For many people, coffee is the magical brew that kickstarts the day, a much-needed pick-me-up in the afternoon, and sometimes even a well-appreciated digestive after dinner. However, how much coffee is too much? A large new study claims to hold the answer.

"What on earth could be more luxurious than a sofa, a book, and a cup of coffee?" So wrote the Victorian writer Anthony Trollope in his 1855 novel The Warden. Whatever it is that draws people to coffee—be it the taste and aroma or effects as a stimulant—it is undeniable that coffee is one of the world's most popular beverages.

In the United States, coffee drinking has even been on the rise. Statistics reports indicate that, in the 2018/2019 fiscal year alone, people in the United States have consumed almost 26.5 million 60-kilogram bags of coffee. According to the same reports, this is significantly more than they consumed during the previous fiscal year. Other statistics show that for 2018, almost half of young adults (aged 18–24) reported drinking coffee, and approximately three-quarters of older adults reported the same.

Many recent studies have suggested that drinking coffee can bring a number of benefits in addition to enhancing focus and productivity. In fact, researchers have argued that coffee can help maintain brain health, increase a person's lifespan, and even slow down prostate cancer. However, as with any food or beverage—even the most nutritious and healthful ones—there is a limit to how much coffee we can consume.

Not only can drinking too much coffee create ill effects in the short-term—some of the symptoms of overcaffeination are headaches, dizziness, and nausea—but consistently having too much of this drink could increase a person's risk of developing cardiovascular disease.

How much is "too much" for the heart? This is the question that scientists at the University of South Australia in Adelaide aimed to answer in their new study, the findings of which now appear in The American Journal of Clinical Nutrition.

Researchers identify 'the tipping point'

The researchers build on previous studies indicating that people with a specific variant of the gene CYP1A2, which plays a key role in caffeine metabolism, metabolize this substance less efficiently. This can put them at an increased risk of developing high blood pressure (hypertension) and cardiovascular disease.

In the new study, the investigators wanted to determine how much coffee would increase the cardiovascular risk of people with and without this genetic variant. To find out, they analyzed the data of 347,077 people aged 37–73, of whom 8,368 had diagnosed cardiovascular disease. The scientists accessed these data through the UK Biobank.

"An estimated 3 billion cups of coffee are enjoyed every day around the world," explains study co-author professor Elina Hyppönen. For this reason, she explains, "[k]nowing the limits of what's good for you and what's not is imperative."

"As with many things," she cautions, "it's all about moderation; overindulge and your health will pay for it."

In their analysis, the scientists looked at how much coffee the participants drank per day, whether or not they had the genetic variant that resulted in slow caffeine metabolism, and how likely they were to develop cardiovascular disease. They found that despite the fact that people without the specific CYP1A2 genetic variant were able to process caffeine four times faster than those with it, this did not appear to significantly affect their cardiovascular risk. However, the amount of coffee they consumed per day did.

In fact, all the people who frequently drank six or more cups of coffee per day—the scientists defined one cup as containing around 75 milligrams of caffeine—had a modest increase in cardiovascular disease risk.

"Most people would agree that if you drink a lot of coffee, you might feel jittery, irritable, or perhaps even [nauseous]—that's because caffeine helps your body work faster and harder, but it is also likely to suggest that you may have reached your limit for the time being," says Hyppönen. "We also know that risk of cardiovascular disease increases with high blood pressure, a known consequence of excess caffeine consumption," she notes.

"In order to maintain a healthy heart and a healthy blood pressure, people must limit their coffees to fewer than six cups a day—based on our data, six was the tipping point where caffeine started to negatively affect cardiovascular risk."

—Elina Hyppönen