Remedy · grain & seed
Flaxseed: what the evidence says about blood pressure and regularity
A seed rich in fiber, lignans and the plant omega-3 ALA. Trials show a small drop in blood pressure; milling it matters if you want the omega-3.
By the editors of Alimentotherapy Last reviewed 3 min read Not medical advice
Flaxseed
Linum usitatissimum
Best-supported use: Slightly lower blood pressure
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The short answer
In a meta-analysis of 14 controlled trials, eating flaxseed lowered blood pressure slightly, by under 2 mm Hg on average for both systolic and diastolic readings Khalesi 2015. Milled (ground) flaxseed raises blood levels of its omega-3 fat, ALA, while whole seeds largely pass through without doing so, and flax does not raise the fish-type omega-3s EPA and DHA Austria 2008.
The evidence, use by use
Each claim graded on human studies only. How we grade
| Use | Grade | What the research found |
|---|---|---|
| Slightly lower blood pressure | Moderate | Pooling 11 studies (14 trials) of at least two weeks, flaxseed lowered systolic pressure by 1.77 mm Hg and diastolic by 1.58 mm Hg. Diastolic effects were larger with whole seed and with 12 weeks or more. Khalesi 2015 |
| Constipation | Limited | In a 12-week trial of 53 constipated adults with type 2 diabetes, 10 g of flaxseed baked into cookies twice a day improved constipation scores compared with placebo cookies. Larger trials are lacking. Soltanian 2018 |
| Raising EPA and DHA (fish-type omega-3s) | Not supported | In a three-month trial, whole seed, milled seed and flax oil all failed to raise plasma EPA or DHA. Austria 2008 |
| Weight loss | Limited | One trial suggests flaxseed mucilage powder mixed with water may help adults with overweight; flaxseed lignan extract and flax oil do not seem to help. NCCIH |
Ground or whole?
Flaxseeds have a hard shell. In a study that baked 30 g of flaxseed a day into muffins for three months, people eating milled seed or flax oil had significantly higher blood ALA levels, while those eating whole seed did not Austria 2008. Whole-seed eaters also struggled to stick with it because of stomach upset.
The blood pressure picture is less tidy: in the meta-analysis, the diastolic effect looked larger with whole seed, though that was a subgroup finding Khalesi 2015. If you want the omega-3, grind it; if you mainly want fiber, either form adds bulk.
In the kitchen
Grind small batches in a coffee or spice grinder and keep the ground seed in a sealed jar in the fridge, because its oil turns rancid with air and warmth. Stir it into oats, yogurt or batter. Drink water with it: it is a bulking fiber.
Amounts used in studies
The trials above used around 20 to 30 g a day: 10 g twice a day in the constipation trial, 30 g a day in the bioavailability study Soltanian 2018; Austria 2008. Start lower and build up, since higher amounts can cause bloating, fullness and diarrhea NCCIH. A tracker such as Cronometer can show how much fiber and ALA your usual meals already provide.
What about flaxseed oil?
Flax oil delivers ALA (it gave the highest blood levels in the muffin study) but none of the seed's fiberAustria 2008. Its supplements may be safe for short-term use of up to six monthsNCCIH. In the same study, none of the three forms raised blood EPA or DHA, which is why flax oil is not a substitute for oily fish Austria 2008.
Before you try it: safety and interactions
Who should avoid it or ask first
- You would eat raw or unripe flaxseeds: they may contain potentially toxic compounds.
- You take an anticoagulant or antiplatelet drug: there are theoretical reasons to suspect an interaction, so ask your prescriber.
- You take oral medicines: as with other fiber, take them at a different time from a large dose of flax.
Possible side effects
- Bloating, fullness and diarrhea, especially at higher amounts or without enough water.
- Stomach discomfort when starting, which tended to settle with milled seed or oil.
Pregnancy and breastfeeding
Some studies suggest flaxseed is possibly unsafe in pregnancy, though the evidence is not conclusive, and little is known about breastfeeding. Check with your clinician before using it beyond normal food amounts. NCCIH; Austria 2008
Medicine interactions
We found no well-documented medicine interactions for flaxseed at food amounts. That is not proof there are none: tell your pharmacist about any herb or supplement you take.
Check another food or medicine in the interactions list.
Common questions
Should flaxseed be ground?
Yes, if you want its omega-3. In a three-month study, milled flaxseed and flax oil raised blood levels of the omega-3 ALA, while whole flaxseed did not. Store ground flax sealed in the fridge so the oil does not go rancid.
Does flaxseed lower blood pressure?
Slightly. A 2015 meta-analysis of 14 controlled trials found flaxseed lowered systolic blood pressure by 1.77 mm Hg and diastolic by 1.58 mm Hg on average, with larger diastolic effects in trials lasting 12 weeks or more.
Is flaxseed a substitute for fish oil?
No. Flaxseed supplies ALA, a plant omega-3, but in a controlled study none of whole seed, milled seed or flax oil raised blood levels of EPA or DHA, the omega-3s found in oily fish.
Sources
Primary research and government sources, each checked on the date this page was reviewed.
- PubMedKhalesi S, Irwin C, Schubert M. Flaxseed consumption may reduce blood pressure: a systematic review and meta-analysis of controlled trials. J Nutr. 2015. PMID 25740909.
- PubMedAustria JA, et al. Bioavailability of alpha-linolenic acid in subjects after ingestion of three different forms of flaxseed. J Am Coll Nutr. 2008. PMID 18689552.
- PubMedSoltanian N, Janghorbani M. A randomized trial of the effects of flaxseed to manage constipation, weight, glycemia, and lipids in constipated patients with type 2 diabetes. Nutr Metab (Lond). 2018. PMID 29760761.
- NCCIHNational Center for Complementary and Integrative Health. Flaxseed and Flaxseed Oil: Usefulness and Safety.
Keep reading
- GoalFood for regularity: fiber, fluids, kiwifruit and prunesMost adults eat well under the fiber they need. Raising it slowly, with fluid, is the first step; kiwifruit, prunes and psyllium all have trial support.
- GoalFood for a healthy heart: blood pressure and cholesterolEating patterns have the strongest heart evidence: DASH for blood pressure, Mediterranean for heart events, and soluble fiber from oats, barley or psyllium for cholesterol.
- RemedyPsylliumThe seed husk sold as a bulk fiber. It has the best evidence of any fiber supplement for chronic constipation and a solid record on LDL cholesterol.
- RemedyOily fishSalmon, sardines, mackerel and their kin are the main food source of the omega-3s EPA and DHA. The case for eating them is stronger than the case for fish oil capsules.