According to new research published today in the Journal of the American Heart Heart Association, drinking two or more cups of coffee daily could double the risk of dying from heart disease among individuals who have extremely high blood pressure (160/100 mm Hg or higher), but not among people with high blood pressure not considered severe.

However, despite the fact that both beverages contain caffeine, the research indicated that drinking one cup of coffee or one cup of green tea every day did not raise the risk of dying from cardiovascular disease at any blood pressure measurement.

The FDA estimates that a cup of green or black tea has 30 to 50 milligrams of caffeine, but a cup of coffee contains more like 80 to 100 mg.

Previous studies have shown that healthy people who consume one cup of coffee per day significantly reduce their chance of having a heart attack or stroke.

Additionally, several studies have found that drinking coffee regularly may lower the risk of contracting chronic diseases like Type 2 diabetes and some cancers, may aid in appetite control, may decrease the likelihood of depression, or may increase alertness, though it is unclear whether these effects are due to the caffeine in coffee or something else.

On the negative side, drinking too much coffee might increase the risk of hypertension, anxiety, heart palpitations, and insomnia.

“Our study aimed to determine whether the known protective effect of coffee also applies to individuals with different degrees of hypertension; and also examined the effects of green tea in the same population,” adds senior author Hiroyasu Iso. “To the best of our knowledge, this is the first study to find an association between drinking 2 or more cups of coffee daily and cardiovascular disease mortality among people with severe hypertension.”

High blood pressure, also called hypertension, happens when the force of the blood against the walls of the blood vessels is always too high. This makes the heart work harder to pump blood through the body. It is measured in mercury millimeters (mm Hg). The current American Heart Association and American College of Cardiology blood pressure recommendations define hypertension as a blood pressure value of 130/80 mm Hg or greater.

The ACC/AHA guidelines and the criteria for this study are a little bit different when it comes to blood pressure. The ideal and normal blood pressure range is less than 130/85 mm Hg; high normal is between 130 and 139/85 and 89 mm Hg; grade 1 hypertension is between 140 and 159/90 and 99 mm Hg; grade 2 is between 160 and 179/100 and 109 mm Hg; and grade 3 is over 180/110 mm Hg. In this research, blood pressure readings in grades 2 and 3 were regarded as severe hypertension.

The study included over 6,570 men and over 12,000 women, aged 40 to 79 years at the beginning of the research. These participants were selected from the Japan Collaborative Cohort Study for Evaluation of Cancer Risk – a large, ongoing study initiated between 1988 and 1990 that included adults living in 45 Japanese communities. The participants provided data through health exams and self-reported questionnaires on lifestyle, diet, and medical history.

In the nearly 19 years of follow-up (until 2009), 842 deaths were linked to heart disease. Results from the statistical analysis of all participants showed:

• People whose blood pressure was 160/100 mm Hg or greater were twice as likely to die from cardiovascular disease if they drank two or more cups of coffee per day compared to those who did not.
• Drinking one cup of coffee a day was not linked to a higher risk of dying from heart disease in any of the blood pressure groups.
• Across all blood pressure groups, drinking green tea was not linked to an increased risk of cardiovascular disease death.

”These findings may support the assertion that people with severe high blood pressure should avoid drinking excessive coffee,” adds Iso. “Because people with severe hypertension are more susceptible to the effects of caffeine, caffeine’s harmful effects may outweigh its protective effects and may increase the risk of death.”

Regardless of blood pressure group, the study found that those who drank coffee more often were more likely to be younger, current drinkers and smokers, consume less vegetables, had higher overall cholesterol levels, and have lower systolic blood pressure (top number).

The presence of polyphenols, which are micronutrients with advantageous antioxidant and anti-inflammatory effects found in plants, may help to explain the advantages of green tea. Even though both green tea and coffee have caffeine, polyphenols may be part of the reason why only coffee consumption was linked to a higher risk of death in people with severe high blood pressure.

There are several limitations to the research: self-reported coffee and tea consumption, blood pressure measured at only one point in time, and the observational nature of the study, which means that it cannot establish a direct cause-and-effect relationship between coffee consumption and cardiovascular disease risk among individuals with severe high blood pressure.

The researchers stated that more research is needed to better understand the effects of coffee and green tea consumption in people with high blood pressure and to confirm the effects of coffee and green tea consumption in other countries.

Source: 10.1161/JAHA.122.026477

Image Credit: Getty