Flaxseed Lignans: Source, Biosynthesis, Metabolism, Antioxidant Activity, Bio-Active Components, and Health Benefits

Authors

  • Alhassane Touré,

    1. Authors are with Chair of Food Technology, Dept. of Chemical Engineering, Inst. Polytechnic, UG, Guinea. Author Touré is also with State Key Laboratory of the School of Food Science and Technology, 1800 Lihu Rd., Wuxi 214122, P.R. China and School of Food Science and Technology, Jiangnan Univ., Wuxi, Jiangsu. Direct inquiries to author Xueming (E-mail: xmxu@jiangnan.edu.cn).
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  • Xu Xueming

    1. Authors are with Chair of Food Technology, Dept. of Chemical Engineering, Inst. Polytechnic, UG, Guinea. Author Touré is also with State Key Laboratory of the School of Food Science and Technology, 1800 Lihu Rd., Wuxi 214122, P.R. China and School of Food Science and Technology, Jiangnan Univ., Wuxi, Jiangsu. Direct inquiries to author Xueming (E-mail: xmxu@jiangnan.edu.cn).
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Abstract

ABSTRACT:  Lignans are compounds found in a variety of plant materials including flaxseed, pumpkin seed, sesame seed, soybean, broccoli, and some berries. The major lignan in flaxseed is called secoisolariciresinol diglucoside (SDG). Once ingested, SDG is converted in the colon into active mammalian lignans, enterodiol, and entero-lactone, which have shown promise in reducing growth of cancerous tumors, especially hormone-sensitive ones such as those of the breast, endometrium, and prostate. Known for their hydrogen-donating antioxidant activity as well as their ability to complex divalent transition metal cations, lignans are propitious to human health. The extraction methods vary from simple to complex depending on extraction, separation, fractionation, identification, and detection of the analytes. Flax lignan is also a source of useful biologically active components found in plant foods, such as phytochemicals, and it is considered a functional food. The safety issues in flaxseed are also briefly discussed.

Introduction

Flax (Linum usitatisimum L.) is grown as either an oil crop or as a fiber crop, with fiber (for linen) derived from the stem of fiber varieties and oil from the seed of linseed varieties (Diederichsen and Richards 2003; Vaisey-Genser and Morris 2003). Freeman (1995) reported that the seed of flax is flat and oval with a pointed tip, and varies in color from dark brown to yellow. Depending on the cultivar and growing conditions, flaxseed contains 40% to 50% oil and meal, comprised of 23% to 34% protein, 4% ash, 5% viscous fiber (mucilage), and lignan precursors (9 to 30 mg/g of defatted meal) (Muir and others 1996; Muir and Westcott 2003). Annual world production of flax was 3.06 million metric tons in 1999 to 2000 with Canada the world's largest producer of flax (about 38% of total production) (Anonymous 2000). Flax is currently the 2nd-most important oilseed crop in Western Canada and is grown primarily in the prairie provinces of Saskatchewan (70%), Manitoba (26%), and Alberta (4%) (Anonymous 2000).

Flax is making its mark in the world's food supply as a functional food. It delivers a health boost beyond what might be expected from their traditional nutrient content. Flax fits this description perfectly, being rich in alpha-linolenic acid (ALA), the essential omega-3 fatty acid, and phytochemicals such as lignans (Morris 2003).

Flaxseed has been the focus of increased interest in the field of diet and disease research due to the potential health benefits associated with some of its biologically active components: oil containing approximately 59% a-linolenic acid) and the presence of plant lignan secoisolariciresinol diglycoside (SDG;).

Lignans are found in most fiber-rich plants, including grains such as wheat, barley, and oats; legumes such as beans, lentils, and soybeans; and vegetables such as garlic, asparagus, broccoli, and carrots (Tham and others 1998; Murphy and Hendrich 2002). Plant lignans are phenolic compounds (Harris and Haggerty 1993). Flax is a particularly rich source of a lignan called secoisolariciresinol diglycoside (SDG). SDG is a plant lignan that is converted by bacteria in the colon of humans (and other animal also) to mammalian lignans known as enterodiol (ED) and enterolactone (EL).

