Not all bad breath comes from ketchup-and-onion sundaes. Around 0.5% to 3% of people get bad breath from places outside the mouth, like the sinuses, esophagus, lungs, or blood. These causes aren't fully understood.
An international team of researchers found another potential cause of this "extra-oral" bad breath: genetics. They researched a protein called "SELENBP1" and suspect that mutations in its parent gene might make the body produce stinky bad-breath molecules. It may even play a role in tumour suppression, too.
The researchers studied five unlucky individuals whose breath had a persistent cabbage-like odor, according to the paper published yesterday in Nature Genetics. They ruled out other potential sources of smell, like diet, and then sent the breath through a gas chromatography machine - essentially a high-tech, man-made nose. The smell came from a series of sulphur-containing molecules, like methanethiol and dimethylsulfide.
What did the patients have in common? The scientists noticed each had a mutation in the SELENBP1 gene. They wanted to figure out if this gene was causing the smell, so they knocked it out of some lab mice. All of these mutant mice had way higher levels of these stinky sulphur compounds in their blood plasma — it looked like the mutation in the SELENBP1 gene could have been causing the stink.
These observations reveal that maybe SELENBP1 produces an enzyme responsible for breaking odorous molecules down. "The function of SELENBP1 might possibly be keeping the breath methanethiol concentration low," the scientists write in the paper, "thus enabling the human nose to detect foul smells from environmental volatile sulphur compounds" instead of the human's own foul breath.
There's more than just bad breath at stake here - the scientists speculated further, based on past research on SELENBP1, that it could play some role in suppressing tumours. They point out that dogs can smell some tumours, and perhaps they're smelling the compounds not broken down by the SELENBP1 proteins. Again, just speculation here, but maybe they have found another gene that plays a role in cancer development, too.
This is just one paper, humans aren't mice, and the research certainly isn't tied up with a bow yet, given how complex the human body is. But if you're about to kiss someone whose breath smells like cabbage, maybe ask if their SELENBP1 gene is ok.