Flaxseed

NutraPedia - Evidence-Based Supplement Research

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Health Goals: heart digestion inflammation

Research Overview

Clinical research on flaxseed has most frequently examined cardiometabolic and vascular risk markers, particularly blood lipids (total cholesterol, LDL-C, HDL-C, triglycerides), inflammatory biomarkers (hs-CRP/CRP, IL-6, TNF-α), and—more recently—lipoprotein(a) and oxidative stress indices (e.g., malondialdehyde, total antioxidant capacity). Much of the evidence base consists of randomized controlled trials synthesized in meta-analyses, with substantial attention to how effects differ by flaxseed form (whole/ground seed vs oil vs isolated lignans), study design, and participant characteristics (baseline lipid levels, sex/menopausal status, BMI, and clinical populations such as coronary artery disease).

Across meta-analyses, the strongest and most consistent signals support modest improvements in selected inflammatory and lipid-related outcomes, with important dependence on intervention type and population. Flaxseed intake is repeatedly associated with reductions in hs-CRP (and in some analyses CRP) and shows evidence for lowering TNF-α in at least one synthesis, while IL-6 findings are mixed and sometimes sensitive to exclusion of individual studies. For lipids, earlier and larger meta-analytic evidence indicates small reductions in total and LDL cholesterol, driven mainly by whole flaxseed and lignan supplements rather than flaxseed oil, and with larger effects in those with higher baseline cholesterol and in females/postmenopausal women. Additional meta-analytic evidence suggests a small but statistically significant reduction in Lp(a), particularly in longer-duration trials. For oxidative stress, flaxseed oil supplementation appears to reduce MDA and increase total antioxidant capacity, though heterogeneity is substantial and effects are not uniform across biomarkers (e.g., no clear effect on glutathione).

Key gaps include the need for more well-designed, longer-duration RCTs that directly compare flaxseed preparations (whole seed vs oil vs lignans), standardize dosing and background diet, and enroll adequately powered, clinically relevant populations. Several outcomes remain equivocal or context-dependent—triglycerides and broader lipid changes in coronary artery disease, adhesion molecules beyond VCAM-1 (ICAM-1, E-selectin), and cytokines such as IL-6 and TNF-α—suggesting potential effect modification by baseline risk, adiposity, and study design. Future work should prioritize clinically meaningful endpoints (e.g., cardiovascular events or validated intermediate outcomes), clarify dose–response relationships, and improve reporting of adherence and biomarker assay methods to reduce heterogeneity and strengthen causal inference.

1. What conditions has Flaxseed been studied for?

  • Systemic inflammation (circulating biomarkers): hs-CRP/CRP, IL-6, TNF-α; also vascular/adhesion markers such as VCAM-1, ICAM-1, E-selectin.

  • Cardiovascular risk / dyslipidemia: total cholesterol (TC), LDL-C, HDL-C, triglycerides (TG).

  • Elevated lipoprotein(a) [Lp(a)] as a cardiovascular risk factor.

  • Oxidative stress / antioxidant status: malondialdehyde (MDA), total antioxidant capacity (TAC), glutathione (GSH).

  • Coronary artery disease (CAD) populations: effects on lipids, fasting blood glucose (FBS), hs-CRP, and weight/BMI.

  • Overweight/obesity and body composition: body weight, BMI, waist circumference (WC).

  • Glycemic control (at least in CAD trials): fasting blood glucose.

2. Does it work in treating those conditions? Summarize the evidence.

  • Inflammation: Evidence suggests small-to-moderate improvements in some inflammatory markers, but results vary by marker and meta-analysis.

    • One meta-analysis of 32 trials found significant reductions in hs-CRP (WMD −0.75) and TNF-α (WMD −0.38), but not IL-6 or standard CRP overall; IL-6 became significant after removing one study in sensitivity analysis.

    • Another meta-analysis of 40 RCTs (n=2520) found reductions in CRP (WMD −0.387 mg/L), IL-6 (WMD −0.154 pg/mL), and VCAM-1 (WMD −22.809 ng/mL), but not TNF-α, ICAM-1, or E-selectin.

