Just the Flax, Ma’am. Does Flaxseed Lower Blood Pressure?

Several foods appear to have a positive impact on blood pressure: dark chocolate, guava fruit, soy, and garlic, to name a few.^1-6  They please both the palate and the prescriber.  Most people won’t turn down chocolate and most clinicians won’t turn down the chance to lower blood pressure in a patient with hypertension.  Enter flaxseed (Linum usitatissimum L) — it’s high-fiber, rich in alpha-linolenic acid (ALA), and may be beneficial for a variety of cardiovascular indications.  Flaxseed may be delightful in muffins or fruit smoothies, but does it lower blood pressure?

Dietary changes including increased fiber intake is a cornerstone for the non-pharmacologic management of blood pressure in patients with hypertension.  The report from the panel members appointed to the eighth Joint National Committee (JNC 8) states that, “for all persons with hypertension, the potential benefits of a healthy diet, weight control, and regular exercise cannot be overemphasized.”⁷ The 2013 American Heart Association/American College of Cardiology (AHA/ACC) Guideline on Lifestyle Management to Reduce Cardiovascular Risk, interventions such as the DASH (Dietary Approaches to Stop Hypertension) diet, increasing whole grain intake, and sodium restriction have notable blood pressure lowering effects in patients with hypertension.^8-11 

The purpose of this secondary post-hoc analysis of the FLAX-PAD¹² Study was to determine the effect of flaxseed on blood pressure.  The FLAX-PAD study was a single-center, prospective, randomized, double blind, placebo-controlled trial. The study set out to evaluate the effects of a diet supplemented with flaxseed (for 12 months) on the risk of myocardial infarction or stroke in patients with peripheral arterial disease. Patients were eligible for inclusion if they were over 40 years of age, were diagnosed with peripheral artery disease (PAD) for more than 6 months, and experienced symptoms secondary to lower-extremity atherosclerotic arterial disease confirmed with an ankle-brachial index of 0.9 or lower or had a previous intervention for PAD. Patients were excluded if they reported resting limb pain, were unable to walk on a treadmill, had history of bowel disease, an impaired ejection fraction (<40%), had moderate-to-severe renal failure, used non-prescribed supplements, had a gluten allergy, or would likely undergo surgery during the course of the trial. 

Patients were randomized to receive either 30 grams of milled flaxseed or placebo (wheat, bran, and molasses to match texture and taste) daily. Subjects received a variety of prepared foods such as bagels, muffins, bars, buns, pasta, or tea biscuits and were instructed to consume one daily. Patients were examined at baseline and at months one, two, six, and twelve with blood pressure measured in a seated position three times per visit. Researchers documented the average of three readings. From a study quality and bias standpoint, the Jadad scale score (Oxford quality scoring system) was a 5 out of 5 – accounting for blinding, randomization methods, description of dropouts, and allocation concealment.

One hundred ten patients were recruited and enrolled in the study; 58 were allocated to the flaxseed group. At baseline, there were no significant differences between study populations. The average age was 67 years. Sixty-six percent of patients were former smokers and 26% continued to smoke. Seventy-five percent of patients were diagnosed with hypertension, 32% with diabetes, 79% with dyslipidemia, and 39% had known coronary artery disease. A majority (79%) were receiving antihypertensive therapy with at least one agent, 74% receiving lipid-lowering therapy, and 90% were receiving anti-thrombotic therapy. The baseline mean blood pressure was 142.9 / 77.5 mmHg.

Eight six patients complete the study. Thirteen patients from the flaxseed group and 11 placebo-treated patients dropped out.  In the flaxseed group, one patient developed unstable angina, another underwent amputation, and one died. Ten were unable to adhere to the diet. In the placebo group, one person experienced increased blood glucose levels, one withdrew due to excessive fatigue, eight were unable to adhere with the diet, and one was lost to follow-up. 

