How we rate
“Strong” = several concordant meta-analyses or systematic reviews. “Moderate” = one synthesis, or several controlled trials. “Limited” = early trials only. The rating is about the quality of the studies, not about how useful the supplement is.
Flaxseeds
Ground flaxseeds: slightly lower blood pressure and cholesterol in meta-analyses, much more so in hypertensive individuals in a 6-month trial. Flaxseed oil does not lower cholesterol, and nothing on hot flashes.
Benefits
- Blood pressure: −2.85 mmHg systolic and −2.39 mmHg diastolic on average, in a meta-analysis of 15 trials (1,302 participants).
- In hypertensives: 30 g of ground seeds per day for 6 months, about 10 mmHg lower systolic than with placebo in 110 patients, and 15 mmHg in those starting at 140 or higher.
- Cholesterol: with whole seeds, −0.21 mmol/L total cholesterol and −0.16 mmol/L LDL in a meta-analysis of 28 trials, especially in postmenopausal women and when cholesterol starts high.
- Triglycerides: in a meta-analysis of 62 trials (3,772 participants), triglycerides, total cholesterol, and LDL decrease slightly; HDL does not budge.
- Weight: −0.99 kg and −0.80 cm waist circumference on average in a meta-analysis of 45 trials, especially with 30 g or more of whole seeds per day for 12 weeks or more.
- Blood glucose: with 13 g of ground seeds per day for 12 weeks, blood glucose and insulin decreased in 25 overweight and prediabetic individuals; 26 g did not do better.
- Constipation: in 77 constipated diabetics, 10 g of seeds twice a day for 12 weeks reduced symptoms, better than psyllium.
- Fatty liver: added to lifestyle habits for 12 weeks, 30 g per day of ground seeds lowered liver enzymes and liver fat further in 50 patients.
Precautions
Antidiabetics: in diabetic patients who kept their treatment, 10 g of flaxseed powder per day lowered fasting blood glucose by 19.7% in one month (study without placebo). Digestion: bloating, gas, diarrhea, and nausea have been reported with flax bars, as with placebo bars, likely due to fiber. HDL cholesterol: 40 g per day for one year lowered it slightly in postmenopausal women, and another trial reports this decrease in men. Flaxseed oil: 3 g per day of alpha-linolenic acid in capsules for 26 weeks increased total cholesterol by 0.45 mmol/L compared to olive oil. Limitations: hot flashes: no better than placebo, neither in a trial of 188 women with 410 mg of lignans per day, nor in a trial of 99 women with 25 g of seeds per day. Inflammation: no effect on C-reactive protein in a meta-analysis, except perhaps in obese individuals. Bones and menopause: no change in bone density or symptoms after one year at 40 g per day, and an effect on cholesterol deemed not clinically significant. Small average effects: −1.77 mmHg systolic in another meta-analysis, and an effect on LDL described as modest and short-lived. Several positive trials are open, without placebo: diabetes, fatty liver, polycystic ovary syndrome.
Recommended dosage
Ground seeds: 10 to 40 g per day depending on the trials, often 30 g; pressure and weight drop more starting from 12 weeks.
How to take it
- Seeds rather than oil: in the meta-analysis of 28 trials, whole seeds lower cholesterol, not flaxseed oil.
- Blood pressure: 30 g of ground seeds per day, incorporated into foods, for 6 months in the trial among hypertensives.
- Cholesterol: 40 g per day of ground seeds in bakery products for 10 weeks; a modest and short-lived effect on LDL.
- Blood glucose: 13 g per day for 12 weeks performed better than 26 g in a trial among prediabetics.
- Duration: blood pressure and weight drops are clearer in trials of 12 weeks or more.
Did you know?
Flaxseeds lower cholesterol slightly, not flaxseed oil: in a 26-week trial, the oil even increased total cholesterol by 0.45 mmol/L compared to olive oil.
What the studies say
This systematic review and meta-analysis of 62 randomized clinical trials, involving 3772 participants, indicates that flaxseed supplementation significantly reduced total cholesterol, triglycerides, and LDL cholesterol. However, no significant effect was observed on HDL cholesterol. These results suggest that flaxseed could improve the lipid profile and potentially slow the progression of heart disease.
Hadi A, Askarpour M, Salamat S, Ghaedi E, Symonds ME, Miraghajani M. — Pharmacological research (2020) — View the source study
For information only: not a substitute for medical advice. Dietary supplements are not a replacement for a varied, balanced diet. Learn more