Flaxseeds (Linum usitatissimum), also known as common flax or linseeds, are small oilseeds that originated in the Middle East thousands of years ago.
Flaxseeds have been linked to health benefits such as improved digestive function and a reduced risk of heart disease, type 2 diabetes and cancer.
They are very easy to incorporate into the diet. Grinding them is the best way to make the most of their health benefits.
Flaxseeds are usually brown or yellow. They are sold whole, ground/milled or roasted, and are often processed into flaxseed oil.
Flaxseeds contain 534 calories per 100 grams, corresponding to 55 calories for each tablespoon of whole seeds (10 grams).
They consist of 42% fat, 29% carbs and 18% protein.
The table below contains information on all the nutrients found in flaxseeds (4).
Flaxseeds are made up of 29% carbs, and a whopping 95% of that amount is fiber.
The net digestible carbs are only 1.5 grams for every 100 grams of seeds, making flax seeds a low-carb friendly food.
Two tablespoons of flaxseeds provide about six grams of fiber. This is roughly 15–25% of the daily recommended intake 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).
When mixed with water, the mucilage gums in flaxseeds become very thick. This, combined with the insoluble fiber content, makes flaxseeds a natural laxative.
Bottom line: Almost all (95%) of the carbs in flaxseeds consist of fiber, both soluble and insoluble. Flaxseeds are a natural laxative and promote regularity.
Despite containing essential amino acids, they are lacking in the amino acid lysine.
They are therefore unable to serve as the sole protein source in the diet (11).
Flaxseed proteins may be useful against fungal infections, high blood pressure, high cholesterol, type 2 diabetes and inflammation (1).
Bottom line: Flaxseeds contain protein and essential amino acids. They may also help prevent heart disease and support immune function.
Flaxseeds contain 42% fat, and there are 4.3 grams of fat in each 10 gram tablespoon.
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.
ALA is an essential fatty acid, which means that the body cannot produce it. Therefore, we need to obtain it from the food we eat.
Flaxseed oil contains the highest amount of ALA, followed by milled seeds. Consuming the seed whole provides the least amount of ALA, because the oil is locked up inside the fibrous structure of the seed (16).
Because of their high content of omega-3 fatty acids, flaxseeds promote a lower ratio of omega-6 to omega-3. A lower ratio of omega-6 to omega-3 fatty acids may significantly reduce the risk of various chronic diseases (17, 18).
There is one variety that is not as nutritious as regular flaxseed, a yellow flax called "solin." It has a very different oil profile and is low in omega-3 fatty acids (22).
Bottom line: Flaxseeds are rich in fat and are one of the best plant-based sources of heart-healthy omega-3 fatty acids.
Flaxseeds are a good source of several vitamins and minerals:
- Vitamin B1: This B-vitamin is also known as thiamine. Vitamin B1 is essential for normal metabolism and nerve function.
- Copper: An essential mineral that is important for growth, development and various functions in the body (23).
- Molybdenum: Flaxseeds are rich in molybdenum. This essential trace mineral is found in seeds, grains and legumes (24).
- Magnesium: An important mineral that has many important functions in the body. It is found in high amounts in grains, seeds, nuts and green leafy vegetables (25).
- Phosphorus: This mineral contributes to bone health and tissue maintenance, and is usually found in protein-rich foods (26).
Bottom line: Flaxseeds are a good source of several vitamins and minerals that are needed for optimal health, including thiamine (B1), copper, molybdenum, magnesium and phosphorus.
Flaxseeds contain several beneficial plant compounds:
- p-Coumaric acid: This polyphenol is one of the main antioxidants found in flaxseeds.
- Ferulic acid: This antioxidant may help to prevent several chronic diseases (27).
- Cyanogenic glycosides: These substances may form compounds called thiocyanates in the 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 in the body (28).
- Lignans: Lignans are present in almost all plants, acting as both antioxidants and phytoestrogens. Flaxseeds are the richest known dietary source of lignans, containing up to 800 times more than other foods (29).
Brown flaxseeds have slightly higher antioxidant activity than yellow flaxseeds (15).
Bottom Line: Flaxseeds are high in several plant compounds, including p-Coumaric acid, ferulic acid, cyanogenic glycosides, phytosterols and lignans.
Flaxseeds are the richest known dietary source 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, since they reduce the amount of fat and glucose in the blood.
Phytoestrogens also help lower blood pressure and reduce oxidative stress and inflammation in the arteries (31).
Bottom line: Lignans are also known as phytoestrogens. They are antioxidants with weak estrogenic properties that are linked with benefits for heart health, metabolic syndrome and several types of hormone-sensitive cancers.
Flaxseeds may be useful as a part of a weight loss diet.
They contain soluble fiber, which becomes highly sticky when mixed with water.
A study on weight loss diets showed that flaxseeds decreased inflammatory markers by 25–46%, compared to weight loss diets without them (35).
Bottom line: Flaxseeds contain soluble fiber, which may promote weight loss by reducing hunger and decreasing cravings.
Flaxseeds 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-cholesterol (36).
