I’ve been researching flaxseed for a while now, and what strikes me is how this tiny seed has been used for thousands of years, yet we’re still figuring out exactly what it does. The Latin name, Linum usitatissimum, literally means “most useful”, which tells you something about how our ancestors viewed it.

Flaxseed has become one of those supplements that gets recommended for almost everything. Heart health, weight loss, inflammation, blood sugar. The question I wanted to answer: does the evidence actually support these claims, or is this another case of traditional use being mistaken for proven benefit?

What is flaxseed?

Flaxseed comes from the flax plant (Linum usitatissimum), one of the oldest cultivated crops in human history. The seeds are small, flat, and range from golden yellow to reddish-brown. They have a mild, nutty taste.

What makes flaxseed interesting nutritionally is its combination of three components:

Alpha-linolenic acid (ALA): This is a plant-based omega-3 fatty acid. A tablespoon of ground flaxseed contains about 1.6 grams of ALA. Your body converts some of this to EPA and DHA (the omega-3s found in fish oil), though the conversion rate is fairly low, around 5-10% according to most estimates.

Lignans: Flaxseed contains 75-800 times more lignans than other plant foods. Lignans are phytoestrogens, meaning they have a weak oestrogen-like effect in the body. This is both potentially beneficial and a reason for caution in certain groups.

Soluble fibre: About 20-40% of flaxseed’s total fibre is soluble, which forms a gel when mixed with water. This affects how quickly food moves through your digestive system and how nutrients are absorbed.

One important point: whole flaxseeds often pass through your digestive system undigested. To get the nutritional benefits, you need to grind them or buy them pre-ground. Ground flaxseed goes rancid fairly quickly though, so store it in the fridge.

What does the research say about flaxseed benefits?

1. Blood pressure reduction

High blood pressure affects roughly a billion people worldwide and remains one of the leading risk factors for heart disease and stroke. Lifestyle changes, including dietary modifications, are often recommended alongside medication.

A 2023 meta-analysis looked at 33 randomised clinical trials with 2,427 participants. The researchers found that flaxseed supplementation significantly reduced both systolic and diastolic blood pressure 1.

What caught my attention was the subgroup analysis. The blood pressure reduction was most pronounced when:

The intervention lasted longer than 20 weeks
The daily dose was 30 grams or more
Participants were overweight (BMI 25-30)
Participants already had hypertension

This suggests flaxseed isn’t a quick fix. You need to take it consistently for several months, and it seems to help most in people who already have elevated blood pressure rather than as a preventive measure in healthy individuals.

The NHS recommends keeping blood pressure below 140/90 mmHg for most adults 2. Flaxseed alone won’t get you there if you’re significantly above this threshold, but it might contribute as part of a broader approach.

My take: The evidence for blood pressure is reasonably solid, but the effect size is modest. Don’t expect dramatic changes.

2. Cholesterol improvement

Dyslipidaemia, particularly elevated LDL cholesterol, is a well-established risk factor for cardiovascular disease. About 39% of men and 40% of women globally have elevated cholesterol levels.

A large systematic review and meta-analysis examined 62 randomised controlled trials with 3,772 participants. Flaxseed supplementation (daily doses ranging from 10g to 60g) significantly reduced total cholesterol, triglycerides, and LDL cholesterol 3.

Interestingly, there was no effect on HDL cholesterol (the “good” cholesterol). So flaxseed appears to lower the harmful components without raising the protective ones.

The mechanism likely involves the soluble fibre, which binds to bile acids in the gut and increases their excretion. Your liver then pulls cholesterol from your blood to make more bile acids.

Similar effects have been observed with other soluble fibre sources like chia seeds, though flaxseed appears to have additional benefits from its ALA content.

My take: This is one of the better-supported benefits. If your cholesterol is borderline, flaxseed might help you avoid medication.

3. Anti-inflammatory effects

Chronic low-grade inflammation is increasingly recognised as a factor in many diseases, from heart disease to diabetes to certain cancers. Markers like C-reactive protein (CRP) and tumour necrosis factor-alpha (TNF-α) are used to measure inflammatory status.

