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Flaxseed

Also known as: Linum usitatissimum, Linseed, Flax, Common flax

Overview

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.

Benefits

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. 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 and might have anti-inflammatory properties. These effects could occur through the reduction of inflammatory markers like C-reactive protein (CRP) and interleukin 6 (IL-6) but the evidence for this is not consistent. 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. 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. 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.

How it works

Flaxseed supplementation could be useful in people with high cholesterol levels. In particular, it could lower LDL cholesterol in people with high cholesterol. 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. 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. 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.

Side effects

Doses used in studies vary, and a specific recommended daily dose is not yet clear. As whole flaxseed, studies have used dosages ranging from 10 to 60 grams per day. As flaxseed oil, dosages range from 1 to 42 grams per day. Lignan — a phytoestrogen in flaxseed — can be supplemented in capsules with dosages ranging from 100 to 600 milligrams per day. Currently there is no clear recommendation on the best form of flaxseed supplement to use. Ground flaxseed and flaxseed oil are more readily absorbed than whole flaxseed, and taking flaxseed in ground form may be more effective for lowering LDL cholesterol levels. High blood pressure might respond best to flaxseed oil, while whole flaxseed had a more significant effect on BMI, weight loss, and waist circumference in one meta-analysis. It is important to remember to store ground flaxseed in an airtight container to prevent it from oxidizing. In warmer climates it is advisable to store it in the refrigerator. Flaxseed oil should be stored in a cool, dark environment, because it can be damaged by both heat and light.

Dosage

Flaxseed can have gastrointestinal side effects such as nausea, constipation, and abdominal cramps. When taken in high doses without sufficient fluids, flaxseed may cause constipation, and a case report of bowel obstruction has also been noted. 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. The effect of phytoestrogens on sexual development in boys and on male fertility is currently unclear.

FAQs

What is flaxseed?

What are flaxseed’s main benefits?

Can flaxseed help with weight loss?

Flaxseed supplementation has been associated with a change in anthropometrics (body measurements). A 2024 meta-analysis found that flaxseed supplementation decreased weight, waist circumference, and BMI. These effects were most notable in people with a BMI above 30 and in trials that lasted more than 10 weeks. Other studies have supported these findings. While the mechanism for this is not yet clear, it could be related to an increase in satiety and a decrease in fat absorption due to an increased fiber intake. Another theory is that flaxseed might have an effect on hormones involved in the regulation of metabolism and appetite, but so far it seems that flaxseed supplementation did not significantly affect leptin or adiponectin levels. Flaxseed supplementation alone is unlikely to result in weight loss, but it could be a useful addition to other interventions.

Does flaxseed reduce cardiovascular disease risk?

Flaxseed has many biologically active compounds that could improve cardiovascular disease (CVD) and reduce the risk of developing CVD. Alpha-linolenic acid (ALA) has anti-inflammatory effects, and several studies have found that flaxseed supplementation can reduce levels of markers of inflammation like C-reactive protein (CRP), interleukin 6 (IL-6), and tumor necrosis factor alpha (TNF-α). Lignans have antioxidant effects and have been observed to increase total antioxidant capacity (TAC) and reduce malondialdehyde (MDA).

Can flaxseed be used to prevent or treat breast cancer?

Flaxseed contains lignans, a type of phytoestrogen that can have weak estrogenic effects but can also block estrogen receptors. Some types of breast cancer are called estrogen-receptor positive (ER+), because the tumor cells have estrogen receptors and can grow in response to estrogen. Because lignans can block estrogen receptors, a flaxseed supplement may slow the growth of certain types of breast cancer. Several studies have found an association between flaxseed supplementation and a decrease in the risk of breast cancer, a slowed growth rate of tumors, an increase in cancer cell death, and a reduction in mortality in people with breast cancer, especially in postmenopausal women and in ER+ tumors. However, these are observational studies, and clinical trials are needed to confirm the findings before flaxseed could be a recommended part of breast cancer treatment.

Can flaxseed affect sex hormones?

The lignans contained in flaxseed are phytoestrogens that can exert weak estrogenic effects and also antiestrogenic effects when binding to estrogen receptors. However, there is no clear research on the effect that flaxseed could have on sex hormones like testosterone or estrogen. A meta-analysis from 2023 found that flaxseed supplementation had no effect on testosterone, sex hormone binding globulin (SHBG), follicle-stimulating hormone (FSH), and dehydroepiandrosterone (DHEA). However, some concerns still exist regarding flaxseed supplementation during pregnancy and whether it could affect male fertility.

Can flaxseed improve the symptoms of menopause?

Flaxseed contains phytoestrogens which could potentially affect the symptoms of menopause, because they can bind to estrogen receptors and have a similar, although less potent, effect to estrogen. However, the research is not conclusive on this yet. Although none of the studies reported adverse effects as a result of flaxseed, the effectiveness of the supplement remains unclear. Some studies have found that flaxseed supplementation could decrease vasomotor symptoms of menopause (e.g., hot flashes, night sweats), but others have concluded that it had no effect or an effect too small to be of clinical significance.

What are flaxseed’s main drawbacks?