Flax has been cultivated and has grown semi-wild wild in various temperate and tropical regions for so many centuries that it is not even clear which hemisphere the plant originated from. Egyptians, Hebrews, Greeks, Romans and New World civilisations used the seeds as food and the fibres (flax) for textiles.

Flax is an annual, 0.3-1 m high, wiry, more or less branching plant with small, unstalked, dark or grayish-green leaves. The flowers are borne on the ends of the stalks. Each flower produces a round capsule in which 1 to 10 seeds develop. There are different cultivars for the production of Linseed oil and flax. Linseed plants are shorter, and their flowers are commonly bright blue, less commonly pale blue or white, while most flax cultivars bear white flowers.

Linseed is a common food in North Europe. It is often used in cereals and breads, including the German Leinsamenbrot. It can be sprouted and served in salads. It is also an ingredient in cattle feeds. Linseed meal may be what is left over from the oil extraction process, or it may be part of the "oil cake" because the hull is highly nutritious.

The seed contains 30-40% oil, which comprises mainly linoleic and linolenic acids. It is used mainly in the preparation of varnish, paint, linoleum, and soap. Linseed oil has recently been used as a laxative.

Flaxseed has experienced millennia of medicinal usage, mainly as a laxative, expectorant and demulcent. Recently, it has been noted that Linseed contains phytoestrogens, compounds with weak estrogenic or antiestrogenic activity, which have purported health benefits such as mitigation of hormone-dependent breast and prostate cancers, osteoporosis, cognitive dysfunction, cardiovascular disease, immune system dysfunction, inflammation, and infertility (1).

Flaxseed has recently gained particular attention in the area of cardiovascular disease. It not only contains lignans, a phytoestrogen. It is also the richest known source of alpha-linolenic acid (ALA), the parent compound of the omega-3 fatty acids. (In comparison, fish contain only trace amounts of ALA, and fish oil can adversely affect the taste and odour of food products.) It is, furthermore, rich in soluble fibre (2).

Allergen description

Linseed contains potent allergens (6), but no allergens from this plant have yet been characterised.

Five allergens with molecular weights of 38, 35, 30, 22, and 20 kDa have been detected (5). However, in another study, allergens with a higher molecular weight (150-175 kDa), intensely bound with IgE, were found, as were other more discrete bindings with lower molecular weight allergens (90-100 kDa). Under reducing conditions, the high-molecular-weight bands disappeared and discrete bands of lower molecular weight (25-100 kDa) were detected (7).

In a study, the major allergen of Linseed was implicated in the case of a 39-year-old woman who developed anaphylaxis to Linseed grains; this allergen was shown to be a dimmer, consisting of a pair of 28 kDa monomers bound by SH2 groups. The authors suggest that the candidate may be malate dehydrogenase MDH-1, found in flax seeds (8).

Potential Cross Reactivity

An extensive cross-reactivity among the different individual species of the genus could be expected but has not been clinically recorded (9).

Clinical Experience

IgE-mediated reactions

Linseed may uncommonly induce symptoms of food allergy in sensitised individuals; but cases could increase in number and variety because of the increased use of Linseed in bread and laxatives, and in a range of products from health food shops (3).

Anaphylaxis induced by Linseed ingestion was described in a 39-year-old woman who developed symptoms immediately after the ingestion of the first spoonful of Linseed grains prescribed as a laxative. Skin reactivity and IgE antibodies to Linseed was demonstrated (3).

A 40-year-old man had had, over a 6-year period, 5 or 6 episodes of intestinal/abdominal pain, vomiting, diarrhoea, generalised urticaria, acute dyspnoea without bronchospasm, hydrorrhoea, successive sneezing, nasal obstruction, pruritis, and intense general malaise which required emergency treatement, all occurring within 2-3 minutes after ingestion of multigrain bread. SPT and IgE antibody determination were positive (7).

Anaphylaxis in a 40-year-old woman was reported to be caused by the ingestion of Linseed oil used as a laxative. Ten minutes after the intake of the first spoonful, she experienced ocular pruritus and weeping, followed by strong palmar pruritus and generalised urticaria, nausea, and vomiting. SPT and IgE antibody determination were positive (5).

In 102 patients initially diagnosed with idiopathic anaphylaxis and then evaluated with a battery of 79 food-antigen skin prick tests, 10 different antigens provoked anaphylaxis, including Linseed. The study concluded that around 7% of the patients in question who were suspected of “idiopathic” anaphylaxis on the basis of history were not truly idiopathic (10).

Occupational dermatitis caused by Sunflower seeds and Linseed has been reported (11).

Other reactions

Flax is an important cause of respiratory disease. Non-IgE-mediated byssinosis, caused by inhalation of dust in the processing of flax, has been described in flax workers (12), as well as IgE-mediated occupational asthma from the processing of Linseed oil (13). (Byssinosis, a respiratory disease also caused by dust of cotton and soft hemp, is classically characterised as shortness of breath, cough, and chest tightness on Mondays or the first day of return to work after other time off (14)). Flaxseed hypersensitivity was described by 2 different authors in the 1930s, 1 describing Flaxseed sensitisation in 6 individuals (4,15).

Linseed may be a source of cyanide exposure. Linseeds are cyanogenic, but toxic effects are unknown from normal conditions of manufacture, which involve high-temperature treatment or from traditional and moderate use by humans. Also, safer cultivars have been developed. (However, unprocessed whole seeds and Linseed cakes processed under low temperature can be toxic to animals.) The potential cyanide yield can vary from 4 to 12 mmol/kg. Linseed contains the same cyanogenic glucosides as Cassava. Authors have suggested that excess intake (via laxatives) may be dangerous (16).

The oil in the seed contains 4% L-glutamic acid, or MSG, and therefore might cause MSG-type reactions.

The seed is hard to digest and provokes flatulence.

Last reviewed: June 2022.