By Dennis Thompson
WEDNESDAY, Nov. 20, 2013 (HealthDay News) -- Coffee seems to offer a mysterious benefit to heart health -- one that doctors have been at pains to explain.
A cup of caffeinated coffee caused a 30 percent increase in blood flow through the small vessels of people's fingertips, compared with a cup of decaf, according to the research, which is scheduled for presentation Wednesday at the American Heart Association's annual meeting in Dallas.
These microvessels regulate the ease with which blood flows through the circulatory system and the body's tissues, said lead researcher Dr. Masato Tsutsui, a cardiologist and professor in the pharmacology department at the University of the Ryukyus, in Okinawa.
Previous studies have shown an association between coffee drinking and lower risk of heart attack, heart disease and stroke, said Dr. Gordon Tomaselli, chief of cardiology at the Johns Hopkins University School of Medicine. Researchers found that high doses of caffeine may improve the function of larger arteries.
But scientists have not been able to figure out why this is, given that coffee also can increase blood pressure. High blood pressure can damage arteries.
"This is an intriguing observation that may help us understand why consumption of coffee may be beneficial," said Tomaselli, former president of the American Heart Association.
The study involved 27 healthy adults, aged 22 to 30, who did not regularly drink coffee. They were asked to drink a 5-ounce cup of either caffeinated or decaffeinated coffee. Researchers then measured their finger blood flow using a noninvasive laser technique for gauging blood circulation.
Two days later, the experiment was repeated with the other type of coffee. Neither the researchers nor the participants knew when they were drinking caffeinated coffee.
The researchers found that blood flow in the small blood vessels improved by nearly one-third among the people who drank caffeinated coffee. The effect continued in those people over a 75-minute period.
Heart rate levels remained the same between the two groups, although caffeinated coffee slightly raised blood pressure.
The improved blood flow is likely because of improved function of the inner lining of the blood vessels, Tsutsui said. Researchers have linked the function of the lining of blood vessels -- also known as endothelial function -- to future heart attacks, heart disease and strokes.
By opening blood vessels and reducing harmful inflammation, caffeine may create favorable conditions for good heart health, he said.
But how much coffee is too much? Tsutsui pointed to a landmark U.S. National Institutes of Health study that showed that, overall, drinking six or more cups of coffee a day reduced men's risk of early death by 10 percent and women's risk by 15 percent.
The new study was co-sponsored by the All Japan Coffee Association, which might raise some healthy skepticism were it not for the large body of evidence that already shows coffee's heart health benefits, Tomaselli said.
That said, the study's small sample size does not conclusively explain why coffee is so good for the heart. "I don't think this answers any questions for us," Tomaselli said.
Data and conclusions presented at meetings typically are considered preliminary until published in a peer-reviewed medical journal.