The cause of your bad breath isn’t what you think

Around 30 per cent of cases of bad breath are though to be linked to the gut. Here’s why – and what you can do about it

Are you embarrassed or concerned about your breath? Most people who are accused of halitosis, the medical term for smelly breath, assume that their mouth hygiene is to blame.
But while oral conditions such as chronic gum disease, dehydration, poor saliva flow and dental problems are thought to drive an estimated 60-70 per cent of cases of bad breath, the remaining 30 per cent are actually thought to be linked in some way to the gut and wider digestive system.
Prof Martin Warren of the Quadram Institute explains that one of the major reasons for this is because the gastrointestinal (GI) tract is essentially one long tube which begins in the oral cavity, and then progresses through the stomach into the intestines. One of the consequences of this is that microbial activity down there has a pathway back to your mouth.
“Some of the microbes in your mouth also reflect some of the microbes that you find in the intestines as well,” he says. “The fact that you can release air from your gut and your stomach back through your mouth and so on just highlights the fact that what happens here can work its way back into your breath.” 
Let’s take a closer look at how bad breath is linked to gut health, the signs, the root causes and what you can do about it. 
Prof Warren explains diplomatically that most of the gases released in the small intestine tend to be released through “another orifice”. As a result, if so-called volatile compounds – chemicals produced by gut bacteria – are being produced in such quantities that they make their way up to the mouth, it tends to be a reflection of something fairly serious going on.
According to the Centre for Gastrointestinal Health in Australia, one common culprit is a condition known as small intestinal bacterial overgrowth (SIBO), which tends to be experienced by people suffering from irritable bowel syndrome or those with intolerances to lactose or fructose. Common symptoms of SIBO include abdominal pain, cramping, bloating, diarrhoea and stomach ulcers. 
People suffering from SIBO can end up with excessive numbers of so-called sulphur-utilising species of bacteria in their small intestine. When they eat protein, these bacteria then metabolise the amino acids within and generate a whole raft of noxious chemicals such as hydrogen sulphide, dimethyl sulphide and methyl mercaptan, which can work their way back into the breath. “Generally, the sulphur-containing compounds have an odour associated with them like rotten eggs or fish,” says Prof Warren.
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It can get even worse. Some species of bacteria associated with SIBO can digest sugar and protein into foul-smelling chemicals such as butyric acid or amines. “Things like butyric acid have a rather vomity smell associated with them,” says Prof Warren. “And amines are associated with rotting flesh.”
According to Dr Federica Amati, the lead nutritionist at Zoe, bad breath can also sometimes be linked to constipation.
“Because you’re constipated, the gases that are made in your gut don’t have a way to escape,” she says. “This means that some of those volatile compounds will be reabsorbed into your bloodstream, and so for people with severe constipation, you can sometimes report that their breath essentially smells like farts.” 
The digestive issues experienced by people with Crohn’s disease and coeliac disease can facilitate the production of sulphurous compounds which can affect breath, according to the Centre for Gastrointestinal Health. 
People with these conditions struggle to break down all the food they’re eating, and so more undigested food particles pass through into the small intestine for bacteria to metabolise, generating greater quantities of gases such as hydrogen sulphide.
The digestive condition gastroesophageal reflux disease – which is caused by a weakening of the valve which separates the oesophagus from the stomach, allowing stomach acid and food contents to seep back up into the oesophagus – can also cause bad breath due to gases being released from the partially digested food.
In some cases, breath issues can be a consequence of rapidly multiplying H. pylori bacteria – a microbe which normally exists within the gut, but can cause damage to the stomach and intestinal linings when out of balance – leading to the production of sulphurous compounds.
There’s also a microscopic parasitic infection called Giardia lamblia, which results in a diarrhoeal disease called giardiasis. It’s caused by consuming contaminated food or water and has also been associated with the production of chemicals which are released as a foul-smelling belch. 
Most gut infections normally clear within two to three weeks but both H. pylori overgrowth and giardiasis can require antibiotic treatment.
Dr Amati says that while gingivitis, or gum disease – which in itself is driven by a combination of diet, poor oral hygiene and a resulting imbalance of bacteria within the mouth – is a very common cause of bad breath, the connection between the mouth and the gut via the gastrointestinal tract means that inflammation can easily spread from one end to the other. 
“If you think of the whole GI tract as like one big planet, inflammation in one part can have effects throughout the whole passageway,” she says. “It’s a bit like when a volcano erupts in Sicily, it disrupts airways across Europe, so people can’t fly. So for people with gum disease, it has repercussions throughout the body and can drive things like oesophageal cancer but also disruption of the gut microbiome.”
According to Dr Amati, there is growing evidence that this two-way connection can slowly spiral out of control, with inflammation from the oral cavity affecting the balance of different bacterial species in the gut and leading to conditions such as SIBO which can lead to the release of more gaseous chemicals in our breath.
“What’s really clear from different studies is that when you have a dysbiosis or imbalance in one of the regions within the gastrointestinal tract, it affects the entire environment,” she says.
While antibiotics undoubtedly save lives and can play a key role in tackling persistent infections, they themselves can cause huge amounts of disruption to the gut microbiome, particularly broad-spectrum drugs which wipe out vast swaths of bacteria indiscriminately.
As a result, Dr Amati says that it’s best not to take them unless strictly necessary. “You need them for conditions like SIBO, but in some cases it’s a matter of balance when antibiotics are necessary, for example with reducing the inflammation from gum disease, and when they’re actually more likely to just cause more disruption to the overall microbiome.”
But there are definitely lifestyle changes which can help minimise the risk of bad breath. As well as flossing and maintaining regular visits to the dentist to keep tabs on oral hygiene, Dr Amati says that diet can go a long way to tackling many forms of gut dysbiosis.
“Inflammation underpins a lot of this issue, leading to things like leaky gut and the passage of chemicals into the bloodstream which shouldn’t be there,” she says. “And for both the gut and oral microbiomes, very similar advice can help. There are probiotic foods which can reduce inflammation like kefir and sauerkraut, as well as lots of fruits and vegetables, fibre-rich foods that don’t contain added sugars, and not eating too late at night. And then just really making sure you brush your teeth twice a day.”
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