The other approximately 10 percent of halitosis bouts are considered to be extra-oral, arising internally from conditions affecting the nose, sinuses, tonsils, esophagus, or blood. These may include infections, diabetes, stomach disorders, or taking certain medications.
New research finds yet another cause to add to the list: People may inherit a genetic mutation that leads to perpetual bad breath. The study, published in the journal Nature Genetics, is the first to link a genetic defect to halitosis.
Co-author Edwin Winkel of the University of Groningen and the Clinic for Periodontology in Amsterdam told Seeker: “Extra-oral halitosis is always present since the odor comes from the blood via the lungs into the breath, while the oral form is caused by bacteria in the oral cavity."
Certain food components, such as the sulfuric compounds allyl methyl sulfide in garlic and methyl propyl sulfide in onions, contribute to halitosis, he added.
Winkel obtained body fluid samples from the patient and sent them to Radboud University Nijmegen Medical Center for analysis. Albert Tangerman, Ron Wevers, and their colleagues found abnormally high concentrations of 4 sulfur-containing metabolites in the samples.
Two of the four are the volatile compounds dimethyl sulfide and methanethiol, which can originate from food and are produced in large amounts in the intestines. It is little wonder that the patient releasing these metabolites had bad breath.
"The odor of methanethiol is described as 'a distinctive putrid smell' and is also part of the smell of human flatus and of French cheese," co-author Huub Op den Camp of Radboud told Seeker. "The smell of dimethylsulfide resembles that released when cooking cauliflower or cabbage."
Op den Camp specializes in the ecology and physiology of microorganisms. He, along with colleagues, next investigated bacteria that break down methanethiol. They discovered a protein, methanethiol oxidase, in the bacterium Hyphomicrobium that does so. The bacterium feeds on contamination in sewage, including sulfur compounds like methanethiol.
"Since we knew the genome of this bacterium, we were able to identify the gene" that encodes the methanethiol oxidase, Op den Camp told Seeker. "The next step was the exciting discovery that humans have an identical gene in their genome."
Op den Camp, lead author Arjan Pol, and their team achieved this latest feat after Wevers asked various colleagues from around the world if they knew of patients with chronic halitosis and especially if the bad breath seemed to affect multiple members of the same family. This led to the identification of three such families: a family in Germany, a Portuguese family, and a Dutch family.
The scientists took breath, blood, and urine from the patients, and determined that all had mutations in the SELENBP1 gene that encodes selenium-binding protein 1, which — as predicted — was found to be a methanethiol oxidase. Individuals harboring the mutation therefore cannot properly break down methanethiol, leading to the malodorous breath.
"In all patients, one of the mutations was maternally inherited and the other came from the father," Wevers said. "In the end, it will turn out that there will be equal numbers of males and females affected."
The mutation may even be more common than presently believed. The researchers calculate that about 1 in 90,000 people carry the mutation. For those who inherit the halitosis disorder, there is no cure, but they are advised to watch their diets to avoid foods that may further worsen the smell of their breath.
The halitosis perhaps affects nearby non-family members more than the patients themselves, since, as Wevers said: "They are used to having this smell around." It also does not seem to hurt their ability to taste food.
A far more worrisome health issue came to the forefront during the research on bad breath, however.
Prior studies have found that SELENBP1 is a possible biomarker for disease progression in several cancers, such as those affecting the breasts, kidney, and colon. Additionally, methanethiol and dimethylsulfide have been identified in numerous of cancers.
"We think that this is the reason dogs can smell cancer, because both metabolites have been found in several cancer types," Wevers said. "Humans can certainly smell each of these, but the dog's nose is simply much better at that."
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