Linoleic Acid-Rich Oil Supplementation Increases Total and High-Molecular-Weight Adiponectin and Alters Plasma Oxylipins in Postmenopausal Women with Metabolic Syndrome

Omega 6 fatty acids: helpful, harmless or harmful?

PURPOSE OF REVIEW

This paper reviews the most recent literature from January 2023 to August 2024 on the physiological effects of n-6 polyunsaturated fatty acids (PUFAs), with a focus on linoleic acid (LA).

RECENT FINDINGS

Contrary to previous concerns that high LA intake may increase inflammation, most recent evidence supports the benefits of LA for cardiometabolic health. Several large studies report that higher blood LA levels correlate with reduced risks of coronary heart disease, stroke, and type 2 diabetes. Potential mechanisms include activation of peroxisome proliferator-activated receptors and modulation of oxylipins involved in glucose and lipid metabolism. The ideal LA intake level remains uncertain, but current intakes around 5-10% of energy appear beneficial. In other areas like cancer, asthma and sleep, the evidence is still inconclusive on LA's effects.

SUMMARY

More research on diverse populations is needed to determine optimal LA levels, effects on specific conditions, and interactions with genetic factors affecting PUFA metabolism. Overall, the review highlights the emerging view that LA, the primary dietary n-6 PUFA, has cardiometabolic benefits rather than harmful effects.

Anti-inflammatory and antioxidant activity of functional lipids extracted through sustainable technologies from Mexican Opuntia ficus-indica seeds

Opuntia ficus-indica (OFI) seeds are a rich source of functional lipids, yet research on Mexican cultivars remains limited. This study evaluated the antioxidant and anti-inflammatory properties of lipids extracted through subcritical fluid and supercritical fluid extraction with carbon dioxide (SCE-CO₂ and SFE-CO₂) from Mexican OFI Villanueva and Rojo Vigor seeds with and without enzymatic pretreatment. SCE OFI Villanueva oil showed higher extraction efficiency of linoleic (45.86 mg/g), and oleic (9.86 mg/g) acids purified more than 5.47 and 1.18 times, respectively. Additionally, SCE oils exhibited the highest antioxidant potential (68 %) and anti-inflammatory activity (45 %) at the evaluated doses. In conclusion, SCE-CO₂ enhanced the extraction efficiency of unsaturated fatty acids, improving their potential biological effects, while enzymatic pretreatment did not positively impact on results, suggesting reduced extraction efficiency and bioactivity. These findings suggest that OFI seeds can serve as a valuable source of functional ingredients for the development of value-added food products.

Effects of Omega-3 Polyunsaturated Fatty Acids on the Formation of Adipokines, Cytokines, and Oxylipins in Retroperitoneal Adi-Pose Tissue of Mice

Oxylipins and specialized pro-resolving lipid mediators (SPMs) derived from polyunsaturated fatty acids (PUFAs) are mediators that coordinate an active process of inflammation resolution. While these mediators have potential as circulating biomarkers for several disease states with inflammatory components, the source of plasma oxylipins/SPMs remains a matter of debate but may involve white adipose tissue (WAT). Here, we aimed to investigate to what extent high or low omega (n)-3 PUFA enrichment affects the production of cytokines and adipokines (RT-PCR), as well as oxylipins/SPMs (liquid chromatography-tandem mass spectrometry) in the WAT of mice during lipopolysaccharide (LPS)-induced systemic inflammation (intraperitoneal injection, 2.5 mg/kg, 24 h). For this purpose, n-3 PUFA genetically enriched mice (FAT-1), which endogenously synthesize n-3 PUFAs, were compared to wild-type mice (WT) and combined with n-3 PUFA-sufficient or deficient diets. LPS-induced systemic inflammation resulted in the decreased expression of most adipokines and interleukin-6 in WAT, whereas the n-3-sufficient diet increased them compared to the deficient diet. The n-6 PUFA arachidonic acid was decreased in WAT of FAT-1 mice, while n-3 derived PUFAs (eicosapentaenoic acid, docosahexaenoic acid) and their metabolites (oxylipins/SPMs) were increased in WAT by genetic and nutritional n-3 enrichment. Several oxylipins/SPMs were increased by LPS treatment in WAT compared to PBS-treated controls in genetically n-3 enriched FAT-1 mice. Overall, we show that WAT may significantly contribute to circulating oxylipin production. Moreover, n-3-sufficient or n-3-deficient diets alter adipokine production. The precise interplay between cytokines, adipokines, and oxylipins remains to be further investigated.