A systematic review and meta-analysis of 32 randomised controlled trials with 1,502 participants found that flaxseed and its derivatives reduced high-sensitivity C-reactive protein (hs-CRP) and TNF-α levels 4.

The effect on interleukin-6 and standard CRP wasn’t significant, which suggests the anti-inflammatory effect is real but selective.

The omega-3 fatty acids in flaxseed likely play a role here. ALA gets converted (inefficiently) to EPA and DHA, which are known to have anti-inflammatory properties.

My take: There’s something there, but it’s not dramatic. If you’re dealing with serious inflammatory conditions, don’t expect flaxseed to replace proper treatment.

4. Blood sugar control

For people with type 2 diabetes or prediabetes, managing blood sugar is critical. Poor glycaemic control can lead to complications including diabetic bladder dysfunction and kidney problems over time.

A meta-analysis of 25 randomised clinical trials with 2,080 participants found that whole flaxseed (not flaxseed oil or lignan extracts) significantly improved several markers of glycaemic control, including fasting glucose, insulin levels, and insulin resistance 5.

The subgroup analysis revealed that the effect was strongest when:

Consumption exceeded 12 weeks
Baseline blood glucose was already elevated

The mechanism probably involves the soluble fibre, which slows carbohydrate absorption and reduces blood sugar spikes after meals. The lignans may also play a role through their effects on insulin sensitivity.

Mayo Clinic notes that fibre-rich foods like flaxseed can help manage blood sugar, though they recommend working with a healthcare provider to adjust any diabetes medications accordingly 6.

My take: For blood sugar, whole ground flaxseed seems to work better than flaxseed oil. The fibre matters.

5. Weight management

Since the 1980s, obesity rates have tripled globally. While flaxseed isn’t a magic weight loss solution, it may help with modest weight management.

A systematic review and meta-analysis of 45 randomised controlled trials with 2,789 participants found that flaxseed supplementation reduced body weight, BMI, and waist circumference 7.

The effects were most pronounced in:

People who took it for more than 12 weeks
Those with a BMI greater than 27

The likely mechanisms involve the soluble fibre (which increases feelings of fullness and delays gastric emptying) and possibly the omega-3 content.

My take: You won’t lose significant weight from flaxseed alone. But if you’re making other dietary changes, adding flaxseed might help you feel fuller and reduce overall intake.

6. Prostate cancer

This one surprised me. A randomised controlled trial looked at 161 men scheduled for prostatectomy (prostate removal surgery). Some were assigned to a diet rich in flaxseed (30g daily) for 30 days before surgery, while others followed a low-fat diet or their usual diet 8.

The flaxseed group showed reduced tumour proliferation rates, measured by something called the Ki-67 index, which indicates how actively cancer cells are dividing.

The researchers suggested this might be related to flaxseed’s effects on hormone metabolism and natural killer cell activity. Flaxseed lignans can affect the metabolism of oestrogen and testosterone, which may be relevant for hormone-sensitive cancers.

For more information about prostate conditions, see our guide on prostate cancer and benign prostatic hyperplasia.

My take: This is preliminary evidence from one trial. Interesting, but I wouldn’t make treatment decisions based on it. More research is needed.

Side effects of flaxseed

For most healthy adults, flaxseed is safe when consumed in moderate amounts (up to 30-50g daily). However, common side effects include:

Bloating and gas (especially when starting out)
Abdominal discomfort
Diarrhoea or loose stools
Constipation (if you don’t drink enough water)
Nausea

These digestive issues are usually temporary and improve as your body adjusts. Starting with a small amount (a teaspoon) and gradually increasing helps most people avoid problems.

The high fibre content means you need to drink plenty of water when consuming flaxseed. Without adequate fluid, the fibre can actually worsen constipation rather than help it.

Safety precautions and contraindications

There are seven situations where you should be cautious with flaxseed or avoid it entirely:

1. Raw or unripe flaxseeds: Don’t eat raw flaxseeds in large quantities. They contain cyanogenic glycosides, which can release small amounts of cyanide when metabolised. Cooking or roasting destroys these compounds. The amounts in normal dietary intake are generally considered safe, but there’s no reason to take the risk when heating solves the problem.

