Many of us swallow a daily multivitamin and assume that we're getting all the vitamin C that we need.

After all, most multivitamins provide 100% of the USDA's recommended Dietary Reference Intake (DRI) per day for vitamin C-75 to 90 milligrams (mg).

So we're all set, right?

Well, a recent analysis from Johns Hopkins University in Baltimore shows that getting even more than the DRI each day might go a long way in terms of reducing blood pressure or maintaining healthy blood pressure.

But how much is enough?

"C" is for controlling pressure

Scouring 45 years of medical literature, lead investigator Stephen Juraschek, MD, and his colleagues looked at 29 clinical trials comparing blood pressure measurements among participants taking vitamin C supplements with those taking placebos. The range of supplementation taken was 60 mg per day to 4,000 mg per day-the median amount was 500 mg per day-so most subjects were taking far more than the USDA's recommended amount. Subjects took the supplements for, on average, eight weeks. Some had high blood pressure and some didn't.

Results: Participants with normal blood pressure who took vitamin C had 3.8 points lower systolic blood pressure (the top number of the reading), on average, than the placebo group and 1.5 points lower diastolic blood pressure (the bottom number of the reading), on average... and those with high blood pressure who took vitamin C had 4.9 points lower systolic, on average, and 1.7 points lower diastolic, on average.

These reductions may not be as significant as the results you might get from blood pressure medications, but if your blood pressure is only slightly high, the vitamin might help keep your pressure in a healthy range or help you take less or no medication.

Juraschek said that the dips in blood pressure are thought to result from vitamin C's action as a diuretic-it prompts the kidneys to excrete more salt and water from the body, which can relax blood vessels.

May help, won't harm

Again, this research was a meta-analysis of many studies, and each study was conducted slightly differently, so Juraschek can't tell us exactly how much vitamin C is the ideal amount to take.

But since the people in the study were taking more than the USDA's recommended amount of vitamin C and their blood pressure was lowered, then should we all be taking more than 75 mg to 90 mg per day?

There's mixed advice from experts on the topic.

Juraschek takes a very cautious approach, saying that more research is needed before people increase how much vitamin C they take. He warned that doses larger than the USDA's recommendation could lead to diarrhea or kidney stones in some people, such as those prone to those problems.

But we're talking about vitamin C here! A vitamin that's good for you that is naturally in many healthy foods. Is so much caution necessary, given that vitamin C is, generally speaking, quite benign?

Excess vitamin C is excreted in urine, so how dangerous could it really be for most people? We spoke to naturopathic doctor Andrew Rubman, ND, medical director of the Southbury Clinic for Traditional Medicines in Southbury, Connecticut, to find out the answers.

Dr. Rubman said that people who are prone to diarrhea or kidney stones might have problems consuming extra vitamin C, so those people, in particular, may want to be cautious. "But that's not most of us," he said. "Chances are that most people-especially those who are prehypertensive (blood pressure between 120/80 and 139/89) or hypertensive (blood pressure of 140/90 or higher)-would benefit from taking more than 75 mg to 90 mg per day."

If you're interested in taking more vitamin C than you already do as a way of controlling blood pressure, discuss it with your doctor... especially if you have diabetes or another chronic condition.

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