We live in a world where free radicals can come from many sources and contribute to the deterioration of health. Sources of free radicals include pollutants, drugs, metal ions, radiation, and high intakes of polyunsaturated fatty acids, and also strenuous exercise, mitochondrial dysfunction, and smoking. These may result in damage to membrane lipids, proteins, nucleic acids, and carbohydrates, which can result in cancer, neurological diseases, lung diseases, diabetes, vascular diseases, autoimmune diseases, premature aging, and eye diseases (Lachance and others 2001).

Lignans are found in many cereals and grains, with the highest amounts occurring in flaxseed. Despite their more widespread occurrence in foods and their greater consumption in Western populations the lignans have received comparatively little attention.

In the United States, flaxseed (FS) and flaxseed meal (FLM, partially defatted FS) have found market acceptability as a component in some cereals, specialty breads, cookies, and salad dressings (Carter 1993; Nesbitt and Thompson 1997). Its growing popularity is due to food components that may provide health benefits beyond basic nutrition (Hasler and others 2000). Among the reported potential health benefits associated with FS and/or FLM are decreased risk of cardiovascular disease (Craig 1999; Jenkins and others 1999), decreased risk of cancer, particularly of the mammary and prostate gland (Craig 1999), antiinflammatory activity (Ranich and others 2001), laxative effect, and alleviation of menopausal symptoms and osteoporosis (Kurzer and Xu 1997).

Phytoestrogen supplementation with flaxseed or soy flour have been reported to increase vaginal cell maturation, an indication of estrogen activity in postmenopausal women (Wilson and others 1990), and to significantly reduce menopause symptom scores, particularly hot flashes and vaginal dryness (Brzezinski and Debi 1999). Dietary studies indicate substantial reduction in breast cancer risk among women with high urinary excretion of phytoestrogens, particularly the isoflavones equol and lignan entero-lactone (Ingram and others 1997). The lower incidence of prostate cancer in Asian men compared to men from North America and Europe has also been speculated to be due to higher dietary intake of isoflavones and lignans (Adlercreutz 1990; Morton and others 1997).

Recent research has demonstrated the ability of SDG to scavenge hydroxyl radicals and shown that SDG have potent antioxidant activity. They are biologically active phytochemicals with apparent anticancer and antioxidant potential. It stands to reason that a review is in order on the extraction, synthesis, metabolism, and antioxidant potentiality of flaxseed lignan, a naturally occurring compound because people everywhere have started to think more about health issues and have taken an interest in natural antioxidant from foods.

Lignans

Sources and structures.

 Lignans are diphenolic compounds of higher plants formed by the coupling of two coniferyl alcohol residues that are present in the plant cell wall (Jenab and others 1999; Westcott and Muir 2003). Bakke and Klosterman (1956) isolated SDG (Figure 1) from a fat-free extract of linseed meal with a 3% yield. SDG was found to be very soluble in water and alcohol. The same authors then isolated SECO [2,3-di-(methoxy-4-hydroxybenzyl) butane-1,4-diol] (Figure 1) by acid hydrolysis of SDG (Bakke and Klosterman 1956). SECO is the aglycone (nonsugar) portion of SDG. Both SDG and SECO have a UV absorption maximum at 280 nm, which is characteristic for lignans. Flax also contains small amounts of the lignans matairesinol (MAT) (11 μg/g of full fat flaxseed), pinoresinol, pinoresinol diglucoside, isolariciresinol, and a diastereomer of SDG (Whiting 1987; Mazur and others 1996).

Figure 1—.

Structure of SDG and SECO (adapted from Muir and Westcott 2003). *Molecular weight.

