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Drinking two or more cups of coffee daily may double the risk of death from heart disease in people with severe hypertension (high blood pressure), according to a new study.

Despite containing caffeine, green tea was not associated with an increased risk of death from heart disease.

“These findings may support the assertion that people with severe high blood pressure should avoid drinking excessive coffee,” Hiroyasu Iso, MD, PhD, MPH, director of the Institute for Global Health Policy Research, Bureau of International Health Cooperation, National Centre for Global Health and Medicine, Tokyo, Japan and professor of public health, Osaka University said in a press release. “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.”

The study, “Coffee and green tea consumption and cardiovascular disease mortality among people with and without hypertension,” was published in Journal of the American Heart Association.

Hypertension occurs when the force of blood pushing against the walls of the arteries is persistently higher than normal. In the United States, it is diagnosed when a person’s blood pressure is at least 130/80 mmHg over several weeks. But in the United Kingdom and Nigeria, it is diagnosed when blood pressure is at least 140/90 mmHg.

Coffee consumption reduces the risk of hypertension and death in people without hypertension but temporarily increases blood pressure in people with diagnosed hypertension, increasing their risk of heart disease. On the other hand, green tea is caffeinated, but it lowers blood pressure in people with prehypertension (130-139 mmHg/85-89 mmHg) and stage 1 hypertension (140-159 mmHg/90-99 mmHg). It also reduces death risk from all causes, including heart disease.

However, whether the protective effects of coffee and green tea apply to all individuals with different stages of hypertension is unclear.

Researchers in Japan enrolled 18609 adults [6574 men and 12035 women] aged 40-79 years. The participants were part of the Japan Collaborative Cohort Study for Evaluation of Cancer Risk, a large, nationwide, community-based prospective study established between 1988 and 1990. All participants provided demographic and medical data and the quantity of coffee and green tea they drank daily. After measuring their blood pressure, participants were grouped into one of four blood pressure categories: Optimal and normal (<130/85 mmHg), high normal (130-139 mmHg/85-89 mmHg), grade 1 hypertension (140-159/90-99 mmHg) and grade 2 or 3 hypertension (≥160/100 mmHg). Grade 2 or 3 hypertension was classified as severe hypertension.

Coffee was not linked to harmful effects in all blood pressure categories

Participants were monitored for a median of 19 years, during which 842 died from heart disease.

Frequent coffee drinkers were more likely to be younger, current smokers, current drinkers, eat fewer vegetables and have higher total cholesterol and lower systolic blood pressure. In contrast, frequent green tea drinkers were more likely to be older and less likely to be unemployed, current smokers or eat fruits.

Compared to non-coffee drinkers, participants with severe hypertension drinking at least two cups of coffee daily had double the risk of death from heart disease. However, frequent coffee drinking was not associated with an increased risk of death in those with normal, high-normal or grade 1 hypertension. There was also no association between green tea consumption and death from heart disease across all blood pressure categories.

“To the best of our knowledge, this is the first study to find a positive association between heavy coffee consumption and people with severe hypertension,” the researchers wrote.

Although caffeinated coffee contains beneficial substances that lower cholesterol and reduce inflammation in women with diabetes, its “harmful effects [raising blood pressure] may outweigh its protective effects and increase the risk of mortality in people with severe hypertension,” the researchers explained.

In contrast, green tea contains epigallocatechin3-gallate – an antioxidant compound that reduces oxidative stress stops inflammation and improves blood fats. “These beneficial effects of green tea catechins may partially explain why only coffee consumption was associated with an increased risk of mortality in people with severe hypertension despite both green tea and coffee containing caffeine.”

The researchers highlighted some limitations to the study, including that participants self-reported their coffee and green tea consumption, and that blood pressure was only measured at the study’s start.

Although this study supports the current advice to limit caffeine with high blood pressure, “more research is needed to confirm the effects of coffee and green tea consumption among people with hypertension.”

The Summary

This study supports the current advice to limit caffeine intake if you have high blood pressure. It is interesting to discover that green tea is not associated with the harmful effects linked to coffee, even though it contains caffeine. Since more research is needed, don’t start drinking green tea without consulting your doctor, especially if you have severe high blood pressure.