    • Overall: likely modest anti-inflammatory effects, but inconsistency across biomarkers and analyses suggests effects depend on study design, supplement type (whole seed vs oil vs lignan), participant characteristics (age/BMI), and study quality (as noted in the studies).

  • Blood lipids (dyslipidemia/cardiometabolic risk): Multiple meta-analyses indicate modest improvements in TC and LDL-C, and sometimes TG, with important differences by product type and population.

    • An older meta-analysis (28 studies, up to 2008) found reductions in TC (−0.10 mmol/L) and LDL-C (−0.08 mmol/L), with stronger effects for whole flaxseed and lignan supplements, but not flaxseed oil; effects were more apparent in females (especially postmenopausal), those with higher baseline cholesterol, and higher-quality studies.

    • A larger meta-analysis (62 RCTs, n=3772) found significant reductions in TC (WMD −5.389 mg/dL), TG (WMD −9.422 mg/dL), and LDL-C (WMD −4.206 mg/dL), with no effect on HDL-C.

    • A 2021-focused review in healthy vs dyslipidemic subjects reported improvements in TC, LDL-C, and TG in dyslipidemic patients, with no significant HDL-C change; it also reported mixed/possibly adverse shifts in some lipids in “healthy” individuals (as summarized in that study), highlighting heterogeneity and the importance of baseline status.

    • In CAD patients (6 RCTs), pooled results showed no clear lipid changes overall, but after sensitivity analysis TG decreased (WMD −18.39 mg/dL).

  • Lipoprotein(a) [Lp(a)]: A meta-analysis of 7 RCTs (n=629) found a small but statistically significant reduction in Lp(a) (MD −2.06 mg/dL), with longer duration showing benefit in subgroup analysis.

  • Oxidative stress / antioxidant status (flaxseed oil): An 8-trial meta-analysis (n=429) found decreased MDA (SMD −0.52) and increased TAC (WMD +82.84), with no significant effect on GSH. Heterogeneity was substantial (I2 ~71% for MDA and ~93% for TAC), so confidence in the exact magnitude is limited.

  • Weight/obesity outcomes: Two meta-analyses of RCTs found small but statistically significant reductions in weight-related measures, with stronger effects in higher-BMI groups and certain durations/doses.

    • Meta-analysis (45 RCTs) found reductions in body weight (WMD −0.99 kg), BMI (−0.30 kg/m2), and waist circumference (−0.80 cm); subgroup analyses suggested greater effects with whole flaxseed ≥30 g/day and longer interventions (abstract truncated but direction stated).

    • Meta-analysis (64 trials/72 arms, up to Aug 2023) found reductions in body weight (WMD −0.63 kg), BMI (−0.24 kg/m2), and WC (−1.43 cm); effects were more significant with 10–20 week interventions and in participants with BMI >30. Evidence certainty for anthropometrics was rated moderate (GRADE) in that study.

    • However, in the CAD-specific meta-analysis, flaxseed did not significantly change weight or BMI, suggesting effects may be smaller or harder to detect in some clinical populations or shorter trials.

  • Glycemic control (CAD): In CAD patients, flaxseed supplementation significantly lowered fasting blood glucose (WMD −8.35 mg/dL) in the pooled analysis.

3. What health benefits does it have?

  • Anti-inflammatory effects (modest): Reductions were reported for hs-CRP and/or CRP, and in some analyses IL-6 and TNF-α (though not consistently across meta-analyses). One meta-analysis also found reduced VCAM-1, suggesting potential vascular inflammation/adhesion benefits.

  • Improved lipid profile (especially TC and LDL-C; sometimes TG): Multiple meta-analyses show small average reductions in TC and LDL-C, with effects more apparent with whole flaxseed or lignan rather than flaxseed oil in at least one analysis.

  • Lower Lp(a) (small effect): A meta-analysis found a statistically significant reduction (MD −2.06 mg/dL), particularly with longer duration.

