Disturbance in uniformly 13C-labelled DHA metabolism in elderly human subjects carrying the apoE ε4 allele

The vascular contribution of apolipoprotein E to Alzheimer's disease

Alzheimer's disease, the most prevalent form of dementia, imposes a substantial societal burden. The persistent inadequacy of disease-modifying drugs targeting amyloid plaques and neurofibrillary tangles suggests the contribution of alternative pathogenic mechanisms. A frequently overlooked aspect is cerebrovascular dysfunction, which may manifest early in the progression of Alzheimer's disease pathology. Mounting evidence underscores the pivotal role of the apolipoprotein E gene, particularly the apolipoprotein ε4 allele as the strongest genetic risk factor for late-onset Alzheimer's disease, in the cerebrovascular pathology associated with Alzheimer's disease. In this review, we examine the evidence elucidating the cerebrovascular impact of both central and peripheral apolipoprotein E on the pathogenesis of Alzheimer's disease. We present a novel three-hit hypothesis, outlining potential mechanisms that shed light on the intricate relationship among different pathogenic events. Finally, we discuss prospective therapeutics targeting the cerebrovascular pathology associated with apolipoprotein E and explore their implications for future research endeavours.

Effects of APOE4 on omega-3 brain metabolism across the lifespan

Omega-3 (n-3) polyunsaturated fatty acids (PUFAs), such as docosahexaenoic acid (DHA), have important roles in human nutrition and brain health by promoting neuronal functions, maintaining inflammatory homeostasis, and providing structural integrity. As Alzheimer's disease (AD) pathology progresses, DHA metabolism in the brain becomes dysregulated, the timing and extent of which may be influenced by the apolipoprotein E ε4 (APOE4) allele. Here, we discuss how maintaining adequate DHA intake early in life may slow the progression to AD dementia in cognitively normal individuals with APOE4, how recent advances in DHA brain imaging could offer insights leading to more personalized preventive strategies, and how alternative strategies targeting PUFA metabolism pathways may be more effective in mitigating disease progression in patients with existing AD dementia.

Omega-3 world map: 2024 update

In 2016, the first worldwide n3 PUFA status map was published using the Omega-3 Index (O3I) as standard biomarker. The O3I is defined as the percentage of EPA + DHA in red blood cell (RBC) membrane FAs. The purpose of the present study was to update the 2016 map with new data. In order to be included, studies had to report O3I and/or blood EPA + DHA levels in metrics convertible into an estimated O3I, in samples drawn after 1999. To convert the non-RBC-based EPA + DHA metrics into RBC we used newly developed equations. Baseline data from clinical trials and observational studies were acceptable. A literature search identified 328 studies meeting inclusion criteria encompassing 342,864 subjects from 48 countries/regions. Weighted mean country O3I levels were categorized into very low ≤4%, low >4-6%, moderate >6-8%, and desirable >8%. We found that the O3I in most countries was low to very low. Notable differences between the current and 2016 map were 1) USA, Canada, Italy, Turkey, UK, Ireland and Greece (moving from the very low to low category); 2) France, Spain and New Zealand (low to moderate); and 3) Finland and Iceland (moderate to desirable). Countries such as Iran, Egypt, and India exhibited particularly poor O3I levels.

Apolipoprotein E potently inhibits ferroptosis by blocking ferritinophagy

Allelic variation to the APOE gene confers the greatest genetic risk for sporadic Alzheimer's disease (AD). Independent of genotype, low abundance of apolipoprotein E (apoE), is characteristic of AD CSF, and predicts cognitive decline. The mechanisms underlying the genotype and apoE level risks are uncertain. Recent fluid and imaging biomarker studies have revealed an unexpected link between apoE and brain iron, which also forecasts disease progression, possibly through ferroptosis, an iron-dependent regulated cell death pathway. Here, we report that apoE is a potent inhibitor of ferroptosis (EC50 ≈ 10 nM; N27 neurons). We demonstrate that apoE signals to activate the PI3K/AKT pathway that then inhibits the autophagic degradation of ferritin (ferritinophagy), thus averting iron-dependent lipid peroxidation. Using postmortem inferior temporal brain cortex tissue from deceased subjects from the Rush Memory and Aging Project (MAP) (N = 608), we found that the association of iron with pathologically confirmed clinical Alzheimer's disease was stronger among those with the adverse APOE-ε4 allele. While protection against ferroptosis did not differ between apoE isoforms in vitro, other features of ε4 carriers, such as low abundance of apoE protein and higher levels of polyunsaturated fatty acids (which fuel ferroptosis) could mediate the ε4 allele's heighted risk of AD. These data support ferroptosis as a putative pathway to explain the major genetic risk associated with late onset AD.

Effects of DHA on cognitive dysfunction in aging and Alzheimer's disease: The mediating roles of ApoE

The prevalence of Alzheimer's disease (AD) continues to rise due to the increasing aging population. Among the various genetic factors associated with AD, apolipoprotein E (ApoE), a lipid transporter, stands out as the primary genetic risk factor. Specifically, individuals carrying the ApoE4 allele exhibit a significantly higher risk. However, emerging research indicates that dietary factors play a prominent role in modifying the risk of AD. Docosahexaenoic acid (DHA), a prominent ω-3 fatty acid, has garnered considerable attention for its potential to ameliorate cognitive function. The intricate interplay between DHA and the ApoE genotype within the brain, which may influence DHA's utilization and functionality, warrants further investigation. This review meticulously examines experimental and clinical studies exploring the effects of DHA on cognitive decline. Special emphasis is placed on elucidating the role of ApoE gene polymorphism and the underlying mechanisms are discussed. These studies suggest that early DHA supplementation may confer benefits to cognitively normal older adults carrying the ApoE4 gene. However, once AD develops, ApoE4 non-carriers may experience greater benefits compared to ApoE4 carriers, although the overall effectiveness of DHA supplementation at this stage is limited. Potential mechanisms underlying these differential effects may include accelerated DHA catabolism in ApoE4 carriers, impaired transport across the blood-brain barrier (BBB), and compromised lipidation and circulatory function in ApoE4 carriers. Thus, the supplementation of DHA may represent a potential intervention strategy aimed at compensating for these deficiencies in ApoE4 carriers prior to the onset of AD.

Synthesis and Preclinical Evaluation of 22-[18F]Fluorodocosahexaenoic Acid as a Positron Emission Tomography Probe for Monitoring Brain Docosahexaenoic Acid Uptake Kinetics

Docosahexaenoic acid [22:6(n-3), DHA], a polyunsaturated fatty acid, has an important role in regulating neuronal functions and in normal brain development. Dysregulated brain DHA uptake and metabolism are found in individuals carrying the APOE4 allele, which increases the genetic risk for Alzheimer's disease (AD), and are implicated in the progression of several neurodegenerative disorders. However, there are limited tools to assess brain DHA kinetics in vivo that can be translated to humans. Here, we report the synthesis of an ω-radiofluorinated PET probe of DHA, 22-[18F]fluorodocosahexaenoic acid (22-[18F]FDHA), for imaging the uptake of DHA into the brain. Using the nonradiolabeled 22-FDHA, we confirmed that fluorination of DHA at the ω-position does not significantly alter the anti-inflammatory effect of DHA in microglial cells. Through dynamic PET-MR studies using mice, we observed the accumulation of 22-[18F]FDHA in the brain over time and estimated DHA's incorporation coefficient (K*) using an image-derived input function. Finally, DHA brain K* was validated using intravenous administration of 15 mg/kg arecoline, a natural product known to increase the DHA K* in rodents. 22-[18F]FDHA is a promising PET probe that can reveal altered lipid metabolism in APOE4 carriers, AD, and other neurologic disorders. This new probe, once translated into humans, would enable noninvasive and longitudinal studies of brain DHA dynamics by guiding both pharmacological and nonpharmacological interventions for neurodegenerative diseases.

