Beware of using mouthwash

I know, I know, mouthwash is no food and this is a blog about food. But bear with me and you will see the connection revealed at the end. Although mouth rinsing has been around for thousands of years it was not until the late 1960’s that effective antibacterial compounds started to be used. Since then commercial interest in mouthwashes has been intense.

New products have been developed that claim effectiveness in reducing bacteria and the associated build-up in dental plaque, a cause of gingivitis. They are also supposed to fight bad breath by controlling anaerobic bacteria that produce unpleasant volatile sulphur compounds.

The downside!

All good then? No, not so fast. Not only will mouthwash not live up to claims in expensive commercials and on product labels, but using a mouthwash can actually make your dental and oral health problems worse.

As the intake of oral antibiotics will disrupt the balance of bacteria in the gut, mouthwash will do the same with the important bacterial balance in the mouth. And just like we need our gut microbiome for general health, we need  our oral microbiome to protect against common issues like cavities, gingivitis and bad breath.

Contrary to popular belief, the common claim of killing “99.9% of germs” does not prevent cavity formation. The oral microbiome actually supports the natural teeth remineralisation and indiscriminately killing the constituent bacteria will eliminate a critical part of the repair mechanism.

Saliva, another key component of the remineralisation process, is typically reduced with mouthwash use. Saliva serves to disturb the oral bacteria that can cause decay, while also depositing important minerals like phosphorous and magnesium onto the teeth.

But there is more!

blood pressure1There is actually a connection between blood pressure and the oral microbiome. Exercise is known to reduce blood pressure, a pleasant bonus of the exertion. But the activity of bacteria in our mouths may determine whether we experience this benefit, according to new research.

Sounds far fetched but there is a plausible explanation.

It was already known that blood vessels open up during exercise, as the production of nitric oxide increases the diameter of the blood vessels, increasing blood flow circulation to active muscles.

What has remained a mystery is how blood circulation remains higher after exercise, in turn triggering a blood-pressure lowering response known as post-exercise hypotension. It might be due to the magic of nitrate metabolism and the influence of the oral microbiome as the nitric oxide in the bloodstream is quickly converted to nitrate within 10 seconds.

Enterosalivary circulation of nitrate

Normally we ingest nitrate with the food we eat; green vegetables like spinach and rocket salad are particularly high in nitrate. In the gastro-intestinal system nitrate is released and enter the blood stream. And here comes the magic. Nitrate is excreted from the bloodstream into the oral cavity by the salivary glands.

Some species of bacteria in the mouth can use nitrate and convert it into nitrite. And when nitrite is swallowed, part of this molecule is rapidly absorbed into the circulation and reduced to nitric oxide. The nitric oxide helps to maintain a widening of blood vessels and a sustained lowering of blood pressure.

Thus the researchers asked the trial participants to rinse their mouths immediately after their exercise with either a mouthwash or water. And they showed that the blood pressure-lowering effect of exercise is significantly reduced when rinsing the mouth with an antibacterial mouthwash, rather than water.

Eating your greens

rucolaThis was new knowledge in relation to exercise but the overall relationship between the oral microbiome and nitrate metabolism was already well known. In 1998, three US scientists received the Nobel prize for their discoveries around the role of nitric oxide.

Several existing studies show that, exercise aside, antibacterial mouthwash can actually raise blood pressure under resting conditions, so this study followed up and showed the mouthwash impact on the effects of exercise.

But in more general terms eating your greens and avoiding the use of a mouthwash will keep your blood pressure under better control, with exercise a bonus.

And there you have your food connection!

Additive ruling on nitrate and nitrites

sausages2IARC (International Agency for Research on Cancer) is sometimes very quick to nominate any chemical as at least a probable carcinogen. And so it is with nitrate and nitrite. Pointing to the endogenous nitrogen cycle in humans (that is ingested nitrate recirculated in saliva and converted to nitrite by microorganisms in the mouth and swallowed) IARC concluded that both nitrate and nitrite are probable human carcinogens as they can generate carcinogenic N-nitroso compounds under acidic gastric conditions.

