Vitamin D achieves its blood pressure lowering effect by addressing one of the major causes of high blood pressure—a substance called angiotensin II.
Angiotensin II is produced by another substance called angiotensin-converting enzyme, or ACE. When ACE is allowed free rein, it sometimes produces too much angiotensin II. Excess angiotensin II constricts blood vessels, which raises blood pressure. But that's not all that excess angiotensin II does.
Among many other bad effects, excess angiotensin II also leads to abnormal thickening of both the heart muscle and blood vessel walls. It increases output of adrenaline and similar substances, increases the output of another blood pressure raising hormone called aldosterone, and increases salt retention by the kidneys. All of this tends to raise blood pressure.
So blocking ACE, and thereby lowering excess angiotensin II, is actually a logical strategy for lowering blood pressure—and, frequently, it works. But until recently, the best way to block ACE was with patented medications called ACE inhibitors. ACE inhibitor names generally end in the syllable "-pril" (enalapril, captopril, etc.), but they're sold under trade names including Vasotec, Lotensin, Zestril, Altace, Capoten, and others. Like most patent medications and synthetic molecules, ACE inhibitors can cause a number of negative side effects-cough, headache, and dizziness at best; skin rash, kidney problems, and swelling of the face, lips, and throat at worst.
But vitamin D might be able to go one step further-without the added disadvantages of the synthetic ACE inhibitors—by preventing the formation of excess angiotensin II in the first place.
Here's how it works: According to a study published in 2002 in the Journal of Clinical Investigation, one of your genes (a tiny part of your DNA) leads to the formation of a molecule called renin. Renin breaks down another molecule, called angiotensinogen, into angiotension I. Angiotensin I is converted into angiotensin II by ACE. Vitamin D persuades the renin-controlling gene to become less active, and the whole process slows down.  The end result is less angiotensin II and lower blood pressure.
Other researchers have found that the higher a person's serum level of vitamin D, the lower his or her blood pressure. And in case studies published in the journal Internal Medicine and the American Journal of Kidney Disease, treatment with vitamin D reduced the subjects' plasma renin, angiotensin II, and blood pressure. http://www.healthiertalk.com/natural-blood-pressure-regulator-017
It sounds like an easy solution, and it can be—but only under the proper guidance. Please don't start taking high doses of vitamin D to control your blood pressure on your own. Since the dosage range needed to lower blood pressure isn't yet known, it's a good idea to contact a physician skilled and knowledgeable in nutritional and natural therapies who can work with you in translating this new research into clinical practice. Plus, you'll need to be monitored for vitamin D safety. For a list of natural medicine physicians in your area, contact the American College for Advancement in Medicine at (800) 532-3688 or www.acam.org.
Don't let that scare you off, though: There's actually a much wider range of safe vitamin D doses than health "authorities" generally admit. In 1999, the American Journal of Clinical Nutrition published an article re-examining the upper limits of vitamin D safety.  The researchers concluded that the often-mentioned upper limit of vitamin D safety, 2,000 IU daily, "is too low by at least 5-fold." Instead, they suggested that 10,000 IU daily might be a better safe upper limit.
The same journal published a follow-up study in 2001 revisiting that recommendation.  This time, the researchers concluded: "We consider 4,000 IU vitamin D3 to be a safe [daily] intake" for adults.
Still, it's smart to have a doctor skilled and knowledgeable in nutritional therapy monitor your vitamin D intake. Back in the 1930s-1950s there were instances of massive vitamin D overdoses. When this happens, calcium is actually leached from bones and appears in the blood in much higher levels than normal—a condition known as hypercalcemia. The initial signs and symptoms of hypercalcemia consist of weakness, fatigue, headache, nausea, vomiting, and diarrhea.
If hypercalcemia persists, calcium is then deposited in soft tissues, most notably in the kidneys. Eventually, this can cause serious kidney damage and osteoporosis.
But you can head these problems off at the pass by keeping a careful eye on your serum calcium level. A doctor skilled and knowledgeable in nutritional therapy will make sure to monitor it, and if your level goes too high, he'll ask you to decrease your vitamin D dosage or stop taking it altogether.