FRIDAY, Jan. 21, 2005 (HealthDayNews) -- Vegetarians tend to have lower blood pressure than the general population, but it hasn't been clear whether their diet or their lifestyle guards them against hypertension.
Now, a new review of previously published studies claims that diet provides the protection.
"It's the diet itself, and it is clearly the diet of choice for people who want to get their blood pressure under control," said Dr. Neal D. Barnard, president and founder of the Physicians Committee for Responsible Medicine and co-author of the report, which appears in the January issue of Nutrition Reviews.
Barnard, a nutritionist and author of the book Breaking the Food Seduction: The Hidden Reasons Behind Food Cravings and Seven Steps to End Them Naturally, concluded that a person who suffers from hypertension and has yet to switch to a vegetarian diet is "really trying to fight their condition with one arm tied behind their back."
About 65 million American adults have high blood pressure, according to the National Heart, Lung, and Blood Institute. Hypertension is often called the "silent killer" because it usually has no symptoms but leads to increased risk for heart disease, congestive heart failure, stroke and kidney failure.
Barnard and committee nutritionist Susan Berkow analyzed 80 scientific studies, including observational studies of individuals on vegetarian diets compared with non-vegetarians and randomized, controlled trials in which outcomes of people who switch to a plant-based diet were compared with control subjects.
"The purpose of our review was to bring together what is known about the effect of the diet, but also what we know about the mechanism and try to explain why this occurs," Barnard explained.
Some of the best observational data, according to the report, come from studies involving Seventh-Day Adventists, who advocate an alcohol-free, tobacco-free, vegetarian lifestyle. About 50 percent of Adventists follow a lacto-ovo-vegetarian diet, which includes dairy products and eggs, the authors noted.
One study involving California Adventists found that vegetarians have about half the prevalence of hypertension compared to non-vegetarian Adventists. When hypertensives were defined as those taking medication intended to reduce their blood pressure, a nearly threefold difference in the prevalence of hypertension was seen between the groups.
Overall, the randomized controlled trials included in the review found that blood pressure is lowered when animal products were replaced with vegetable products in both people with normal blood pressure and those who are hypertensive.
To understand the blood-pressure-lowering effects of a plant-based diet, the authors examined changes in body weight and intake of specific food groups and nutrients.
Studies show that vegetarians tend to be slimmer, on average, which may help explain their lower incidence of hypertension. A vegetarian diet also is significantly lower in saturated fat, reducing the viscosity, or thickness, of the blood.
Blood becomes "less like oil, more like water," Barnard explained.
And because vegetarian diets are generally high in fruits and vegetables, people who follow this diet consume more potassium than those who eat a diet of meat and vegetables. The analysis cites two reviews involving a total of 52 randomized clinical trials showing potassium supplementation significantly lowered blood pressure in people with normal and elevated blood pressure.
There are those who disagree with the finding, however.
Dr. Lawrence J. Appel, a nutrition specialist at the Johns Hopkins Bloomberg School of Public Health, said the paper fails to establish a clear cause-and-effect relationship between consuming a plant-based diet and lowering one's blood pressure.
"It's a good review, but there are still unanswered questions," he said.
He also noted that very few clinical trials have been conducted, and that those that have been done are small and not tightly controlled. Much of the data is observational.
So, he said, it remains unclear whether a vegetarian diet alone is responsible for lowering blood pressure or whether some aspect of a vegetarian regimen -- such as eating lots of fruits and vegetables rich in potassium
and fiber while maintaining a desirable body weight -- could have the same effect.
And then there there is the fact that not everyone who has high blood pressure eats poorly or is overweight; genetic factors significantly influence a person's risk for hypertension.
Still, Barnard insists a vegetarian diet is healthy for everyone, whether or not they have high blood pressure.
He offers this caveat for people taking blood pressure medication: "Don't throw your medication in the trash." High blood pressure is a serious medical condition requiring immediate medical attention. Even if you switch to a vegetarian diet to trim down, you won't lose the weight overnight, he said. It could take more than a year for a person who is 60 pounds overweight to drop that excess baggage.
Barnard hopes the review will prompt more doctors to recommend a vegetarian diet. Many are reluctant to do because they fear that patients won't stick with it, but there's no reason to believe patients would be less likely to go vegetarian than to comply with other diets, he said.
"They may not hit the mark 100 percent, but they'd do much better if a doctor recommended it," he said.
Barnard's group, the physicians committee, is a Washington, D.C.-based nonprofit that promotes good nutrition, opposes unethical human experimentation and advocates alternatives to animal research.
More information
The National Heart, Lung, and Blood Institute can tell you more about how to lower your blood pressure.
