memory, and brain disorders associated with impaired cognitive functions.

Quantitative and carbon isotope ratio analysis of fatty acids isolated from human brain hemispheres

Our knowledge surrounding the overall fatty acid profile of the adult human brain has been largely limited to extrapolations from brain regions in which the distribution of fatty acids varies. This is especially problematic when modeling brain fatty acid metabolism, therefore, an updated estimate of whole-brain fatty acid concentration is necessitated. Here, we sought to conduct a comprehensive quantitative analysis of fatty acids from entire well-characterized human brain hemispheres (n = 6) provided by the Douglas-Bell Canada Brain Bank. Additionally, exploratory natural abundance carbon isotope ratio (CIR; δ¹³ C, ¹³ C/¹² C) analysis was performed to assess the origin of brain fatty acids. Brain fatty acid methyl esters (FAMEs) were quantified by gas chromatography (GC)-flame ionization detection and minor n-6 and n-3 polyunsaturated fatty acid pentafluorobenzyl esters by GC-mass spectrometry. Carbon isotope ratio values of identifiable FAMEs were measured by GC-combustion-isotope ratio mass spectrometry. Overall, the most abundant fatty acid in the human brain was oleic acid, followed by stearic acid (STA), palmitic acid (PAM), docosahexaenoic acid (DHA), and arachidonic acid (ARA). Interestingly, cholesterol as well as saturates including PAM and STA were most enriched in ¹³ C, while PUFAs including DHA and ARA were most depleted in ¹³ C. These findings suggest a contribution of endogenous synthesis utilizing dietary sugar substrates rich in ¹³ C, and a combination of marine, animal, and terrestrial PUFA sources more depleted in ¹³ C, respectively. These results provide novel insights on cerebral fatty acid origin and concentration, the latter serving as a valuable resource for future modeling of fatty acid metabolism in the human brain.

Proteomics-Compatible Fourier Transform Isotopic Ratio Mass Spectrometry of Polypeptides

Measuring the relative abundances of heavy stable isotopes of the elements C, H, N, and O in proteins is of interest in environmental science, archeology, zoology, medicine, and other fields. The isotopic abundance measurements of the fine structure of immonium ions with ultrahigh resolution mass spectrometry obtained in gas-phase fragmentation of polypeptides have previously uncovered anomalous deuterium enrichment in (hydroxy)proline of bone collagen in marine mammals. Here, we provide a detailed description and validation of this approach and demonstrate per mil-range precision of isotopic ratio measurements in aliphatic residues from proteins and cell lysates. The analysis consists of proteomics-type experiment demanding sub-microgram amounts of a protein sample and providing concomitantly protein sequence data allowing one to verify sample purity and establish its identity. A novel software tool protein amino acid-resolved isotopic ratio mass spectrometry (PAIR-MS) is presented for extracting isotopic ratio data from the raw data files acquired on an Orbitrap mass spectrometer.

Serum lipid analysis and isotopic enrichment is suggestive of greater lipogenesis in young long-term cannabis users: A secondary analysis of a case-control study

Cannabis is now legal in many countries and while numerous studies have reported on its impact on cognition and appetite regulation, none have examined fatty acid metabolism in young cannabis users. We conducted an exploratory analysis to evaluate cannabis impact on fatty acid metabolism in cannabis users (n = 21) and non-cannabis users (n = 16). Serum levels of some saturated and monounsaturated fatty acids, including palmitic, palmitoleic, and oleic acids were higher in cannabis users compared to nonusers. As palmitic acid can be derived from diet or lipogenesis from sugars, we evaluated lipogenesis using a de novo lipogenesis index (palmitate/linoleic acid) and carbon-specific isotope analysis, which allows for the determination of fatty acid ¹³ C signature. The significantly higher de novo lipogenesis index in the cannabis users group along with a more enriched ¹³ C signature of palmitic acid suggested an increase in lipogenesis. In addition, while serum glucose concentration did not differ between groups, pyruvate and lactate were lower in the cannabis user group, with pyruvate negatively correlating with palmitic acid. Furthermore, the endocannabinoid 2-arachidonoylglycerol was elevated in cannabis users and could contribute to lipogenesis by activating the cannabinoid receptor 1. Because palmitic acid has been suggested to increase inflammation, we measured peripheral cytokines and observed no changes in inflammatory cytokines. Finally, an anti-inflammatory metabolite of palmitic acid, palmitoylethanolamide was elevated in cannabis users. Our results suggest that lipogenic activity is increased in cannabis users; however, future studies, including prospective studies that control dietary intake are required.

