Flaxseed, also known as linseed, has been used as a food and industrial fiber source since ancient times. Flaxseed contains several macronutrients including lipid's, protein, dietary fiber, and other bioactives such as eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and secoisolariciresinol diglucoside (SDG) to which health benefits are attributed to. These health benefits span cardiovascular, hormonal, and metabolic systems.
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The composition of flaxseed
Flaxseed is comprised of 40% lipid, 21% protein, 28% dietary fiber, 4% ash, and lignans which comprise approximately 6%. The remaining 1% is comprised of other soluble components which include sugars phenolic acids. Flaxseed has a notable oil content and represents between 29 and 45% of the seed; the exact value depends on the location, agroclimatic conditions, and cultivar.
Alpha-linolenic acid (ALA): A key bioactive molecule
The main nutritional benefit of flaxseed oil is derived from the high level of alpha-linolenic acid (ALA). ALA constitutes 50-60% of the total oil content. ALA, however, is not as readily available in flaxseed; typically, flaxseed must be ground to facilitate efficient nutrient absorption. ALA Is an essential omega-three fatty acid as it cannot be synthesized de novo.
ALA is also converted into longer-chain polyunsaturated fatty acids which include eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). A study has shown that subjects (two groups; 18–29 or 45–69 years) consuming 6 g/day of an ALA in the form of ground flaxseed or flaxseed oil showed a significant increase in their plasma DHA and EPA concentration over one month.
However, there is a variation in the response of conversion of ALA conversion to these longer-chain polyunsaturated fatty acids which may reflect differences in the age and gender of the subject studies as well as their differing diets. The consumption of ALA has clinical and public health implications; a significant proportion of the population does not consume fish or have access to fish rich in the essential EPA.
Flaxseed and cardiovascular disease (CVD)
High ALA consumption is associated with a moderately reduced risk of CVD. However, it should be noted that there is a wide variation in results across several studies, and additional research is necessary before conclusive evidence of its beneficial effects on cardiovascular health is demonstrated.
Overall, ALA appears to protect against CVD by modifying the omega-three fat content of cell membranes by improving endothelial function and blood lipid profile. ALAs may also produce significant anti-inflammatory and anti-thrombotic effects. A meta-analysis of prospective studies concluded that increasing the intake of ALA by two teaspoons of milled flaxseed (equivalent to 1.2g/day) – may decrease the risk of fatal coronary heart disease by at least 20%.
Dietary fiber, alongside lignans and ALA, may work in tandem to reduce the lipid-lowering effect of flaxseed. More studies are necessary to clarify the specific components responsible for exerting flaxseed’s mechanistic effects, however, they may be summarized as follows:
Cardioprotective mechanisms as
determined in human studies |
CVD risk factors and processes affected by components of flaxseed |
- Decreased total LDL-cholesterol
- Decrease blood pressure in long term studies (> three months)
- Decreased blood glucose, insulin resistance, and insulin levels
|
CVD |
- Improved endothelial function due to high levels of ALA
- No effect on oxidative stress
- Decreased cell adhesion molecules (cell adhesion molecules, including E-selectin, vascular cell adhesion molecule type 1 (VCAM-1), and intercellular adhesion molecule type 1 (ICAM-1) affect the tendency for leukocytes to adhere to the endothelium, which is a characteristic of endothelial dysfunction, the earliest detectable stage in the development of atherosclerosis)
- Increased anticoagulant activated protein C (APC)
|
Endothelial function |
- Decreased inflammatory molecules such as eicosanoids, acute phase proteins, and cytokines
|
Inflammation |
- Decreased platelet aggregation
|
Platelet aggregation |
- Reduced risk of myocardial infarction ischemic caught disease and stroke which results from atherosclerosis (based on epidemiological studies of diets rich in ALA)
|
Atherosclerosis |
Adapted from Fitzpatrick KC, 2011.
The effect of flaxseed on diabetes
Owing to the soluble fiber, protein, SGD, an antioxidant, and ALA content, flaxseed may cause a modest improvement in insulin secretion and the maintenance of plasma glucose homeostasis (as measured by fasting plasma hemoglobin A1c [HbA1c], glucose, and insulin).
In other trials in which participants were given flaxseed-derived lignin, participants showed significant improvement in glycemic control as measured by HbA1c; however, no changes in fasting glucose, and in turn, internal insulin resistance, and blood lipid profiles were seen.
