LINACEAE

The Linaceae or flax family consists of about 14 genera of annual or perennial herbs or sometimes shrubs, of mainly temperate regions worldwide. Leaves simple, entire and usually alternate. Flowers mostly in cymes though sometimes corymbose, racemose or panicled; regular, in 4 or 5 parts. Fruit usually a capsule but sometimes a drupe.

Linum usitatissimum

Flaxseed, linseed

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Family Linaceae

Description

A thin annual growing from 30–130 cm tall in flower. Stems erect, usually glabrous and simple but sometimes branching at the base. Leaves alternate, sessile, simple, linear to lanceolate, entire, glaucous green, 3–5 cm long and marked with 3 veins. Flowers pale to dark blue, sometimes white, 5-petaled, to 3 cm across in terminal leafy panicles, sepals ovate, half as long as the petals. Capsule longer than the calyx, containing 5 or 10 cells packed with shiny brown oblong, oval seeds narrowed at one end, 4–5 mm long. The capsule does not shatter to release seed. Flowers from late spring to early autumn.

Habitat and cultivation

Flax has been cultivated for its fibre and linseed oil for so long that no one knows its origin. It is an introduced crop in the southern hemisphere. It is also widely naturalised. It is always grown from seed sown where the plants are to stand and prefers a rich, moist soil and sunny situation. It is drought and frost tender. An improved type of L. usitatissimum containing omega 3 fatty acids is also available and is grown in the same way.

Parts used

The seed harvested when ripe.

Active constituents

1) Fixed oil (30–40%) mainly based on α-linolenic (55%) and linoleic acids—the proportions change according to growing conditions.1 As the seed develops the former increases and the latter decreases2.

2) Cyanogenic glycosides including mainly linustatin and neolinustatin3,4

3) Mucilage (up to 10%) mainly arabino-xylans5,6 and galactoglucan7 as part of dietary fibre

4) Lignans (0.7–1.5%)8–11 mainly secoisolariciresinol diglucoside12,13 in the form of a polymer, also glycosides of matairesinol,14 isolariciresinol and pinoresinol15

Also contains dietary fibre (both soluble and insoluble up to 28%),16 cinnamic glycosides8 (p-coumaric and ferulic acids),17 proteins (25%) and cyclic peptides,18 a phenylpropanoid (linusitamarin),19 β-sitosterol,19 wax and sugar.

Nutritional constituents

Vitamins: B3 (nicotinamide)19

Minerals: Calcium and potassium

Actions

1) Laxative

2) Demulcent

3) Antitussive

4) Emollient

Scientific information

Linum was an ancient Roman medicine although its origins are likely to pre-date this period. It has been an official medicine throughout the world, being used internally as a demulcent to treat coughs and as a bulk laxative and externally as a poultice to reduce inflammation.20 German Commission E has approved its internal use for the treatment of chronic constipation, irritable bowel, diverticulitis, gastritis and enteritis and externally for local inflammation.

In traditional medicine linseed is used for conditions that benefit from the action of mucilage. The seeds which contain high levels of unsaturated fatty acid, particularly α-linolenic acid (ALA), have good nutritional value also. However the lignans and their metabolites have been studied in recent times as they have been linked to benefits for the cardiovascular system, reduction of aromatase activity (an enzyme involved in hormone production) and possible inhibition of prostate and breast cancer.21 As linseed has become a commercially successful health supplement it has been preferentially referred to as flaxseed possibly to disassociate it from its other uses—protecting wood and as a component of paint.

Cardiovascular effects

The oil has a very high level of ALA, one of the omega-3 fatty acids, which can be converted in vivo to the longer chain poly-unsaturated fatty acids associated with protection against cardiovascular disease.

In vivo—Long chain fatty acid levels in erythrocytes and plasma rose after 4 weeks supplementation of 50 g a day of ground linseed.74 ALA supplementation from linseed has resulted in:-

There appears to be little benefit from using plant derived unsaturated omega-3 oils to treat raised serum lipids22,27–29 although one study in which flaxseed oil was used with an appropriate diet, lipid profiles in overweight men did improve.30

The use of whole ground flaxseed to alter cardiovascular disease risk factors has yielded mixed results. Positive benefits in serum lipid levels were shown in:-

Other trials showed there was no benefit to lipid profiles or cardiovascular health in already healthy people, from supplementing with lignans or ground flaxseed.36–40

Phyto-oestrogen

Linseed contains the highest levels of plant lignans in the edible plants so far studied.41 These are metabolised by gut flora to the “mammalian lignans”, predominantly enterodiol and enterolactone, via intermediate metabolites that are also likely to be bioactive.42 These compounds are structurally similar to oestrogens and tamoxifen and appear to be both weakly oestrogenic and anti-oestrogenic.43