The Majority of Brain Palmitic Acid is Maintained by Lipogenesis from Dietary Sugars and is Augmented in Mice fed Low Palmitic Acid Levels from Birth

Previously, results from studies investigating if brain palmitic acid (16:0; PAM) was maintained by either dietary uptake or lipogenesis de novo (DNL) varied. Here, we utilize naturally occurring carbon isotope ratios (¹³ C/¹² C; δ¹³ C) to uncover the origin of brain PAM. Additionally, we explored brain and liver fatty acid concentration, total brain metabolomic profile, and behaviour. BALB/c dams were equilibrated onto either a low PAM diet (LP; <2%) or high PAM diet (HP; >95%) prior to producing one generation of offspring. Offspring stayed on the respective diet of the dam until 15-weeks of age, at which time the Open Field test was conducted in the offspring, prior to euthanasia and tissue lipid extraction. Although liver PAM was lower in offspring fed the LP diet, as well as female offspring, brain PAM was not affected by diet or sex. Across offspring of either sex on both diets, brain ¹³ C-PAM revealed compared to dietary uptake, DNL from dietary sugars contributed 68.8%-79.5% and 46.6%-58.0% to the total brain PAM pool by both peripheral and local brain DNL, and local brain DNL alone, respectively. DNL was augmented in offspring fed the LP diet, and the ability to upregulate DNL in the liver or the brain depended on sex. Anxiety-like behaviours were decreased in offspring fed the LP diet and were correlated with markers of LP diet consumption including increased liver ¹³ C-PAM, warranting further investigation. Altogether, our results indicate that DNL from dietary sugars is a compensatory mechanism to maintain brain PAM in response to a LP diet.

A Scoping Review of Clinical Studies in Infants Fed Formulas Containing Palm Oil or Palm Olein and Sn-2 Palmitate

BACKGROUND

Palmitic acid (PA; 16:0) is added to infant formula in the form of palm oil/palm olein (PO/POL) and stereospecific numbered-2 palmitate (SN2). Several studies have examined the effects of PO/POL and or SN2 in formulas on health outcomes, mainly growth, digestion, and absorption of nutrients. However, the roles of PA, PO/POL, and SN2 on neurodevelopment remains unknown.

OBJECTIVES

The objective of this scoping review was to map out studies in infants fed formula with PO/POL or SN2 to identify current knowledge on the role of PA in infant nutrition, specifically neurodevelopment.

METHODS

Data sources, including Medline, Embase, CAB Abstracts, and the Cochrane Database, were searched. Eligible articles were randomized controlled trials (RCTs) and observational studies examining outcomes in term singleton infants fed formula containing PO/POL or SN2. Studies examining preterm infants or infants with infections, mixed-feeding interventions, or outcomes not concerned with PO/POL or SN2 were excluded. Screening and data extraction were performed by 2 independent reviewers, and results were charted into 10 outcome categories.

RESULTS

We identified 28 RCTs and 2 observational studies. Only 1 RCT examined a neurodevelopmental outcome, reporting infants fed SN2 formula had higher fine motor skill scores compared to those fed a vegetable oil formula with a lower amount of SN2; however, only after adjustment for maternal education and at an earlier, but not a later time point. Anthropometric measures do not appear to be influenced by PO/POL or SN2 within formulas. Alternatively, it was reported that infants fed PO/POL within formulas had a decreased absorption of calcium, total fat, and PA compared to those fed vegetable oil formulas. However, studies were heterogenous, making it difficult to isolate the effects of PO/POL or SN2 in formulas.

CONCLUSIONS

Our review reiterates the need for future studies to address the effects of PO/POL and SN2 on neurodevelopment in infants. This study is registered at Open Science Framework as osf.io/697he.

Dairy product consumption is associated with a lowering of linoleic acid within serum triglycerides in adolescent females with overweight or obesity: a secondary analysis

Dairy fat is rich in saturated fatty acids such as palmitic acid (16:0) but low in linoleic acid (18:2n-6). The natural carbon 13 enrichment (δ13C) of 16:0 is higher in dairy fat than in most of the food supply. In adults, serum levels of pentadecanoic acid (15:0) and heptadecanoic acid (17:0) are recognized as biomarkers of dairy intake. In adolescents, no study has evaluated serum fatty acid levels or δ13C in response to chronic dairy consumption. The objectives of this study were to evaluate whether increased dairy product consumption can modulate 1) serum fatty acid levels and 2) 16:0 δ13C in adolescents with overweight/obesity who followed a 12-week weight management program. This secondary analysis of a RCT included two groups of adolescent females: recommended dairy (RDa; n=23) and low dairy (LDa; n=23). The RDa group was given 4 servings/d of dairy products while the LDa group maintained dairy intakes at ≤2 servings/d. Blood was sampled before and after the intervention. Lipids were extracted, separated, and fatty acids were quantified by gas chromatography. Isotope ratio mass spectrometry was used to assess 16:0 δ13C. There were no group differences on serum changes of 15:0 or 17:0. Within triglycerides, 18:2n-6 was lowered by 7.4% only in the RDa group (p = 0.040). The difference in delta 16:0 δ13C between the LDa and RDa group did not reach statistical significance (p = 0.070). Reductions in serum 18:2n-6 by dairy consumption could have positive health implications but more studies are needed to confirm this assertion.

