Coffee and the risk of kidney stones: What do studies say?

Coffee and the risk of kidney stones: What do studies say, and should we drink it to prevent the disease?

Kidney stones form when some substances in urine reach a high enough concentration to crystallize. In studies on diet and the risk of urolithiasis, coffee is one of the beverages of interest. Some results show that coffee drinkers have a lower risk of kidney stones, but that does not mean coffee is a preventive medication.

To understand these results correctly, we need to distinguish between the observed association in the population and evidence of a causal relationship, and also consider the role of fluid intake and caffeine.

How do kidney stones form?

Urine contains many dissolved substances such as calcium, oxalate, phosphate, and uric acid. When the concentration of some substances exceeds the ability to keep them in a dissolved state, crystals can form and develop into stones.

The risk is influenced by many factors: daily urine volume, urine composition, diet, family history, metabolic disorders, and some health conditions.

What do coffee studies record?

A meta-analysis published in International Urology and Nephrology in 2022 pooled 7 studies, including 772,290 participants and 9,707 cases of kidney stones. The group with the highest caffeine intake had a lower risk of kidney stones than the group with the lowest intake; the relative risk was reported to be 0.68, with a 95% confidence interval from 0.61 to 0.75.

This is an interesting result, but we need to understand it correctly: this meta-analysis mainly pooled observational data. It shows a statistical association, but does not prove that increasing caffeine will directly prevent kidney stones in individuals.

A Mendelian randomization study published in American Journal of Kidney Diseases in 2022 used genetic data from the UK Biobank and FinnGen. The results showed that coffee and caffeine consumption predicted by genetics were associated with a lower risk of kidney stones. This design helps to evaluate the causal relationship better than usual observational studies, but still has its own assumptions and limitations, such as the impact of genetic variants through other pathways beyond coffee drinking habits.

Why can coffee be related to a lower risk?

An important hypothesis is the amount of fluid intake. When drinking a fluid-containing beverage, the total fluid intake may increase and contribute to increasing urine volume. Dilute urine may reduce the degree of supersaturation of some stone-forming substances.

Caffeine also has a mild diuretic effect in some conditions. However, we should not infer that this effect means severe dehydration or increased stone risk in all coffee drinkers. The actual effect depends on the amount of caffeine consumed, drinking habits, and individual tolerance.

It is currently not possible to determine accurately the proportion of the effect of water, caffeine, or other components in coffee on the observed association.

Can coffee replace filtered water?

We should not consider coffee as the sole means of hydration. Although coffee contributes to total fluid intake, filtered water is still a suitable choice to maintain a daily hydration habit.

For individuals with kidney stone risk, the important goal is to create enough urine volume to prevent crystallization. Specific guidance should be based on the type of stone, underlying disease, climate, level of physical activity, and healthcare professional assessment.

Cases requiring caution

People sensitive to caffeine: may experience anxiety, restlessness, or insomnia when consuming large amounts.
Individuals with uncontrolled hypertension: should discuss caffeine intake with their doctor.
People with kidney disease or fluid restriction: should not increase fluid or coffee intake without medical guidance.
Individuals who have had kidney stones: need to determine the stone composition and risk factors to adjust their diet, rather than applying a general recommendation.

What can be concluded from current evidence

Existing studies suggest that coffee or caffeine consumption may be associated with a lower risk of kidney stones. However, there is insufficient evidence to recommend that non-coffee drinkers start drinking coffee for disease prevention, or to advise individuals with a history of stones to increase their caffeine intake.

For kidney health, controlling fluid intake, reducing sodium when necessary, maintaining an appropriate diet, and managing metabolic risk factors remain more important than relying on any single beverage.

Coffee can be part of the diet of healthy adults, but it is not an established treatment or preventive measure for kidney stones.

References

Zhao, J., Huang, Y. & Yu, X. (2022). Caffeine intake and the risk of incident kidney stones: a systematic review and meta-analysis. International Urology and Nephrology, 54, 2457–2466. https://doi.org/10.1007/s11255-022-03295-1
Yuan, S. & Larsson, S. C. (2022). Coffee and Caffeine Consumption and Risk of Kidney Stones: A Mendelian Randomization Study. American Journal of Kidney Diseases, 79(1), 9–14.e1. https://doi.org/10.1053/j.ajkd.2021.04.018
National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Kidney Stones.