Beneficial effects of linoleic acid on cardiometabolic health: an update

Linoleic acid (LA), as a part of the wider debate about saturated, omega-6 and omega-3 fatty acids (FAs) and health, continues to be at the center of controversy in the world of fatty acid research. A robust evidence base, however, demonstrates that higher intakes and blood levels of LA are associated with improved cardiometabolic health outcomes. LA lowers total and low-density lipoprotein cholesterol when compared with saturated fatty acids and carbohydrates. Using large prospective datasets, higher blood levels of LA were associated with lower risk of coronary heart disease, stroke and incident type-2 diabetes mellitus compared with lower levels, suggesting that, across the range of typical dietary intakes, higher LA is beneficial. Recent trials of LA-rich oils report favorable outcomes in people with common lipid disorders. However, an LA intake that is too high can impair endogenous synthesis of eicosapentaenoic acid (EPA) from alpha-linolenic acid (ALA), but the threshold at which this becomes clinically relevant is not known. In the absence of a significant intake of EPA and docosahexaenoic acid, an ideal dietary ratio of LA and ALA may be theoretically useful as it provides insight into the likely extent of endogenous EPA synthesis from ALA. Updating dietary reference intakes (DRIs) for LA and ALA is needed; however, there are insufficient data to establish RDAs for these fatty acids. The omega-6 (n-6) to omega-3 (n-3) PUFA ratio is not informative and does not shed meaningful insight about the amount of individual fatty acids in each class needed to confer health benefits.

Aging, sex, metabolic and life experience factors: Contributions to neuro-inflammaging in Alzheimer's disease research

Alzheimer's disease (AD) is prevalent around the world, yet our understanding of the disease is still very limited. Recent work suggests that the cornerstone of AD may include the inflammation that accompanies it. Failure of a normal pro-inflammatory immune response to resolve may lead to persistent central inflammation that contributes to unsuccessful clearance of amyloid-beta plaques as they form, neuronal death, and ultimately cognitive decline. Individual metabolic, and dietary (lipid) profiles can differentially regulate this inflammatory process with aging, obesity, poor diet, early life stress and other inflammatory factors contributing to a greater risk of developing AD. Here, we integrate evidence for the interface between these factors, and how they contribute to a pro-inflammatory brain milieu. In particular, we discuss the importance of appropriate polyunsaturated fatty acids (PUFA) in the diet for the metabolism of specialised pro-resolving mediators (SPMs); raising the possibility for dietary strategies to improve AD outlook.

Important roles of linoleic acid and α-linolenic acid in regulating cognitive impairment and neuropsychiatric issues in metabolic-related dementia

Metabolic syndrome (MetS), which is characterized by insulin resistance, high blood glucose, obesity, and dyslipidemia, is known to increase the risk of dementia accompanied by memory loss and depression. The direct pathways and specific mechanisms in the central nervous system (CNS) for addressing fatty acid imbalances in MetS have not yet been fully elucidated. Among polyunsaturated acids, linoleic acid (LA, n6-PUFA) and α-linolenic acid (ALA, n3-PUFA), which are two essential fatty acids that should be provided by food sources (e.g., vegetable oils and seeds), have been reported to regulate various cellular mechanisms including apoptosis, inflammatory responses, mitochondrial biogenesis, and insulin signaling. Furthermore, inadequate intake of LA and ALA is reported to be involved in neuropathology and neuropsychiatric diseases as well as imbalanced metabolic conditions. Herein, we review the roles of LA and ALA on metabolic-related dementia focusing on insulin resistance, dyslipidemia, synaptic plasticity, cognitive function, and neuropsychiatric issues. This review suggests that LA and ALA are important fatty acids for concurrent treatment of both MetS and neurological problems.

Causal relationship between linoleic acid and type 2 diabetes and glycemic traits: a bidirectional Mendelian randomization study

Objective

To investigate the causal relationships between linoleic acid and type 2 diabetes, and between linoleic acid and glycemic traits in European populations.

Methods

This study employed a two-sample Mendelian randomization approach to infer causality between linoleic acid and type 2 diabetes, as well as between linoleic acid and glycemic traits, leveraging genetic variations. Data were sourced from genome-wide association study summary datasets. Random-effects inverse-variance weighted, weighted median, and MR-Egger methods were used for the two-sample Mendelian randomization analyses. Results were presented as odds ratios with a 95% confidence interval. Multiple sensitivity analyses were conducted to assess result robustness.

Results

MR findings indicated a correlation between linoleic acid levels and the risk of type 2 diabetes, fasting blood glucose, and glycated hemoglobin (HbA1c), but not with fasting insulin. Specifically: type 2 diabetes (OR: 0.811, 95% CI: 0.688-0.956, P=0.013<0.05),fasting blood glucose (β_IVW): -0.056, 95% CI: (-0.091,-0.021), P=0.002< 0.0125), glycated hemoglobin (β_IVW: -0.032, 95% CI: (-0.048,-0.015), P=0.0002< 0.0125) and Fasting insulin (β_IVW: -0.024, 95% CI: (-0.056,-0.008), P=0.136 >0.05).Reverse MR analyses showed a correlation between type 2 diabetes and reduced levels of linoleic acid (β_IVW: -0.033, 95% CI: (-0.059,-0.006), P=0.014<0.05). Multiple sensitivity analyses also detected study heterogeneity but found no evidence of horizontal pleiotropy.

