More than three cups of coffee a day may harm kidney function in some people, study finds

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Many people rely on coffee, one of the most widely consumed beverages in the world, to get their daily caffeine fix. The stimulant boosts mental alertness and physical energy.

Drinking caffeinated coffee is also thought to influence kidney health.

While some research has suggested that it protects against chronic kidney disease, other studies have linked it to impaired kidney function.

Now, new findings from the University of Toronto and Italy’s University of Padova suggest the link between coffee and kidney dysfunction depends on your genetics – and how much of it you drink.

Caffeine and the CYP1A2 gene

The CYPIA2 gene is responsible for making an enzyme that metabolizes, or breaks down, caffeine. Variations of this gene effect how quickly we clear caffeine from our body.

Roughly 50 per cent of populations of European ancestry have the gene variant that makes them slow caffeine metabolizers. In South Asians and Black people, the prevalence of slow caffeine metabolizers can be more than 70 per cent.

The latest research, published Jan. 26 in the journal JAMA Network Open, set out to determine if individual differences in caffeine metabolism could explain the conflicting findings of previous studies that investigated coffee intake and kidney health.

About the new study

The study included 1,180 males and females, ages 18 to 45, with untreated stage 1 hypertension who participated in the Hypertension and Ambulatory Recording Venetia Study (HARVEST), a separate study in Italy that began in the 1990s and is still going on.

Stage 1 hypertension, according to guidelines when the study began, was indicated by a blood pressure reading, measured in millimetres of mercury (mm Hg), of 140 to 159 (systolic) over 90 to 99 (diastolic). Current guidelines identify stage 1 hypertension as a systolic blood pressure of 130 to 139 mm Hg or a diastolic pressure between 80 and 89 mm Hg.

During the 7.5-year follow-up period, blood pressure and lifestyle habits – including diet and coffee consumption – were assessed regularly. DNA testing was done to identify CYPIA2 gene variants.

Participants underwent annual blood and urine tests, which included two measures of kidney dysfunction, albuminuria and hyperfiltration.

Albuminuria means that you have too much albumin, a blood protein, in your urine. A healthy kidney prevents albumin from passing into the urine from the blood.

Hyperfiltration is a sign of stressed kidneys; filtration measures the rate the kidneys filter your blood.

The findings

Albuminuria and hyperfiltration were three times higher in heavy coffee drinkers (more than three cups of Italian espresso a day) with the CYPIA2 gene variant that makes them slow caffeine metabolizers, compared with heavy coffee drinkers who were fast caffeine metabolizers.

Heavy coffee drinkers who were slow caffeine metabolizers were also more likely to experience a worsening of blood pressure.

This study was observational and uncovered associations only. (The researchers accounted for other risk factors including age, sex and body mass index.) The findings don’t prove that having the CYPIA2 gene variant for slow caffeine metabolism causes kidney dysfunction.

How much caffeine is too much?

For the study, a total of three cups of Italian espresso was equivalent to 300 milligrams of caffeine. It’s not difficult to consume that amount: Consider that a Starbucks grande (16 ounce) dark roast and a Tim Hortons large (18 ounce) dark roast each have 260 milligrams.

Health Canada recommends adults consume no more than 400 milligrams of caffeine a day. Women who are pregnant or breastfeeding, or planning a pregnancy, are advised to limit daily caffeine intake to 300 milligrams.

According to Dr. Sara Mahdavi, the study’s lead author: “It would be prudent to reduce the current recommendation to less than 200 mg of caffeine a day as the current guidelines are safe only for fast metabolizers.”

What to do?

If you want to know whether you’re a fast or slow caffeine metabolizer, many personal genetic tests include the CYP1A2 gene.

Consider also that grapefruit juice decreases caffeine metabolism by 25 per cent, whereas cruciferous vegetables (such as broccoli, cauliflower and Brussels sprouts) can increase caffeine metabolism by roughly 30 per cent.

To reduce your caffeine intake, trying ordering your coffee half regular, half decaffeinated. You could also replace some of your daily coffee with green or black tea, which have considerably less caffeine (30 to 45 milligrams in eight ounces).

Leslie Beck, a Toronto-based private practice dietitian, is director of food and nutrition at Medcan. Follow her on Twitter @LeslieBeckRD

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