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For Patients & Caregivers
Bottom Line: Flaxseed appears to be effective in reducing menopausal symptoms, however there is mixed evidence of its ability to reduce cholesterol levels.
Flaxseed is a very concentrated source of phytoestrogenic compounds called lignans, which have hormone-like effects on the body (soybeans are another source of phytoestrogens). These lignans are likely the reason why flaxseed can affect menstrual cycle length and menopausal symptoms. Flaxseed has also been shown to affect intracellular signals within the body that may play a role in breast and prostate cancer growth. It contains alpha-linolenic acid (ALA), a building block of omega-3 fatty acids. ALA has been shown to have numerous effects on the body including protecting the kidneys from damage. Results from clinical studies are mixed on flaxseed’s ability to reduce cholesterol.
- Cancer prevention
Studies of postmenopausal women showed that flaxseed supplementation improved the ratio of hormones that are thought to help prevent breast cancer. Studies in animals have shown promising results, however human data are lacking.
- High cholesterol
Study results are mixed.
- Menopausal symptoms
A study showed flaxseed to be as effective as hormone replacement therapy in the management of menopausal symptoms.
One study indicated that flaxseed was not effective for mucositis.
- Periodontal disease
Flaxseed was shown ineffective against periodontal disease in a study.
Fifty-five individuals with high cholesterol were given flaxseed-derived lignan extracts (300 or 600 mg/day) or placebo for 8 weeks after which fasting blood glucose levels and lipid profiles were tested. Subjects who received 600 mg of extract had reductions in total cholesterol, low density lipoprotein (LDL or bad cholesterol), and fasting blood glucose levels.
For this study, 68 type II diabetic participants received lignan supplements (360 mg/day) or placebo for 12 weeks. Subjects then underwent an 8-week washout period before receiving placebo. Glycemic control was modestly affected by lignan supplementation; however, there was no change in fasting glucose and insulin levels, insulin resistance, and blood lipid concentrations. Because the influence of lignan on glycemic control was modest, more studies are needed.
In this study, 46 postmenopausal women received a placebo, soy (25 g soy flour), or flaxseed (25 g ground flaxseed) muffins for 16 weeks. Blood and 24-hour urine samples were checked for estrogen metabolites, serum hormones, and biochemical markers of bone metabolism. Results showed that the urinary concentrations of 2-hydroxyestrone, but not of 16alpha-hydroxyestrone increased significantly in the flaxseed group. There was no effect on the biochemical markers of bone metabolism.
- Allergic reactions
- Increase of the luteal phase of the menstrual cycle
- Increased bowel movement
- Anaphylaxis has been reported following intake of flaxseeds.
- Dietary flaxseed was shown to worsen dextran sodium sulfate-induced colonic injury and inflammation in mice.
For Healthcare Professionals
Flaxseed has been used in traditional medicine to treat coughs, colds, constipation, urinary tract infections, as a topical demulcent and as an emollient (1). It is rich in omega-3 fatty acids and phytoestrogenic lignans.
Flaxseed was shown to have chemoprotective effects in postmenopausal women (2), conferred renoprotection in patients with lupus nephritis (3), and improved mild menopausal symptoms (11).
Supplementation with a major lignan derived from flaxseed improved glycemic control in Type 2 diabetic patients (13). Although flaxseed-derived lignan reduces blood glucose levels in hypercholesterolemic individuals (14), data on the cholesterol-lowering effects of flaxseed are mixed (11)(15)(26). Flaxseed supplementation may benefit women with polycystic ovarian syndrome by reducing androgen levels (23); a moderate reduction of estrogens and androgens was also seen in postmenopausal women (24). However, lignan supplementation was ineffective in reducing hot flashes in postmenopausal women with or without breast cancer (28).
Flaxseed has been investigated for its anticancer potential. It was shown to inhibit the growth and metastasis of human breast cancer (6)(29), prostate cancer (7) and melanoma (8) in vitro and in mice. It also reduced radiation therapy-induced lung damage and improved survival (27).
In other studies, flaxseed was shown to lower tumor biomarkers in men with prostate cancer (9)(25) and in patients with breast cancer (10), but a flaxseed extract was ineffective in preventing oral infection following radiation treatment for head and neck cancer (12).
Flaxseed ingestion can increase urinary lignan excretion (4) and the length of the luteal phase of the menstrual cycle (5).
