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That morning cup of coffee usually leads to a bathroom run later. You did just ingest liquid, but it’s also because the caffeine in coffee is a diuretic—a substance that stimulates the kidneys to release more urine. We know this effect of caffeine well from our daily routine but how caffeine interacts with our kidneys’ physiology is actually still a scientific mystery.

Urine is mostly leftover water that didn’t get absorbed in the kidneys by the body. As mentioned in an earlier post, one way the body absorbs water is through sodium. After sodium is filtered from the blood into the kidneys’ ducts, it’s moved back into the body through pumps and channels along the ducts’ walls. Water is attracted to sodium so it follows sodium into the body. Changing the amount of sodium reentering the body changes the amount of water absorbed and urine produced.

A recent study in the American Journal of Physiology-Renal Physiology looked at whether the diuretic effect of caffeine occurs because it interacts with the pumps that transport sodium, specifically one called sodium-hydrogen exchanger 3 (NHE-3). Mice missing this sodium transporter in their kidneys urinated the same amount as normal mice when given caffeine, telling the researchers that caffeine’s diuretic effect was not through NHE-3. The researchers also considered other types of sodium transporters and found that blocking another group, the sodium-bicarbonate co-transporters, did make the mice urinate less. The study showed that caffeine’s diuretic effect could be because it blocks a sodium transporter, although not NHE-3 like the researchers first thought. There’s still no definitive answer on why caffeine makes you go, but this research helps narrow down what’s not the cause.

Source: Fenton RA, Poulsen SB, de la Mora Chavez S, Soleimani M, Busslinger M, Dominguez Rieg JA, Rieg T. Caffeine-Induced Diuresis and Natriuresis Is Independent of Renal Tubular NHE3. American Journal of Physiology – Renal Physiology. April 29, 2015.

Maggie Kuo, PhD, is the former Communications and Social Media Coordinator for APS. Catch more of her writing in the Careers Section of Science Magazine.

Reviewed by Timo Rieg, MD