  • Oxidative stress/antioxidant status (flaxseed oil): Lower MDA and higher TAC were observed in pooled analyses, suggesting improved antioxidant defense, though heterogeneity was high.

  • Small improvements in body weight and central adiposity: Meta-analyses show modest reductions in weight, BMI, and waist circumference, especially in people with obesity and with interventions around 10–20 weeks and/or higher whole-flax doses (≥30 g/day in one analysis).

  • Lower fasting blood glucose in CAD populations: A CAD-focused meta-analysis found a significant reduction in fasting glucose.

4. Does it have any downsides or side effects?

  • Safety/side effects were not reported in the studies reviewed here. The studies focus on efficacy outcomes and do not summarize adverse event rates, tolerability, or discontinuations.

  • Quality and product-safety limitations were noted: The 2023 anthropometrics meta-analysis reported that only 3 studies tested purity, 40 reported potency, and risk of heavy-metal contamination was not mentioned in the included studies. This is not evidence of harm, but it is a limitation relevant to real-world supplement safety/quality control.

  • Inconsistent effects and heterogeneity: Several analyses reported contradictory findings and/or high heterogeneity (e.g., oxidative stress outcomes), implying that responses may vary by formulation (whole seed vs oil vs lignan), dose, duration, baseline health status, and study design.

Because adverse effects are not described in these studies, no evidence-based conclusion about specific side effects (e.g., GI effects, bleeding risk, interactions) can be drawn from the provided text alone.

5. Is it beneficial or harmful for any genetic variations (pharmacogenomics)?

The studies reviewed here do not report pharmacogenomic analyses or outcomes stratified by genotype. Therefore:

  • No specific genetic variants can be identified from these studies as predicting benefit or harm from flaxseed, flaxseed oil, or lignan supplementation.

  • Given the observed heterogeneity by participant characteristics (e.g., BMI, age, baseline lipids) and supplement type, it is plausible that biology (including genetics) could influence response, but this is not directly evaluated in the cited studies.

