Health effects with consumption of the flax lignan secoisolariciresinol diglucoside

Flaxseed is the richest source of the lignan secoisolariciresinol diglucoside (SDG). After ingestion, SDG is converted to secoisolariciresinol, which is further metabolised to the mammalian lignans enterodiol and enterolactone. A growing body of evidence suggests that SDG metabolites may provide health benefits due to their weak oestrogenic or anti-oestrogenic effects, antioxidant activity, ability to induce phase 2 proteins and/or inhibit the activity of certain enzymes, or by mechanisms yet unidentified. Human and animal studies identify the benefits of SDG consumption. SDG metabolites may protect against CVD and the metabolic syndrome by reducing lipid and glucose concentrations, lowering blood pressure, and decreasing oxidative stress and inflammation. Flax lignans may also reduce cancer risk by preventing pre-cancerous cellular changes and by reducing angiogenesis and metastasis. Thus, dietary SDG has the potential to decrease the incidence of several chronic diseases that result in significant morbidity and mortality in industrialised countries. The available literature, though, makes it difficult to clearly identify SDG health effects because of the wide variability in study methods. However, the current evidence suggests that a dose of at least 500 mg SDG/d for approximately 8 weeks is needed to observe positive effects on cardiovascular risk factors in human patients. Flaxseed and its lignan extracts appear to be safe for most adult populations, though animal studies suggest that pregnant women should limit their exposure. The present review discusses the potential health benefits of SDG in humans, with supporting evidence from animal studies, and offers suggestions for future research. (Authors abstract)
In this article, the authors provide a comprehensive review of lignans in flaxseed � their composition, amounts, health effects etc. Flaxseed contains secoisolariciresinol diglucoside (SDG, about 1% of dry weight). New methods are available to quantify SDG content in flaxseed extract sources. After ingestion, the plant lignan SDG is converted to mammalian lignans by bacteria in the human colon. SDG first undergoes hydrolysis to yield the aglycone plant lignin secoisolariciresinol (SECO). SECO is then converted to enterodiol (ED) and enterolactone (EL). The structural similarity of EL and ED to the most predominant and active form of oestrogen in the body, estradiol, allows these lignans to bind to estrogen receptors and exert weak estrogenic or anti-estrogenic Diets high in plant lignans may provide health benefits by decreasing the risk of hormone-sensitive cancers and diseases associated with increased inflammation and oxidative damage. ED and EL also inhibit the activity of certain enzymes such as 5a-reductase which may help to relieve lower urinary tract symptoms in patients with benign prostatic hyperplasia. Also, the ability of EL to inhibit aromatase may be beneficial in estrogen-responsive breast cancers. Another mechanism by which SDG may provide health benefits is through induction of phase 2 proteins. Phase 2 enzymes are generally characterised by either promoting the scavenging of oxidants or decreasing the probability of oxidant formation. The mechanism(s) through which lignans mediate the putative health benefits and the actual bioactive lignan form (i.e. SDG, SECO, ED and/or EL) is not known. The subject characteristics, sources of SDG and lack of product quality assurance, dosage, methods, data analysis and the duration of the study vary greatly between published studies.  Animal studies using rat, mice and rabbit models suggest that SDG supplementation protects against the development of chronic diseases such as CVD, cancer and diabetes. The outcomes are complicated by differences in sex, age and species strain. The wide variability in the methods used in human trials also complicates the interpretation of results though there is growing evidence that SDG- enriched flaxseed products offer health benefits. More randomised controlled trials are needed before it can be elucidated whether or not SDG supplementation protects against disease in humans. (Editors comments)