SECO is the major lignan present in flaxseed, which is found as the conjugate diglycoside SDG (Ford and others 1999). Early studies have demonstrated that SDG is part of a larger complex (Bakke and Klosterman 1956). An oligomer of SDG, which is ester-linked via 3-hydroxy-3-methylglutaric acid (HMGA), has been identified (Muir and others 2000a, 2000b; Ford and others 2001; Westcott and Muir 2003). A straight chain oligomeric structure composed of 5 SDG residues interconnected by 4 HMGA residues (molecular weight of about 4000 Da) was also reported (Kamal-Eldin and others 2001). SDG may comprise greater than 35% (by weight or mole %) of the polymer (Kamal-Eldin and others 2001; Westcott and Paton 2001). Among foods, flaxseed is the richest source of SDG (7 mg/g or 3.7 mg SECO 2/g). It contains 75 to 800 times more SDG than any other foods (Mazur and others 1996; Westcott and Muir 1996). Variation in flaxseed lignan concentrations depend on the variety, location, and crop year (Westcott and Muir 1996).

Whole seed and ground flax typically contain between 0.7% and 1.9% SDG, which is approximately 77 to 209 mg SDG/tbsp of whole seed or 56 to 152 mg SDG/tbsp of ground flaxseed (Morris 2004). Flax oil containing added lignans has been available for several years. One such product contains 0.1% SDG or about 14 mg SDG/tbsp flax oil (Morris 2004).

Other food sources of lignans are seeds, legumes, cereals, vegetables, berries, seaweed, tea, and alcoholic beverages (Thompson and others 1991; Namiki 1995; Mazur and others 1996, 1998a, 1998b, 2000; Mazur 1998; Liggins and others 2000; Nurmi and others 2003). Quantitative data on some lignans in food are presented in Table 1.

Table 1—.  Levels of lignans in different food plant products.
Plant sourceLignanLevel (mg/kg; dry weight)Reference
  1. aObtained by enzymatic or acid hydrolysis.

  2. bNot reported whether it is of fresh weight or dry weight.

  3. cObtained by alkaline hydrolysis.

  4. dIn oil.

FlaxseedSECOa3699Mazur and others (1996)
Matairesinol10.9 
Matairesinol7 to 28.5bKraushofer and Sontag (2002)
SDG11900 to 25900CEliasson and others (2003)
Sesame seedSesamin1547 to 8852dNamiki (1995)
Sesamolin1235 to 4765d 
CerealsSECO0.1 to 1.3Mazur and Adlercreutz (1998)
Matairesinol0 to 1.7 
VegetablesSECO0.1 to 38.7Mazur and Adlercreutz (1998)
MatairesinolTrace-0.2 
LegumesSECO0 to 15.9Mazur and others (1998a)
Matairesinol0 to 2.6 
FruitsSECOTrace-30.4Mazur (1998); Mazur and Adlercreutz (1998)
Matairesinol0 to 0.2 
BerriesSECO1.4 to 37.2Mazur and others (2000)
Matairesinol0 to 0.8 
TeaSECO15.9 to 81.9Mazur and others (1998b)
Matairesinol1.6 to 11.5 

Biosynthesis of lignans.

 The biosynthesis of lignan has recently been revised based on the discovery of the dirigent proteins that guide phenolic radical coupling (Davin and others 1997; Davin and Lewis 2000). Lignans are derived mainly via differential partitioning of the monolignol, coniferyl alcohol, to yield the lignan pinoresinol, which in turn serves as the precursor of both secoisolariciresinol and matairesinol (Figure 2). Two genes encoding the corresponding protein involved in the formation of pinoresinol and lariciresinol have been obtained from developing flaxseed (Ford and others 2001).

Figure 2—.

Biosynthesis of lignan (SDG) in flaxseed (adapted from Ford and others 2000).

Elucidation of the pathway leading to the cancer chemopreventive agent secoisolariciresinol (SDG) was first accomplished using Forsythia intermedia, a plant that both produces and further enantiometrically metabolizes pure pinoresinol, a dimeric lignan formed from (E)-coniferyl alcohol. It involves the region and stereoselective intermolecular phenoxy radical coupling of 2 molecules of (E)-coniferyl alcohol by a pinoresinol dirigent protein to yield pinoresinol (Ford and others 2001). Sequential enantiospecific reduction of this intermediate then occurs by a reductase to consequently generate lariciresinol and then SDG. Glycosilation of SDG is accomplished by secoisolariciresinol diglucosyl transferase that appears to be localized mainly in the seed (Ford and others 2001). Elucidation of the lignan biosynthetic pathway leads to the development of strategies for enhancing the levels of SDG in staple dietary foodstuffs and maximizing yields of lignans used to treat or protect against human disease (Jung and others 2000).

