You have to see this. Just 4 tablespoons of ground flax seed a day lowered their blood pressure as good as drugs. The top number (systolic) went down by 10 to 15 points, the bottom (diastolic) down by 7 points:

Potent Antihypertensive Action of Dietary Flaxseed in Hypertensive Patients, Hypertension, December 2013

It was a very good, gold-standard study:

In this prospective, double-blinded, placebo-controlled, randomized trial, patients (110 in total) ingested a variety of foods that contained 30 g of milled flaxseed or placebo each day over 6 months.

Results:

SBP was ≈10 mm Hg lower, and DBP was ≈7 mm Hg lower in the flaxseed group compared with placebo after 6 months. Patients who entered the trial with a SBP ≥140 mm Hg at baseline obtained a significant reduction of 15 mm Hg in SBP and 7 mm Hg in DBP from flaxseed ingestion.

It wasn’t because the flaxseed caused weight loss:

Patient body weights were not significantly different between the 2 groups at any time.

Conclusion:

In summary, flaxseed induced one of the most potent antihypertensive effects achieved by a dietary intervention.

Look at this drop. You can see a significant drop after just one month:

Figure 4. Mean systolic and diastolic blood pressure among hypertensive patients (BP ≥140/90 mm Hg) on flaxseed (FX) at baseline, 1 month, and 6 months. *P=0.04 baseline vs 1 month; †P=0.002 baseline vs 6 months; ‡P=0.01 baseline vs 1 month; §P=0.003 baseline vs 6 months. PL indicates placebo.

Notes:

• One tablespoon of ground flaxseed weighs about 7 grams, so 30 grams is about 4 tablespoons. That’s about 1/4 cup ground.
• Hypertension or high blood pressure is anything over 140/90 mm Hg. A good goal is to get that top number down to 120 or below.
• “The dietary factors that influence blood pressure (BP) have traditionally included Na+, K+, caloric content, caffeine, and alcohol.” They all raise BP except for K+ (potassium) which lowers it.
• “Compliance was carefully monitored through plasma ALA and enterolignan levels.”
• “A reduction of 7 mm Hg in DBP would be expected to result in a 46% and 29% decrease in the incidence of stroke and coronary heart disease, respectively. A 10 mm Hg decrease in SBP would result in a 36% and 27% decrease in the incidence of stroke and myocardial infarctions, respectively.”
• “The magnitude of this decrease in BP demonstrated by dietary flaxseed, therefore, is as good or better than other nutritional intervention and comparable to many drugs.”
• “Patients with BP values in the normal range did not appear to respond to flaxseed with a decrease in BP, which may be dangerous.” You don’t want healthy people fainting just because they eat flaxseed.
• “Four components within flaxseed may be responsible for the changes in BP: ALA, lignans, fiber, peptides, or a synergistic action of all 4 components together.” ALA is an omega-3 fatty acid.