Flax seeds (Linum usitatissimum) — also known as common flax or linseeds — are small oil seeds that originated in the Middle East thousands of years ago.
Lately, they have gained popularity as a health food. This is due to their high content of heart-healthy omega-3 fats, fiber and other unique plant compounds (1, 2, 3).
Flax seeds have been linked to health benefits, such as improved digestion and a reduced risk of heart disease, type 2 diabetes and cancer.
They’re easily incorporated into your diet — grinding them is the best way to make the most of their health benefits.
Flax seeds are usually brown or yellow. They’re sold whole, ground/milled or roasted — and are often processed into flaxseed oil.
This article tells you everything you need to know about flax seeds.
Flaxseeds have 534 calories per 3.5 ounces (100 grams) — corresponding to 55 calories for each tablespoon (10 grams) of whole seeds.
They consist of 42% fat, 29% carbs and 18% protein.
One tablespoon (10 grams) of whole flax seeds provides the following nutrients (4):
- Calories: 55
- Water: 7%
- Protein: 1.9 grams
- Carbs: 3 grams
- Sugar: 0.2 grams
- Fiber: 2.8 grams
- Fat: 4.3 grams
Carbs and Fiber
Flax seeds are made up of 29% carbs — a whopping 95% of which is fiber.
This means that they’re low in net digestible carbs — the number of total carbs minus the amount of fiber — making them a low-carb food.
Two tablespoons (20 grams) of flax seeds provide about 6 grams of fiber. This is roughly 15–25% of the Reference Daily Intake (RDI) for men and women, respectively (5).
The fiber content is composed of (6):
- 20–40% soluble fiber (mucilage gums)
- 60–80% insoluble fiber (cellulose and lignin)
Soluble fiber helps regulate blood sugar and cholesterol levels. It also promotes digestive health by feeding your beneficial gut bacteria (7, 8).
When mixed with water, the mucilage gums in flax seeds become very thick. Combined with the insoluble fiber content, this makes flax seeds a natural laxative.
Consuming flax seeds can help promote regularity, prevent constipation and reduce your risk of diabetes (2, 9, 10).
Flax seeds are made up of 18% protein. Their amino acid profile is comparable to soybeans.
Despite containing essential amino acids, they’re lacking in the amino acid lysine.
Therefore, they’re considered an incomplete protein (11).
Still, flax seeds are high in the amino acids arginine and glutamine — both of which are important for heart and immune system health (12, 13).
Flax seeds contain 42% fat, with 1 tablespoon (10 grams) providing 4.3 grams.
This fat content is composed of (14):
- 73% polyunsaturated fatty acids, such as omega-6 fatty acids and the omega-3 fatty acid alpha-linolenic acid (ALA)
- 27% monounsaturated and saturated fatty acids
Flax seeds are one of the richest dietary sources of ALA. In fact, they’re only exceeded by chia seeds(15).
ALA is an essential fatty acid, which means that your body cannot produce it. Thus, you need to obtain it from the food you eat.
Flaxseed oil contains the highest amount of ALA, followed by milled seeds. Eating the seeds whole provides the least amount of ALA, as the oil is locked up inside the fibrous structure of the seed (16).
Due to their high content of omega-3 fatty acids, flax seeds have a lower ratio of omega-6 to omega-3than many other oil seeds.
A lower ratio of omega-6 to omega-3 fatty acids has been linked to a lower risk of various chronic diseases (17, 18).
However, flax seeds don’t contain as much omega-3 as fish oils.
What’s more, your body needs to convert the ALA in flax seeds to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) — a process that is often inefficient (19, 20, 21).
One type of flax seeds — solin, the yellow variety — is not as nutritious as regular flax seed. It has a very different oil profile and is low in omega-3 fatty acids (22).
Flax seeds are very high in fiber and provide good amounts of protein. They’re also rich in fat and one of the best plant-based sources of heart-healthy omega-3 fatty acids.
Vitamins and Minerals
Flax seeds are a good source of several vitamins and minerals:
- Thiamine. This B vitamin is also known as vitamin B1. It’s essential for normal metabolism and nerve function.
- Copper. An essential mineral, copper is important for growth, development and various bodily functions (23).
- Molybdenum. Flax seeds are rich in molybdenum. This essential trace mineral is abundant in seeds, grains and legumes (24).
- Magnesium. An important mineral that has many functions in your body, magnesium is occurs in high amounts in grains, seeds, nuts and green leafy vegetables (25).
- Phosphorus. This mineral is usually found in protein-rich foods and contributes to bone health and tissue maintenance (26).
Flax seeds are a good source of several vitamins and minerals needed for optimal health. These include thiamine (vitamin B1), copper, molybdenum, magnesium and phosphorus.