Analysis of phenolic compounds in flaxseed.

 Extraction methods vary widely depending on the sample and the phytoestrogen of interest. Following the discovery of SDG by Bakke and Klosterman (1956) and its connection to the mammalian lignans, several methods for the analysis of lignans and other constituents of the fat free portion of flaxseed have been developed, although only a few quantitative ones. The polymeric powder obtained by ethanol : dioxane extraction of defatted flaxseed flour (DFF) was found to release hydroxymethyl glutaric acid (HMGA; Figure 3), 4-O-β-D-glucopyranosyl coumaric acid, and SDG upon alkaline hydrolysis (Klosterman and Smith 1954; Klosterman and others 1955; Bakke and Klosterman 1956), suggesting that these compounds are bound in ester-linked polymeric structure(s) in flaxseed. The compounds may also be released from the polymeric material as aglycones by enzyme or acid hydrolysis (Mazur and others 1996).

Figure 3—.

Hydroxymethyl glutaric acid.

Extractions of flaxseed phenols have usually been carried out with organic solvents (Bakke and Klosterman 1956; Bambagiotti-Alberti and others 1994; Rickard and others 1996; Chimichi and others 1999) sometimes mixed with water (Axelson and others 1982; Kozlowska and others 1983; Dabrowski and Sosulski 1984; Amarowicz and others 1994; Westcott and Muir 1996; Meagher and others 1999; Ford and others 2001; Charlet and others 2002; Degenhardt and others 2002; Sicilia and others 2003), but the use of supercritical fluid (SCF) extraction has also been reported (Harris and Haggerty 1993). SDG and cinnamic acids absorb light in the UV-region and have been detected and quantified by column chromatography, HPLC, GC, and NMR techniques. The differences in efficiency of different modes of extractions and hydrolyses have resulted in a wide variation of yields. Methods for the quantification of SDG in flaxseed have been compiled in Table 2.

Table 2—.  Extraction systems for lignan isolation from flaxseed and flaxseed containing foods.
ExtractionHydrolysisPurificationLevel SDG (μmol/g seed)References
  1. nr = not reported.

  2. Source: Adapted from Muir and others (2000a).

MeOH-dioxane (1:1) 24 hBa methoxideCellulose column3.15Bakke and Klosterman (1956)
In vitro FermentationNa methoxideSilical gel (CHCl3-MeOH-H2O)0.96 to 3.15Thompson and others (1991)
β-Glucuronidaseβ-GlucuronidaseC18 SPE1.19 to 1.97Obermeyer and others (1995)
β-Glucuronidaseβ-Glucuronidase 2 M HCl, 2.5 h, 100°CEther extraction/DEAE-Sephadex OHQAE-SephadexAC9.05 to 10.21Mazur and Adlercreutz (1998)
Reflux, 80% MeOH, 2 hβ-GlucuronidaseC18 SPE + Lipophilic gel chromatography0.22 to 3.41Setchel and others (1999)
70% Aqueous alcoholNaOHC18 SPE5.24 to 15.74Westcott and Muir (1998)
95% EtOH-dioxane (1:1) 8 hnrnr0.001 to 0.004Harris and others (1994)
SCO2+ THF-H2O (1:1)nrnr7.15Wilson and others (1993)
Shaker, 80% MeOH, 4 h, 55°C1 M HCl, 1 h 100°CEtOAC-hexane (1:1)nrMeagher and others (1999)

Hydrolysis of glycosidic bonds of lignans.