Transcriptomic Downregulation of APOE, Polymorphic Variations of APOE, Diet, Social Isolation, and Co-morbidities as Contributing Factors to Alzheimer's Disease: a Case-Control Study of Kashmiri Population

Alzheimer's disease (AD) is the most common form of dementia, generally affecting elderly people in the age group of above 60-65 years. Amyloid deposition has been found to be a possible cause and a characteristic feature of Alzheimer's disease. Mutations, variant genotypes, or downregulation that reduce amyloid clearance or accelerate amyloid accumulation can lead to Alzheimer's disease. This study involved clinically confirmed AD patients, age matched controls of similar ethnicity, and patients who had no history of cancer or any other chronic disease. DNA and RNA extractions of samples were done as per Saguna et al. [45] and TRIzol method, respectively. Frequencies of variant genotypes were observed using the RFLP technique, whereas, for expression analysis, qPCR was performed. The association between diet, smoking status, family history, and co-morbidities was calculated using statistical tools. Expression analysis showed downregulation in more than 65% of AD cases. Hypertension and diabetes also had a significant association with AD. Allelic isoforms ε2:ε2 and ε2:ε3 tend to be less frequent among AD cases compared to controls (2.85% vs 26.15% and 11.42% vs 21.43%, respectively). Among individuals (AD cases) with ε2:ε3 and ε2:ε4, 37.5% of the patients were having severe dementia and 62.5% were having mild to moderate dementia, whereas, among individuals with ε3:ε4 and ε4:ε4, 57% were having severe dementia and 43% were having mild to moderate dementia. Besides this, all early-onset Alzheimer's patients were found to have at least one ε4 allele. The percentage of individuals with family history (cases vs controls) was 34.17% vs 3.75%, without family history 64.55% vs 95%. On comparing AD cases against controls for smoking status, the results observed are the following: chain smokers, 12.65% vs 18.75%; moderate smokers, 16.45% vs 6.25%; ex-smokers, 36.70% vs 22.50%; non-smokers, 34.17% vs 52.50%. On comparing dietary habits in AD cases against controls, the results were as follows: individuals with generally fatty diet 26.58% vs 11.25%, with mixed diet 36.70% vs 78.75%, with generally vegetarian diet 34.17% vs 10.00%, data not available 2.53% among AD cases. Family history, dietary habits, genetics, and socioeconomic status are strongly associated with the development of Alzheimer disease. Although family history or genetic makeup cannot be changed, eating habits can be changed quite easily. We simply need to go from a high-fat diet to one that is lower in fat. Regarding socioeconomic status, which includes stress of both kinds, including economic stress, stress brought on by the loss of loved ones through death or separation, and co-morbidities (hypertension and diabetes), all are manageable and even modifiable through counseling, positive behavior, and physical activity like exercise, walking, cycling, and playing games.

Associations of ApoE4 status and DHA supplementation on plasma and CSF lipid profiles and entorhinal cortex thickness

Apolipoprotein ε allele 4 (APOE4) influences the metabolism of polyunsaturated fatty acids (PUFAs) such as docosahexaenoic acid (DHA). The entorhinal cortex (EC) in the brain is affected early in Alzheimer's disease and is rich in DHA. The purpose of this study is to identify the effect of APOE4 and DHA lipid species on the EC. Plasma and cerebrospinal fluid (CSF) lipidomic measurements were obtained from the DHA Brain Delivery Pilot, a randomized clinical trial of DHA supplementation (n = 10) versus placebo (n = 12) for six months in nondemented older adults stratified by APOE4 status. Wild-type C57B6/J mice were fed a high or low DHA diet for 6 months followed by plasma and brain lipidomic analysis. Levels of phosphatidylcholine DHA (PC 38:6) and cholesterol ester DHA (CE 22:6) had the largest increases in CSF following supplementation (P < 0.001). DHA within triglyceride (TG) lipids in CSF strongly correlated with corresponding plasma TG lipids, and differed by APOE4, with carriers having a lower increase than noncarriers. Changes in plasma PC DHA had the strongest association with changes in EC thickness in millimeters, independent of APOE4 status (P = 0.007). In mice, a high DHA diet increased PUFAs within brain lipids. Our findings demonstrate an exchange of DHA at the CSF-blood barrier and into the brain within all lipid species with APOE having the strongest effect on DHA-containing TGs. The correlation of PC DHA with EC suggests a functional consequence of DHA accretion in high density lipoprotein for the brain.

The Influence of APOE Genotype, DHA, and Flavanol Intervention on Brain DHA and Lipidomics Profile in Aged Transgenic Mice

The apolipoprotein E4 (APOE4) genotype is predictive of Alzheimer's disease (AD). The brain is highly enriched with the omega-3 polyunsaturated fatty acid (n3-PUFA), docosahexaenoic acid (DHA). DHA's metabolism is defective in APOE4 carriers. Flavanol intake can play a role in modulating DHA levels. However, the impact of flavanol co-supplementation with fish oil on brain DHA uptake, status and partitioning, and according to APOE genotype is currently unknown. Here, using a humanised APOE3 and APOE4 targeted replacement transgenic mouse model, the interactive influence of cocoa flavanols (FLAV) and APOE genotype on the blood and subcortical brain PUFA status following the supplementation of a high fat (HF) enriched with DHA from fish oil (FO) was investigated. DHA levels increased in the blood (p < 0.001) and brain (p = 0.001) following supplementation. Compared to APOE3, a higher red blood cell (RBC) DHA (p < 0.001) was evident in APOE4 mice following FO and FLAV supplementation. Although FO did not increase the percentage of brain DHA in APOE4, a 17.1% (p < 0.05) and 20.0% (p < 0.001) higher DHA level in the phosphatidylcholine (PC) fraction in the HF FO and HF FO FLAV groups, and a 14.5% (p < 0.05) higher DHA level in the phosphatidylethanolamine (PE) fraction in the HF FO FLAV group was evident in these animals relative to the HF controls. The addition of FLAV (+/- FO) did not significantly increase the percentage of brain DHA in the group as a whole. However, a higher brain: RBC DHA ratio was evident in APOE3 only (p < 0.05) for HF FLAV versus HF. In conclusion, our data shows only modest effects of FLAV on the brain DHA status, which is limited to APOE3.

New perspectives on randomized controlled trials with omega-3 fatty acid supplements and cognition: A scoping review

Long chain polyunsaturated omega-3 fatty acids (n-3 FA), such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are known to be important components in a healthy diet and contribute to healthy functioning of the heart and the brain, among other organs. Although there are epidemiological studies on the strong relationship between fish or n-3 FA consumption and lower risk of cognitive decline, results from randomized controlled trials (RCTs) are less consistent. Here, we performed a scoping review on RCTs with n-3 FA supplementation where cognition was evaluated. Seventy-eight RCTs published before April 2022 were included in this review. Among these RCTs, 43.6% reported a positive cognitive outcome after the consumption of n-3 FA compared to the placebo. However, there was a large diversity of populations studied (age ranges and health status), wide range of doses of EPA + DHA supplemented (79 mg/day - 5200 mg/day) and a multitude of tests evaluating cognition, mainly diagnostic tests, that were used to assess cognitive scores and overall cognitive status. RCTs were thereafter categorized into non-cognitively impaired middle-aged adults (n = 24), non-cognitively impaired older adults (n = 24), adults with subjective memory complaints (n = 14), adults with mild cognitive impairments (MCI, n = 9) and people with diagnosed dementia or other cognitive changes (n = 7). Among these categories, 66.7% of RCTs conducted with MCI adults reported a positive cognitive outcome when supplemented with n-3 FA vs. the placebo. Therefore, this scoping review provides rationale and questions to a) strengthen the design of future RCTs with n-3 FA for cognitive outcomes, and b) generate more informative data to support clinicians in their practice in assessing cognition before and after a nutritional intervention.

Alterations in Peripheral Metabolites as Key Actors in Alzheimer's Disease

Growing evidence supports that Alzheimer's disease (AD) could be regarded as a metabolic disease, accompanying central and peripheral metabolic disturbance. Nowadays, exploring novel and potentially alternative hallmarks for AD is needed. Peripheral metabolites based on blood and gut may provide new biochemical insights about disease mechanisms. These metabolites can influence brain energy homeostasis, maintain gut mucosal integrity, and regulate the host immune system, which may further play a key role in modulating the cognitive function and behavior of AD. Recently, metabolomics has been used to identify key AD-related metabolic changes and define metabolic changes during AD disease trajectory. This review aims to summarize the key blood- and microbial-derived metabolites that are altered in AD and identify the potential metabolic biomarkers of AD, which will provide future targets for precision therapeutic modulation.

