While watching the documentary Forks Over Knives I heard a physician explain that it was possible that a vegan diet may lower blood pressure by lowering the blood viscosity (thickness). He stated this in passing but to me it was earth-shaking. I have had thousands of patients with high blood pressure and am treating it on a daily basis. I have sat through countless lectures on hypertension and still to this day we do not really know what causes it. The leading explanations include increased sympathetic activity (think high adrenaline, anxiety), decrease kidney function with age, increased hormone called angiotensin II, or complex interaction of genetic factors. However all of these fail to really explain why obesity, inactivity and diet raise blood pressure.
When I heard that it could be the viscosity (thickness) of the blood I was overwhelmed by the simplicity of it. If the blood is thicker it will take higher pressures to move it. Think of the pressure needed to move water through a pipe as opposed to oil. Thick liquids are harder to move though pipes so a higher pressure is needed. So maybe that is why we are having such a hard time finding a cause for high blood pressure. Because we view it as the problem. However, high blood pressure may not be a problem at all, but instead is the body’s adaptive response to keep the blood flowing when it gets thicker. The problem may be thick blood. However, none of the medications in my arsenal for hypertension treat high viscosity.
As usual I went to the research to see what foresighted researcher was working on this question. And yet again I found that it was looked at LONG ago. In 1981 in the American Journal of Medicine Dr. Letcher et al. showed a direct relationship between blood pressure and blood viscosity. As the blood gets thicker the blood pressure goes up. And those patients with high blood pressure have high blood viscosity. This correlation was further supported by another study by Dr. Letcher in 1983 published in the journal Hypertension.
It seems armed with this information pharmaceutical companies would have marched forward to find the magic drug to decrease blood viscosity and researchers would be toiling night and day to find the underlying cause of the high viscosity blood. However, that did not happen. Today hypertension drugs mostly act by dilating blood vessels (making the pipe wider) or decreasing the pumping action of the heart to decrease the pressure. This does give health benefits likely because it decreases the shear forces on the walls of the vessels. However, none of these medications fix the underlying issue that the body may have been trying to overcome, pushing thick blood around effectively.
And despite there being a simple test to measure blood viscosity it is rarely used and when it is it is not for hypertensive patients. In fact in all my years of practicing medicine I can only think of one patient who had their blood viscosity checked.
One of the culprits that may be causing this increased blood thickness is fibrinogen, a protein involved in the formation of blood clots. Fibrinogen seems to be closely correlated with blood viscosity and it is logical that blood clotting agents may thicken the blood. Also, it is known that a vegan diet lowers your levels of fibrinogen so this may be the pathway through which a vegan diet lowers blood pressure. Also, smoking is shown to raise fibrinogen levels and smoking raises blood pressure. Also there is a class of cholesterol medications called fibrates that are known to decrease fibrinogen levels and studies have shown that they also lower blood pressure.
I can attest personally to the blood pressure lowering effects of a vegetarian diet. My mother has very high blood pressure (she was 220/110 when she was diagnosed). She is on 4 separate blood pressure medications. I had this tendency as well and would run high even in college, my blood pressure was close to 140. After watching a documentary and hearing a piece on NPR about the conditions at cattle and pig farms and slaughter houses I decided to be vegetarian. Within months my blood pressure came down to an average of about 115.
To leave the question of what causes high blood pressure unanswered when the explanation may be so close at hand and has already been put forth 30 years ago seems negligent. Millions are suffering with high blood pressure and resultant strokes and heart attacks. If we could find the answer and a treatment that fixes the true underlying cause, the potential benefit to society and savings to our healthcare system would be almost impossible to measure.
Here is another great article summing up the research. Well what little there is.