Higher Increase in Plasma DHA in Females Compared to Males Following EPA Supplementation May Be Influenced by a Polymorphism in ELOVL2: An Exploratory Study

Young adult females have higher blood docosahexaenoic acid (DHA), 22:6n-3 levels than males, and this is believed to be due to higher DHA synthesis rates, although DHA may also accumulate due to a longer half-life or a combination of both. However, sex differences in blood fatty acid responses to eicosapentaenoic acid (EPA), 20:5n-3 or DHA supplementation have not been fully investigated. In this exploratory analysis, females and males (n = 14-15 per group) were supplemented with 3 g/day EPA, 3 g/day DHA, or olive oil control for 12 weeks. Plasma was analyzed for sex effects at baseline and changes following 12 weeks' supplementation for fatty acid levels and carbon-13 signature (δ¹³ C). Following EPA supplementation, the increase in plasma DHA in females (+23.8 ± 11.8, nmol/mL ± SEM) was higher than males (-13.8 ± 9.2, p < 0.01). The increase in plasma δ¹³ C-DHA of females (+2.79 ± 0.31, milliUrey (mUr ± SEM) compared with males (+1.88 ± 0.44) did not reach statistical significance (p = 0.10). The sex effect appears driven largely by increased plasma DHA in the AA genotype of females (+58.8 ± 11.5, nmol/mL ± SEM, n = 5) compared to GA + GG in females (+4.34 ± 13.5, n = 9) and AA in males (-29.1 ± 17.2, n = 6) for rs953413 in the ELOVL2 gene (p < 0.001). In conclusion, EPA supplementation increases plasma DHA levels in females compared to males, which may be dependent on the AA genotype for rs953413 in ELOVL2.

Natural abundance carbon isotope ratio analysis and its application in the study of diet and metabolism

Due to differences in carbon assimilation pathways between plants, there are subtle but distinct variations in the carbon isotope ratios of foods and animal products throughout the food supply. Although it is well understood that the carbon isotope ratio composition of the diet influences that of the consumers' tissues, the application of natural abundance carbon isotope ratio analysis in nutrition has long been underappreciated. Over the past decade, however, several studies have investigated the utility of carbon isotope ratio analysis for evaluation of nutritional biomarker status, primarily focusing on its application as an objective indicator of sugar and animal protein intake. More recently, research investigating the application of natural abundance measurements has been extended to study fatty acid metabolism and has yielded encouraging results. Collectively, data from large-scale observational studies and experimental animal studies highlight the potential for carbon isotope ratio analysis as an additional and effective tool to study diet and metabolism. The purpose of this review is to provide an overview of natural abundance carbon isotope ratio analysis, its application to studying nutrition, and an update of the research in the field.

EPA and DHA have divergent effects on serum triglycerides and lipogenesis, but similar effects on lipoprotein lipase activity: a randomized controlled trial

BACKGROUND

Comparative studies suggest that DHA may have stronger serum triglyceride-lowering effects than EPA; however, the molecular basis for this differential effect remains unexplored in humans. Differential regulation of lipogenesis and triglyceride clearance are 2 possible mechanisms of action.

OBJECTIVES

We compared the effects of EPA and DHA supplementation on serum triglycerides, markers of lipogenesis, and lipoprotein lipase (LPL) activity in adults participating in a double-blind, multiarm, placebo-controlled parallel-group randomized trial. Lipogenesis was assessed with the lipogenic index and compound specific isotope analysis (CSIA).

METHODS

Young, healthy normolipidemic men and women (n = 89; 21.6 ± 0.23 y; mean ± SEM) were randomly allocated into 1 of 3 supplement groups for 12 wk: 1) olive oil, 2) ∼3 g EPA/d, and 3) ∼3 g DHA/d. Omega-3 supplements were provided in triglyceride form. Blood was collected before and after supplementation for the analysis of fatty acids and preheparin LPL activity. Variations in the 13C:12C ratio (δ13C) of palmitate (16:0) and linoleate (18:2n-6) were measured by CSIA.

RESULTS

DHA supplementation reduced blood triglycerides (0.85 ± 0.04 mmol/L to 0.65 ± 0.03 mmol/L; P < 0.01), with no change seen with EPA supplementation. DHA supplementation did not change the lipogenic index or δ13C-16:0, whereas EPA supplementation increased the lipogenic index by 11% (P < 0.01) and δ13C-16:0 (P = 0.03) from -23.2 ± 0.2 to -22.8 ± 0.2 milliUrey ± SEM.

CONCLUSIONS

Reduced triglyceride concentrations after DHA supplementation are associated with increased LPL activity, whereas the null effect of EPA supplementation on blood triglycerides may stem from the concomitant increases in lipogenesis and LPL activity. Further investigation of the differential triglyceride-lowering effects of EPA and DHA is warranted in both normolipidemic and hyperlipidemic individuals. This trial was registered at clinicaltrials.gov as NCT03378232.

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.