Uncovering mechanisms of brain inflammation in Alzheimer's disease with APOE4: Application of single cell-type lipidomics

A chronic state of unresolved inflammation in Alzheimer's disease (AD) is intrinsically involved with the remodeling of brain lipids. This review highlights the effect of carrying the apolipoprotein E ε4 allele (APOE4) on various brain cell types in promoting an unresolved inflammatory state. Among its pleotropic effects on brain lipids, we focus on APOE4's activation of Ca²⁺ -dependent phospholipase A2 (cPLA2) and its effects on arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid signaling cascades in the brain. During the process of neurodegeneration, various brain cell types, such as astrocytes, microglia, and neurons, together with the neurovascular unit, develop distinct inflammatory phenotypes that impact their functions and have characteristic lipidomic fingerprints. We propose that lipidomic phenotyping of single cell-types harvested from brains differing by age, sex, disease severity stage, and dietary and genetic backgrounds can be employed to probe mechanisms of neurodegeneration. A better understanding of the brain cellular inflammatory/lipidomic response promises to guide the development of nutritional and drug interventions for AD dementia.

ApoE in Alzheimer’s disease: pathophysiology and therapeutic strategies

Alzheimer’s disease (AD) is the most common cause of dementia worldwide, and its prevalence is rapidly increasing due to extended lifespans. Among the increasing number of genetic risk factors identified, the apolipoprotein E (APOE) gene remains the strongest and most prevalent, impacting more than half of all AD cases. While the ε4 allele of the APOE gene significantly increases AD risk, the ε2 allele is protective relative to the common ε3 allele. These gene alleles encode three apoE protein isoforms that differ at two amino acid positions. The primary physiological function of apoE is to mediate lipid transport in the brain and periphery; however, additional functions of apoE in diverse biological functions have been recognized. Pathogenically, apoE seeds amyloid-β (Aβ) plaques in the brain with apoE4 driving earlier and more abundant amyloids. ApoE isoforms also have differential effects on multiple Aβ-related or Aβ-independent pathways. The complexity of apoE biology and pathobiology presents challenges to designing effective apoE-targeted therapeutic strategies. This review examines the key pathobiological pathways of apoE and related targeting strategies with a specific focus on the latest technological advances and tools.

DHA-Enriched Fish Oil Ameliorates Deficits in Cognition Associated with Menopause and the APOE4 Genotype in Rodents

Female APOE4 carriers have a greater predisposition to developing Alzheimer’s disease (AD) compared to their male counterparts, which may partly be attributed to menopause. We previously reported that a combination of menopause and APOE4 led to an exacerbation of cognitive and neurological deficits, which were associated with reduced brain DHA and DHA:AA ratio. Here, we explored whether DHA-enriched fish oil (FO) supplementation mitigated the detrimental impact of these risk factors. Whilst DHA-enriched fish oil improved recognition memory (NOR) in APOE4 VCD (4-vinylcyclohexene diepoxide)-treated mice (p < 0.05), no change in spatial working memory (Y-maze) was observed. FO supplementation increased brain DHA and nervonic acid and the DHA:AA ratio. The response of key bioenergetic and blood−brain barrier related genes and proteins provided mechanistic insights into these behavioural findings, with increased BDNF protein concentration as well as mitigation of aberrant Erβ, Cldn1 and Glut-5 expression in APOE4 mice receiving fish oil supplementation (p < 0.05). In conclusion, supplementation with a physiologically relevant dose of DHA-enriched fish oil appears to offer protection against the detrimental effects of menopause, particularly in “at-risk” APOE4 female carriers.

Long-chain Omega-3 fatty acids supplementation and cognitive performance throughout adulthood: A 6-month randomized controlled trial

OBJECTIVE

To investigate whether omega-3 polyunsaturated fatty acids (n-3 PUFA) supplementation improve cognitive performance and if apolipoprotein E (APOE) genotype or age were effect modifiers.

METHODS

Healthy adults of 20 to 80 years old (n = 193) were completed a 6-month double-blind randomized controlled trial with two groups: 2.5 g/day of n-3 PUFA or a placebo. Primary outcomes were visuospatial ability and working memory and secondary outcomes were episodic memory and executive function, measured at baseline and 6 months.

RESULTS

Cognitive performances did not significantly differ between groups on primary or secondary outcomes after 6 months of treatment. APOE carriers and age were not effect modifiers for any outcomes. Those with low episodic memory scores and taking the n-3 PUFA supplement, significantly improved their scores (p = 0.043).

CONCLUSIONS

A 6-month n-3 PUFA supplementation did not improve cognitive performance in cognitively healthy adults and APOE status or age were not effect modifiers.

Use of fish oil supplements is differently related to incidence of all-cause and vascular dementia among people with the distinct APOE ε4 dosage

BACKGROUNDS &AIMS

Previous studies have shown that marine omega-3 PUFAs (fish oil) supplements was associated with improved cognitive function, whereas the association between use of fish oil supplements and risk of incident dementia was still unclear. We aimed to prospectively assess the relations between use of fish oil supplements and risks of all-cause and disease-specific dementia according to the apolipoprotein E (APOE) ε4 dosage.

METHODS

A total of 445,961 participants from UK biobank, who were free of dementia at baseline and completed data on supplement use and genetic information were analyzed in this study. Cox proportional hazards models were used to calculate the hazard ratios (HRs) comparing incident dementia rates in participants who did and did not use fish oil.

RESULTS

During a median of 12.2 years of follow-up, a total of 5795 incident cases of dementia were documented, including 1266 cases of vascular dementia and 2382 cases of AD. After adjustment for covariates, use of fish oil supplements was significantly associated with lower risks of all-cause dementia (Hazard ratios, HR, 95% CI, 0.90, 0.85-0.96) and vascular dementia (HR, 0.85; 95% CI, 0.75-0.97), but not AD (HR, 0.99; 95% CI, 0.91-1.09). For all-cause dementia and vascular dementia, we found that the protective associations appeared to be attenuated by the increasing APOE ε4 dosage (P-interaction = 0.002 and 0.002, respectively). Notably, the use of fish oil supplements was significantly associated with an 86.0% higher risk of vascular dementia in participants with two APOE-ε4 alleles (HR, 1.86, 95%CI, 1.23-2.80).

CONCLUSIONS

Our results indicate that use of fish oil supplements is differently associated with risks of all-cause dementia and vascular dementia according to the APOE ε4 dosage.

Factors Influencing Alzheimer's Disease Risk: Whether and How They are Related to the APOE Genotype

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease featuring progressive cognitive impairment. Although the etiology of late-onset AD remains unclear, the close association of AD with apolipoprotein E (APOE), a gene that mainly regulates lipid metabolism, has been firmly established and may shed light on the exploration of AD pathogenesis and therapy. However, various confounding factors interfere with the APOE-related AD risk, raising questions about our comprehension of the clinical findings concerning APOE. In this review, we summarize the most debated factors interacting with the APOE genotype and AD pathogenesis, depict the extent to which these factors relate to APOE-dependent AD risk, and discuss the possible underlying mechanisms.

Why Have the Benefits of DHA Not Been Borne Out in the Treatment and Prevention of Alzheimer's Disease? A Narrative Review Focused on DHA Metabolism and Adipose Tissue

Docosahexaenoic acid (DHA), an omega-3 fatty acid rich in seafood, is linked to Alzheimer's Disease via strong epidemiological and pre-clinical evidence, yet fish oil or other DHA supplementation has not consistently shown benefit to the prevention or treatment of Alzheimer's Disease. Furthermore, autopsy studies of Alzheimer's Disease brain show variable DHA status, demonstrating that the relationship between DHA and neurodegeneration is complex and not fully understood. Recently, it has been suggested that the forms of DHA in the diet and plasma have specific metabolic fates that may affect brain uptake; however, the effect of DHA form on brain uptake is less pronounced in studies of longer duration. One major confounder of studies relating dietary DHA and Alzheimer's Disease may be that adipose tissue acts as a long-term depot of DHA for the brain, but this is poorly understood in the context of neurodegeneration. Future work is required to develop biomarkers of brain DHA and better understand DHA-based therapies in the setting of altered brain DHA uptake to help determine whether brain DHA should remain an important target in the prevention of Alzheimer's Disease.