 Two basic approaches have been used to hydrolyze the glycosidic bonds between plant lignans and carbohydrates, and both methods have been used to produce analytical results on lignan concentrations in various foods (Thompson and others 1991; Mazur and others 1996). The Thompson method employs anaerobic in vitro fermentation of food containing plant lignans with gut bacteria producing entero-lactone and enterodiol, the concentration of which is subsequently measured by GC and flame ionization detection. The 2nd method, described by Mazur and others (1996), employs hot acid to break the glycoside bond and is a multistepped process using both enzyme and hot acid to hydrolytically remove the carbohydrate component of lignan in food. In this procedure, the food extract is first subjected to enzyme hydrolysis, liberating the lignan from their respective glycosides.

Since the lignans are not completely hydrolyzed by enzymes, hot acid is used to liberate the aglycones that are partitioned off with organic solvents. The organic fractions of lignan aglycones are then purified, derivatized, and analyzed by gas chromatography-mass spectrometry (GC-MS).

However, the stability of products from acid hydrolysis has recently been questioned (Liggins and others 2000). It appears that a naturally occurring lignan called shonanin (3,4-divanillyltetrahydrofuran) is also liberated from its glycosides alongside SDG as a result of acid hydrolysis. To account for the presence of shonanin and its hydrolytic product enterofuran in foods, Liggins and others (2000) modified the procedure of Mazur and others (1996) and simplified it for the quantification of lignans, SDG, matairesinol, and shonanin in food after hydrolytic removal of any conjugated carbohydrate. The modification includes acid hydrolysis of food samples for 1, 2, and 3 h, followed by neutralization with sodium hydroxide, separation of the aglycones from aqueous to organic phase, and quantification of the aglycones by GC-MS after the formation of trimethylsilyl derivatives of the lignans.

The content of lignans in reference foods is reported as the combined concentrations of SDG and shonanin, alongside the concentration of matairesinol after 1, 2, and 3 h of hydrolysis, since optimum time of hydrolysis for the maximum yield of lignans varies between foodstuffs.

Metabolism of flax lignans

Conversion of plant lignans to mammalian lignans.

Studies in which flaxseed was fed to rats, monkeys, or humans have found that the urinary excretion of the lignans enterodiol (END) and enterolactone (ENL) are significantly increased (Axelson and others 1982; Westcott and Muir 1997). Excretion of END and ENL increased 3- to 285-times after flaxseed consumption (5 to 10 g daily for 6 wk) in the urine of 18 healthy young women, 31 healthy postmenopausal women, and 6 healthy young men (Lampe and others 1994; Hutchins and others 2000).

The first mammalian lignans, END (MW = 302) and ENL (MW = 298), were identified in humans and animals by Setchell and others (1980). Mammalian lignans are formed in the human body in the gastrointestinal (GI) tract, where GI bacteria hydrolyze the sugar moiety of SDG to release SECO (Muir and others 1996, 1997, 2000b; Ford and others 1999; Thompson, 1999). This is followed by dehydroxylation and demethylation by the colonic microflora to give the mammalian lignan END (Figure 4). END is presumed to be oxidized by the GI microbial flora to give ENL. ENL may also be formed directly from matairesinol, although this is likely to be a minor metabolic route if other lignans are present in the diet (Setchell and others 1980; Borriello and others 1985; Westcott and Muir 1997; Muir and others 1996, 2000a). The mammalian lignans differ from plant lignans in that mammalian lignans have hydroxyl groups at the 3’ position while plant lignans have their oxygenated substituents at the 3’ and 4’ positions (Axelson and Setchell 1981; Thompson 1999; Muir and Westcott 2003). A scheme outlining the proposed biosynthetic pathway of END and ENL from the flaxseed lignans SECO and SDG is shown in Figure 4. Concentrations of mammalian lignans in urine are typically greater than in plasma, thus most analytical methods target the measurement of urinary lignan levels (Muir and Westcott 2003).

Figure 4—.

Biosynthesis pathway of flaxseed lignans SDG, SECO, and their corresponding mammalian lignans END and ENL (adapted from Ford and others 2001).

Role of gut flora in the oxidation of plant lignans to mammalian lignans.

