Lignans are polyphenolic compounds found in a variety of plant-based foods, including seeds (especially flax and sesame), whole grains, legumes, and certain fruits and vegetables. In the body, gut bacteria convert these plant lignans into mammalian lignans, enterodiol and enterolactone, which have weak estrogen-like properties. While generally safe and even beneficial for health in moderate amounts, high intake from foods or concentrated supplements can cause negative side effects.
Gastrointestinal distress
One of the most commonly reported side effects of high lignan intake, particularly from flaxseed, is digestive upset. This is largely due to the high fiber content of the seeds that are the primary source of lignans. When consumed in large quantities without sufficient water, this can lead to:
- Bloating and gas: The sudden increase in fiber can overwhelm the digestive system, leading to gas buildup.
- Diarrhea or constipation: The high fiber content can act as a laxative, causing diarrhea in some individuals, while insufficient fluid intake can lead to blockages or constipation in others.
- Stomach cramps: Accompanying bloating and gas can be uncomfortable stomach cramps.
How to mitigate digestive issues
- Start slowly: Introduce lignan-rich foods gradually to allow your digestive system to adjust.
- Increase water intake: Ensure you drink plenty of fluids when increasing fiber consumption.
- Consider ground vs. whole: For better absorption and potentially fewer issues, use ground flaxseed, as whole seeds can pass through the body undigested.
Hormonal fluctuations
As phytoestrogens, lignans can mimic the effects of the hormone estrogen in the body, although with a much weaker effect. This can be a concern for certain groups.
- Interference with hormone-sensitive conditions: Women with a history of hormone-sensitive cancers, such as certain breast or uterine cancers, should exercise caution with high lignan intake. The estrogenic effect could potentially stimulate cancer cells, though research is mixed.
- Reproductive health: During pregnancy, high amounts of lignans are potentially unsafe. Animal studies have shown that high maternal intake can alter offspring's reproductive organs. For this reason, and due to a lack of sufficient human studies, pregnant and breastfeeding women are often advised to avoid high-dose lignan supplements.
- Possible effect on men: Some limited research suggests high phytoestrogen intake might influence testosterone levels in men, though the evidence is not conclusive.
Potential drug interactions
Lignans and high-fiber foods that contain them can interact with certain medications, affecting their absorption and effectiveness.
- Blood thinners (anticoagulants): Some sources suggest that flaxseed, a major source of lignans, can have blood-thinning effects, which may increase the risk of bleeding if combined with anticoagulant medications.
- Hormone-sensitive medications: Because lignans can exert estrogenic effects, they could interfere with medications containing estrogen, such as hormone replacement therapy or certain birth control pills.
- Thyroid medication: The high fiber content of flaxseed can interfere with the absorption of thyroid medications like levothyroxine. It is recommended to take these medications at least 30 to 60 minutes apart from lignan-rich foods.
Thyroid function impairment
Flaxseeds contain cyanogenic glycosides, compounds that can form thiocyanates in the body and potentially impair thyroid function. While moderate portions are unlikely to affect healthy individuals, those with pre-existing thyroid problems should be aware of this risk, especially if their iodine intake is low. Excessive consumption of flaxseed could theoretically worsen conditions like hypothyroidism.
Comparison of Potential Lignan Side Effects
| Side Effect Category | Common Symptoms | Cause | Risk Level (Moderate Use) | Risk Level (High Use/Sensitivity) |
|---|---|---|---|---|
| Gastrointestinal | Bloating, gas, diarrhea, constipation, cramps | High fiber intake, insufficient water | Low to Moderate | High |
| Hormonal | Fluctuations, interference with hormone therapy | Phytoestrogenic activity (weak estrogen mimic) | Very Low | Moderate to High (for sensitive groups) |
| Drug Interactions | Reduced medication effectiveness, increased bleeding | High fiber, blood-thinning potential, hormonal effects | Low | Moderate to High (dependent on medication) |
| Thyroid | Impaired thyroid function | Cyanogenic glycosides (in flaxseed) | Very Low | Moderate (for individuals with thyroid issues) |
Conclusion
Lignans, primarily consumed through foods like flaxseed, offer many health benefits in moderation, but they are not without potential negative side effects, particularly when intake is high. The most common issues relate to digestive discomfort due to high fiber content. Of greater concern for specific populations are the lignans' weak estrogenic effects, which can be problematic for those with hormone-sensitive conditions, as well as pregnant or breastfeeding women. It is also crucial to be mindful of drug interactions, especially for those on blood thinners or thyroid medication. The key to safe lignan consumption lies in moderation and understanding individual sensitivities. Consulting a healthcare provider before starting high-dose supplementation is highly recommended, especially for sensitive groups, to ensure the benefits outweigh the risks. Learn more about the biology and health effects of lignans on the Linus Pauling Institute website.
