While research has shown that fish oil pills may lower high blood pressure, getting those heart-healthy fats from food might also help, a study published Monday suggests.

The study of middle-aged adults in China, Japan, Great Britain and the U.S. found that people who ate more foods rich in omega-3 fatty acids tended to have lower blood pressure. This relationship was seen with fish, as well as with vegetable sources, such as canola oil, soybean oil, flaxseeds and walnuts, according to lead author Dr. Hirotsugu Ueshima of Shiga University, Japan, and associates.

The findings, published online in the journal Hypertension, add to evidence of the heart benefits of omega-3 fatty acids.

Oily fish, such as salmon, mackerel and sardines, contain two types of omega-3 fats -- DHA and EPA -- that have been shown to reduce blood fats called triglycerides and possibly prevent dangerous heart-rhythm disturbances. Because of this, the American Heart Association recommends that adults eat fatty fish twice a week, and that people with heart disease consider taking fish oil pills to get extra DHA and EPA.

The effects of omega-3 fats on blood pressure are less clear. And it has not been known whether omega-3-rich foods, specifically, are helpful; this includes plant sources, which contain an omega-3 known as ALA, a precursor to DHA and EPA.

The new findings suggest that both fish and vegetable sources of omega-3 do offer a modest blood pressure benefit, according to the study authors.

Among 4,680 adults ages 40 to 59, those who ate relatively more omega-3 fats from food tended to have somewhat lower blood pressure. The results lend "modest support" to the general recommendation that adults bump up their omega-3 intake, the researchers conclude.

The study participants had their blood pressure measured several times over 3 weeks and were interviewed at each visit about their food intake for the previous day.

In general, the researchers found, blood pressure levels tended to dip as participants' omega-3 intake climbed -- even with a range of other factors taken into account, such as age, weight, exercise habits and cholesterol and saturated fat intake.

The benefit was most clear among people without high blood pressure and those who were not taking medication or following a special diet to control their elevated blood pressure.

The researchers suggest that people eat their omega-3 fats as part of an overall diet that helps control blood pressure. That includes limiting salt, alcohol and saturated fat, while getting enough vegetable protein and nutrients such as potassium and calcium.

"With blood pressure, every millimeter counts," Ueshima said in a statement. "The effect of each nutrient is apparently small but independent, so together they can add up to a substantial impact on blood pressure."