As is the case in CVD, the role of dietary fiber, lignans, soluble fiber, and ALA may all contribute to glycemic control. Not all analyses are conclusive, and many are conflicting, hence further research is necessary to determine the mechanistic link between flaxseed and its effect on glycemic control and diabetes.
Flaxseed consumption and hormonal control
ALA is also an intermediate in the biosynthesis of hormone-like eicosanoids, which regulate inflammation and immune function in higher animals. EPA and DHA, which result from further ALA metabolism can be converted endogenously into different metabolites known as resolvins, neuroprotectins, and protectins.
- Resolvins have (1) a strong anti-inflammatory effect, limiting the extent of inflammation by blocking the actions of prostanoids (subclass of eicosanoids) which are mediators of inflammatory and anaphylactic reactions, and (2) an immune site clearing effect
- Protectins, alongside resolvins, promote the resolution of several types of organ inflammation, working to activate specific mechanisms to maintain homeostasis
- Neuroprotectins are derived from DHA and maintain the integrity of neurons by inhibiting oxidative, stress-induced pro-inflammatory gene expression, and therefore promote cell survival
Anti-cancer potential of flaxseed
Flaxseed consumption has been linked to a decreased risk of cancer, in particular breast and prostate cancer. Overall, results of randomized control trials suggest that flaxseed consumption is associated with biological alterations which may be protective for prostate and breast cancer.
The anti-cancer effects attributed to flaxseed in the context of breast cancer may be a result of the flaxseed lignans which act to compete with estrogen and testosterone for binding to their respective receptors and inhibiting aromatase, an enzyme that is implicated in estrogen production.
In the context of breast cancer, flax consumption is associated with decreased risk and the reduced growth and size of tumors. Again, opposing trials and studies which included premenopausal and postmenopausal women did not show the same results. Consequently, more studies are still necessary, to determine the net effect of flaxseed on breast cancer.
This cautionary conclusion applies in the context of prostate cancer. There is conflicting evidence from epidemiological studies that suggest that ALA is associated with an increased risk of prostate cancer. These contradictory findings may be due to different methods of prostate cancer classification and population demographics.
Overall, flaxseed contains several bioactive components that encompass anti-inflammatory, antioxidant and hormonal, and metabolic effects leading to health benefits concerning cardiovascular diseases, diabetes, and menopause. T
he variation in the benefits of consumption of whole flaxseed components and formats (oil, mucilage, and protein) indicate that consideration of the whole complement of bioactive must be considered before firm conclusions that associate biological activity and health outcomes with specific compounds are drawn. Nonetheless, flaxseed is a valuable food source associated with net positive effects on health.
References
- Fitzpatrick KC. Chapter 10 - Health Benefits of Flaxseed. In: Ernesto M. (2011) Hernandez, Masashi Hosokawa (Eds.) Omega-3 Oils (pp. 213-264). Academic Press and AOCS Press
- Brouwer IA, et al. Dietary alpha-linolenic acid is associated with reduced risk of fatal coronary heart disease, but increased prostate cancer risk: a meta-analysis. J Nutr. 2004;134(4):919-22. doi: 10.1093/jn/134.4.919.
- Pan A, et al. Meta-analysis of the effects of flaxseed interventions on blood lipids. Am J Clin Nutr. 2009;90(2):288-97. doi: 10.3945/ajcn.2009.27469.
- Kajla P, et al. Flaxseed-a potential functional food source. J Food Sci Technol. 2015;52(4):1857-1871. doi:10.1007/s13197-014-1293-y.
- Thompson LU, et al. Dietary flaxseed alters tumor biological markers in postmenopausal breast cancer. Clin Cancer Res. 2005;11(10):3828-35. doi: 10.1158/1078-0432.CCR-04-2326.
- Bidoli E, et al. Macronutrients, fatty acids, cholesterol, and prostate cancer risk. Ann Oncol. 2005;16(1):152-7. doi: 10.1093/annonc/mdi010.
- De Stéfani E, et al. Alpha-linolenic acid and risk of prostate cancer: a case-control study in Uruguay. Cancer Epidemiol Biomarkers Prev. 2000;9(3):335-8.
- Ramon JM, et al. Dietary fat intake and prostate cancer risk: a case-control study in Spain. Cancer Causes Control. 2000;11(8):679-85. doi: 10.1023/a:1008924116552.
- Pan A, 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. doi: 10.1371/journal.pone.0001148.
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