Many people across the world begin their day with a hot cup of coffee. Most people would agree that this is not a dangerous practice and may, in fact, have some benefits. However, a new study has now shown that those who go on to drink large amounts of coffee during the day may be putting their lives at risk.

The study, to be published in the journal Mayo Clinic Proceedings, shows that those who drink more than 28 cups of coffee per week (an average of four cups per day) have a significantly higher mortality risk than those who do not. This result, however, was only found for those under the age of 55.

“Significantly, the results did not demonstrate any association between coffee consumption and all-cause mortality among older men and women,” said Steven Blair, a co-author of the study and a biostatistics and epidemiology professor at the University of South Carolina. “It is also important to note that none of the doses of coffee in either men or women whether younger or older had any significant effects on cardiovascular mortality.”

Researchers looked at more than 43,000 patients in the Aerobics Center Longitudinal Study, which followed people between the ages of 20 and 87 from 1979 to 1998. Those who drank large amounts of coffee were more likely to smoke, and were found to have lower overall cardiorespiratory fitness levels. Both men and women who drank excessive amounts of coffee were found to have a greater than 50% increase in their mortality risks.

The study points to research showing that coffee can inhibit insulin, increase blood pressure, release epinephrine, and increase homocysteine levels as possible reasons for the higher mortality risk. Also suggested is that addiction to coffee may have genetic associations with other risk factors.

“Research also suggests that heavy coffee drinkers may experience additional risk through potential genetic mechanisms or because of confounding through the deleterious effects of other risk factors with which coffee drinking is associated,” said the study’s authors. “Therefore, we hypothesize that the positive association between coffee and mortality may be due to the interaction of age and coffee consumption, combined with a component of genetic coffee addiction.”