Fish, n-3 fatty acids, cognition and dementia risk: not just a fishy tale

With growing and ageing populations, the incidence of dementia is expected to triple globally by 2050. In the absence of effective drugs to treat or reverse the syndrome, dietary approaches which prevent or delay disease onset have considerable population health potential. Prospective epidemiological studies and mechanistic insight from experimental models strongly support a positive effect of a high fish and long chain n-3 fatty acid (EPA and DHA) intake on a range of cognitive outcomes and dementia risk, with effect sizes equivalent to several years of ageing between the highest and lowest consumers. As reviewed here, an effect of EPA and DHA on neuroinflammation and oxylipin production is likely to in part mediate the neurophysiological benefits. However, randomised controlled trials (RCTs) with EPA and DHA supplementation have produced mixed findings. Insight into the likely modulators of response to intervention and factors which should be considered for future RCTs are given. Furthermore, the impact of APOE genotype on disease risk and response to EPA and DHA supplementation is summarised. The prevalence of dementia is several-fold higher in APOE4 females (about 13% Caucasian populations) relative to the general population, who are emerging as a subgroup who may particularly benefit from DHA intervention, prior to the development of significant pathology.

APOE4 genotype exacerbates the impact of menopause on cognition and synaptic plasticity in APOE-TR mice

The impact of sex and menopausal status in Alzheimer's disease remains understudied despite increasing evidence of greater female risk, particularly in APOE4 carriers. Utilizing female APOE-TR mice maintained on a high-fat diet background we induced ovarian failure through repeated VCD injections, to mimic human menopause. At 12 months of age, recognition memory and spatial memory were assessed using object recognition, Y-maze spontaneous alternation, and Barnes maze. A VCD*genotype interaction reduced the recognition memory (P < .05), with APOE4 VCD-treated animals unable to distinguish between novel and familiar objects. APOE4 mice displayed an additional 37% and 12% reduction in Barnes (P < .01) and Y-maze (P < .01) performance, indicative of genotype-specific spatial memory impairment. Molecular analysis indicated both VCD and genotype-related deficits in synaptic plasticity with BDNF, Akt, mTOR, and ERK signaling compromised. Subsequent reductions in the transcription factors Creb1 and Atf4 were also evident. Furthermore, the VCD*genotype interaction specifically diminished Ephb2 expression, while Fos, and Cnr1 expression reduced as a consequence of APOE4 genotype. Brain DHA levels were 13% lower in VCD-treated animals independent of genotype. Consistent with this, we detected alterations in the expression of the DHA transporters Acsl6 and Fatp4. Our results indicate that the combination of ovarian failure and APOE4 leads to an exacerbation of cognitive and neurological deficits.

LPC-DHA/EPA-Enriched Diets Increase Brain DHA and Modulate Behavior in Mice That Express Human APOE4

Compared with APOE3, APOE4 is associated with greater age-related cognitive decline and higher risk of neurodegenerative disorders. Therefore, development of supplements that target APOE genotype-modulated processes could provide a great benefit for the aging population. Evidence suggests a link between APOE genotype and docosahexaenoic acid (DHA); however, clinical studies with current DHA supplements have produced negative results in dementia. The lack of beneficial effects with current DHA supplements may be related to limited bioavailability, as the optimal form of DHA for brain uptake is lysophosphatidylcholine (LPC)-DHA. We previously developed a method to enrich the LPC-DHA content of krill oil through lipase treatment (LT-krill oil), which resulted in fivefold higher enrichment in brain DHA levels in wild-type mice compared with untreated krill oil. Here, we evaluated the effect of a control diet, diet containing krill oil, or a diet containing LT-krill oil in APOE3- and APOE4-targeted replacement mice (APOE-TR mice; treated from 4 to 12 months of age). We found that DHA levels in the plasma and hippocampus are lower in APOE4-TR mice and that LT-krill oil increased DHA levels in the plasma and hippocampus of both APOE3- and APOE4-TR mice. In APOE4-TR mice, LT-krill oil treatment resulted in higher levels of the synaptic vesicle protein SV2A and improved performance on the novel object recognition test. In conclusion, our data demonstrate that LPC-DHA/EPA-enriched krill oil can increase brain DHA and improve memory-relevant behavior in mice that express APOE4. Therefore, long-term use of LT-krill oil supplements may on some level protect against age-related neurodegeneration.

The role of docosahexaenoic acid (DHA) in the prevention of cognitive impairment in the elderly

Aging is an inevitable and progressive biological process that leads to irreversible physiological and functional changes, also in the nervous system. Cognitive decline occurring with age can significantly affect the quality of life of older people. Docosahexaenoic acid (DHA) is necessary for the proper functioning of the nervous system; it can affect its action directly through its impact on neurogenesis and neuroplasticity, but also indirectly by affecting the functioning of the cardiovascular system or anti-inflammatory effect. Literature analysis shows that good nutritional status of n-3 fatty acids, determined on the basis of their level in blood plasma or erythrocytes, is associated with a lower risk of cognitive decline in selected cognitive domains, as well as a lower risk of dementia or Alzheimer’s disease, although studies are also available where the above relationship has not been confirmed. Apart from this, studies on DHA and EPA diet intake, as well as in the form of dietary supplements, show their beneficial effects in the context of cognitive functioning and the risk of dementia. Also, the results of intervention studies, although not explicit, suggest that high doses of DHA and EPA in the form of dietary supplements may slow down the process of deteriorating the cognitive functioning of the elderly within selected domains. Based on the review of the literature, it can be concluded that DHA and EPA play an essential role in the prevention of cognitive impairment.

Associations of Omega-3 fatty acids with brain morphology and volume in cognitively healthy older adults: A narrative review

INTRODUCTION

Human neurodevelopment is complete by the 4th decade of life at which point brain atrophy ensues with variable rate and regionality into old age. Literally all regions of the brain experience atrophy with older age, however the pattern and rate of atrophy can dictate the behavioral consequences (i.e., cognitive impairment, Alzheimer's disease). Substantial research has aimed to discover the reasons why some people experience greater morphologic changes that produce undesirable consequences with aging and how it may be prevented. One possible explanation is diet, particularly fish consumption and the intake of omega-3 polyunsaturated fatty acids (omega 3) concentrated in fish oil. This narrative review examines the available evidence on the association between omega-3 and brain volume in non-demented older adults.

METHODS

A PubMed search of the literature was conducted in search of studies that investigated the associations of omega-3 on brain morphology and volume in cognitively intact older adults. Inclusion criteria were: populations of adults aged 45 years or over, who were cognitively intact, free of any central nervous system disease, and free of advanced structural brain atrophy. Study participants had to have DHA and EPA levels measured either by blood testing or scoring of dietary intake. There were no restrictions to dates of publication. Studies including demented participants, or participants with substantial white or grey matter atrophy visible on magnetic resonance imaging were excluded.

RESULTS AND CONCLUSION

The search identified only 12 studies, 8 of which were cross-sectional observational studies, 3 longitudinal observational studies, and 1 randomized controlled trial published between 2007 and 2019. The largest amount of evidence indicated that the hippocampus was most frequently involved in this association, with a higher volume associated with higher omega-3 levels. Larger total grey matter, total brain volume, and lower white matter lesion volume were also associated with higher omega-3 among four of the reviewed studies. However, most studies reviewed provided mixed findings regarding the presence or absence of the association of interest, and the findings were observed to be brain region-dependent. Current evidence is still insufficient to formulate recommendations for omega-3 intake to support brain health specifically.

APOE and Alzheimer's Disease: From Lipid Transport to Physiopathology and Therapeutics

Alzheimer’s disease (AD) is a devastating neurodegenerative disorder characterized by extracellular amyloid β (Aβ) and intraneuronal tau protein aggregations. One risk factor for developing AD is the APOE gene coding for the apolipoprotein E protein (apoE). Humans have three versions of APOE gene: ε2, ε3, and ε4 allele. Carrying the ε4 allele is an AD risk factor while carrying the ε2 allele is protective. ApoE is a component of lipoprotein particles in the plasma at the periphery, as well as in the cerebrospinal fluid (CSF) and in the interstitial fluid (ISF) of brain parenchyma in the central nervous system (CNS). ApoE is a major lipid transporter that plays a pivotal role in the development, maintenance, and repair of the CNS, and that regulates multiple important signaling pathways. This review will focus on the critical role of apoE in AD pathogenesis and some of the currently apoE-based therapeutics developed in the treatment of AD.