Human studies have shown that the daily consumption of flaxseeds, or flaxseed oil, may lower cholesterol levels by 6–11%.
Flaxseeds may be very useful when consumed along with cholesterol-lowering medication. In one 12-month study, flaxseeds caused an additional 8.5% reduction in LDL-cholesterol, when compared to those not consuming flaxseeds (45).
This cholesterol-lowering effect is thought to be caused by the high fiber and lignan content found in flaxseeds.
The fiber and lignans bind with cholesterol-rich bile acids and carry them down the digestive tract. This reduces cholesterol levels in the 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.
Flaxseeds are very high in the omega-3 fatty acid alpha-linolenic acid (ALA).
They have been shown to decrease the risk of heart disease in animal studies by reducing inflammation in the arteries (47).
Several studies have linked 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, when compared to those with lower ALA intake (48, 49, 50, 51).
In one study, patients with heart disease were given 2.9 grams per day of ALA for one year. Patients receiving the supplement had significantly lower rates of death and heart attacks than those not taking ALA (52).
In a six-month study of individuals with elevated blood pressure, the systolic blood pressure (SBP) was reduced by 10 mmHg. In this same study, the diastolic pressure (DBP) was reduced by 7 mmHg.
Patients who entered the study with an SBP greater than 140 mmHg experienced a reduction of 15 mmHg. A 7 mmHg reduction in DBP was also recorded (56).
Bottom line: Flaxseeds may help fight heart disease by lowering blood pressure, regulating blood cholesterol, and increasing the levels of heart-healthy omega-3 fatty acids.
Flaxseeds have been shown to benefit many aspects of human health.
Diarrhea and constipation cause major distress and may even threaten health.
About 2–7% of people in the USA suffer from chronic diarrhea, while reoccurring constipation affects 12–19% of the population. This rate can be as high as 27% in Europe, with women at twice the risk of men (62, 63).
It has also been proposed that the soluble fiber binds to water in the digestive tract. This causes it to swell and increase the bulk of the stool, therefore preventing diarrhea (65).
According to the World Health Organization, 1 in 10 adults had diabetes in the year 2012 (68).
However, not all studies have found flaxseeds to be effective in regulating blood glucose and insulin levels (71).
Although the link between flaxseeds and type 2 diabetes is still unclear, they may be considered a safe and healthy addition to the diet for individuals with type 2 diabetes (72).
Increased blood levels of sex hormones have been linked with an increased risk of several cancers (75, 76, 77). Flaxseeds may modestly lower serum levels of sex hormones in overweight women, and decrease the risk of breast cancer (78, 79).
Bottom line: Flaxseeds may improve digestion by relieving diarrhea and constipation. They may also reduce fasting blood sugar in diabetics and reduce the risk of several cancers.
Dry flaxseeds are usually well tolerated by the body, and allergy is rare (82).
However, it is recommended to drink plenty of water when eating these seeds.
Flaxseeds naturally contain plant compounds called cyanogenic glycosides. These substances can bind with sulfur compounds in the body to form thiocyanates.
Excessive amounts of thiocyanates may impair the function of the thyroid gland (83).
Moderate portions are highly unlikely to cause any adverse effects in healthy individuals. However, those who suffer from thyroid problems should consider avoiding high amounts of flaxseeds (84).
The intake limit of flaxseeds is about 50 grams per day (5 tablespoons) in healthy individuals. Higher levels may cause adverse effects, and may be toxic in some cases (14).
Similar to other seeds, flaxseeds contain phytic acid.
Phytic acid is often referred to as an anti-nutrient since it may reduce the absorption of minerals like iron and zinc (85).
However, phytic acid only impairs mineral absorption in a single meal, and does not affect any subsequent meals.
Therefore, this should not be a major concern except for individuals who are deficient in minerals like iron.
For people who are not used to eating a lot of fiber, incorporating flaxseeds too quickly can cause mild digestive problems. This includes bloating, gas, abdominal pain and nausea.
It is best to start with small doses and work your way up to one or two tablespoons daily.
Adding flaxseeds to the diet may also increase bowel movement frequency, since flaxseeds are a natural laxative.
Risks During Pregnancy
Although there are limited human studies on this, many health professionals fear that consuming flaxseeds during pregnancy may have undesirable effects.
This is due to the phytoestrogens that are present in flaxseeds, which may act similarly to the female sex hormone estrogen.
Animal studies have shown that flaxseeds 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 is unlikely that smaller doses of flaxseeds will have an adverse effect. However, during pregnancy and lactation it is recommended to limit the intake of flaxseeds and other dietary sources of phytoestrogens. This also includes some soy products.
Blood Thinning Effects
Large doses of omega-3 fatty acids may have blood thinning effects (88).
Bottom line: Flaxseeds may cause mild digestive issues. They contain plant compounds that may adversely affect some people, and they can have blood thinning effects that affect some medications.
Flaxseeds have become popular due to their high content of omega-3 fatty acids, fiber and other plant compounds. They also have many health benefits.
They have been shown to affect a range of risk factors, and are associated with improved overall health and wellness.
Adding flaxseeds to the diet is also ideal to increase fiber intake.