The researchers, led by Professor Barry Halliwell from the National University of Singapore, have previously reported that dark soy sauce (DSS) has an antioxidant activity 150 times that of vitamins C and E, and ten times that of wine.
The new study is the first to report positive results of a clinical trial on levels of antioxidants and risk factors for cardiovascular disease (CVD).
CVD causes almost 50 per cent of deaths in Europe, and is reported to cost the EU economy an estimated €169bn ($202bn) per year. According to the American Heart Association, 34.2 per cent of Americans (70.1m people) suffered from some form of cardiovascular disease (CVD) in 2002.
The observer-blind, placebo-controlled, crossover study, randomly assigned 24 young healthy volunteers (average age of 22.8, BMI of 21.6 kg per sq. metre, and blood pressure of 109/65 mmHg) to eat a single dose (30 mL) of dark soy sauce (Tiger Brand, Cheun Chong Food Industries, Singapore) with a 200 gram serving of plain boiled rice, or the rice plus food colouring (placebo).
A three-day washout period followed and then the volunteers were crossed-over to receive the other meal. Measurements were taken every hour for four hours after eating, including blood and urine samples, blood pressure, and heart rates.
The researchers found that levels of compounds called F2-isoprostanes, a product of free radical oxidation of arachidonic acid and related to oxidative stress from smoking, CVD, obesity and diabetes, were lower in the plasma of the DSS group than placebo. After three hours, the DSS group had a 13 per cent lower level of F2-isoprostanes. The difference grew to 16 per cent after four hours.
No significant difference between the placebo and intervention group was observed for heart rate, or for systolic blood pressure. Diastolic blood pressure however was lower in the DSS group than placebo (62.63 mmHg versus 65.75 mmHg after two hours).
"Diastolic blood pressure decreased further after DSS than placebo, consistent with the notion that a decrease in oxidative stress could have physiologically relevant effects on the vasculature," wrote the researchers in the journal Biochemical and Biophysical Research Communications (Vol. 344, pp. 906-911).
"We find that dark soy sauce has a rapid (3 to 4 hours) antioxidant effect against lipid peroxidation in vivo, and that these effects are accompanied by vasodilatory hemodynamic changes in vascular function consistent with antioxidant effects on the endothelium or vascular smooth muscle," they said.
The next step, said the scientists, was to investigate if long-term feeding of dark soy sauce could extend the antioxidant activity and help combat the harm caused by oxidative stress.
Work should also focus on identifying the active component(s) in the sauce that have the protective effects. The antioxidant activity of soy products has often been linked to the isoflavone content, although this seems unlikely in this instance due to the low concentration of isoflavones in the sauce.
A recent study in The American Journal of Clinical Nutrition (Vol. 83, pp. 244-251) reported that soy proteins and not the isoflavones were responsible for lowering serum lipid levels and are responsible for the cardiovascular benefits.
It should also be stressed that commercial dark soy sauce has a significant content of salt, which has been shown to raise blood pressure and is linked to an increased risk of CVD and stroke.
The dark soy sauce industry saw sales fall in 2002 amid fears about carcinogenic contaminants in the sauces, with national food agencies quick to remove the small number of offending products from supermarket shelves.