The role of peripheral fatty acids as biomarkers for Alzheimer's disease and brain inflammation

Alzheimer's disease (AD) is a complex and heterogeneous neurodegenerative disease. A wide range of techniques have been proposed to facilitate early diagnosis of AD, including biomarkers from the cerebrospinal fluid and blood. Although phosphorylated tau and amyloid beta are amongst the most promising biomarkers of AD, other peripheral biomarkers have been identified and in this review we synthesize the current knowledge on circulating fatty acids. Fatty acids are involved in different biological process including neurotransmission and inflammation. Interestingly, some fatty acids appear to be modulated during disease progression, including long chain saturated fatty acids, and polyunsaturated fatty acids, such as docosahexaenoic acid . However, discrepant results have been reported in the literature and there is the need for further validation and method standardization. Nonetheless, our literature review suggests that fatty acid analyses could potentially provide a valuable source of data to further inform the pathology and progression of AD.

A Scoping Review of Dietary Factors Conferring Risk or Protection for Cognitive Decline in APOE ε4 Carriers

Alzheimer's disease (AD) is a progressive and fatal neurodegenerative disease. The strongest genetic risk factor for sporadic AD is carriage of the ε4 allele of the Apolipoprotein E (APOE) gene. Strategies to slow the progression of AD, including dietary interventions, may be modified by the pathogenic effect of this polymorphism. Our objective in this review was to determine the extent and quality of the literature investigating how dietary factors and interventions interact with the APOE ε4 genotype to impact cognitive decline in AD. To that end, we performed a systematic scoping review of published English-language articles involving human subjects. We found evidence suggesting that adherence to a Mediterranean diet may reduce cognitive decline among APOE ε4 carriers, whereas ketogenic agents appear to be ineffective. Diets high in saturated fats may be particularly harmful for APOE ε4 carriers. We identified several topics, including the use of ω-3 fatty acid and antioxidant supplements, for which additional high level evidence is needed.

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.

Brain delivery of supplemental docosahexaenoic acid (DHA): A randomized placebo-controlled clinical trial

BACKGROUND

Past clinical trials of docosahexaenoic Acid (DHA) supplements for the prevention of Alzheimer's disease (AD) dementia have used lower doses and have been largely negative. We hypothesized that larger doses of DHA are needed for adequate brain bioavailability and that APOE4 is associated with reduced delivery of DHA and eicosapentaenoic acid (EPA) to the brain before the onset of cognitive impairment.

METHODS

33 individuals were provided with a vitamin B complex (1 mg vitamin B12, 100 mg of vitamin B6 and 800 mcg of folic acid per day) and randomized to 2,152 mg of DHA per day or placebo over 6 months. 26 individuals completed both lumbar punctures and MRIs, and 29 completed cognitive assessments at baseline and 6 months. The primary outcome was the change in CSF DHA. Secondary outcomes included changes in CSF EPA levels, MRI hippocampal volume and entorhinal thickness; exploratory outcomes were measures of cognition.

FINDINGS

A 28% increase in CSF DHA and 43% increase in CSF EPA were observed in the DHA treatment arm compared to placebo (mean difference for DHA (95% CI): 0.08 µg/mL (0.05, 0.10), p<0.0001; mean difference for EPA: 0.008 µg/mL (0.004, 0.011), p<0.0001). The increase in CSF EPA in non-APOE4 carriers after supplementation was three times greater than APOE4 carriers. The change in brain volumes and cognitive scores did not differ between groups.

INTERPRETATION

Dementia prevention trials using omega-3 supplementation doses equal or lower to 1 g per day may have reduced brain effects, particularly in APOE4 carriers.

TRIAL REGISTRATION

NCT02541929.

FUNDING

HNY was supported by R01AG055770, R01AG054434, R01AG067063 from the National Institute of Aging and NIRG-15-361854 from the Alzheimer's Association, and MGH by the L. K. Whittier Foundation. This work was also supported by P50AG05142 (HCC) from the National Institutes of Health. Funders had no role in study design, data collection, data analysis, interpretation, or writing of the report.

APOE and metabolic dysfunction in Alzheimer's disease

The strongest genetic risk factor for sporadic Alzheimer's disease (AD) is carriage of the E4 allele of APOE. Metabolic dysfunction also increases risk of dementia and AD. Facing a need for effective therapies and an aging global population, studies aimed at uncovering new therapeutic targets for AD have become critical. Insight into the biology underlying the effects of E4 and metabolic impairment on the brain may lead to novel therapies to reduce AD risk. An understudied hallmark of both AD patients and E4 individuals is a common metabolic impairment-cerebral glucose hypometabolism. This is a robust and replicated finding in humans, and begins decades prior to cognitive decline. Possession of E4 also appears to alter several other aspects of cerebral glucose metabolism, fatty acid metabolism, and management of oxidative stress through the pentose phosphate pathway. A critical knowledge gap in AD is the mechanism by which APOE alters cerebral metabolism and clarification as to its relevance to AD risk. Facing a need for effective therapies, studies aimed at uncovering new therapeutic targets have become critical. One such approach is to gain a better understanding of the metabolic mechanisms that may underlie E4-associated cognitive dysfunction and AD risk.

Biosynthesis of uniformly carbon isotope-labeled docosahexaenoic acid in Crypthecodinium cohnii

Docosahexaenoic acid (DHA) enriched in brain can yield many important degradation products after the attack of hydroxyl radicals, which is known to serve as a nutraceutical and neuroprotective effects. Oxidative stress is a commonly observed feature of Alzheimer's disease (AD). Therefore, uniformly radiolabeled DHA plays an important role in studying the oxidative fate of DHA in vivo and vitro. However, carbon isotope labeled DHA isn't commercially available now. The heterotrophic microalgae Crypthecodinium cohnii (C. cohnii) has been identified as a prolific producer of DHA. In this study, the growth rate and DHA production in C. cohnii were optimized in a new defined media, and the biosynthesis of U-13C-DHA from U-13C-glucose and U-14C-DHA from U-14C-glucose were analyzed by HPLC-MS/MS. Approximately 40 nmoles of U-13C-DHA with higher isotopic purity of 96.8% was produced in a 300 μL batch, and ~ 0.23 μCi of U-14C-DHA with significant specific activity of 5-6 Ci/mol was produced in a 300 μL batch. It was found that C. cohnii had the optimal growth and DHA accumulation at 25 °C in this defined media (C/N = 10). An efficient protocol for the biosynthesis of U-13C-DHA and U-14C-DHA were set up firstly, which provides the basic support for the analysis of oxidative degradation products of DHA in AD.

Effect of APOE Genotype on Plasma Docosahexaenoic Acid (DHA), Eicosapentaenoic Acid, Arachidonic Acid, and Hippocampal Volume in the Alzheimer's Disease Cooperative Study-Sponsored DHA Clinical Trial

BACKGROUND

Docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), and arachidonic acid (AA) play key roles in several metabolic processes relevant to Alzheimer's disease (AD) pathogenesis and neuroinflammation. Carrying the APOEɛ4 allele (APOE4) accelerates omega-3 polyunsaturated fatty acid (PUFA) oxidation. In a pre-planned subgroup analysis of the Alzheimer's Disease Cooperative Study-sponsored DHA clinical trial, APOE4 carriers with mild probable AD had no improvements in cognitive outcomes compared to placebo, while APOE 4 non-carriers showed a benefit from DHA supplementation.

OBJECTIVE

We sought to clarify the effect of APOEɛ4/ɛ4 on both the ratio of plasma DHA and EPA to AA, and on hippocampal volumes after DHA supplementation.

METHODS

Plasma fatty acids and APOE genotype were obtained in 275 participants randomized to 18 months of DHA supplementation or placebo. A subset of these participants completed brain MRI imaging (n = 86) and lumbar punctures (n = 53).