At 6 months, the mean systolic and diastolic blood pressures were significantly lower in the flaxseed group when compared to the placebo group (See Table 1).  At 12 months, these differences appeared to be sustained but the authors did not report whether these differences were statistically significant. 

Table 1. Blood Pressure Changes (mmHg)

*indicates statistically significant (p<0.05) compared to baseline
blood pressure measurements at 12 months were not assessed for statistical significance
SBP=systolic blood pressure; DBP=diastolic blood pressure

The results of this study suggest that the addition of 30 grams of flaxseed to the diet of patients with hypertension and peripheral arterial disease can significantly decrease both systolic and diastolic blood pressure. Although the authors attempt to extrapolate findings to changes in morbidity and mortality, we shouldn’t assume that clinical outcomes will be favorable without data.  Despite this limitation, the study highlights and reinforces non-pharmacologic, dietary changes as an effective means to reduce blood pressure.  Foods such as dark chocolate, guava fruit, and soy have been shown to reduce systolic blood pressure by 3 mmHg, 8 mmHg, and 8 mmHg, respectively.^1-4  The sustained reduction observed with flaxseed (7 mmHg) was similar.

Flaxseed produced a greater blood pressure reduction in patients with a baseline SBP ≥ 140 mmHg.  Unfortunately, the authors do not state the number of patients who had a SBP ≥ 140 mmHg at baseline, the proportion of this group treated with an antihypertensive medication, or the proportion who achieved a SBP ≤ 140 mmHg by study’s end.  While flaxseed demonstrated benefit in all patients, those with elevated SBP and treated with medications might benefit from flaxseed the most, thus avoiding additional medications.

Several issues may leave clinicians reluctant to recommend flaxseed. Per the study protocol, researchers did not prohibit prescribers (primary care or others) altering the patient’s antihypertensive regimen. Thus, antihypertensive medications could be adjusted, added, or discontinued.  However, this potential confounder probably did not influence the study findings.  Approximately 80% of participants in both groups remained on the same regimen and dose throughout the study. In the flaxseed group, 8% of patients de-escalated their antihypertensive therapy compared to 3.5% in the placebo group. The authors did not comment on whether participants had their therapy intensified – but presumably it was a very small percentage of patients.

Another concern — the mean blood pressure of participants in the placebo group increased over the course of the study by approximately 3 mmHg. This is not consistent with observations in placebo-treated subjects in other antihypertensive studies.¹³ It is unclear if this small change would increase the risk of negative clinical outcomes or if it influenced the statistically significant difference between the flaxseed and control group.

Finally, the flaxseed-fortified meals were prepared ahead of time for the participants.  On the surface, it does not appear to be difficult for a patient to prepare a batch of food to be stored for a week or so based on recipes formulated to result in the recommended flaxseed intake.  However, cost, time, and food choices might make the intervention prohibitive for many patients.

Were the decreases in blood pressure observed in this study due to flaxseed consumption, other changes in the patient’s diet, a positive interaction with the antihypertensive medications, or a combination these?  In the case of the DASH diet, researchers believe that the observed BP reductions are due to dietary modifications.  However, this and other studies suggest that certain foods may have a direct and positive impact on blood pressure.

Funding for this research was provided by Flax2015 (Flax Council of Canada).  However, in fairness, grant support was also provided by the Canola Council of Canada, the Agri-food Research Development Initiative, and the Canadian Institutes for Health Research. 

On the surface, the blood pressure lowering effects of flaxseed cannot be ignored because they are as impactful as sodium restriction and even some medications. However, unlike the guideline recommend first line antihypertensive agents, flaxseed lacks long-term morbidity and mortality outcome data.  Where does flaxseed fit in the treatment of a patient with hypertension?  Given the significant blood pressure lowering potential of flaxseed, should it be used in place of or only in conjunction with prescription antihypertensive medications? Should it be introduced with the first antihypertensive agent or reserved for patients who require multiple medications?  Do you recommend flaxseed to your patients?