Medical investigators have for decades looked to validate their addiction to one of the world’s most powerful stimulants: caffeine.

Now, a small study funded partially by the All Japan Coffee Association shows the drug may confer cardiovascular health benefits by dilating small blood vessels. Blood flow measured in a subject’s finger increased by 30 percent over a 75-minute period compared to those whose coffee was — no joke — secretly switched with the decaffeinated version. Those measurements provide a good indication of the workings of the inner lining of the body’s smaller blood vessels, researchers say.

“This gives us a clue about how coffee may help improve cardiovascular health,” lead investigator Masato Tsutsui said in a statement. “If we know how the positive effects of coffee work, it could lead to a new treatment strategy for cardiovascular disease in the future.”

Previous study has shown numerous health benefits to drinking coffee, the most widely consumed beverage in the world. People who drink coffee tend to enjoy a lower risk of death from heart disease and stroke, with evidence that high doses of caffeine — usually consumed by drinking coffee and other beverages — may improve the function of larger arteries.

In the study, investigators measured the effects of caffeinated coffee on subjects, ages 22-30, who did not regularly drink the beverage. On one day, the subjects drank one five-ounce cup of either regular or decaffeinated coffee, after which the researchers measured finger blood flow with a non-invasive laser, microscopically gauging blood circulation. Two days later, the subjects repeated the test — but with caffeine conditions reversed.

In either case, test subjects were not told whether their coffee contained caffeine or not.

Noting blood pressure, heart rate, and vascular resistance levels, investigators also took blood samples to analyze levels of caffeine in the body and to eliminate the influence of hormones as a possibly confounding factor in the test.

Compared to those drinking decaf, coffee drinkers boosted with caffeine experienced slight upticks in blood pressure as well as improved vessel inner lining function. Importantly, heart rate levels remained the same between the two groups. However, Tsutsui said the study still leaves unclear how caffeine works to dilate small blood vessels.

A review of three dozen studies announced earlier this month by Harvard University investigators linked moderate consumption of coffee with reduced risk for cardiovascular disease, with the lowest risk seen at three to five cups per day. Anything more, they found, produced no further benefit in lowering risk.