Flaxseed is the soluble fibre mucilage obtained from the fully developed seed of Linus usitatis-simum.
Flaxseed is a rich source of alpha-linolenic acid and lignans. Alpha-linolenic acid is an essential fatty acid of the n-3 (omega-3) series, which must be supplied by the diet. It can be converted into longer-chain fatty acids of the n-3 series, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), the fatty acids found in fish oils (see Fish oils).
Fatty acids of the n-3 series (like those of the n-6 series) are precursors to a range of prostaglandins, thromboxanes and leuko-trienes, and those formed from the n-3 series are generally less pro-inflammatory and atherogenic than those formed from the n-6 series. In addition, there is competition between the fatty acids of both series for the enzyme systems that effect chain elongation, suggesting that a balance between the two types of fatty acids is important; it is estimated that an optimal ratio between n-3 and n-6 fatty acid is 1:4. However, changes in food production and advice to consume polyunsaturated fats has led to an increase in the n-6 fatty acids at the expense of the n-3 fats, and the ratio of n-3 to n-6 fats is between 1:10 and 1:30. Flaxseed oil, which contains approximately three times more n-3 than n-6 fatty acids, may help to reverse the imbalance between n-3 and n-6 fats.
Lignans are a type of phytoestrogen. They are digested in the colon to produce enterodiol and enterolactone, lignans that are thought to be protective against cancer.
Traditionally, flaxseed has been used for constipation and other bowel disorders such as diverticulitis and irritable bowel syndrome (IBS). In theory, it may also be useful as a source of n-3 fatty acids for the same conditions (e.g. CVD and other inflammatory disorders) where fish oil has benefit. It is also a useful oil for vegetarians, to help improve the balance between omega-3 and omega-6 fatty acids, because vegetarians tend to consume significant quantities of omega-6 fatty acids.
In a small trial, 11 healthy male subjects were randomly assigned to receive 40 g flaxseed oil or sunflower seed oil for 23 days.1 A statistically significant reduction in platelet aggregation response to collagen was found in subjects supplemented with flaxseed but not with sunflower seed. The authors noted that the higher percentage of energy from fat in the sunflower oil might have skewed the results. In spite of this limitation, the conclusion was that oils rich in alpha-linolenic acid may offer increased protection in CVD (via reduction in platelet aggregation) compared with oils rich in linoleic acid.
In a randomised, double-blind study, 32 healthy subjects received either 35 mg flaxseed oil or 35 mg fish oil for 3 months.2 In contrast to fish oil, flaxseed had no effect on serum triglyceride or cholesterol levels, but the authors concluded that the dose may have been insufficient or that conversion of alpha-linolenic acid to EPA was inadequate.
In a double-blind crossover study, 29 subjects with hyperlipidaemia were given (in random order) muffins containing either 50 g partially defatted flaxseed or wheat bran (control) each day for 3 weeks. Flaxseed significantly reduced total serum cholesterol by 4.6% and LDL cholesterol by 7.6%, but there were no effects on HDL cholesterol.3
In a double-blind, placebo-controlled study, 11 patients with well-controlled type 2 diabetes were given flaxseed oil or fish oil capsules for 3 months each, in random order. Fish oil, but not flaxseed oil, reduced serum triglycerides, but there were no significant differences between the two oils in relation to total, LDL and HDL cholesterol levels.4
In a study involving post-menopausal women who were not on HRT, flaxseed supplementation (40 g with 1 g of calcium and 400 units of vitamin D daily) lowered both serum total cholesterol and non-HDL cholesterol by 6% whereas the comparative control regime had no such effect. Flaxseed reduced serum levels of both LDL and HDL cholesterol by 4.7% and triglyceride by 12.8% (although these did not reach statistical significance). Serum apolipoprotein B concentrations were significantly reduced by 6.0% and 7.5% respectively by the flaxseed regimen. Markers of bone formation and resorption were not affected.5
A trial in 25 menopausal women with total cholesterol > 6.2 mmol/L, a cholesterol:HDL cholesterol ratio > 4.5 and triglycerides 3.5 mmol/L compared 40 g crushed flaxseed daily with 0.625 mg of conjugated equine oestrogens alone or combined with progesterone. Both HRT and flaxseed produced similar decreases in menopausal symptoms and glucose and insulin levels. However, only HRT significantly improved cholesterol profile and favourably modified markers related to cardiovascular health.6
A meta-analysis of five cohort studies and three clinical trials on alpha-linolenic acid and heart disease, together with nine cohort and case-control studies on alpha-linolenic acid (ALA) and prostate cancer found that high ALA intake was associated with reduced risk of fatal heart disease in prospective cohort studies. Three open-label trials also indicated that ALA may protect against heart disease. However, epidemiological studies in this meta-analysis also showed an increased risk of prostate cancer in men with a high intake or blood level of ALA.7
More recently, an RCT in 199 menopausal women found that flaxseed (40 g daily) reduced serum total cholesterol and HDL cholesterol compared with placebo. There was no significant change in BMD or menopausal symptoms.8 A further RCT in 57 men with an atherogenic lipoprotein phenotype found that fish oil produced predictable changes in plasma lipids and small dense LDL that were not reproduced by a diet enriched with ALA.9
A systematic review of 14 studies found that ALA supplementation may cause small dereases in fibrinogen concentrations and plasma glucose, but most cardiovascular risk markers do not appear to be affected. The reviewers concluded that supplementation with ALA to reduce CVD cannot be recommended.10
There is some evidence from animal studies that flaxseed inhibits tumour growth, particularly mammary tumours,11,12 but clinical trials are needed to assess whether flaxseed has anti-cancer properties in humans.