RESULTS

After the intervention, DHA-treated APOEɛ3/ɛ3 and APOEɛ2/ɛ3 carriers demonstrated significantly greater increase in plasma DHA/AA compared to ɛ4/ɛ4 carriers. APOEɛ2/ɛ3 had a greater increase in plasma EPA/AA and less decline in left and right hippocampal volumes compared to compared to ɛ4/ɛ4 carriers. The change in plasma and cerebrospinal fluid DHA/AA was strongly correlated. Greater baseline and increase in plasma EPA/AA was associated with a lower decrease in the right hippocampal volume, but only in APOE 4 non-carriers.

CONCLUSION

The lower increase in plasma DHA/AA and EPA/AA in APOEɛ4/ɛ4 carriers after DHA supplementation reduces brain delivery and affects the efficacy of DHA supplementation.

Effect of n-3 long-chain polyunsaturated fatty acids on mild cognitive impairment: a meta-analysis of randomized clinical trials

N-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFAs) have positive effect on cognitive function with mild cognitive impairment (MCI) is still controversial. The aim for this meta-analysis was to assess the scientific evidence published in the last 10 years on the effects of n-3 LC-PUFAs intake on MCI patients to explore whether n-3 LC-PUFAs have positive effective. A comprehensive literature search was developed using the Google Scholar, EMBASE, and PubMed database. The pooled effect for all studies was calculated using random-effects model. And the terms of weighted mean difference (WMD) with 95% confidence interval (CI) was pooled and indicated the effects. Heterogeneity was assessed by I² statistics. A total of seven randomized clinical trials involving 213 cases of intervention and 221 cases of placebo were included in this analysis. Compared with placebo, n-3 LC-PUFAs supplements effectively improved cognition in elders with MCI (WMD = 0.85, 95% CI: 0.04-1.67, Z = 2.05, P = 0.04). Slight heterogeneity was detected across studies. Our results provided further evidence that n-3 LC-PUFAs may have beneficial effect in elderly with MCI.

Omega-3 PUFA metabolism and brain modifications during aging

In Canada, 5.5 million (16% of Canadians) adults are >65 years old and projections suggest this number will be approximately 20% of Canadians by 2024. A major concern regarding old age is a decline in health, especially if this entails a loss of self-sufficiency and independence caused by a decline in cognition. The brain contains 60% of fat and is one of the most concentrated organs in long chain omega-3 fatty acids such as docosahexaenoic acid (DHA). During aging, there are physiological modifications in the metabolism of lipids that could also have consequences on brain structure and levels of DHA. This review will hence discuss the physiological modifications in the metabolism of lipids during aging with a focus on long chain omega-3 and omega-6 fatty acids and also outline the structural and functional modifications of the brain during aging including brain lipid modifications and its relation to higher levels of DHA and cognition. Therefore, in this review, we outline the importance of collecting more data on the biology of aging since it might highly improve our understanding about what are «normal» modifications occurring during aging and what can become pathological.

Apolipoprotein E4 Alters Astrocyte Fatty Acid Metabolism and Lipid Droplet Formation

Lipid droplets (LDs) serve as energy rich reservoirs and have been associated with apolipoprotein E (APOE) and neurodegeneration. The E4 allele of APOE (E4) is the strongest genetic risk factor for the development of late onset Alzheimer's disease (AD). Since both E4 carriers and individuals with AD exhibit a state of cerebral lipid dyshomeostasis, we hypothesized that APOE may play a role in regulating LD metabolism. We found that astrocytes expressing E4 accumulate significantly more and smaller LDs compared to E3 astrocytes. Accordingly, expression of perilipin-2, an essential LD protein component, was higher in E4 astrocytes. We then probed fatty acid (FA) metabolism and found E4 astrocytes to exhibit decreased uptake of palmitate, and decreased oxidation of exogenously supplied oleate and palmitate. We then measured oxygen consumption rate, and found E4 astrocytes to consume more oxygen for endogenous FA oxidation and accumulate more LD-derived metabolites due to incomplete oxidation. Lastly, we found that E4 astrocytes are more sensitive to carnitine palmitoyltransferase-1 inhibition than E3 astrocytes. These findings offer the potential for further studies investigating the link between astrocyte lipid storage, utilization, and neurodegenerative disease as a function of APOE genotype.

Effects of apolipoprotein E on nutritional metabolism in dementia

PURPOSE OF REVIEW

Various groups have explored the effect of apolipoprotein E (APOE) on neurodegeneration through nutritional and metabolic alterations. In this review, we hope to summarize recent findings in humans as well as preclinical APOE models.

RECENT FINDINGS

Metabolic pathways including lipid metabolism appear to play a large role in the pathophysiology of Alzheimer's disease. Carrier status of the E4 variant of the APOE gene is the strongest genetic risk factor for Alzheimer's disease, and increasing evidence suggests that E4 carriers may respond differently to a host of dietary and metabolic-related treatments. A new appreciation is forming for the role of APOE in cerebral metabolism, and how nutritional factors may impact this role.

SUMMARY

Considering the role dietary factors play in APOE-associated cognitive decline will help us to understand how nutritional interventions may facilitate or mitigate disease progression.

Non-dietary factors associated with n-3 long-chain PUFA levels in humans - a systematic literature review

Numerous health benefits are attributed to the n-3 long-chain PUFA (n-3 LCPUFA); EPA and DHA. A systematic literature review was conducted to investigate factors, other than diet, that are associated with the n-3 LCPUFA levels. The inclusion criteria were papers written in English, carried out in adult non-pregnant humans, n-3 LCPUFA measured in blood or tissue, data from cross-sectional studies, or baseline data from intervention studies. The search revealed 5076 unique articles of which seventy were included in the qualitative synthesis. Three main groups of factors potentially associated with n-3 LCPUFA levels were identified: (1) unmodifiable factors (sex, genetics, age), (2) modifiable factors (body size, physical activity, alcohol, smoking) and (3) bioavailability factors (chemically bound form of supplements, krill oil v. fish oil, and conversion of plant-derived α-linolenic acid (ALA) to n-3 LCPUFA). Results showed that factors positively associated with n-3 LCPUFA levels were age, female sex (women younger than 50 years), wine consumption and the TAG form. Factors negatively associated with n-3 LCPUFA levels were genetics, BMI (if erythrocyte EPA and DHA levels are &lt;5·6 %) and smoking. The evidence for girth, physical activity and krill oil v. fish oil associated with n-3 LCPUFA levels is inconclusive. There is also evidence that higher ALA consumption leads to increased levels of EPA but not DHA. In conclusion, sex, age, BMI, alcohol consumption, smoking and the form of n-3 LCPUFA are all factors that need to be taken into account in n-3 LCPUFA research.

System biology approach intersecting diet and cell metabolism with pathogenesis of brain disorders

The surge in meals high in calories has prompted an epidemic of metabolic disorders around the world such that the elevated incidence of obese and diabetic individuals is alarming. New research indicates that metabolic disorders pose a risk for neurological and psychiatric conditions including stroke, Alzheimer's disease, Huntington's disease, and depression, all of which have a metabolic component. These relationships are rooted to a dysfunctional interaction between molecular processes that regulate energy metabolism and synaptic plasticity. The strong adaptive force of dietary factors on shaping the brain during evolution can be manipulated to transform the interaction between cell bioenergetics and epigenome with the aptitude to promote long-lasting brain healthiness. A thorough understanding of the association between the broad action of nutrients and brain fitness requires high level data processing empowered with the capacity to integrate information from a multitude of molecular entities and pathways. Nutritional systems biology is emerging as a viable approach to elucidate the multiple molecular layers involved in information processing in cells, tissues, and organ systems in response to diet. Information about the wide range of cellular and molecular interactions elicited by foods on the brain and cognitive plasticity is crucial for the design of public health initiatives for curtailing the epidemic of metabolic and brain disorders.