Other Plant Compounds
Flax seeds contain several beneficial plant compounds:
- p-Coumaric acid. This polyphenol is one of the main antioxidants in flax seeds.
- Ferulic acid. This antioxidant may help prevent several chronic diseases (27).
- Cyanogenic glycosides. These substances may form compounds called thiocyanates in your body, which can impair thyroid function in some people.
- Phytosterols. Related to cholesterol, phytosterols are found in the cell membranes of plants. They have been shown to have cholesterol-lowering effects (28).
- Lignans. Lignans are present in almost all plants, acting as both antioxidants and phytoestrogens. Flax seeds are exceptionally rich in lignans, containing up to 800 times more than other foods (29).
Brown flax seeds have slightly higher antioxidant activity than yellow varieties (15).
Flax seeds are one of the richest known dietary sources of lignans. These nutrients function as phytoestrogens (2).
Phytoestrogens are plant compounds that are similar to the female sex hormone estrogen. They have weak estrogenic and antioxidant properties (30).
They have been linked to a decreased risk of heart disease and metabolic syndrome, as they reduce levels of fat and glucose in your blood.
Flax lignans also help reduce blood pressure, oxidative stress and inflammation in your arteries (31).
Lignans are fermented by bacteria in your digestive system and may reduce the growth of several cancers — especially hormone-sensitive types, such as breast, uterus and prostate cancer (31, 32).
Flax seeds are high in several plant compounds, including p-Coumaric acid, ferulic acid, cyanogenic glycosides, phytosterols and lignans. In particular, the last two have been linked to various benefits.
Flax seeds may be useful as a part of a weight loss diet.
They contain soluble fiber, which becomes highly sticky when mixed with water.
This fiber has been shown to be effective at suppressing hunger and cravings, potentially promoting weight loss (33, 34).
A review of controlled studies concluded that flax seeds promote weight loss in overweight and obese people. Those who added the seeds to their diet lost an average of 2.2 pounds (1 kg), compared to the control group (35).
The analysis also showed that weight loss tended to be greater in studies lasting for more than 12 weeks and among those who consumed more than 30 grams of flax seeds per day (35).
Flax seeds contain soluble fiber, which may promote weight loss by reducing hunger and decreasing cravings.
Flax seeds have been associated with major benefits for heart health, mainly attributed to their content of omega-3 fatty acids, lignans and fiber.
High blood cholesterol is a well-known risk factor for heart disease. This is especially true for oxidized LDL (bad) cholesterol (36).
Human studies note that daily consumption of flax seeds — or flaxseed oil — may lower cholesterol by 6–11%.
These studies also indicate a 9–18% reduction in the number of LDL (bad) cholesterol particles (7, 37, 38, 39).
This is supported by animal studies showing that flax seeds may improve cholesterol levels and the composition of blood fats (40, 41, 42, 43, 44).
These seeds may be very useful when consumed along with cholesterol-lowering medication.
One 12-month study found that flax seeds caused an additional 8.5% reduction in LDL (bad) cholesterol, compared to a control group (45).
This cholesterol-lowering effect is thought to be caused by the high fiber and lignan content in flax seeds.
These substances bind with cholesterol-rich bile acids and carry them down your digestive tract. This reduces cholesterol levels in your body (46).
Omega-3 Fatty Acids
Omega-3 fatty acids are essential. They may have benefits for various aspects of heart health, including blood platelet function, inflammation, and blood pressure.
Flax seeds are very high in the omega-3 fatty acid alpha-linolenic acid (ALA).
They have been shown to decrease heart disease risk in animal studies by reducing inflammation in the arteries (47).
Several studies link ALA with a lower risk of stroke, heart attacks, and chronic kidney disease. These studies observed a 73% lower risk of sudden death as well, compared to people with lower ALA intake (48, 49, 50, 51).
In one study, people with heart disease were given 2.9 grams of ALA per day for one year. Those receiving the supplement had significantly lower rates of death and heart attacks than people in the control group (52).
Plant-based ALA fatty acids seem to benefit heart health similarly to fish oils, which are rich in EPA and DHA (53, 54, 55).
Eating flax seeds is an effective way to lower blood pressure (31, 56, 57, 58, 59).
In a 6-month study in people with elevated blood pressure, those consuming 3 tablespoons (30 grams) of flax seeds daily experienced a 10 and 7 mm Hg reduction in systolic and diastolic blood pressure, respectively.
People with a systolic level — the top number in a blood pressure reading — greater than 140 mm Hg at the start of the study observed an even greater reduction of 15 mm Hg (56).
For each 5 mm Hg reduction in systolic and 2–5 mm Hg reduction in diastolic blood pressure, your risk of stroke is estimated to be reduced by 11–13% and your risk of heart disease by 34% (60, 61).