In a double-blind, placebo-controlled study involving 22 patients with rheumatoid arthritis, 30 g flaxseed powder daily for 3 months had no effect on clinical subjective parameters of the disorder compared with sunflower oil.13
A small study in eight patients with SLE, present as lupus nephritis, found that flaxseed in a dose of 30 g daily improved renal function and inflammatory mediators and was well tolerated.14
No problems have been reported.
Pregnancy and breast-feeding
No problems have been reported, but there have not been sufficient studies to guarantee the safety of flaxseed in pregnancy and breast-feeding.
There are no known toxicity or serious side-effects, but no long-term studies have assessed the safety of flaxseed. Flaxseed contains cyanogenic glycosides, which are naturally-occurring toxicants. The long-term effects of these compounds are unknown, but high doses could increase plasma thiocyanate levels.
Flaxseed oil is available in the form of a liquid and in capsules.
The dose is not established. Manufacturers suggest one tablespoon of flaxseed oil daily.
Theoretically, flaxseed as a source of n-3 fatty acids could benefit the same conditions as those indicated for fish oil. However, research data are currently insufficient to make recommendations for supplements for reduction in risk of heart disease and cancer or management of rheumatoid arthritis and other inflammatory conditions.
Allman MA, Pena NM, Pang D. Supplementation with flaxseed oil versus sunflower oil in healthy young men consuming a low fat diet: effects on platelet composition and function. Eur J Clin Nutr 1995; 49: 169–178.
Layne KS, Goh YK, Jumpsen JA, et al. Normal subjects consuming physiological levels of 18:3(n-3) and 20:5(n-3) from flaxseed or fish oils have characteristic differences in plasma lipid and lipoprotein fatty acid levels. J Nutr 1996; 126: 2130–2140.
Jenkins DJA, Kendall CWC, Vidgen E, et al. Health aspects of partially defatted flaxseed, including effects on serum lipids, oxidative measures, and ex vivo androgen and progestin activity: a controlled crossover trial. Am J Clin Nutr 1999; 69: 395–402.
McManus RM, Jumpson J, Finegood DT. A comparison of the effects of n-3 fatty acids from linseed oil and fish oil in well-controlled type II diabetes. Diabetes Care 1996; 19: 463–467.
Lucas AE, Wild RD, Hammond LJ, et al. Flaxseed improves lipid profile without altering biomarkers of bone metabolism in postmenopausal women. J Clin Endocrinol Metab 2002; 87: 1527–1532.
Lemay A, Dodin S, Kadri N, et al. Flaxseed dietary supplement versus hormone replacement therapy in hypercholesterolemic menopausal women. Obstet Gynecol 2002; 100: 495–504.
Brouwer IA, Katan MB, Zock PL. Dietary alpha-linolenic acid is associated with reduced risk of fatal coronary heart disease, but increased prostate cancer: a meta-analysis. J Nutr 2004; 134: 919–922.
Dodin S, Lemay A, Jaques H, et al. The effects of flaxseed dietary supplement on lipid profile, bone mineral density, and symptoms in menopausal women: a randomized, double-blind, wheat germ placebo-controlled clinical trial. J Clin Endocrinol Metab 2005; 90: 1390–1397.
Wilkinson P, Leach C, Ah-Sing EE, et al. Influence of alpha-linolenic acid and fish-oil on markers of cardiovascular risk in subjects with an atherogenic lipoprotein profile. Atherosclerosis 2005; 181: 115–124.
Wendland E, Farmer A, Glasziou P, Neil A. Effect of alpha linolenic acid on cardiovascular risk markers: a systematic review. Heart 2006; 92: 166–169.
Serraino M, Thompson LU. The effect of flaxseed supplementation on the initiation and promotional stages of mammary tumorigenesis. Nutr Cancer 1992; 17: 153–159.
Thompson LU, Rickard SE, Orcheson LJ, et al. Flaxseed and its lignan and oil components reduce mammary tumour growth at a late stage of carcinogenesis. Carcinogenesis 1996; 17: 1373–1376.
Nordstrom DC, Honkanen VE, Nasu Y, et al. Alpha-linolenic acid in the treatment of rheumatoid arthritis. A double-blind placebo controlled and randomized study; flaxseed versus safflower seed. Rheumatol Int 1995; 14: 231–234.
Clark WF, Parbtani A, Huff MW, et al. Flaxseed: a potential treatment for lupus nephritis. Kidney Int 1995; 48: 475–480.