Short Communication: Implications of Flaxseed on Urinary Hormone Testing

By: Clifford Morris, Ph.D. Cand., Chief Chemist and Research Scientist


Flaxseed and Phytoestrogens

Flaxseed is a powerful plant that has achieved massive interest, yet much of what people know about flaxseed is simply misunderstood. Flaxseed is a popular dietary source of lignans, which are a type of phytoestrogen.1,2 A phytoestrogen is a plant nutrient with structural and biochemical similarity to estrogens. Many plants including soy, sunflower, and sesame contain phytoestrogens. Whole flaxseed is the richest plant source of alpha-linolenic acid (ALA) and the lignan, secoisolaricirescinol diglucoside (SDG).3 Flaxseed also contains a significant number of other macronutrients, fiber, and minerals. ALA is converted to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) which are anti-inflammatory and cardioprotective, while SDG is converted to enterolactone (EL) and enterodiol (ED) which have anti-estrogenic and antioxidant properties.4–6 Phytoestrogen compounds found in flaxseed act closely to estrogen blockers; by modulating the production, availability, and metabolism of endogenous estrogen, specifically 2-methoxyestradiol, estradiol and 17 beta-estradiol.7–10 SDG in flaxseed can lower the production of estradiol by blocking the aromatase enzyme, similarly to aromatase inhibitors, as well as having a higher affinity for estrogen receptors than almost all the endogenous estrogens.11


Flaxseed and Breast Cancer

Lignans are the center of the controversy regarding whether it is safe for women with breast cancer to consume flaxseeds. Since the phytoestrogens in flaxseed can act as or block estrogen, this has raised concerns about whether phytoestrogens may not be safe for people with a history of hormone-linked cancers, such as prostate cancer, endometrial cancer, or estrogen receptor positive breast cancer.  In postmenopausal women, lignans can cause the body to produce less active forms of estrogen. This is believed to potentially reduce breast cancer risk.1,7 There is evidence that adding ground flaxseeds into the diet decreases cell growth in breast tissue as well.3,5 Again, this would be the type of change that would be expected to decrease breast cancer risk. Research provides evidence that flaxseed can increase apoptosis. Cell and animal studies have shown that two specific phytoestrogens found in lignans, EL and ED, may help suppress breast tumor growth. Animal studies have shown that both flaxseed oil and lignans can reduce breast tumor growth. 4,6,7,12 Combined, this suggests that flaxseeds may have anti-cancer benefits. It is recommended that human intake should be through diet only, not supplementation. If you plan to add flaxseeds into your nutrition plan, please talk to your doctor first to ensure it is an appropriate choice for you.


Flaxseed and Urinary Hormone Testing

Consumption of flaxseed influences estrogen metabolism, as indicated by both urinary metabolite excretion and serum hormone concentrations. Studies have shown that flaxseed lignans moderately inhibit the enzymes aromatase and 17 beta-hydroxysteroid dehydrogenases, which catalyze the conversion of androgens to estrogens and balance between estrogens, respectively.2,9,10 Flaxseed consumption has also been shown to affect both endocrine and growth factor pathways by modifying steroid hormone metabolism of IGF and EGFR. Since flaxseed alters estrogen metabolism and could change the clinical interpretation of an estrogen profile, it is strongly advised that flaxseed and phytoestrogen consumption be avoided for at least 3 days prior to urinary hormone testing with Physicians Lab.




(1)         Flower, G.; Fritz, H.; Balneaves, L. G.; Verma, S.; Skidmore, B.; Fernandes, R.; Kennedy, D.; Cooley, K.; Wong, R.; Sagar, S.; et al. Flax and Breast Cancer : A Systematic Review. 2014.

(2)          Brooks, J. D.; Ward, W. E.; Lewis, J. E.; Hilditch, J.; Nickell, L.; Wong, E.; Thompson, L. U. Supplementation with Flaxseed Alters Estrogen Metabolism in Postmenopausal Women to a Greater Extent than Does Supplementation with an Equal Amount of Soy 1 – 3. 2004, No. 1, 318–325.

(3)          Mora, C.; Tomaz, C.; Ana, S.; Gustavo, A.; Costa, V.; Ibrahim, P.; Maria, N.; Costa, B. Comparative Effects of Brown and Golden Flaxseeds on Body Composition , Inflammation and Bone Remodelling Biomarkers in Perimenopausal Overweight Women Q. J. Funct. Foods 2020, 33 (2017), 166–175.

(4)          Lindahl, G.; Saarinen, N.; Abrahamsson, A.; Dabrosin, C. Tamoxifen , Flaxseed , and the Lignan Enterolactone Increase Stroma- and Cancer Cell – Derived IL-1Ra and Decrease Tumor Angiogenesis in Estrogen-Dependent Breast Cancer. 2011, 51–61.

(5)          Gray, S. L.; Lackey, B. R. Optimizing a Recombinant Estrogen Receptor Binding Assay for Analysis of Herbal Extracts. J. Herb. Med. 2018, No. August, 100252.

(6)          Truan, J. S.; Chen, J.; Thompson, L. U. Flaxseed Oil Reduces the Growth of Human Breast Tumors ( MCF-7 ) at High Levels of Circulating Estrogen. 2010, 2, 1414–1421.

(7)          Jelodar, G.; Masoomi, S.; Rahmanifar, F. Hydroalcoholic Extract of Flaxseed Improves Polycystic Ovary Syndrome in a Rat Model. 2017.

(8)          Hutchins, A. M.; Martini, M. C.; Olson, B. A.; Thomas, W.; Slavin, J. L. Flaxseed Consumption Influences Endogenous Hormone Concentrations in Postmenopausal Women. 2001, 39 (1), 58–65.

(9)          Sturgeon, S. R.; Volpe, S. L.; Puleo, E.; Bertone-johnson, E. R.; Heersink, J.; Sabelawski, S.; Kristina, W.; Bigelow, C.; Kurzer, M. S. Effect of Flaxseed Consumption on Urinary Levels of Estrogen Metabolites in Postmenopausal Women. 2010, 62 (2), 175–180.

(10)        Sturgeon, S. R.; Heersink, J. L.; Volpe, S. L.; Bertone-johnson, E. R.; Puleo, E.; Stanczyk, F. Z.; Kristina, W.; Kurzer, M. S.; Bigelow, C. Effect of Dietary Flaxseed on Serum Levels of Estrogens and Androgens in Postmenopausal Women. 2008, 60 (5), 612–618.

(11)        Mccann, S. E.; Edge, S. B.; Hicks, D. G.; Thompson, L. U.; Morrison, C. D.; Andrews, C. A Pilot Study Comparing the Effect of Flaxseed , Aromatase Inhibitor , and the Combination on Breast Tumor Biomarkers Kim Clark and John Wilton. 2014, 66 (4), 566–575.

(12)        Dikshit, A.; Hales, K.; Hales, D. B. Whole Flaxseed Diet Alters Estrogen Metabolism to Promote 2-Methoxtestradiol-Induced Apoptosis in Hen Ovarian Cancer ☆ , ☆☆. J. Nutr. Biochem. 2017, 42, 117–125.