APOE and Alzheimer's Disease: Neuroimaging of Metabolic and Cerebrovascular Dysfunction

Apolipoprotein E4 (ApoE4) is the strongest genetic risk factor for late onset Alzheimer’s Disease (AD), and is associated with impairments in cerebral metabolism and cerebrovascular function. A substantial body of literature now points to E4 as a driver of multiple impairments seen in AD, including blunted brain insulin signaling, mismanagement of brain cholesterol and fatty acids, reductions in blood brain barrier (BBB) integrity, and decreased cerebral glucose uptake. Various neuroimaging techniques, in particular positron emission topography (PET) and magnetic resonance imaging (MRI), have been instrumental in characterizing these metabolic and vascular deficits associated with this important AD risk factor. In the current mini-review article, we summarize the known effects of APOE on cerebral metabolism and cerebrovascular function, with a special emphasis on recent findings via neuroimaging approaches.

Association of oily fish intake, sex, age, BMI and APOE genotype with plasma long-chain n-3 fatty acid composition

n-3 Fatty acids are associated with better cardiovascular and cognitive health. However, the concentration of EPA, DPA and DHA in different plasma lipid pools differs and factors influencing this heterogeneity are poorly understood. Our aim was to evaluate the association of oily fish intake, sex, age, BMI and APOE genotype with concentrations of EPA, DPA and DHA in plasma phosphatidylcholine (PC), NEFA, cholesteryl esters (CE) and TAG. Healthy adults (148 male, 158 female, age 20-71 years) were recruited according to APOE genotype, sex and age. The fatty acid composition was determined by GC. Oily fish intake was positively associated with EPA in PC, CE and TAG, DPA in TAG, and DHA in all fractions (P≤0·008). There was a positive association between age and EPA in PC, CE and TAG, DPA in NEFA and CE, and DHA in PC and CE (P≤0·034). DPA was higher in TAG in males than females (P<0·001). There was a positive association between BMI and DPA and DHA in TAG (P<0·006 and 0·02, respectively). APOE genotype×sex interactions were observed: the APOE4 allele associated with higher EPA in males (P=0·002), and there was also evidence for higher DPA and DHA (P≤0·032). In conclusion, EPA, DPA and DHA in plasma lipids are associated with oily fish intake, sex, age, BMI and APOE genotype. Such insights may be used to better understand the link between plasma fatty acid profiles and dietary exposure and may influence intake recommendations across population subgroups.

Brain docosahexaenoic acid uptake and metabolism

Docosahexaenoic acid (DHA) is the most abundant n-3 polyunsaturated fatty acid in the brain where it serves to regulate several important processes and, in addition, serves as a precursor to bioactive mediators. Given that the capacity of the brain to synthesize DHA locally is appreciably low, the uptake of DHA from circulating lipid pools is essential to maintaining homeostatic levels. Although, several plasma pools have been proposed to supply the brain with DHA, recent evidence suggests non-esterified-DHA and lysophosphatidylcholine-DHA are the primary sources. The uptake of DHA into the brain appears to be regulated by a number of complementary pathways associated with the activation and metabolism of DHA, and may provide mechanisms for enrichment of DHA within the brain. Following entry into the brain, DHA is esterified into and recycled amongst membrane phospholipids contributing the distribution of DHA in brain phospholipids. During neurotransmission and following brain injury, DHA is released from membrane phospholipids and converted to bioactive mediators which regulate signaling pathways important to synaptogenesis, cell survival, and neuroinflammation, and may be relevant to treating neurological diseases. In the present review, we provide a comprehensive overview of brain DHA metabolism, encompassing many of the pathways and key enzymatic regulators governing brain DHA uptake and metabolism. In addition, we focus on the release of non-esterified DHA and subsequent production of bioactive mediators and the evidence of their proposed activity within the brain. We also provide a brief review of the evidence from post-mortem brain analyses investigating DHA levels in the context of neurological disease and mood disorder, highlighting the current disparities within the field.

Anti-Inflammatory Effects of Omega-3 Fatty Acids in the Brain: Physiological Mechanisms and Relevance to Pharmacology

Classically, polyunsaturated fatty acids (PUFA) were largely thought to be relatively inert structural components of brain, largely important for the formation of cellular membranes. Over the past 10 years, a host of bioactive lipid mediators that are enzymatically derived from arachidonic acid, the main n-6 PUFA, and docosahexaenoic acid, the main n-3 PUFA in the brain, known to regulate peripheral immune function, have been detected in the brain and shown to regulate microglia activation. Recent advances have focused on how PUFA regulate the molecular signaling of microglia, especially in the context of neuroinflammation and behavior. Several active drugs regulate brain lipid signaling and provide proof of concept for targeting the brain. Because brain lipid metabolism relies on a complex integration of diet, peripheral metabolism, including the liver and blood, which supply the brain with PUFAs that can be altered by genetics, sex, and aging, there are many pathways that can be disrupted, leading to altered brain lipid homeostasis. Brain lipid signaling pathways are altered in neurologic disorders and may be viable targets for the development of novel therapeutics. In this study, we discuss in particular how n-3 PUFAs and their metabolites regulate microglia phenotype and function to exert their anti-inflammatory and proresolving activities in the brain.

Retroconversion is a minor contributor to increases in eicosapentaenoic acid following docosahexaenoic acid feeding as determined by compound specific isotope analysis in rat liver

Dietary docosahexaenoic acid (DHA, 22:6n-3) not only increases blood and tissue levels of DHA, but also eicosapentaenoic acid (EPA, 20:5n-3). It is generally believed that this increase is due to DHA retroconversion to EPA, however, a slower conversion of α-linolenic acid (ALA, 18:3n-3) derived EPA to downstream metabolic products (i.e. slower turnover of EPA) is equally plausible. In this study, 21-day old Long Evans rats were weaned onto an ALA only or DHA + ALA diet for 12 weeks. Afterwards, livers were collected and the natural abundance ¹³C-enrichment was determined by compound specific isotope analysis (CSIA) of liver EPA by isotope ratio mass-spectrometry and compared to dietary ALA and DHA ¹³C-enrichment. Isotopic signatures (per mil, ‰) for liver EPA were not different (p > 0.05) between the ALA only diet (−25.89 ± 0.39 ‰, mean ± SEM) and the DHA + ALA diet (−26.26 ± 0.40 ‰), suggesting the relative contribution from dietary ALA and DHA to liver EPA did not change. However, with DHA feeding estimates of absolute EPA contribution from ALA increased 4.4-fold (147 ± 22 to 788 ± 153 nmol/g) compared to 3.2-fold from DHA (91 ± 14 to 382 ± 13 nmol/g), respectively. In conclusion, CSIA of liver EPA in rats following 12-weeks of dietary DHA suggests that retroconversion of DHA to EPA is a relatively small contributor to increases in EPA, and that this increase in EPA is largely coming from elongation/desaturation of ALA.

Docosahexaenoic acid (DHA), a fundamental fatty acid for the brain: New dietary sources

Docosahexaenoic acid (C22: 6n-3, DHA) is a long-chain polyunsaturated fatty acid of marine origin fundamental for the formation and function of the nervous system, particularly the brain and the retina of humans. It has been proposed a remarkable role of DHA during human evolution, mainly on the growth and development of the brain. Currently, DHA is considered a critical nutrient during pregnancy and breastfeeding due their active participation in the development of the nervous system in early life. DHA and specifically one of its derivatives known as neuroprotectin D-1 (NPD-1), has neuroprotective properties against brain aging, neurodegenerative diseases and injury caused after brain ischemia-reperfusion episodes. This paper discusses the importance of DHA in the human brain given its relevance in the development of the tissue and as neuroprotective agent. It is also included a critical view about the ways to supply this noble fatty acid to the population.