2. Intestinal conditions: If you have bowel obstruction, intestinal inflammation, or oesophageal stricture, avoid flaxseed. The high fibre content can worsen these conditions by adding bulk and potentially causing blockages.

3. Low blood pressure or blood pressure medications: Since flaxseed can lower blood pressure, combining it with blood pressure medications might cause your pressure to drop too low. Monitor your blood pressure and talk to your doctor if you’re on antihypertensives.

4. Hormone-sensitive conditions: The lignans in flaxseed have weak oestrogenic effects. If you have or have had breast cancer, uterine cancer, ovarian cancer, endometriosis, or uterine fibroids, consult your doctor before using flaxseed. The same applies to pregnant and breastfeeding women, where the safety hasn’t been established.

5. Bleeding disorders or anticoagulants: Flaxseed may have mild anticoagulant effects. If you’re taking blood thinners like warfarin (Coumadin), clopidogrel (Plavix), or even aspirin, discuss flaxseed with your healthcare provider first.

6. Diabetes medications: Since flaxseed can lower blood sugar, combining it with diabetes medications might cause hypoglycaemia. Monitor your blood sugar levels more closely if you add flaxseed to your diet.

7. Hormonal contraceptives or hormone replacement therapy: The phytoestrogens in flaxseed may theoretically interfere with hormonal medications. While this hasn’t been definitively proven to cause problems, it’s worth mentioning to your prescriber.

How to use flaxseed

For most benefits, research suggests 20-30g (about 2-3 tablespoons) of ground flaxseed daily. Some studies used up to 50-60g, but digestive side effects become more common at higher doses.

Ground flaxseed is preferable to whole seeds (better absorption) or flaxseed oil (you miss out on the fibre and lignans). You can add it to:

Smoothies
Porridge or yoghurt
Baked goods (though heat may reduce some benefits)
Salads as a topping

Store ground flaxseed in an airtight container in the refrigerator for up to 3 months. Whole seeds last longer but need to be ground before use.

Summary

Flaxseed has reasonable evidence supporting benefits for blood pressure, cholesterol, and blood sugar control. The effects are modest but consistent across multiple studies. It’s not a replacement for medication in people who need it, but it might be a useful addition to a healthy diet.

The main caveats are around hormone-sensitive conditions (due to the lignans) and potential interactions with blood thinners, blood pressure medications, and diabetes medications. If you fall into any of these categories, talk to your doctor before adding significant amounts of flaxseed to your diet.

For most healthy adults, 2-3 tablespoons of ground flaxseed daily is a reasonable amount that’s supported by research and unlikely to cause problems. Start slowly to let your digestive system adjust.

References

Kavyani Z, et al. The effect of flaxseed supplementation on blood pressure: A systematic review and meta-analysis. Nutr Metab Cardiovasc Dis. 2023. PubMed
NHS. High blood pressure (hypertension). NHS Website
Hadi A, et al. Effect of flaxseed supplementation on lipid profile: An updated systematic review and dose-response meta-analysis. Eur J Nutr. 2020. PubMed
Askarpour M, et al. Effect of flaxseed supplementation on markers of inflammation and endothelial function. Cytokine. 2019. PubMed
Mohammadi-Sartang M, et al. The effect of flaxseed supplementation on body weight and body composition: A systematic review and meta-analysis of 45 randomised placebo-controlled trials. Obes Rev. 2017. PubMed
Mayo Clinic. Diabetes diet: Create your healthy-eating plan. Mayo Clinic Website
Mohammadi-Sartang M, et al. Effects of flaxseed intervention on glycaemic parameters: A systematic review and meta-analysis of clinical trials. Br J Nutr. 2018. PubMed
Demark-Wahnefried W, et al. Flaxseed supplementation (not dietary fat restriction) reduces prostate cancer proliferation rates in men presurgery. Cancer Epidemiol Biomarkers Prev. 2008. PubMed