References

  1. Effects of Flaxseed Interventions on Circulating Inflammatory Biomarkers: A Systematic Review and Meta-Analysis of Randomized Controlled Trials
    Mehran Rahimlou, Nasrin Banaie Jahromi, Nazila Hasanyani, Amirhossein Ramezani Ahmadi (2019)
  2. Impact of flaxseed supplementation on plasma lipoprotein(a) concentrations: A systematic review and meta-analysis of randomized controlled trials
    Amir Hadi, Moein Askarpour, Rahele Ziaei, Kamesh Venkatakrishnan, Ehsan Ghaedi (2020)
  3. Flaxseed Oil Supplementation Augments Antioxidant Capacity and Alleviates Oxidative Stress: A Systematic Review and Meta-Analysis of Randomized Controlled Trials
    Vali Musazadeh, Jaber Jafarzadeh, Majid Keramati, Meysam Zarezadeh, Mahshid Ahmadi (2021)
  4. Effect of flaxseed supplementation on markers of inflammation and endothelial function: A systematic review and meta-analysis
    Moein Askarpour, Mohammadreza Karimi, Amir Hadi, Ehsan Ghaedi, Micheal E Symonds (2020)
  5. Effects of flaxseed supplementation on weight loss, lipid profiles, glucose, and high-sensitivity C-reactive protein in patients with coronary artery disease: A systematic review and meta-analysis of randomized controlled trials
    Hamid Reza Sabet, Mohammad Ahmadi, Mehdi Akrami, Mahsa Motamed, Omid Keshavarzian (2024)
  6. Meta-analysis of the effects of flaxseed interventions on blood lipids
    An Pan, Danxia Yu, Wendy Demark-Wahnefried, Oscar H Franco, Xu Lin (2009)
  7. Effects of Flaxseed on Blood Lipids in Healthy and Dyslipidemic Subjects: A Systematic Review and Meta-Analysis of Randomized Controlled Trials
    Motahareh Sadat Masjedi, Pardis Mohammadi Pour, Yalda Shokoohinia, Sedigheh Asgary (2022)
  8. Effect of flaxseed supplementation on lipid profile: An updated systematic review and dose-response meta-analysis of sixty-two randomized controlled trials
    Amir Hadi, Moein Askarpour, Shekoufeh Salamat, Ehsan Ghaedi, Micheal E Symonds (2020)
  9. The effects of flaxseed (Linum usitatissimum) supplementation on anthropometric indices: An updated systematic review and meta-analysis of randomized clinical trials
    Vali Musazadeh, MohammadReza Abolghasemian, Zeynab Kavyani, Amir Hossein Moridpour, Ahmad Nazari (2024)
  10. The effect of flaxseed supplementation on body weight and body composition: a systematic review and meta-analysis of 45 randomized placebo-controlled trials
    M Mohammadi-Sartang, Z Mazloom, H Raeisi-Dehkordi, R Barati-Boldaji, N Bellissimo (2017)
  11. Can flaxseed supplementation affect circulating adipokines in adults? An updated systematic review and meta-analysis of randomized controlled trials
    Shaghayegh Abbasi, Kiana Karimi, Amir Hossein Moridpour, Vali Musazadeh, Amir Hossein Faghfouri (2023)
  12. The effect of flaxseed supplementation on circulating adiponectin and leptin concentration in adults: A systematic review and meta-analysis of randomized controlled trials
    Cyrus Jalili, Mehran Pezeshki, Moein Askarpour, Wolfgang Marx, Bahar Hassani (2020)
  13. Flaxseed supplementation on glucose control and insulin sensitivity: a systematic review and meta-analysis of 25 randomized, placebo-controlled trials
    Mohsen Mohammadi-Sartang, Zahra Sohrabi, Reza Barati-Boldaji, Hamidreza Raeisi-Dehkordi, Zohreh Mazloom (2018)
  14. Flaxseed supplementation significantly reduces hemoglobin A1c in patients with type 2 diabetes mellitus: A systematic review and meta-analysis
    Huihui Xi, Wei Zhou, Muhammad Sohaib, Yirou Niu, Ruiting Zhu (2023)
  15. Effect of flaxseed (Linum usitatissimum) supplementation on glycemic control and insulin resistance in prediabetes and type 2 diabetes: A systematic review and meta-analysis of randomized controlled trials
    Andrea Isabel Villarreal-Renteria, Dulce Daniela Herrera-Echauri, Norma Patricia Rodríguez-Rocha, Laura Yareni Zuñiga, José Francisco Muñoz-Valle (2022)
  16. Can flaxseed supplementation alleviate blood pressure in patients with metabolic diseases? Evaluating the strength of evidence using a meta-analysis and GRADE approach
    Vali Musazadeh, Niloofar Hamidi, Mahsa Mahmoudinezhad, Ali Jafari, Farzad Shidfar (2025)
  17. Effect of flaxseed supplementation on blood pressure: a systematic review, and dose-response meta-analysis of randomized clinical trials
    Lingou Li, Hanzhang Li, Yuzhen Gao, Somayeh Vafaei, Xingyu Zhang (2023)
  18. Flaxseed consumption may reduce blood pressure: a systematic review and meta-analysis of controlled trials
    Saman Khalesi, Christopher Irwin, Matt Schubert (2015)
  19. Flaxseed Lowers Blood Pressure in Hypertensive Subjects: A Meta-Analysis of Randomized Controlled Trials
    Ezatollah Fazeli Moghadam, Leili Khaghani, Parivash Shekarchizadeh-Esfahani (2024)
  20. The effect of flaxseed supplementation on sex hormone profile in adults: a systematic review and meta-analysis
    Vali Musazadeh, Ahmad Nazari, Mohammad Natami, Zahra Hajhashemy, Kimia Sadat Kazemi (2023)


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This article summarizes published research for educational purposes. It is not medical advice. Talk to your doctor before starting, stopping, or changing any supplement or medication.

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