Omega-3 fatty acids: Small but important antihypertensive effect

June 04, 2007

Shiga, Japan - Omega-3 polyunsaturated fatty acids (PFAs) from foods such as fish, nuts, seeds, and vegetable oils appear to have a small antihypertensive effect, according to the first comprehensive population-based study to look at this issue [1].

Dr Hirotsugu Ueshima (Shiga University, Japan) and colleagues conducted INTERMAP, an international cross-sectional epidemiologic study, in almost 5000 men and women in China, Japan, the UK, and the US. They report their findings online June 4, 2007 in Hypertension.

"A large percentage of people between the ages of 20 and 60 have a rise in blood pressure, and by middle age many have high blood pressure. We're looking at dietary factors that may help prevent that rise, and omega-3 fatty acids are a small but important piece of the action," says coauthor Dr Jeremiah Stamler (Feinberg School of Medicine at Northwestern University, Chicago, IL) in an American Heart Association media release.

Japanese have highest consumption of omega-3 PFAs

The researchers explain that short-term studies had already indicated that taking dietary supplements of omega-3 PFAs could lower blood pressure in people with hypertension, but the impact of omega-3 PFAs in food and in people without hypertension was previously unknown.

INTERMAP is a large study of lifestyle factors, particularly diet habits and nutrients, and their influence on blood pressure in 4680 men and women, ages 40 to 59, living in 17 different areas of Japan, China, the UK, and the US, constituting an ethnically diverse population from a variety of socioeconomic backgrounds. All participants completed four in-depth 24-hour dietary recalls and two timed 24-hour urine collections, supplied information on their use of alcohol, dietary supplements, and medications, and had their blood pressure measured twice at each of four study visits.

The amount of omega-3 PFAs in the diet for all 4680 participants was calculated and found to average around 2.0 g per day. The researchers then compared blood pressure in people consuming higher vs lower amounts of omega-3 PFAs (about 0.7 percentage points higher as a percentage of their daily calorie intake; about 1.9 grams per day higher).

Of the four countries studied, the people in Japan had the greatest intake of omega-3 PFAs, both from vegetable sources and fish.

In general, the effect size of consuming more omega-3 PFAs was small. After multivariate adjustment for factors such as age, gender, height, weight, exercise, alcohol consumption, salt intake, and other dietary factors, diets rich in omega-3 PFAs were associated with <1 mm Hg lower systolic and diastolic blood pressures.

The effect size was larger for nonhypertensive people and for those not reporting lifestyle modification (eg, special diet, use of nutritional supplements), cardiovascular disease, or diabetes, or prescribed medication for major chronic diseases.

But despite the effect being small, it is nevertheless important, say the authors, citing studies that show that a decrease of 2 mm Hg of blood pressure reduces the populationwide average death rate from stroke by an estimated 6% and from coronary heart disease by 4%.

"With blood pressure, every millimeter counts. The effect of each nutrient is apparently small but independent, so together they can add up to a substantial impact on blood pressure," says Ueshima. "If you can reduce blood pressure a few millimeters from eating less salt; losing a few pounds; avoiding heavy drinking; eating more vegetables, whole grains, and fruits and getting more omega-3 fatty acids, then you've made a big difference."

Vegetables and nuts just as important as fish

Ueshima et al say their data from INTERMAP on food omega-3 PFAs and blood pressure "are concordant with results from meta-analyses of randomized trials assessing whether omega-3 PFA supplements (mostly fish oils) influence BP." In particular, "our data are similar qualitatively and quantitatively in indicating a low-order favorable BP effect, including in nonhypertensive persons," they note.

They also found that omega-3 PFAs from nuts, seeds, and vegetable oils—such as walnuts and flaxseed—had just as much impact on blood pressure as omega-3 PFAs from fish sources.

The authors say people should increase their intake of unsalted, cooked fish such as trout, mackerel, herring, and sardines and consume vegetable products high in linolenic acid (an omega-3 PFA), such as unsalted walnuts, flax seed, canola oil, and soy bean oil.

"We want to emphasize that you can get plenty of the omega-3 fatty acids by eating modest portions with a reasonable amount of calories and fat," Stamler says.

And the benefits of omega-3 fatty acids may extend beyond blood-pressure lowering, they add, noting that they have also been shown to favorably influence dyslipidemia and have anticoagulant and antiarrhythmic effects.

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