More than half of American adults drink coffee daily. But how does that caffeine affect us, biologically? It depends on genetics.

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Is morning just not morning without a steaming mug of coffee in your hand? You’re not alone, as a little more than half of American adults drink coffee daily, for the taste, the aroma, the pick-me-up or all of the above. I suspect that number’s even higher in Seattle. As an added bonus, moderate coffee consumption is linked to a number of health benefits, including reduced risk of developing type 2 diabetes. But for some people, the caffeine in even moderate amounts of coffee could have a downside.

The safe caffeine limit for healthy nonpregnant adults is 400 milligrams (mg) per day, the amount in four average 8-ounce cups of brewed coffee. The average caffeine intake in this country is about 180 mg per day, but those averages are just that — average. Some people get less caffeine than that, and others get more, sometimes much more, because not all coffee is “average.”

For example, a 12-ounce medium roast at Starbucks has about 235 mg. Get a refill and you’ve exceeded your limit. Then there’s the growing trend of super-high-octane coffee, with brands like Black Insomnia and Death Wish competing for the title of “world’s strongest coffee” and clocking in at about 700 mg of caffeine for a 12-ounce cup. While that’s not optimal for anyone, for some people it might be risky — the collision of genetic differences in how we metabolize caffeine with the latest generation of high-caffeine beverages may have unintended consequences.

How caffeine makes you feel is partly attributed to your tolerance level, but it’s largely linked to genetics. Of greatest concern is one gene — CYP1A2 — that alters how caffeine affects us, but in ways we don’t actually feel. Depending on which version of the CYP1A2 gene you inherited, you metabolize (break down) caffeine slow or fast. Let’s look at why that matters.

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For a long time, drinking more than a few cups of coffee was linked to an increased risk of heart attack. But when researchers looked closer several years ago, they found that risk was only increased among people who were slow caffeine metabolizers.

For slow metabolizers, one cup per day wasn’t a problem, but after that, the risk started to rise. Slow metabolizers younger than 50 who drink four cups per day or more had quadruple the risk of having a heart attack.

By contrast, fast metabolizers — especially those younger than 59 — actually saw reduced risk of heart attack with a moderate one to three cups per day.

“Four cups of coffee for one person might be the biological equivalent of one cup for someone else, just depending on how much of that caffeine sticks around in their system,” said Ahmed El-Sohemy, Ph.D., one of the study’s authors, at the 2017 Nutrition & Health conference in Phoenix in May. He said more recent studies have found similar effects for risk of pre-diabetes, hypertension and kidney disease in slow metabolizers.

El-Sohemy said that slow metabolizers don’t self-regulate caffeine because they don’t feel the difference. So what does this mean for you? You could get tested for the CYP1A2 gene, but insurance might not cover it.

Or, you could play it safe and enjoy a moderately sized cup or two and leave it at that.

You could go decaf or “half-caf” — decaffeinated coffee contains the same health-promoting compounds, including phytonutrients, found in regular coffee.

Finally, if you find you rely on coffee to get through the day, maybe it’s time to cultivate a more beneficial source of energy — sleep!