Flaxseed is the seed of the Linum usitatissimum plant, which has been used for textile production (linen) and as animal feed for many years. More recently, flaxseed has been noted to have several health benefits in humans and has become more common in human diets and as a supplement.

Flaxseed is high in fiber, with about 28% of its mass being fiber, a third of which is soluble fiber. It also contains around 30% protein, 22% alpha-linolenic acid (ALA), and phenolic compounds, including a type of phytoestrogen called lignan.^[14][1]

Flaxseed supplementation could be useful in people with high cholesterol levels. In particular, it could lower LDL cholesterol in people with high cholesterol.^[1][2][15] The effect of flaxseed supplements on other types of cholesterol — like HDL cholesterol and triglycerides — is inconsistent, although some studies do find a decrease in triglycerides and an increase in HDL cholesterol.^[15][1][16]

Flaxseed supplements may also lower both systolic and diastolic blood pressure in people with high blood pressure. The effect is most notable when whole or ground flaxseed is used.^{[10][9][17][18]}

Flaxseed has been used in people with high blood glucose. Supplementation with flaxseed seems to lower fasting blood glucose levels and insulin levels but does not consistently improve HbA1c levels.^{[19][20][3][14]}

Flaxseed can have gastrointestinal side effects such as nausea, constipation, and abdominal cramps.^[10] When taken in high doses without sufficient fluids, flaxseed may cause constipation, and a case report of bowel obstruction has also been noted.^[16]

The phytoestrogens in flaxseed have weak estrogenic activity. While this is likely to be safe, it is best not to start taking flaxseed during pregnancy, because the effects of these hormones on gestation are uncertain.^[16]

The effect of phytoestrogens on sexual development in boys and on male fertility is currently unclear.^[19]

Flaxseed contains several different biological compounds that are responsible for its effect on cholesterol, blood pressure, high blood glucose, and several other positive effects.

Flaxseed contains both soluble and insoluble fiber. Soluble fiber helps to modulate glucose levels, particularly just after eating. Insoluble fiber can increase insulin sensitivity. Fiber also increases satiety, reduces the rate of glucose absorption through the intestine, and increases the excretion of cholesterol.^[19][16]

Alpha-linolenic acid (ALA) is the omega-3 fatty acid found in flaxseed. ALA is used by the body in several cellular processes, and ALA can also be converted into eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), although this process is not efficient. EPA and DHA are the omega-3 fatty acids that are found predominantly in seafood sources. Flaxseed can therefore be a useful way to supplement omega-3 fatty acids, especially in people who do not regularly eat seafood or choose to follow a vegetarian or vegan diet. ALA may have some benefits for cardiovascular disease^[21] and might have anti-inflammatory properties.^[22] These effects could occur through the reduction of inflammatory markers like C-reactive protein (CRP) and interleukin 6 (IL-6),^[6] but the evidence for this is not consistent.^[19]

Lignans are phenolic compounds that are found in flaxseed, and they are converted from plant lignans to biologically active mammalian lignans by the intestinal microbiota. They are thought to have several positive effects, including antioxidant, anti-inflammatory, and weak estrogenic effects.^[19]

Flaxseed also contains several minerals, such as calcium, magnesium, and phosphorus, all of which are important in blood pressure regulation, muscle contraction and relaxation, and several cellular functions. Along with the small amount of vitamin E found in flaxseed, these may also contribute to the anti-inflammatory and antioxidant effects.^[19]

Phytoestrogens (e.g., lignans) and vitamin E play an important role in cellular regulation, in particular in angiogenesis (the growth of new blood vessels) and cell apoptosis (natural cell death). This could result in flaxseed having an anticancer effect. These effects have been observed in animal models and have not yet been confirmed in human studies.^[19]