Summary
Cardiovascular health is negatively affected by a blood pressure that is too high. This causes damage to the heart and blood vessels, but also increases the risk of cardiovascular health events such as heart attacks and strokes. DHA levels in the plasma are associated with blood pressure, where higher levels correlate with lower blood pressure. DHA supplementation is reported to cause a reduction in blood pressure in healthy individuals and men suffering from hypertension or elevated cholesterol levels.
 
Introduction
Blood pressure during resting state is an important indicator of cardiovascular health, meaning the health of the heart and blood vessels. Two ‘phases’ of blood pressure exist, systolic and diastolic blood pressure, the values of which are expressed as millimeters of mercury (Hg). The systolic pressure is the top measure in a blood pressure reading, when the heart actively pumps blood and maximum force is exerted on the blood vessels. The blood pressure in between pumps of the heart, when the force exerted on the blood vessels is the lowest, is called the diastolic pressure. The American Heart Association dictates that a blood pressure over 130 systolic  mm Hg & 80 diastolic mm Hg is considered too high (referred to as hypertension). Common health conditions such as hyperlipidemia (cholesterol levels that are too high) and stress are associated with hypertension. Besides being harmful to cardiovascular health, hypertension can also damage other organs like the eyes and liver. Additionally, high blood pressure increases the risk of sexual dysfunction and cardiovascular health events such as heart attacks and strokes. Omega-3 fatty acids have been implicated as benefactors of cardiovascular health for decades now.  Many studies conducted on this subject have used fish oil, which contains various types and concentrations of omega-3 fatty acids such as EPA and DHA. This has led to a lack of consensus amongst certain scientists regarding which omega-3 fatty acid exerts which effect on cardiovascular health. Listed below are research findings that have shown the positive isolated effects of DHA on blood pressure.
 
Research findings
Highlighting the importance of DHA in regard to blood pressure, Liu and colleagues reported that plasma DHA levels are inversely correlated with diastolic blood pressure. This means that persons with higher levels of DHA in their plasma on average have a lower resting blood pressure (Liu et al., 2011).
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In a 2011 study, Sagara and colleagues examined the effects of supplementation of 2g DHA / day for 5 weeks in men aged 45-59  with hypertension or hyperlipidemia, with olive oil as a control (Sagara et al., 2011). DHA supplementation caused a decrease of 5.8% in systolic blood pressure and 3.7% in diastolic blood pressure. The greatest decrease in blood pressure took place in the persons that had the lowest concentration of DHA in their blood plasma at the start of DHA supplementation.

Studies conducted on young healthy individuals report similar findings on the effects of DHA supplementation on men; the effects on women were less uniform. Singhal and colleagues reported that supplementation of 1.6g DHA / day for 4 months in subjects aged 18-37 resulted in a decreased blood pressure in men, but not in women (Singhal et al., 2013).

Interestingly, a 2019 study found that DHA supplementation of 1.6g / day for 12 weeks in both men and women aged 18-30 caused a significant reduction in diastolic and systolic blood pressure (Lee et al., 2019). This study also reported that EPA supplementation caused an increase in systolic and diastolic blood pressure.
 
Conclusion
A resting blood pressure that is too high has negative effects on cardiovascular health and other organs. DHA has been shown to affect blood pressure in healthy individuals, though the effects on men are more clear than those on women. Furthermore, men with hypertension or hyperlipidemia experience a beneficial decrease in blood pressure after taking DHA supplements. In conclusion, DHA supplementation may be beneficial for healthy individuals and those with elevated resting blood pressure or hypertension.   
 
References
Lee, J. B., Notay, K., Klingel, S. L., Chabowski, A., Mutch, D. M., & Millar, P. J. (2019). Docosahexaenoic acid reduces resting blood pressure but increases muscle sympathetic outflow compared with eicosapentaenoic acid in healthy men and women. American Journal of Physiology-Heart and Circulatory Physiology, 316(4), H873–H881. https://doi.org/10.1152/ajpheart.00677.2018
 
Liu, J. C., Conklin, S. M., Manuck, S. B., Yao, J. K., & Muldoon, M. F. (2011). Long-Chain Omega-3 Fatty Acids and Blood Pressure. American Journal of Hypertension, 24(10), 1121–1126. https://doi.org/10.1038/ajh.2011.120
 
Sagara, M., Njelekela, M., Teramoto, T., Taguchi, T., Mori, M., Armitage, L., Birt, N., Birt, C., & Yamori, Y. (2011). Effects of Docosahexaenoic Acid Supplementation on Blood Pressure, Heart Rate, and Serum Lipids in Scottish Men with Hypertension and Hypercholesterolemia. International Journal of Hypertension, 2011, e809198. https://doi.org/10.4061/2011/809198
 
Singhal, A., Lanigan, J., Storry, C., Low, S., Birbara, T., Lucas, A., & Deanfield, J. (2013). Docosahexaenoic Acid Supplementation, Vascular Function and Risk Factors for Cardiovascular Disease: A Randomized Controlled Trial in Young Adults. Journal of the American Heart Association, 2(4), e000283. https://doi.org/10.1161/JAHA.113.000283