Caffeine is one of the most popular stimulants in the world. Millions of people begin their day with a cup of coffee or tea, but for years cardiologists and the American Heart Association warned that, for some, its stimulating effects could raise blood pressure and the risk of an irregular heartbeat.

That guidance is changing. Recent studies have found that caffeine may have cardiovascular benefits, prompting the American Heart Association to release a new scientific statement about the effects of caffeine on the cardiovascular system.

Caffeinated coffee without additives like sugar, sweetener, milk, cream or flavored syrups lowered the risk of type 2 diabetes, heart disease, stroke, heart failure and some types of arrhythmias.

The findings of recent high-quality studies such as randomized clinical trials have challenged conventional wisdom, Gregory Marcus, chair of the statement writing committee, told TheDoctor. Clinical practice guidelines are also starting to incorporate those findings into formal recommendations.

“We thought because a scientific statement on this topic had not been put together before, it was time to provide a summary of the latest and best evidence about the effect of caffeine on cardiovascular disease,” said Marcus, a professor of medicine at the University of California in San Francisco.

Coffee is the main source of caffeine in most studies, so the statement included a lot of comments on caffeinated coffee, concluding that 400 milligrams of caffeine a day is generally safe for most adults. Regular brewed coffee usually contains 9.5 to 20.6 milligrams of caffeine per fluid ounce, equal to three-to-five eight-ounce cups.

Synthetic caffeine, however, and the higher levels of caffeine found in energy drinks and energy shots may harm the cardiovascular system and increase the risk of hypertension and arrhythmia.

Caffeinated coffee without additives like sugar, sweetener, milk, cream or flavored syrups lowered the risk of type 2 diabetes, heart disease, stroke, heart failure and some types of arrhythmias. It may also lower the risk of heart disease, heart failure and stroke for some people. More studies are needed to find out how additives like sweeteners affect these health benefits.

Synthetic caffeine and the higher levels of caffeine found in energy drinks and energy shots may harm the cardiovascular system and increase the risk of hypertension and arrhythmia.

Fewer studies focus on decaffeinated coffee, but some evidence suggests decaf may have cardiovascular benefits, Marcus said. Decaffeinated coffee usually has caffeine in it, just in very low amounts, he pointed out. This suggests that other compounds in coffee, not just caffeine, may have health benefits. “When we study caffeinated coffee, we often cannot tell what effects may be due to caffeine specifically versus the other compounds in coffee,” Marcus said.

A compound found in both caffeinated and decaffeinated coffee, cafestol, is associated with higher levels of low-density lipoprotein, the bad cholesterol. Cafestol is found in unfiltered coffee, like espresso, Turkish coffee, boiled coffee and French press, but not in coffee brewed using paper filters or instant coffee. Further studies are needed to learn which products have high levels of cafestol and how it might raise LDL levels, the researchers said.

Studies are also needed to learn more about the health effects of caffeine in products like soda, chocolate and tea. Caffeinated teas seem to have the same benefits as caffeinated coffee, but high-quality studies that compare the two have not been done, Marcus explained.

The American Heart Association's statement is published in Circulation.