 Incubation of flaxseed by bacteria present in stools, at a concentration of 103 to 104 bacteria/g of stool resulted in the formation of END and ENL (Borriello and others 1985). This study showed that plant lignans are converted into END and ENL and the conversion is not reversible, thus END and ENL cannot convert to plant lignans in vivo. Furthermore, incubation of flaxseed in a human fecal bacterial culture at 1000 to 10000 bacteria/g converted END to ENL, but sterile fecal cultures could not (Setchell and others 1980). These results suggest that human gut floras are responsible for the conversion of plant lignans to mammalian lignans. A time course study of the metabolism of SDG by human fecal cultures shows initial demethylation (Figure 5), occurs prior (20 to 30 h) to dehydroxylation (48 h) (Wang and others 2000). Urinary excretion of END and ENL has been used as an index of plant lignan intake (Thompson 1998).

Figure 5—.

Metabolism of SDG by human fecal bacteria (Muir and Westcott 2003) -OH = dehydroxylation, -Me = demethylation.

Antioxidant scavenging activity of lignans.

 Natural antioxidants can be classified as primary (chain-breaking) antioxidants, which can react directly with lipid radicals and convert them into stable products, or as secondary (preventive) antioxidants, which can lower the rate of oxidation by different mechanisms (Decker and others 2005). Primary antioxidants most often act by donating a hydrogen atom, while secondary antioxidants may act by binding metal ions able to catalyze oxidative processes, by scavenging oxygen, by absorbing UV radiation, by inhibiting enzymes or by decomposing hydroperoxides (Schwarz and others 2001). It is known that different natural phenolic compounds function as both primary and secondary antioxidants by different mechanisms. Monitoring of either the decrease of the radical or the antioxidant, or the formation of products can be used for assessing the antioxidant activity (Decker and others 2005).

The antioxidant activity of SDG was lower than END and ENL in both lipid and aqueous in vitro model systems (Kitts and others 1999). The antioxidant activity was monitored by hydroxyl and peroxyl radical scavenging activity of SDG, END and ENL using a lipid emulsion system and inhibition of deoxyribose (Kitts and others 1999). In this study, the deoxyribose assay was used to evaluate the nonsite-specific and sitespecific Fenton reactant-induced ·OH-scavenging activity (Kitts and others 1999). The degree of oxidation of Fe2+ to Fe3+ (in the Fenton reaction) by peroxyl radicals can be measured by the ammonium thiocyanate assay (Kitts and others 1999).

Beneficial effects of SDG in cancer and lupus nephritis showed that these beneficial effects could be due to the ability of SDG to scavenge ·OH radicals (Prasad 1997).

The antioxidant activity of the flaxseed lignans and metabolites have been to exert protective effects against AAPH-induced oxidation, particularly in the recent literature (Hosseinian and others 2006, 2007; Chun and others 2007), evaluating the efficacy of these plant and mammalian lignans in protecting against AAPH peroxyl radical-induced damage and studies with DPPH (Eklund and others 2005).

The antioxidant activities of SECO, SDG, END, and ENL have also been suggested to contribute in reduction of hypercholesterolemia, atherosclerosis, and diabetes (Prasad 2000a, b). The antioxidant activity of SECO, END, and ENL was investigated by Prasad (2000b) using chemiluminescence (CL) of zymosan-activated polymorphonuclear leukocytes (PMNL). This study demonstrated that SDG, SECO, END, ENL, and vitamin E, at a concentration of 2.5 mg/mL, produced a reduction of zymosan-activated PMNL by 23.8%, 91.2%, 94.2%, 81.6%, and 18.7%, respectively (Prasad 2000b). The antioxidant activity was highest with SECO and END and lowest with vitamin E. The relative antioxidant potency of SECO, END, ENL, and SDG was 4.9, 5, 4.3, and 1.3, respectively, as compared to vitamin E (Prasad 2000b). This study suggested SECO, END, and ENL are, respectively, 3.82, 3.95, and 3.43 times more potent than SDG (Prasad 2000b). Activation of PMNL is known to generate oxygen-free-radicals. Oxygen-free radical-producing activity of PMNL was monitored by measuring luminol-dependent CL. The reduction of PMNL, therefore, reflects the antioxidant activity of the compounds studied (Prasad 2000b). Zymosan is a polysaccharide that is capable of stimulating inflammatory cytokine production and PMNL. Zymosan may serve as a model for the study of innate immune responses (Prasad and others 1991).

