Kidney Stone Risk: How Much Caffeine Is Too Much?

If you’ve had kidney stones—or worry you might—you’ve likely been told to watch what you eat and drink. But one surprising factor may be the amount of caffeine in your daily cup. New research highlights a clear link between caffeine consumption and increased risk for those already susceptible to kidney stones.

The short answer? If you have a history of kidney stones, limit yourself to less than two 16-ounce cups of coffee per day. That’s the threshold, backed by research, beyond which your urine calcium levels—and thus your kidney stone risk—begin to climb.

What the Study Found

Researchers at the University of Washington, led by Dr. Linda Massey, discovered that people with a history of kidney stones showed a notable increase in urinary calcium after consuming the amount of caffeine found in just two cups of coffee. That’s roughly 16 ounces—the equivalent of two standard coffee servings or 'coffee pots' as described in the study.

The experiment revealed that this level of caffeine intake raised urinary calcium levels, a key component in kidney stone formation. The more calcium in urine, the greater the risk stones will develop.

How the Experiment Was Conducted

Dr. Massey and Dr. Roger Sutton from Columbia University designed a controlled test. They took 39 participants: some with a known history of kidney stones, others with no such history. All were asked to fast for 14 hours, then given water with added caffeine equivalent to that in two cups of coffee.

Urine samples were taken before caffeine intake and again two hours afterward. In both groups—those with and without prior kidney stones—researchers observed increased levels of calcium, sodium, magnesium, and citrate in the urine post-caffeine. This indicates that even people not predisposed to kidney stones experience a rise in urinary calcium after caffeine intake.

Why Caffeine Raises Kidney Stone Risk

Caffeine is known to act as a diuretic and can influence the way your kidneys handle certain minerals. In particular, it can lead to higher urinary calcium excretion. Since calcium is a major building block of most kidney stones (especially calcium oxalate stones), more calcium in the urine can mean a higher chance those minerals will crystallize into stones.

For those already prone to forming stones, this extra calcium output can be especially risky. The study suggests that individuals with a history of kidney stones may be more sensitive to caffeine’s calcium-raising effects than the general population.

How Much Caffeine Is Safe?

The study specifically defines the upper safe limit as the caffeine found in two 16-ounce cups of coffee. That’s approximately 32 ounces total, or two standard 'coffee pots'. Any intake above this level—from coffee or other sources like tea, soda, or energy drinks—may further raise urinary calcium and increase kidney stone risk.

To put it simply: if you've had kidney stones, keep your total daily caffeine from all beverages below the amount in two 16-ounce coffees.

Frequently Asked Questions

How much caffeine is in two 16-ounce cups of coffee?

The article does not specify the exact milligrams of caffeine, but defines the limit as the amount present in two 16-ounce (approx. 473 ml each) cups of coffee. That is considered the threshold for increased urinary calcium and kidney stone risk.

Does caffeine affect everyone the same way regarding kidney stones?

No. While both people with and without a history of kidney stones showed increased urinary calcium after caffeine intake, those with a history of stones are more likely to be negatively affected and are advised to limit intake more strictly.

Are other sources of caffeine also risky?

Yes. The 16-ounce coffee limit refers to total daily caffeine intake, not just coffee. Caffeine from tea, soda, energy drinks, or other sources contributes to the same risk if the combined total exceeds that found in two 16-ounce cups of coffee.

What urine components increased after caffeine intake?

In the study, researchers found increased levels of calcium, sodium, magnesium, and citrate in urine after participants consumed caffeine. Elevated calcium is the primary concern for kidney stone formation.

Should I stop drinking coffee completely if I have had kidney stones?

Not necessarily. The study recommends limiting coffee (and all caffeine sources) to less than the amount found in two 16-ounce cups per day—not eliminating it entirely. Staying below that threshold may help manage your risk.

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