Conclusion

High levels linoleic acid can reduce the risk of type 2 diabetes, fasting blood glucose, and glycated hemoglobin, but has no significant relation with fasting insulin. Type 2 diabetes can lower linoleic acid levels; however, no significant causal relationship was observed between the three glycemic traits and reduced levels of linoleic acid.

The Impact of Linoleic Acid on Infant Health in the Absence or Presence of DHA in Infant Formulas

Both linoleic acid (LA) and α-linolenic acid (ALA) are essential dietary fatty acids, and a balanced dietary supply of these is of the utmost importance for health. In many countries across the globe, the LA level and LA/ALA ratio in breast milk (BM) are high. For infant formula (IF), the maximum LA level set by authorities (e.g., Codex or China) is 1400 mg LA/100 kcal ≈ 28% of total fatty acid (FA) ≈ 12.6% of energy. The aims of this study are: (1) to provide an overview of polyunsaturated fatty acid (PUFA) levels in BM across the world, and (2) to determine the health impact of different LA levels and LA/ALA ratios in IF by reviewing the published literature in the context of the current regulatory framework. The lipid composition of BM from mothers living in 31 different countries was determined based on a literature review. This review also includes data from infant studies (intervention/cohort) on nutritional needs regarding LA and ALA, safety, and biological effects. The impact of various LA/ALA ratios in IF on DHA status was assessed within the context of the current worldwide regulatory framework including China and the EU. Country averages of LA and ALA in BM range from 8.5-26.9% FA and 0.3-2.65% FA, respectively. The average BM LA level across the world, including mainland China, is below the maximum 28% FA, and no toxicological or long-term safety data are available on LA levels > 28% FA. Although recommended IF LA/ALA ratios range from 5:1 to 15:1, ratios closer to 5:1 seem to promote a higher endogenous synthesis of DHA. However, even those infants fed IF with more optimal LA/ALA ratios do not reach the DHA levels observed in breastfed infants, and the levels of DHA present are not sufficient to have positive effects on vision. Current evidence suggests that there is no benefit to going beyond the maximum LA level of 28% FA in IF. To achieve the DHA levels found in BM, the addition of DHA to IF is necessary, which is in line with regulations in China and the EU. Virtually all intervention studies investigating LA levels and safety were conducted in Western countries in the absence of added DHA. Therefore, well-designed intervention trials in infants across the globe are required to obtain clarity about optimal and safe levels of LA and LA/ALA ratios in IF.

Esterified Oxylipins: Do They Matter?

Oxylipins are oxygenated metabolites of fatty acids that share several similar biochemical characteristics and functions to fatty acids including transport and trafficking. Oxylipins are most commonly measured in the non-esterified form which can be found in plasma, free or bound to albumin. The non-esterified form, however, reflects only one of the possible pools of oxylipins and is by far the least abundant circulating form of oxylipins. Further, this fraction cannot reliably be extrapolated to the other, more abundant, esterified pool. In cells too, esterified oxylipins are the most abundant form, but are seldom measured and their potential roles in signaling are not well established. In this review, we examine the current literature on experimental oxylipin measurements to describe the lack in reporting the esterified oxylipin pool. We outline the metabolic and experimental importance of esterified oxylipins using well established roles of fatty acid trafficking in non-esterified fatty acids and in esterified form as components of circulating lipoproteins. Finally, we use mathematical modeling to simulate how exchange between cellular esterified and unesterified pools would affect intracellular signaling.. The explicit inclusion of esterified oxylipins along with the non-esterified pool has the potential to convey a more complete assessment of the metabolic consequences of oxylipin trafficking.

Linoleic Acid-Rich Oil Alters Circulating Cardiolipin Species and Fatty Acid Composition in Adults: A Randomized Controlled Trial

SCOPE

Higher circulating linoleic acid (LA) and muscle-derived tetralinoleoyl-cardiolipin (LA4 CL) are each associated with decreased cardiometabolic disease risk. Mitochondrial dysfunction occurs with low LA4 CL. Whether LA-rich oil fortification can increase LA4 CL in humans is unknown. The aims of this study are to determine whether dietary fortification with LA-rich oil for 2 weeks increases: 1) LA in plasma, erythrocytes, and peripheral blood mononuclear cells (PBMC); and 2) LA4 CL in PBMC in adults.

METHODS AND RESULTS

In this randomized controlled trial, adults are instructed to consume one cookie per day delivering 10 g grapeseed (LA-cookie, N = 42) or high oleate (OA) safflower (OA-cookie, N = 42) oil. In the LA-cookie group, LA increases in plasma, erythrocyte, and PBMC by 6%, 7%, and 10% respectively. PBMC and erythrocyte OA increase by 7% and 4% in the OA-cookie group but is unchanged in the plasma. PBMC LA4 CL increases (5%) while LA3 OA1 CL decreases (7%) in the LA-cookie group but are unaltered in the OA-cookie group.

CONCLUSIONS

LA-rich oil fortification increases while OA-oil has no effect on LA4 CL in adults. Because LA-rich oil fortification reduces cardiometabolic disease risk and increases LA4 CL, determining whether mitochondrial dysfunction is repaired through dietary fortification is warranted.