Health benefits

Lignans.

 The health benefits of flaxseed lignans are thought to be due to antioxidant activity, primarily as hydroxyl radical scavengers (Prasad 1997; Kitts and others 1999), and also as estrogenic and antiestrogenic compounds due, in part, to the structural similarity to 17-b-estradiol (Waters and Knowler 1982; Adlercreutz and others 1992). The behavior of the lignans depends on the biological levels of estradiol. At normal estradiol levels, the lignans act as estrogen antagonists, but in postmenopausal women (at low estradiol levels) they can act as weak estrogens (Rickard and Thompson 1997; Hutchins and Slavin 2003). Other activities related to estrogen include the in vivo synthesis of 2-hydroxy estrogen, a compound that may protect against cancer (Haggans and others 1999) and inhibit the binding of estrogen and testosterone to receptors on sex-binding globulin (Martin and others 1996).

The presence of the oxidized metabolites is unique and may provide additional reasons for the health benefits of lignans. Classical antioxidant mechanisms show that the addition of an ortho-hydroxyl group to a monophenol enhances the antioxidant activity of the original monophenol. Thus, some of the mammalian lignan metabolites may actually have greater or different activity than the parent lignan. Kitts and others (1999) reported that enterolactone and enterodiol had greater antioxidant activity than the parent.

Role of flaxseed lignan in cancer prevention

Breast cancer.

 Lignans could be a significant part of a treatment regimen for cancer based on the large number of small-scale studies. The presence of flaxseed lignans in MCF-7 tumors and the observed lignan binding to ER suggests that the lignan function may be ER-mediated (Adlercreutz and others 1992; Saarinen and others 2000). Although the lignans have been shown to be protective against breast cancer, minor structural alterations may influence overall activity (Saarinen and others 2005). Thus, many of the aforementioned benefits might be the results of specific structural features needed for lignans to bind to ER.

Flaxseed was among the best food sources in the prevention of in vivo spontaneous chromosomal damage in mice (Trentin and others 2004). The exact reason for the chromosomal damage prevention has not seen identified; however, the mechanism may be related to the antioxidant function of flaxseed components.

Lignans have antioxidant activity and thus may contribute to the anticancer activity of flaxseed (Prasad 1997; Kitts and others 1999; Yuan and others 1999; Kangas and others 2002).

Prostate cancer.

 Lignan, enterodiol, and enterolactone were believed to be partly responsible for the growth inhibition of 3 human prostate cancer cell lines (Lin and others 2001). Morton and others (1997) reported that higher enterolactone levels in prostatic fluid were associated with populations with a low risk of prostate cancer. In a small clinical study, prostate cancer cell proliferation decreased and apoptosis increased in men fed 30 g of flaxseed per day (Demark-Wahnefried and others 2001). A significant factor which may have influenced their study was that the subjects were on a low-fat diet.

A subsequent study by those authors further supported the role of flaxseed in combination with a low-fat diet as a means to control prostate growth (Demark-Wahnefried and others 2004). In this study, prostate-specific antigen level and cell proliferation both decreased from baseline after only 6 mo on the dietary regime.

Colon and skin cancer.

 Although not extensively evaluated, flaxseed has been shown to inhibit colon and skin cancers in cell cultures and in animal studies as reviewed by Thompson (2003) and Morris (2003).

Danbara and others (2005) reported that a 10 mg/kg dose of enterolactone, by subcutaneous injection 3 times per week, reduced the expression of colon 201 human colon cancer cells in athymic mice. Using various testing protocols, Danbara and others (2005) concluded that the tumor suppression was due to apoptosis and decreased cell proliferation. In general, flaxseed may be a valuable tool in the fight against various cancers. Further research is needed in clinical settings to support the role of flaxseed in cancer prevention in human populations.

Diabetes prevention.

