High blood pressure is a major risk factor for heart disease, and recent evidence suggests that it is increasing across the globe, with more than 1.5 billion people expected to have high blood pressure by 2025, or around one in three adults over the age 20.
Several small-scale clinical trials have previously suggested that short-term soy consumption can lower blood pressure but it is not yet known whether the usual intake of soyfoods has any effect on blood pressure in general populations.
A new trial, assessing the effect of the protein-rich food over two to three years, has found that soy intake was inversely associated with both systolic and diastolic blood pressure, particularly among elderly women.
A team from the Vanderbilt University Medical Center in the US and the Shanghai Cancer Institute in China measured the usual intake of soyfoods at baseline among 45,694 participants of the Shanghai Women's Health Study. The women were aged 40-70 years old and had no history of hypertension, diabetes, or cardiovascular disease when they started the trial.
Blood pressure levels were measured two to three years after the initial data were collected.
Analysis of the new data revealed that soy protein intake was inversely associated with blood pressure, after adjustment for age, body mass index, and lifestyle and other dietary factors.
In women who consumed at least 25g of soy protein each day the adjusted mean systolic blood pressure was 1.9 mm Hg lower and the diastolic BP was 0.9 mm Hg lower than in women consuming less than 2.5g per day, report the researchers in this month's issue of the American Journal of Clinical Nutrition (vol 81, no 5, pp1012-1017).
They also found that the inverse associations became stronger with increasing age. Among women who were over 60 years old, the corresponding differences were -4.9 mm Hg for systolic BP and -2.2 mm Hg for diastolic BP.
Older women are especially susceptible to hypertension and it is thought that more than half of women over 60 have high blood pressure.
Researchers believe that the isoflavones in soy may increase levels of nitric oxide in the blood, which causes blood vessels to relax, and therefore lowers blood pressure.
The US government is funding a new study on the effect of isoflavones and blood pressure at Baylor College of Medicine. Results from this could boost the use of soy in heart health foods even further. It is already one of the most common ingredients marketed for this target health area.