When you grab a cup of coffee, there's a good chance that you're more concerned about how fast it will wake you up than what else it might be doing to your body. That's totally understandable, of course. Who doesn't love the zing that makes your brain light up and your body move when the precious caffeine rush kicks in? Granted, beyond that, you might experience a number of surprising side effects if you drink coffee every day—some that are good and some that aren't so good. In fact, a new study has found that drinking three or more cups of coffee a day may damage your kidneys.
The study, which was recently published in JAMA Network Open, involved 1,180 adults who were between 18 and 45 years old and had untreated stage 1 hypertension (high blood pressure). While the study lasted roughly 16 years, the participants were each checked out during a follow-up that occurred within a span of seven and a half years. At that time, those behind the study kept an eye out for hyperfiltration (a rate of glomerular filtration that is considered higher than normal), albuminuria (too much albumin in the urine), and hypertension—all of which are common signs of kidney dysfunction and kidney disease.
While taking a look at the results, they first noted that the rs762551 variant of the CYP1A2 gene was present in around half of the participants, which means that their body doesn't metabolize caffeine as quickly as they might otherwise. They also found that those who metabolized caffeine slower were 2.7 times more likely to end up dealing with kidney dysfunction if they drank three cups of coffee or more each day. Specifically, they were 2.5 times more likely to find themselves with hyperfiltration, 2.7 times more likely to develop albuminuria, and 2.8 times more likely to face hypertension.
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"This implicates caffeine, specifically, as the component in coffee that can damage the kidneys," said Dr. Sara Mahdavi, the lead author of the study as well as a researcher with the Department of Community and Family Medicine at the University of Toronto, according to Medical News Today.
"This is certainly an interesting study and highlights how important it is that nutrition be individualized for each person," Melanie Betz MS, RD, CSR, CSG at the Kidney Foundation, tells Eat This, Not That! "We are only beginning to understand the role genetics play in the response to any medication or nutrition intervention."
At the same time, Betz points out, "It is important to remember that this study found a higher risk of kidney dysfunction in people who drank more than 3 cups of coffee per day. That is quite a bit!"
"The results of this one study do not tell me that everyone should be avoiding coffee altogether," Betz says. "But, perhaps more than 1 to 2 cups of coffee isn't ideal. Also, most other studies have found a reduced risk of kidney disease in people who drink coffee. It is important to interpret all findings together."
Beyond that, she points out that "the study did not adjust for other dietary factors we know impact kidney health." Because of this, she explains that "it is quite possible that people who drink more coffee tend to eat fewer fruits and vegetables and more animal protein, all things we consistently see have a negative impact on kidney health."
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Does this mean that you're safe from the potentially harmful effects coffee might have on your kidneys if you drink less than 3 cups each day?
"There has been a 'U-shaped' curve to caffeine consumption," Jen Hernandez RDN, CSR, LDN, renal dietitian and CEO of Plant-Powered Kidneys Inc. explains to Eat This, Not That! "Those that consume about 1 cup or less per day have seen benefits including blood pressure protection, whereas those consuming 3+ cups per day have also seen benefits." So, at the end of the day, it really does depend on the individual.
"Coffee contains antioxidants and micronutrients like potassium, which can be helpful to protecting kidney function as well as maintaining good blood pressure," Hernandez says while noting that "a 2020 review found coffee to be beneficial in kidney protection."
That surely means that it's best to consume coffee in moderation if you want to safely enjoy the benefits it has to offer. However, if you're still concerned and would rather be safe than sorry, Dr. Mahdavi has a suggestion, saying, "[D]ecaffeinated coffee is virtually devoid of caffeine, those who consume decaf would not have a higher risk of kidney dysfunction, regardless of their genetics."
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