The Cancer Research
Flaxseed’s potential influence on breast, prostate, and other hormone-related cancers has led to an interest in its role for the prevention and survivorship of these cancers. Results have shown mixed findings, and much more human research is needed.
Interpreting the data
After a systematic review of the global scientific literature, AICR/WCRF analyzed how vegetables and different foods and its nutrients affect the risk of developing cancer.
- Evidence categorized as “convincing” and “probable” means there is strong research showing a causal relationship to cancer—either decreasing or increasing the risk. The research must include quality human studies that meet specific criteria and biological explanations for the findings.
- A convincing or probable judgment is strong enough to justify recommendations.
- There is probable evidence that foods with dietary fiber DECREASE the risk of:
- Colorectal cancer
• Evidence categorized as “limited suggestive” means results are generally consistent in overall conclusions, but it’s rarely strong enough to justify recommendations to reduce risk of cancer.
Ongoing Areas of Investigation
Interest in flaxseed and cancer prevention often focuses on its ALA (alpha-linolenic acid) and lignans, although evidence is clearest regarding its role as a source of dietary fiber.
Alpha-linolenic acid (ALA) is a form of omega-3 fat that out bodies convert into another omega-3 fat, called EPA (eicosapentaenoic acid). EPA is a source of protective, anti-inflammatory compounds. Only a small proportion of ALA is converted into EPA, yet moderate amounts of flaxseed in limited animal and human studies have been shown to increase EPA levels.
In cell studies, lignans increase antioxidant, anti-inflammatory and carcinogen-deactivating enzymes. They also decrease growth and increase self-destruction of cancer cells. In studies with mice, lignans and flaxseed decrease cancer development and growth.
- Lignans are sometimes categorized as phytoestrogens because they have a chemical structure similar to estrogen. Earlier, this led to fears that lignans could promote cancers fueled by excess estrogen, such as estrogen receptor-positive (ER+) breast cancer. There was also concern that lignans could interfere with tamoxifen and certain other anti-estrogen cancer treatments. But in studies with mice, flaxseed and lignans isolated from flaxseed reduce development and growth of both ER+ and estrogen-negative (ER-) breast cancer. The lower levels of several growth factors that promote breast cancer and increase the expression of tumor suppressor genes.
Based on limited animal studies that found flaxseed and flaxseed oil does not interfere with actions of tamoxifen or trastuzumab (medications used for breast cancer treatment) and may even enhance their effectiveness. Most of the animal studies have focused on breast cancer. The relatively few animals addressing other cancers have shown some decrease in markers of inflammation and a decrease in the number and size of colon cancer tumors, and in the growth and spread of prostate cancer.
In cell and animal studies, phenolic acids increase cells’ antioxidant and anti-inflammatory defenses against damage that could lead to cancer. Emerging evidence in animal studies suggests they may also improve glucose metabolism, decrease insulin resistance, and alter the gut microbiota (microbes living in the colon), creating an environment in the body less likely to support cancer.
Gamma-tocopherol is one of eight compounds that make up vitamin E. Earlier research focused on the related alpha-tocopherol compound, the form listed on nutrition labels and the only form currently recognized to meet human requirements. Yet cell and animal studies suggest that gamma-tocopherol may provide even stronger anti-inflammatory protection than alpha-tocopherol. In both cell and animal studies, gamma-tocopherol decreases cancer cell growth.
Overall, research in flaxseed and cancer prevention with humans show mixed results. The studies are relatively small and short term. In one month-long trial of about 30 post-menopausal women with newly diagnosed breast cancer, daily flaxseed decreased signs of cancer cell growth.7 In several studies of healthy women consuming flaxseed daily, estrogen levels decreased or estrogen shifted to more of a relatively inactive form. The result was less estrogen in the form that seems to promote breast cancer growth. However, studies show unexplained variability,which may partly reflect effects of individual differences in hormones, overall diet and genetics.
Population studies are not clear about the impact of ALA on prostate cancer risk, but most recent overall analyses show no significant effect. In a study of men with prostate cancer who ate flaxseed, followed a low-fat diet or both for about 30 days before surgery, each of these strategies reduced cancer cell growth compared to a control group, although the combination strategy produced best results.
Dietary Fiber: AICR/WCRF found strong evidence, including population studies, linking high dietary fiber consumption with reduced risk of colorectal cancer.
Analysis for the AICR/WCRF Third Expert Report found the evidence too limited to support a conclusion for an association of dietary fiber to several other cancers. Other analyses combining 16 to 20 prospective cohort studies have linked dietary fiber with lower risk of breast cancer.
- Viscous fiber is a type of soluble dietary fiber that forms a gel in the digestive tract, slowing digestion and thus, the rise in blood sugar after eating. Having healthier blood sugar and insulin levels could potentially help reduce cancer risk. Viscous fiber’s actions in the gut may also contribute to reducing circulating estrogen levels. Such effects could help explain why some studies link greater soluble dietary fiber to lower breast cancer risk.
- AICR/WCRF found strong evidence that foods containing dietary fiber reduce the risk of weight gain, overweight and obesity. Some studies suggest that viscous fiber like that in flaxseed may contribute to this effect by increasing a feeling of fullness. By supporting a healthy weight, foods rich in fiber indirectly contribute to a lower risk of the 12 or more cancers related to excess body fat.
