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What's in urine? 3,000 chemicals and counting

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Looking for an encyclopedia of pee? Scientists have laid out the entire chemical composition of human urine, revealing that more than 3,000 compounds are found in the fluid, and have published it all in an online database.

In the study, which took seven years to complete, the researchers found that at least 3,079 compounds can be detected in urine. Seventy-two of these compounds are made by bacteria, while 1,453 come from the body itself. Another 2,282 come from diet, drugs, cosmetics or environmental exposure (some compounds belong to more than one group).

"Urine is an incredibly complex biofluid. We had no idea there could be so many different compounds going into our toilets," said study researcher David Wishart, professor of biology and computing science at the University of Alberta. [Pee Rainbow: From Red to Indigo, What Urine Colors Mean]

The complete list of all metabolites that can be detected in human urine using current technologies has been placed into an online public database called the Urine Metabolome Database.The word metabolome refers to the complete collection of metabolites, which are the products of metabolism and include hormones, vitamins and other molecules.

A favorite among fluids

"Urine has long been a 'favored' biofluid among metabolomics researchers," because it is sterile and can be obtained easily in large volumes, the scientists wrote in their study published Wednesday (Sept. 4) in the journal PLOS ONE.

However, the chemical complexity of urine has made it a difficult substance to fully understand, the researchers said. As abiological waste material, urine typically contains metabolic breakdown products from a wide range of foods, drinks, drugs, environmental contaminants, waste metabolites of the body and bacterial by-products.

Compared to other body fluids such as saliva or cerebrospinal fluid, urine contains about five to 10 times more compounds, and shows a larger chemical diversity, the researchers found. The compounds found in human urine fall into 230 different chemical classes.

"Given that there are only 356 chemical classes in the entire human metabolome, this certainly demonstrates the enormous chemical diversity found in urine," the researchers said.

The researchers also found that more than 480 compounds in urine were not previously reported to be in blood, contrary to the long-standing idea that the collection of chemicals in urine is a subset of compounds found in the blood.

Why so many chemicals?

"The fact that so many compounds seem to be unique to urine likely has to do with the fact that the kidneys do an extraordinary job of concentrating certain metabolites from the blood," the researchers said.

To find the chemicals in urine, the researchers used a variety of techniques, including nuclear magnetic resonance spectroscopy, gas chromatography, mass spectrometry and liquid chromatography. They analyzed urine samples from 22 healthy people, and scoured more than 100 years of scientific literature about human urine to supplement their findings.

The chemical composition of urine  may be of interest to physicians, nutritionists and environmental scientists because it reveals medical conditions, as well as information about what a person has consumed, and what chemicals she or he has been exposed to in the environment.

The database of urine chemical composition will continue to grow as new techniques and instruments are developed to identify additional compounds, the scientists said.

"This is certainly not the final word on the chemical composition of urine," Wishart said.

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