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Nitrates May Explain Success of Blood-Pressure Diet

Small Swedish study suggests leafy vegetables could be source of nutrient

WEDNESDAY, Dec. 27, 2006 (HealthDay News) -- A small Swedish study could help explain why the U.S. government-recommended DASH diet helps fight high blood pressure.

DASH -- Dietary Approaches to Stop Hypertension -- recommends eating lots of vegetables and fruits. The Swedish study indicates that the nitrates in foods such as spinach, beet root and lettuce spur production in the body of nitric oxide, a molecule that relaxes blood vessels.

"What this study suggests is that the well-known beneficial effects of vegetables on cardiovascular disease may at least partly depend on nitrate," said Dr. Eddie Weitzberg, professor of anesthesiology and intensive care at the Karolinska Institute in Stockholm, and a leader of the study.

Weitzberg and his colleagues had 17 young adults alternately take dietary supplements of sodium nitrate or a placebo, an inactive substance. There was no reduction in systolic blood pressure (the higher number of a 120/80 reading) when they took the nitrate, but a 3.7-point reduction in diastolic pressure (the lower number) when they did.

"This reduction is in the same magnitude as in the cited DASH study, where those subjects were put on a fruit and vegetable diet for 11 weeks," Weitzberg noted.

The findings are published in the Dec. 28 issue of the New England Journal of Medicine.

The study suggests a new pathway for nitric oxide production in the body, Weitzberg said. "Ingested nitrate is reduced by oral, commensal, bacteria to nitrite, which can further be reduced to nitric oxide," he explained.

It's too early to recommend nitrate supplements, Weitzberg said. "One should be extremely cautious to draw strong conclusions from our small trial, since we have not yet investigated if the observed effects are withstanding after a longer period of dietary nitrate," he said.

And nitrate is considered "a substance that should be avoided or at least reduced in our drinking water and food," Weitzberg noted, because it is believed to increase the risk of a rare blood condition, methemoglobulinemia, and perhaps of cancer.

"Everyone has been asking why this [the DASH] diet worked," said Dr. Steven Nissen, chairman of cardiovascular medicine at the Cleveland Clinic, adding that the study does provide evidence for the possible effect of the sodium nitrate in green, leafy vegetables.

But Nissen was quick with a caveat -- there isn't all that much nitrate in those vegetables. "Too get as much nitrate as was given in the study, you would have to eat about half a pound of spinach," he said. "As a practical issue, it's hard to ingest 250 grams of nitrate-rich foods."

Weitzberg said research on nitrates and high blood pressure will continue at the Karolinska Institute, in part because the researchers aren't sure about the mechanism of the nitrate effect.

"It is not very easy to unravel the exact underlying mechanisms," he said. "We are naturally planning to look into this more deeply."

More information

Get full details of the DASH diet from the U.S. National Heart, Lung, and Blood Institute.

SOURCES: Eddie Weitzberg, M.D., Ph.D., professor of anesthesiology and intensive care, Karolinska Institute, Stockholm, Sweden; Steven Nissen, M.D., chairman of cardiovascular medicine, Cleveland Clinic; Dec. 28, 2006, New England Journal of Medicine
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