Flaxseed is the most concentrated food source of the plant lignan, secoisolariciresinol, a precursor for enterolactone. It is thought that phytoestrogenic lignans contribute to the plant’s hormonal effects (4). Flaxseed has been shown to affect the length of the menstrual cycle in premenopausal women (5). It may also alter estrogen metabolism, increasing the ratio of 2-hydroxyestrogen to 16 alpha-hydroxyestrone in a dose dependent fashion (2). Flaxseed’s renoprotective effects are thought to be via high concentration of alpha-linolenic acid, an omega-3 fatty acid precursor (3) or through inhibition of angiogenesis, tyrosine protein kinases and cytokine-induced activation of transcription factors (16). The laxative effects of flaxseed are likely due to its fiber content (22).
In addition, flaxseed’s inhibition of growth of human breast cancer, and metastasis in mice is due in part to the down-regulation of insulin-like growth factor I and expression of epidermal growth factor receptor (6). Flaxseed was also shown to induce apoptosis by significantly upregulating p53 mRNA in breast cancer cell lines (29). In another study, flaxseed oil enhanced the effects of trastuzumab in reducing HER2 signaling via the Akt and mitogen-activated protein kinase (MAPK) pathways, resulting in reduction in cell proliferation and an increase in apoptosis (33).
The antiproliferative activity of flaxseed against prostate cancer in mice is attributed to its inhibition of cellular proliferation (7).
The hormonal effects of flaxseed may also play a role in its ability to modulate prostate cancer biology and associated biomarkers (9), and lower serum lipid levels (10).
Absorption: Flaxseed ingestion resulted in a dose-dependent urinary lignan response. The plasma lignan concentration was greater than baseline, nine hours after flaxseed ingestion. No plateau in serum lignan concentration was observable in dosages up to 25 grams (20).
Common: Increased bowel movements (16), constipation and flatulence (9)
Anaphylaxis has been reported following ingestion of flaxseeds. (17)(30)
A case of false polyposis coli on double contrast barium enema (18), and an increase in the luteal phase of the menstrual cycle (5) were observed after flaxseed supplementation.
Dietary flaxseed was shown to exacerbate dextran sodium sulfate-induced colonic injury and inflammation in mice (31).
Zhang W, Wang X, Liu Y, et al. Dietary flaxseed lignan extract lowers plasma cholesterol and glucose concentrations in hypercholesterolaemic subjects. Br J Nutr. 2008;99:1301-9.
Fifty-five individuals with high cholesterol participated in this randomized, double-blind, placebo-controlled study. Participants were given flaxseed-derived lignan extracts (300 or 600 mg/day) or placebo for 8 weeks after which fasting blood glucose levels and lipid profiles were determined. Subjects who received 600 mg of extract had reductions in total cholesterol, LDL, and fasting blood glucose levels. Although other studies have reported discrepant effects on cholesterol levels by flaxseed, the authors postulate that these inconsistencies are due at least in part to the variable quantity of lignan within whole or defatted flaxseeds, a complication that is overcome with use of an extract.
Pan A, Sun J, Chen Y, et al. Effects of a flaxseed-derived lignan supplement in type 2 diabetic patients: a randomized, double-blind, cross-over trial. PLoS ONE 2007; 2(11):e1148.
Effects of flaxseed-derived lignan supplementation in 68 type II diabetic participants were analyzed in this randomized, double-blind, cross-over study. After 12 weeks of receiving lignan supplements (360 mg/day) or placebo, the participants underwent an 8-week washout period before completing the remaining treatment arm. Glycemic control (as determined by HbA1c) was modestly affected by lignan supplementation; however, fasting glucose and insulin levels, insulin resistance, and blood lipid concentrations were unaltered. Because the influence of lignan on glycemic control was modest, additional studies are required to determine if it is clinically meaningful.
Brooks JD, et al. Supplementation with flaxseed alters estrogen metabolism in postmenopausal women to a greater extent than does supplementation with an equal amount of soy. Am J Clin Nutr 2004;79:318-25.
In this study, 46 postmenopausal women were randomized to receive a placebo, soy (25 g soy flour), or flaxseed (25 g ground flaxseed) muffin for 16 weeks. Blood and 24-h urine samples were analyzed at baseline and at the endpoint for estrogen metabolites, serum hormones, and biochemical markers of bone metabolism. Results showed that the urinary concentrations of 2-hydroxyestrone, but not of 16alpha-hydroxyestrone increased significantly in the flaxseed group. There was no effect on the biochemical markers of bone metabolism. But since the study was limited by a short treatment time and fewer subjects, long-term studies with larger treatment groups are warranted.