Metabolism of uniformly labeled 13C-eicosapentaenoic acid and 13C-arachidonic acid in young and old men

Background: Plasma eicosapentaenoic acid (EPA) and arachidonic acid (AA) concentrations increase with age.Objective: The aim of this study was to evaluate EPA and AA metabolism in young and old men by using uniformly labeled carbon-13 (¹³C) fatty acids.Design: Six young (∼25 y old) and 6 old (∼75 y old) healthy men were recruited. Each participant consumed a single oral dose of 35 mg ¹³C-EPA and its metabolism was followed in the course of 14 d in the plasma and 28 d in the breath. After the washout period of ≥28 d, the same participants consumed a single oral dose of 50 mg ¹³C-AA and its metabolism was followed for 28 d in plasma and breath.Results: There was a time × age interaction for ¹³C-EPA (P_{time × age} = 0.008), and the shape of the postprandial curves was different between young and old men. The ¹³C-EPA plasma half-life was ∼2 d for both young and old men (P = 0.485). The percentage dose recovered of ¹³C-EPA per hour as ¹³CO₂ and the cumulative β-oxidation of ¹³C-EPA did not differ between young and old men. At 7 d, however, old men had a >2.2-fold higher plasma ¹³C-DHA concentration synthesized from ¹³C-EPA compared with young men (P_age = 0.03). ¹³C-AA metabolism was not different between young and old men. The ¹³C-AA plasma half-life was ∼4.4 d in both young and old participants (P = 0.589).Conclusions: The metabolism of ¹³C-AA was not modified by age, whereas ¹³C-EPA metabolism was slightly but significantly different in old compared with young men. The higher plasma ¹³C-DHA seen in old men may be a result of slower plasma DHA clearance with age. This trial was registered at clinicaltrials.gov as NCT02957188.

Carriers of an apolipoprotein E epsilon 4 allele are more vulnerable to a dietary deficiency in omega-3 fatty acids and cognitive decline

Carriers of an epsilon 4 allele (E4) of apolipoprotein E (APOE) develop Alzheimer's disease (AD) earlier than carriers of other APOE alleles. The metabolism of plasma docosahexaenoic acid (DHA, 22:6n-3), an omega-3 fatty acid (n-3 FA), taken up by the brain and concentrated in neurons, is disrupted in E4 carriers, resulting in lower levels of brain DHA. Behavioural and cognitive impairments have been observed in animals with lower brain DHA levels, with emphasis on loss of spatial memory and increased anxiety. E4 mice provided a diet deficient in n-3 FA had a greater depletion of n-3 FA levels in organs and tissues than mice carrying other APOE alleles. However, providing n-3 FA can restore levels of brain DHA in E4 animals and in other models of n-3 FA deficiency. In E4 carriers, supplementation with DHA as early as possible might help to prevent the onset of AD and could halt the progression of, and reverse some of the neurological and behavioural consequences of their higher vulnerability to n-3 FA deficiency.

Association of Docosahexaenoic Acid Supplementation With Alzheimer Disease Stage in Apolipoprotein E ε4 Carriers: A Review

Importance

The apolipoprotein E ε4 (APOE4) allele identifies a unique population that is at significant risk for developing Alzheimer disease (AD). Docosahexaenoic acid (DHA) is an essential ω-3 fatty acid that is critical to the formation of neuronal synapses and membrane fluidity. Observational studies have associated ω-3 intake, including DHA, with a reduced risk for incident AD. In contrast, randomized clinical trials of ω-3 fatty acids have yielded mixed and inconsistent results. Interactions among DHA, APOE genotype, and stage of AD pathologic changes may explain the mixed results of DHA supplementation reported in the literature.

Observations

Although randomized clinical trials of ω-3 in symptomatic AD have had negative findings, several observational and clinical trials of ω-3 in the predementia stage of AD suggest that ω-3 supplementation may slow early memory decline in APOE4 carriers. Several mechanisms by which the APOE4 allele could alter the delivery of DHA to the brain may be amenable to DHA supplementation in predementia stages of AD. Evidence of accelerated DHA catabolism (eg, activation of phospholipases and oxidation pathways) could explain the lack of efficacy of ω-3 supplementation in AD dementia. The association of cognitive benefit with DHA supplementation in predementia but not AD dementia suggests that early ω-3 supplementation may reduce the risk for or delay the onset of AD symptoms in APOE4 carriers. Recent advances in brain imaging may help to identify the optimal timing for future DHA clinical trials.

Conclusions and Relevance

High-dose DHA supplementation in APOE4 carriers before the onset of AD dementia can be a promising approach to decrease the incidence of AD. Given the safety profile, availability, and affordability of DHA supplements, refining an ω-3 intervention in APOE4 carriers is warranted.

Docosahexaenoic acid prevents cognitive deficits in human apolipoprotein E epsilon 4-targeted replacement mice

At a population level, dietary consumption of fish rich in docosahexaenoic acid (DHA) is associated with prevention of cognitive decline but this association is not clear in carriers of the apolipoprotein E epsilon 4 allele (E4). Plasma and liver DHA concentrations show significant alterations in E4 carriers, in part corrected by DHA supplementation. However, whether DHA sufficiency in E4 carriers has consequences on cognition is unknown. Mice expressing human E4 or apolipoprotein E epsilon 3 allele (E3) were fed either a control diet or a diet containing DHA for 8 months and cognitive performance was tested using the object recognition test and the Barnes maze test. In E4 mice fed the control diet, impaired memory was detected and arachidonic acid concentrations were elevated in the hippocampus compared to E3 mice fed the control diet. DHA consumption prevented memory decline and restored arachidonic acid concentrations in the hippocampus of E4 mice. Our results suggest that long-term high-dose DHA intake may prevent cognitive decline in E4 carriers.

DHA brain uptake and APOE4 status: a PET study with [1-11C]-DHA

BACKGROUND

The apolipoprotein E ɛ4 (APOE4) allele is the strongest genetic risk factor identified for developing Alzheimer's disease (AD). Among brain lipids, alteration in the ω-3 polyunsaturated fatty acid docosahexaenoic acid (DHA) homeostasis is implicated in AD pathogenesis. APOE4 may influence both brain DHA metabolism and cognitive outcomes.

METHODS

Using positron emission tomography, regional incorporation coefficients (k*), rates of DHA incorporation from plasma into the brain using [1-¹¹C]-DHA (J _in), and regional cerebral blood flow using [¹⁵O]-water were measured in 22 middle-aged healthy adults (mean age 35 years, range 19-65 years). Data were partially volume error-corrected for brain atrophy. APOE4 phenotype was determined by protein expression, and unesterified DHA concentrations were quantified in plasma. An exploratory post hoc analysis of the effect of APOE4 on DHA brain kinetics was performed.

RESULTS

The mean global gray matter DHA incorporation coefficient, k*, was significantly higher (16%) among APOE4 carriers (n = 9) than among noncarriers (n = 13, p = 0.046). Higher DHA incorporation coefficients were observed in several brain regions, particularly in the entorhinal subregion, an area affected early in AD pathogenesis. Cerebral blood flow, unesterified plasma DHA, and whole brain DHA incorporation rate (J _in) did not differ significantly between the APOE groups.

CONCLUSIONS

Our findings suggest an increase in the DHA incorporation coefficient in several brain regions in APOE4 carriers. These findings may contribute to understanding how APOE4 genotypes affect AD risk.

The effect of dietary fish oil on weight gain and insulin sensitivity is dependent on APOE genotype in humanized targeted replacement mice

We investigated the independent and interactive impact of the common APOE genotype and marine n-3 polyunsaturated fatty acids (PUFAs) on the development of obesity and associated cardiometabolic dysfunction in a murine model. Human APOE3 and APOE4 targeted replacement mice were fed either a control high-fat diet (HFD) or an HFD supplemented with 3% n-3 PUFAs from fish oil (HFD + FO) for 8 wk. We established the impact of intervention on food intake, body weight, and visceral adipose tissue (VAT) mass; plasma, lipids (cholesterol and triglycerides), liver enzymes, and adipokines; glucose and insulin during an intraperitoneal glucose tolerance test; and Glut4 and ApoE expression in VAT. HFD feeding induced more weight gain and higher plasma lipids in APOE3 compared to APOE4 mice (P < 0.05), along with a 2-fold higher insulin and impaired glucose tolerance. Supplementing APOE3, but not APOE4, animals with dietary n-3 PUFAs decreased body-weight gain, plasma lipids, and insulin (P < 0.05) and improved glucose tolerance, which was associated with increased VAT Glut4 mRNA levels (P < 0.05). Our findings demonstrate that an APOE3 genotype predisposes mice to develop obesity and its metabolic complications, which was attenuated by n-3 PUFA supplementation.-Slim, K. E., Vauzour, D., Tejera, N., Voshol, P. J., Cassidy, A., Minihane, A. M. The effect of dietary fish oil on weight gain and insulin sensitivity is dependent on APOE genotype in humanized targeted replacement mice.