Bioaccessibility of lignans from flaxseed (Linum usitatissimumL.) determined by single batch invitro simulation of the digestive process

Flaxseed is an important source of lignan secoisolariciresinol diglucoside (SDG) and its aglycone, secoisolariciresinol (SECO). These phenolic compounds can be metabolized to the mammalian lignans enterodiol (ED) and enterolactone (EL) by human intestinal microflora. Flaxseed lignans are known for their potential health benefits, which are attributed to their antioxidant and phytoestrogenic properties. The focus of this study was to determine the bioaccessibility of plant and mammalian lignans in whole flaxseed (WF) and flaxseed flour (FF) throughout the entire digestive process. Moreover, the metabolic activity of intestinal microflora was evaluated. A single batch in vitro simulation of the digestive process was performed, including fermentation by the intestinal microflora in the colon. Bioaccessibility was calculated as (free lignan) to (total lignan). In digested WF, the bioaccessibility values of SECO, ED and EL were 0.75 percent, 1.56 percent and 1.23 percent, respectively. Conversely, in digested FF, the bioaccessibility values of SDG, ED and EL were 2.06 percent, 2.72 percent and 1.04 percent, respectively. The anaerobic count and short chain fatty acids indicate that bacteria survival and carbohydrate fermentation occurred. The contents of both SDG and ED were significantly higher in digested FF than in digested WF. FF facilitated the action of intestinal bacteria to release SDG and metabolize ED. (Authors abstract)
Flaxseed (Linum usitatissimum L) is known for its high content of lignans relative to other grains and legumes, of which secoisolariciresinol diglucoside (SDG) is the most important.  SDG can be metabolized to mammalian lignans by the gut micro flora through a series of reactions: first, SDG undergoes hydrolysis to generate the aglycone secoisolariciresinol (SECO),which is then dehydrated and demethylated to produce enterodiol (ED), and, finally, ED can be oxidized to enterolactone (EL).   Flaxseed lignans and their mammalian metabolites are known to have a number of potential health benefits, including reducing the risk of breast, prostate and colon cancers, which has been attributed to its (anti) estrogenic and antioxidant properties.  Furthermore, they can lower the total cholesterol, low density lipoprotein (LDL) cholesterol and glucose concentrations in the blood, which could prevent cardiovascular diseases.  Depending on the rate of SDG metabolism by the intestinal microflora, intestinal epithelial cells can be exposed to relatively high concentrations of SECO,ED and EL.   Several in vitro gastrointestinal models have been designed to study the reactions that occur during digestion. The simulator of the human intestinal microbial ecosystem (SHIME) involves five or six bioreactors with controlled pH conditions that simulate the stomach, the small intestine and the ascending, transverse and descending colon. Recently, a new system was designed that uses a single bioreactor to study the passage of food through the stomach and small intestine. This system simulates the upper gastrointestinal tract and can be used to determine the survival of probiotics in different food matrixes. Studies on the effects of the intake of lignans derived from flaxseed consumption, either as whole seeds or as flour, are not reported. The aim of this study was to evaluate the metabolism of SDG from whole flaxseed and flaxseed flour during the digestive process using a single batch in vitro model. The digestive simulation included mastication, the stomach, and the small and large intestine. Further, the metabolic activity of intestinal microflora exposed to flaxseed was evaluated through short chain fatty acid (SFCA) production, anaerobic counts and amino acid profile.
The flaxseed lignans from FF and WF were bio accessible during in vitro simulation of the digestive process. The main difference between the FF and WF digestion processes was the release of lignans at different digestion stages. When consuming FF, a larger amount of ED could be exposed than when consuming WF. In addition, FF digestion would release more SDG to be absorbed and/or metabolized later.  However, WF digestion can also provide nutritional benefits from the release of lignans that can be absorbed in the human bowel.  Furthermore, the intestinal bacteria are able to metabolize these plant lignans to enterolignans. Although the bio accessibility was not as high as expected, these results are satisfactory and allowed the development of a single batch in vitro simulation of the digestive process. The results suggested that, to increase the bio accessibility of lignans, it is necessary to consume flaxseed for a prolonged period, so that gut microflora can then adapt to metabolize lignans. Further experiments are needed, including a study of probiotic bacteria that might metabolize the flaxseed lignans in food before consumption, which could increase the bio accessibility of the SDG in whole flaxseed. (Editors comments)