Lignans are a group of naturally occurring compounds in plants that belong to the family of polyphenols, which are micronutrients known for their antioxidant properties. These plant compounds are primarily recognized as phytoestrogens, meaning they possess a chemical structure that allows them to interact with the body’s hormonal systems. Unlike other plant nutrients, lignans are not directly active upon consumption but function as precursors to biologically potent metabolites. The health benefits attributed to dietary lignans are directly dependent on their transformation by the human body after ingestion.
Chemical Identity and Dietary Sources
Lignans are distinct in their chemical makeup, characterized as diphenolic compounds formed by the dimerization of two phenylpropane units. This structure classifies them as phytoestrogens, which are plant-derived substances that exhibit a weak similarity to human estrogen. The most commonly studied precursor lignan is secoisolariciresinol diglucoside, frequently abbreviated as SDG.
Flaxseed stands out as the single richest dietary source of lignans, containing concentrations that are 75 to 800 times greater than those found in most other plant foods. The majority of the lignan content in flaxseed is the precursor SDG, which is concentrated mostly within the seed coat. Lignans are also widely distributed across the plant kingdom, albeit in much lower amounts.
A varied diet provides a steady intake of various lignan precursors from multiple sources. Other notable food sources include sesame seeds, which are particularly rich, along with various whole grains like rye and barley. Lesser amounts are found in certain fruits, vegetables, and legumes, contributing to the overall dietary intake of these polyphenols.
The Role of Gut Microbiota in Lignan Conversion
Native lignans found in plant materials are generally considered biologically inactive in their ingested form. They must survive the upper digestive tract and reach the colon before they can exert physiological effects. This necessitates the involvement of the body’s resident intestinal bacteria.
Specific species of gut microbiota metabolize precursor compounds, such as SDG, through enzymatic reactions. This biotransformation begins with the hydrolysis of glycosidic bonds on the SDG molecule, yielding the intermediate compound secoisolariciresinol. Bacterial action then converts this intermediate into the final, biologically active end products.
The key metabolites produced are enterodiol (ED) and enterolactone (EL), collectively known as enterolignans or mammalian lignans. These enterolignans are absorbed into the bloodstream and distributed throughout the body to initiate biological activity. The efficiency of this conversion varies significantly between individuals, meaning the same dietary intake can result in different circulating levels depending on the composition of a person’s gut flora.
Phytoestrogenic and Antioxidant Mechanisms
Once produced by the gut microbiota, enterolignans exert their influence through two primary biological mechanisms. The first is their phytoestrogenic activity, stemming from their structural similarity to the body’s natural estrogen, estradiol. This analogous shape allows them to bind to estrogen receptors located on cells throughout the body.
Enterolignans preferentially bind to estrogen receptor beta (ER-beta), which is distinct from estrogen receptor alpha (ER-alpha). Their binding affinity to these receptors is significantly weaker than that of endogenous estrogen, which leads to a modulatory effect on hormonal signaling. In environments with high natural estrogen levels, lignans may act as weak antagonists, while they may function as weak agonists in low estrogen environments, thus helping to balance hormonal activity.
The second major mechanism is their function as antioxidants. The polyphenol structure of lignans and their metabolites enables them to scavenge free radicals, unstable molecules that cause cellular damage. This activity reduces oxidative stress, a process implicated in the development of various chronic conditions. Lignans contribute to the protection of cellular components like DNA and lipids from oxidative damage.
Lignans in Supplementation and Fortified Foods
Beyond natural food consumption, lignans are commercially utilized in various consumer products to provide a concentrated dose. Commercial preparation primarily involves extracting lignans from flaxseed hulls, yielding a product highly enriched in secoisolariciresinol diglucoside (SDG). This extract is commonly formulated into dietary supplements, offering a standardized intake of the precursor compound.
For consumers seeking a precise and consistent level of lignans, supplementation provides an alternative to whole foods, particularly for those with limited access to flaxseed. Another source for supplements is the Norwegian Spruce tree, which yields 7-hydroxymatairesinol (HMR), a different lignan that is also efficiently converted to enterolactone.
In the food industry, lignans are incorporated as fortifiers to enhance the nutritional profile of common products. They are often added to baked goods like breads, cereals, and snack bars. While fortified foods and extracts offer standardized lignan content, consuming ground flaxseed still provides the additional benefits of dietary fiber and omega-3 fatty acids, which are absent in isolated lignan extracts.