Flax seeds may help fight heart disease by lowering blood pressure, regulating blood cholesterol, and increasing your levels of heart-healthy omega-3 fatty acids.
Other Health Benefits of Flax Seeds
Flax seeds have been shown to benefit many aspects of human health.
Diarrhea and constipation cause major distress and may even threaten your health.
About 2–7% of people in the United States experience chronic diarrhea, while recurring constipation affects 12–19% of the population. Constipation rate can be as high as 27% in Europe, with women at twice the risk of men (62, 63).
Several studies have found that flax seeds prevent both diarrhea and constipation (64, 65, 66).
The insoluble fiber content in flax seeds adds bulk to your digestive waste, acting as a laxative and relieving constipation (32, 67).
Soluble fiber is also thought to bind to water in your digestive tract. This causes it to swell and increase the bulk of your stool, preventing diarrhea (65).
According to the World Health Organization (WHO), 1 in 10 adults had diabetes in 2012 (68).
Studies in people with type 2 diabetes show that supplementing with 10–20 grams of flaxseed powder per day for 1–2 months may reduce fasting blood sugar by up to 19.7% (69, 70).
However, not all studies find flax seeds to be effective in regulating blood glucose and insulin levels (71).
Though the link between flax seeds and type 2 diabetes is still unclear, they may be considered a safe and healthy addition to your diet if you have type 2 diabetes (72).
Test-tube and animal studies show that flax seeds may suppress the formation of several types of cancer, such as those of the colon, breast, skin, and lungs (73, 74).
Increased blood levels of sex hormones have been linked to an increased risk of several cancers (75, 76, 77).
Flax seeds may modestly lower serum levels of sex hormones in overweight women, potentially decreasing breast cancer risk (78, 79).
These seeds have also been shown to protect against prostate cancer (80, 81).
Flax seeds may improve digestion by relieving diarrhea and constipation. They may also reduce fasting blood sugar in people with diabetes and lower your risk of several cancers.
Adverse Effects and Individual Concerns
Dry flax seeds are usually well tolerated, and allergy is rare (82).
Still, it’s recommended to drink plenty of water when eating these seeds.
Flax seeds naturally contain plant compounds called cyanogenic glycosides. These substances can bind with sulfur compounds in your body to form thiocyanates.
Excessive amounts of thiocyanates may impair the function of your thyroid gland (83).
Moderate portions are highly unlikely to cause any adverse effects in healthy individuals. However, those with thyroid problems should consider avoiding high amounts of flax seeds (84).
Though the safe upper limit of flaxseed intake has not been determined, one study concluded that 5 tablespoons (50 grams) per day is safe and beneficial for most healthy people (14).
Similar to other seeds, flax seeds contain phytic acid.
Phytic acid is often referred to as an antinutrient, as it may reduce the absorption of minerals like iron and zinc (85).
Still, phytic acid doesn’t cause any lasting reduction in mineral absorption and does not affect any subsequent meals.
Therefore, this should not be a major concern — except for people who are deficient in minerals like iron and/or follow an imbalanced diet.
For people who are not used to eating a lot of fiber, incorporating flax seeds too quickly can cause mild digestive problems. These include bloating, gas, abdominal pain and nausea.
It’s best to start with small doses and work your way up to 1–2 tablespoons (10–20 grams) daily.
Adding flax seeds to your diet may also increase bowel movement frequency, as flax seeds are a natural laxative.
Risks During Pregnancy
Though human studies are limited, many health professionals fear that consuming flax seeds during pregnancy may have undesirable effects.
This is due to the phytoestrogens in the seeds, which may act similarly to the female sex hormone estrogen.
Animal studies show that flax seeds and flaxseed lignans may cause lower birth weight and affect the development of the offspring’s reproductive system — especially if consumed during early pregnancy (86, 87).
It’s unlikely that smaller doses of flax seeds will have an adverse effect.
However, during pregnancy and breastfeeding, it’s recommended to limit your intake of flax seeds and other dietary sources of phytoestrogens. This also includes some soy products.
Large doses of omega-3 fatty acids may have blood-thinning effects (88).
If you have a bleeding disorder or are taking blood thinners or other medications, consult with your healthcare professional before incorporating large amounts of flax seeds into your diet (89, 90).
Flax seeds may cause mild digestive issues. They contain plant compounds that may adversely affect some people and are not considered safe for high-dose consumption in early pregnancy.
The Bottom Line
Flax seeds have become popular due to their high content of omega-3 fatty acids, fiber, and other plant compounds, which are responsible for many of the seeds’ benefits.
They may aid weight loss and improve blood sugar control, as well as heart and digestive health.
If you want to boost your health with these tiny powerhouses, you can buy them locally or online.