Tips for Selection, Storage and Preparation
- Whole flaxseed provides plenty of fiber, but our bodies can’t digest it enough to access its other healthful components. Ground flaxseed offers more potential health benefits, but it doesn’t stay fresh as long as whole flaxseed.
- Buy whole flaxseed to grind in a coffee or spice grinder.
- If ground flaxseed or flaxseed meal is more convenient, buy either type, refrigerated or in a vacuum-sealed package.
- Flaxseed oil is quite perishable. Buy it in refrigerated opaque bottles.
- Store whole flaxseed up to a year in an airtight container in a dry, cool cabinet.
- Store ground flaxseed in an airtight container in the refrigerator up to 30 days or in freezer up to four months.
- Add ground flaxseed to hot or cold cereal, yogurt or smoothies.
- Sprinkle ground flaxseed on salads or on top of cooked vegetables for a nutty flavor.
- Include ground flaxseed in baked muffins or other quick breads.
- Flaxseed may decrease absorption of medications, so take it one hour before or two hours after any prescription or non-prescription medicine. Talk to your physician or healthcare provider about use if taking fish oil, omega-three supplements or anticoagulant medications.
- Drizzle flaxseed oil on salads or vegetables or in smoothies. Do not cook with flaxseed oil.
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- Mason JK, Thompson LU. Flaxseed and its lignan and oil components: can they play a role in reducing the risk of and improving the treatment of breast cancer? Appl Physiol Nutr Metab. 2014;39(6):663-678.
- Taibi A, Lin Z, Tsao R, Thompson LU, Comelli EM. Effects of Flaxseed and Its Components on Mammary Gland MiRNome: Identification of Potential Biomarkers to Prevent Breast Cancer Development. Nutrients. 2019;11(11):2656.
- Del Rio D, Rodriguez-Mateos A, Spencer JP, Tognolini M, Borges G, Crozier A. Dietary (poly)phenolics in human health: structures, bioavailability, and evidence of protective effects against chronic diseases. Antioxid Redox Signal. 2013;18(14):1818-1892.
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- Thompson LU, Chen JM, Li T, Strasser-Weippl K, Goss PE. Dietary flaxseed alters tumor biological markers in postmenopausal breast cancer. Clin Cancer Res. 2005;11(10):3828-3835.
- Haggans CJ, Hutchins AM, Olson BA, Thomas W, Martini MC, Slavin JL. Effect of flaxseed consumption on urinary estrogen metabolites in postmenopausal women. Nutr Cancer. 1999;33(2):188-195.
- Haggans CJ, Travelli EJ, Thomas W, Martini MC, Slavin JL. The effect of flaxseed and wheat bran consumption on urinary estrogen metabolites in premenopausal women. Cancer Epidemiol Biomarkers Prev. 2000;9(7):719-725.
- Sturgeon SR, Volpe SL, Puleo E, et al. Effect of flaxseed consumption on urinary levels of estrogen metabolites in postmenopausal women. Nutr Cancer. 2010;62(2):175-180.
- McCann SE, Wactawski-Wende J, Kufel K, et al. Changes in 2-hydroxyestrone and 16alpha-hydroxyestrone metabolism with flaxseed consumption: modification by COMT and CYP1B1 genotype. Cancer Epidemiol Biomarkers Prev. 2007;16(2):256-262.
- Sturgeon SR, Heersink JL, Volpe SL, et al. Effect of dietary flaxseed on serum levels of estrogens and androgens in postmenopausal women. Nutr Cancer. 2008;60(5):612-618.
- Carayol M, Grosclaude P, Delpierre C. Prospective studies of dietary alpha-linolenic acid intake and prostate cancer risk: a meta-analysis. Cancer Causes Control. 2010;21(3):347-355.
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- Demark-Wahnefried W, Polascik TJ, George SL, et al. Flaxseed supplementation (not dietary fat restriction) reduces prostate cancer proliferation rates in men presurgery. Cancer Epidemiol Biomarkers Prev. 2008;17(12):3577-3587.
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- World Cancer Research Fund / American Institute for Cancer Research. Continuous Update Project Expert Report 2018. Diet, nutrition, physical activity and colorectal cancer. Available at: dietandcancerreport.org.
- Aune D, Chan DS, Greenwood DC, et al. Dietary fiber and breast cancer risk: a systematic review and meta-analysis of prospective studies. Ann Oncol. 2012;23(6):1394-1402.
- Norat T, Chan D, Vingeliene S, et al. The Associations Between Food, Nutrition and Physical Activity and the Risk of Breast Cancer. WCRF/AICR Systematic Literature Review Continuous Update Project Report. London: World Cancer Research Fund / American Institute for Cancer Research;2017.
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- Li J, Gu Z, Pan Y, et al. Dietary supplementation of α-linolenic acid induced conversion of n-3 LCPUFAs and reduced prostate cancer growth in a mouse model. Lipids in Health and Disease. 2017;16(1):136.
- Edel AL, Patenaude AF, Richard MN, et al. The effect of flaxseed dose on circulating concentrations of alpha-linolenic acid and secoisolariciresinol diglucoside derived enterolignans in young, healthy adults. Eur J Nutr. 2016;55(2):651-663.
- Taylor CG, Noto AD, Stringer DM, Froese S, Malcolmson L. Dietary milled flaxseed and flaxseed oil improve N-3 fatty acid status and do not affect glycemic control in individuals with well-controlled type 2 diabetes. J Am Coll Nutr. 2010;29(1):72-80.
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