In vitro—Enterolactone and/or lignans induce apoptosis and inhibit the growth of colon cancer cells44 and hormone dependent tumour cells.45

In vivo—The potential for the phyto-oestrogenic lignans to act as a natural HRT in post-menopausal women has been assessed. Studies failed to find any advantage in healthy post-menopausal women in reducing menopausal symptoms36,46 or improving bone density31,36,47 from supplementation with Linum. However flaxseed may be as effective as HRT in women with mild symptoms48 and did reduce blood pressure in post-menopausal women with cardiovascular disease who were subjected to stress.49

Flaxseed alters hormone metabolism. Post-menopausal women who supplemented with it daily had decreased levels of 17 β-oestradiol and oestrone sulphate, increased prolactin levels50 and increased excretion of oestrogen metabolites51 all of which could be protective against breast cancer. (Diets rich in ALA and dietary fibre have been associated with a decreased incidence of some cancers.)52,53 Flaxseed supplements given to women with primary breast cancer had reduced levels of proliferation and invasiveness of their tumours.52

Phyto-oestrogens are also potentially useful for prostate disease. A pilot study using a low fat diet and flaxseed supplement reduced proliferation of prostate cancers53 and benign prostatic hyperplasia, growth rates were reduced without changing testosterone levels.54 More information on these potential benefits is required as another study failed to corroborate benefits for prostate cancer treatment.55

Glucose control

In vivo—Flax meal increased bowel function (bulk laxative), improved postprandial blood glucose48,56,74 and reduced insulin levels48 whether taken in baked products or as ground meal.

Other

In vitro—Other actions for Linum's constituents include:-

Ex-vivo—ALA from flaxseed oil does not seem to have the same beneficial anti-inflammatory potential as γ-linolenic acid of animal or plant origin.65

In vivo—Additional clinical data:-

Medicinal uses

Respiratory tract

Gastro-intestinal tract

As a tonic laxative it may be used safely and long term for an over-relaxed bowel. Also for:

Externally

As a poultice Linum is used to soothe:

Pharmacy

Three times daily  
Crushed or entire seeds –   3–6 g
Infusion of the seeds –   3–6 g

Ground flaxseed doses used in the above clinical studies have varied between 5–50 g per day with most using 20–40 g daily. German Commission E recommends 20–30 g daily.

The seeds of Linum can be used either whole or crushed however the bio-availability of lignans is greatly increased by crushing or better still by grinding them.71 The lignans are heat stable, appear to store well, survive being baked72 and if eaten with dietary fibre, their absorbance from the digestive tract is not impeded.73 The fatty acids are also very well absorbed from the ground seeds.74

To achieve the bulk laxative effect the seeds should be taken whole or crushed, followed by drinking at least 1–2 glasses of fluid as this releases their mucilage. The seeds may need to be taken for a long time before bowel tone is restored and the immediate laxative effects too may require several days continuous intake to become effective.

The other way of using Linum is to first soak the seeds to extract the mucilage and then to use this internally as a demulcent or externally as an emollient. They may be applied after soaking in hot water (as a poultice), to draw boils.

CONTRAINDICATIONS—Stenoses of gastrointestinal tract or ileus.

Pharmacokinetics

After ingestion of flaxseed the gut flora convert lignans to enterolignans, the main one being enterolactone. They are absorbed by the intestinal cells and conjugated there or in the liver, mainly with glucuronide, becoming detectable in plasma some 8–10 hours later75 and reaching peak plasma levels after 24 hours.76 They occur with a variety of their metabolites in this conjugated form in plasma, serum and urine77,78 but are present in faeces in their free form.79 The plasma level of enterolignans attained varies from person to person75 but is doserelated.80

Precautions and/or safety

There have been cautions issued regarding flaxseed use and studies have examined their veracity:-

1) The cyanogenic glycosides can degrade to release cyanide—these levels were non-toxic to humans81

2) Long chain fatty acids could delay spontaneous births—flaxseed oil did not effect delivery dates82

3) The lignans may alter female hormone levels—serum hormone levels of pre-menopausal women were unchanged after consuming flaxseed73

Flaxseed oil fed to fruit flies was genotoxic although the significance of this for humans is uncertain.83 There are reported cases of anaphylaxis to the herb.84,85

Interactions

There is an anecdotal report of haematuria after flaxseed oil was used with aspirin, omega-3 fatty acids could have altered prostaglandin levels.86

Historical uses

In coughs and colds; for urinary tract irritations.