Abstract
Secoisolariciresinol diglucoside (SDG), found mainly in flaxseed, is one of the essential lignans. SDG, as well as the beneficial fatty acid composition and high fiber content, has made flaxseed an important source of functional food or nutraceutical ingredients. Various studies have shown that SDG offers several health benefits, including protective effects against cardiovascular diseases, diabetes, cancer, and mental stress. These health benefits have been attributed to the antioxidant properties of SDG. Additionally, SDG metabolites, namely mammalian lignans, enterodiol and enterolactone, have shown promising effects against cancer. Therefore, understanding the biosynthetic pathway of SDG and its molecular mechanisms is a key to enable the production of new flaxseed cultivars rich in nutraceutical content. The present review highlights studies on the different health benefits of SDG, as well as lignan biosynthesis in flaxseed and genes involved in the biosynthetic pathway. Since SDG, the predominant lignan in flaxseed, is a glycosylated lignan, we also focus on studies investigating the genes involved in secoisolariciresinol glycosylation. These genes can be used to produce new cultivars with a novel level of glycosylation or lignan composition to maximize the yields of lignans with a therapeutic or protective potential.
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Key Points
Secoisolariciresinol (SECO) diglucoside (SDG) is one of the essential dietary lignans, found in high levels in flaxseed. Along with α-linolenic acid, lignans, mainly SDG, have made flaxseed derivatives (flax oil and lignan extracts) important sources of functional food or nutraceutical ingredients. Although flaxseed possesses beneficial fatty acid composition and high fiber content, the phytoestrogenic, anticarcinogenic, and antiatherogenic effects have been attributed to its lignan content. Moreover, ingested SDG is converted into mammalian lignans, enterodiol (END) and enterolactone (ENL), by the gut microflora enzymes, and these mammalian lignans have been found to negatively correlate with the incidence of breast cancer. The main lignan in flaxseed is SDG, which is found in a linked macromolecular structure (lignan macromolecule). In flaxseed, the hull fraction contains higher amounts of lignans than that in the whole, suggesting that lignans are mainly biosynthesized in the hull.
Various studies on the health effects of flaxseed lignans have shown some potential beneficial effects on human and animal health. The conversion of SDG into mammalian lignans, ENL and END, by the intestinal microflora contributes to the bioavailability of SDG; however, other factors, such as diet, antibiotics, and obesity, also affect the circulating lignan levels in the body. This review summarizes the health beneficial effects of SDG, the most predominant lignan in flaxseed, where it can protect against several diseases, including cardiovascular diseases, cancer, diabetes, and mental stress, and affect the reproductive system. Additionally, the biosynthetic pathways of SDG in plants is described.