 Low-glycemic-index foods containing soluble fiber may not only prevent certain metabolic ramifications of insulin resistance, but also reduce insulin resistance (Reaven and others 1993). Soluble fiber and other components of flaxseed fractions could potentially affect insulin secretion and its mechanisms of action in maintaining plasma glucose homeostasis. Flaxseed was shown to reduce the postprandial blood glucose response in humans (Cunnane and others 1993; Jenkins and others 1999). A consumption of 50 g/d ground flaxseed by young females over a 4-wk period caused a reduction in blood glucose levels (Cunnane and others 1993). Similar findings were observed in postmenopausal women fed a 40 g/d flaxseed fortification diet (Lemay and others 2002). Bread containing 25% flaxseed gave a glycemic response that was 28% lower than the control (no flaxseed) bread (Jenkins and others 1999).

Prasad and others (2000) reported that rats fed 22 mg SDG/kg and treated with the diabetes-promoting chemical streptozotocin had 75% lower incidence of type-1 diabetes than the streptozotocin-treated control group. However, the serum glucose of the SDG plus streptozotocin-treated rats had significantly higher serum glucose levels than the streptozotocin-treated control group.

Antinutrients.

 Flaxseed has several compounds that may negatively influence health and well-being. In some cases, the negative impact might simply be an assumption based on literature reports of similar compounds from other foods. The 2 components that have been questioned most frequently are the cyanogenic glycosides and linatine, an antipyridoxine factor.

Cyanogenic glycosides are not exclusive to flaxseed. These compounds can be found in a number of plants including brassica vegetables and especially cassava. Many of the health concerns regarding cyanogenic glycosides stem from studies showing that cassava was toxic to animals and humans (McMahon and others 1995). However, cassava contains significantly more cyanogenic glycosides than flaxseed. Furthermore, the release of hydrogen cyanide from flaxseed would be minimal and below the toxic or lethal dose. At the recommend daily intake of about 1 to 2 tablespoons, approximately 5 to 10 mg of hydrogen cyanide is released from flaxseed, which is well below the estimated acute toxic dose for an adult of 50 to 60 mg inorganic cyanide and below the 30 to 100 mg/d humans can routinely detoxify (Roseling 1994). Daun and others (2003) reported that a person would have to consume 8 cups (1 kg) of ground flaxseed to achieve acute cyanide toxicity.

In addition to cyanogenic glycosides, trypsin inhibitor, linatine, and phytic acid are other antinutrients contained in flaxseed. Trypsin inhibitor activity (TIA) in flaxseed was low when compared to those in soybean and canola seeds. Bhatty (1993) reported laboratory-prepared flaxseed meals containing 42 to 51 units of TIA, which was slightly higher than 10 to 30 units observed by Madusudhan and Singh (1983) and commercially obtained flaxseed meal (14 to 37 units). The contents of phytic acid were significantly different among cultivars. AC Linora has the lowest phytic acid content of 2280 mg/100 g and low ALA yellow-seeded cultivar Linola 947 has the highest content (3250 mg/100 g seed) among the 8 cultivars examined (Oomah and others 1996).

Kratzer (1946) reported that pyridoxine supplementation in chicks on diets containing linseed meal was necessary to counteract their vitamin B6 deficiency. Klosterman and others (1967) identified the antipyridoxine factor linatine. Although linatine is a problem in chicks, flaxseed has not been associated with a vitamin B6 deficiency in humans. In fact, no effect on serum pyridoxine levels in subjects consuming 45 grams of flaxseed per day over 5 wk has been observed (Dieken 1992).

Conclusions

Phytoestrogens, such as lignans, which act as either estrogen agonists or antagonists have generated interest because of their potential use in hormone replacement therapy and cancer prevention. Mammalian lignans are produced by the action of gut microflora on precursors such as the plant lignan SDG. Flaxseed lignans have potential antioxidant advantages in that they are natural antioxidants with potential health benefits. This review provides a better understanding of the flaxseed antioxidant activities and suggests that flaxseed lignans may be used as natural antioxidants. More in vivo studies are needed to ascertain the propitious effects of lignans secoisolariciresinol and to see if there are any dangers in possible overdoses.

Ancillary