Flax Seed Side Effects: 6 Ways It May Cause Harm

From constipation to allergies - beware of the side effects of flaxseeds before consuming them.

Medically Reviewed by Staci Gulbin, RD, LDN
By Sindhu Koganti, BTech (Biotechnology), Diploma In Nutrition

Flax seeds are a fiber-rich vegetarian food source derived from the Linum usitatissimum plant. This ancient crop has been used in traditional medicine for its therapeutic value. However, intake of excessive amounts of flax seeds may cause adverse effects.

The side effects of flax seeds include allergic reactions, slowed blood clotting, constipation, and increased risk of cancer in susceptible individuals. In addition, the anti-nutrients in flax seeds are also considered a risk factor.

In this article, we have discussed the possible side effects of flax seeds and the recommended dosage. Keep reading.

What Are Flax Seeds?

Flax seeds are rich in oils while the stems are replete with high-quality fiber. The seeds contain about 55% ALA, 28–30% protein, 35% fiber, and phytoestrogens.

The alpha-linolenic acid (ALA) and the fiber content of flax seeds are responsible for their health benefits. ALA is an essential polyunsaturated fatty acid that has anti-inflammatory and antithrombotic (anti-clotting) properties.

In addition to the beneficial nutrients, flax seeds also contain certain molecules called anti-nutrients. These anti-nutrients form toxic intermediates in your body and prevent the absorption of minerals.

Flax seeds or flaxseed oil are often used as fish oil substitutes. You can also find the seeds or their products in fiber bars (granola bars), bread, muffins, cereals, and meat extenders (1).

How Can Flax Seeds Be Harmful?

Extensive research has shown the presence of anti-nutrients in flax seeds. Anti-nutrients are molecules that may have adverse effects on your health. Cyanogenic glycosides and phytic acid are the most abundant anti-nutrients found in these seeds (1), (2).

Anti-nutrients are heat-sensitive. They break down when you roast or cook flax seeds. However, the presence of these components in raw seeds results in various side effects. As discussed, the anti-nutrients form lethal intermediates. One such intermediate is hydrogen cyanide, which can interfere with the absorption of minerals such as calcium, zinc, magnesium, copper, and iron.

Excess consumption of flax seeds can further enhance the adverse effects of anti-nutrients. These can lead to side effects.

What Are The Side Effects Of Flax Seeds?

Overconsumption of flaxseeds can cause bloating, gas, stomach ache, and nausea. There are also several chronic problems associated with flaxseed intake.

1. May Cause An Allergic Reaction

Like other nuts and seeds, flax seeds (and flaxseed oil) can act as allergens. As per anecdotal evidence, the seeds may cause skin rashes and severe constipation. In a study on children, certain food products containing flax seeds caused skin rashes, vomiting, and a tingling sensation (3).

Shortness of breath (dyspnea) and hives were reported in a 42-year-old female who had flax flour. Anaphylaxis is another risk associated with the intake of these seeds. In some cases, you may also notice swelling on the face (facial edema) (4).

2. May Increase Bleeding

Flax seeds contain high levels of omega-3 fatty acids. These fatty acids are vital for the proper functioning of the brain, heart, and body. However, they may also cause bleeding disorders. Through direct research is limited, some studies link the disorders to omega-3 fatty acids, which flax seeds are rich in.

Omega-3 fatty acids are known to slow down blood clotting and increase bleeding. They inhibit the aggregation of blood platelets, which is a crucial step in the formation of a blood clot (5).

Hence, it is crucial to watch your flaxseed intake if you are on blood-thinning or heart medication. Do not consume Aspirin or other herbal medicines along with these seeds (6). These drugs may interact with the fatty acids and prolong the bleeding time and blood loss.

3. May Lead To Fiber Overload And Constipation

Flax seeds are rich in dietary fiber (7). Sudden overconsumption of these seeds increases the fiber load in your body. Too much fiber can clog your stomach and intestines (8).

You may experience stomach aches, cramps, and constipation in such cases. Insufficient water intake may also worsen your condition. Eating small portions of flax seeds with sufficient water is the right way to go.

Moreover, people with irritable bowel syndrome (IBS) or constipation are at a higher risk of experiencing such side effects. It is best to consult a healthcare provider to learn more about the dosage and safety of flax seeds.

4. May Increase Cancer Risk

Research is unclear in this regard. Flaxseeds are rich in phytoestrogens. In studies, phytoestrogens did not show any protective effect against prostate cancer (9). Some believe the lignans and phytoestrogens present in flax seeds may increase the risk of prostate cancer, although more studies are required to understand this mechanism.

5. May Cause Problems During Pregnancy And Lactation

There is limited research in this aspect. During the lactation period, a flaxseed diet may affect the metabolism of thyroid hormones (10). However, more research is required to understand the effect of the flax seeds during pregnancy. Consult your doctor before consuming flax seeds during pregnancy.

6. May Cause Cyanide Toxicity

Nuts and seeds like almonds, legumes, and flax seeds have traces of cyanide compounds. It is alright to consume these in small quantities (11), (12).

Moreover, most of the cyanogen glycosides are heat sensitive. They get degraded into smaller molecules that evaporate or become less toxic when you roast or cook flax seeds.

The problem arises when you eat large amounts of raw flax seeds. Since raw flax seeds have the harmful cyanide residues intact, they could cause cyanide toxicity. However, research suggests that the recommended dietary amounts of omega-3 fatty acids (ALA) can be obtained by consuming about 9 g of flaxseed per day (13). Sticking to this limit will do no harm.

What Is The Safe Dosage Of Flax Seeds?

It is recommended to consume about 1-2 tablespoons of ground flax seeds or 1 tablespoon of flaxseed oil every day to reap its health benefits. Having 9-10 g or 1 tablespoon of these seeds a day can provide the required levels of ALAs (1.6 g/day for males and 1.1 g/day for females) (13).

You can use flax seeds to make baked goods, energy bars/granola bars, gums, and thickeners.

But remember to drink enough water (2-4 liters/day at least) to prevent the side effects of these seeds.

The side effects of flax seeds are to be blamed on the presence of certain anti-nutrients — phytic acid and cyanogenic glycosides — that prevent the absorption of nutrients by the body. Some individuals may be allergic to these seeds and experience vomiting, rashes, tingling, and facial swelling. Their high omega-3 fatty acid content may also cause bleeding disorders by slowing down blood clotting. Flax seeds are rich in fiber, and excess intake of this macronutrient can be counterproductive, leading to constipation. Moreover, taking these seeds beyond the recommended dosage may lead to cyanide toxicity, so exercise caution.

Frequently Asked Questions

Is flaxseed good for kidneys?

Yes. Animal studies have suggested that flaxseed consumption is beneficial for renal health (14).

What is the best time to eat flax seeds?

While you can include flaxseed in your diet in several ways at various mealtimes, having them first thing in the morning in your breakfast offers the most benefits.

Which is better – flax or chia seeds?

Both the seeds are nutrient-dense and highly beneficial to your health. You can pick either one, depending on your wellness goals and preference.

Can we eat flax seeds on an empty stomach?

Yes, you can eat flaxseeds on an empty stomach. However, it makes you feel full, thus reducing your hunger. So depending on why you want to incorporate flaxseeds into your diet, you can have them before or after meals.

Sources

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Sindhu Koganti is a health and wellness writer with over 5 years of experience. She has a bachelor’s degree in... more

Staci Gulbin

(MS, MEd, RD, LDN)
Staci Gulbin is a registered dietitian, freelance writer, health editor, and founder of LighttrackNutrition.com. She has been a registered dietitian... more

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