Now let’s make it clear, EFSA in two recently published opinions on nitrate and nitrite partially supported the IARC conclusions on the link between nitrates and nitrites in processed meat and an increased risk of colorectal cancer. But, and this is a big BUT….

Both additives are considered safe to use at current approved levels without any concerns. So why is that?

Nitrate and nitrite cleared as additive at current levels

EFSA, in contrast to IARC, not only looks at the potential for a substance to cause cancer, but also the exposure level necessary. That is practical life conditions. And concluded that existing safe levels for nitrates and nitrites intentionally added to meat and other foods are sufficiently protective for consumers.

Case closed? Well, not so fast. Let’s look at some of the detail.

EFSA’s Panel on Food Additives and Nutrient Sources Added to Food said that there is some evidence in epidemiological studies of a link between dietary nitrite, preformed N-nitroso compounds and gastric cancers and also for the combination of nitrate plus nitrite from processed meat and colorectal cancers. However, they stressed that this included only very limited evidence.

Using refined exposure assessment scenarios, the Panel calculated that exposure to nitrites as a food additive accounts for 17% of total exposure to nitrite and exposure to nitrates as a food additive only accounts for up to 5% of total exposure to nitrates. Other sources making up the balance of exposure include their natural presence in other food products and environmental contamination.

The main contributors to exposure are vegetables and vegetable-based foods, such as starchy roots, leafy vegetables, such as spinach and lettuce, and prepared salads. Nitrates also contaminate water as a result of intensive farming, fertilisers and sewage discharge.

The remaining problem


Although the use of nitrate and nitrite as additives have little influence on overall exposure, there is still a remaining overall concern.

If all sources of dietary nitrate are considered, such as food additive use, natural presence in food and environmental contaminants, the Acceptable Daily Intake may be exceeded for individuals of all age groups with medium to high exposure.

If all sources of dietary nitrite are considered, the Acceptable Daily Intake may be exceeded for infants, toddlers and children with average exposure, and for highly exposed individuals of all age groups.

However, the estimated formation in the body of N-nitroso compounds from nitrites added to food items at the approved level were far below those that could be considered to be of risk to human health.

To further reduce uncertainties, the Panel made several recommendations, including:

  • additional studies to measure the excretion of nitrate into human saliva, its conversion to nitrites, and the resulting methaemoglobin formation (a potential problem in babies);
  • further studies on the levels of N-nitroso compounds formed in different meat products based on known amounts of added nitrites/nitrates;
  • large-scale epidemiological studies on nitrite, nitrate and nitrosamine intake and risk of certain cancer types.

In the meantime an Acceptable Daily Intake of 0.07 mg/kg body weight for nitrite and 3.7 mg/kg body weight for nitrate as food additives would be of no concern as most people would not exceed it through eating food to which the additives had been added and only some children would slightly exceed nitrite additive exposure.

Rucola – good, bad or ugly?

Rucola is the vegetable with the highest nitrate content

Actually all of the above. Let’s look at them one at a time starting from the back, but first a brief description of the plant.

Rucola, arugula or rocket, or to be very specific Eruca sativa in latin, are all different names for one member of the brassica family that is most often consumed as a leafy green vegetable. It is native to the Mediterranean area, but are now grown in many parts of the world. It was very popular during Roman times and considered an aphrodisiac, but was forgotten for a long time. It has lately seen a renaissance and is growing in popularity. It is most often served as an ingredient in mixed green salads, but in Italy it can also be found as the sole topping on a rucola pizza or the sole ingredient of a green salad. It has a rich, peppery taste, and an exceptionally strong flavour for a leafy green vegetable.

And why ugly? Well, it looks very similar to the leaves of the public enemy number one in the garden, the dandelion weed. The leaves are deeply pinnately lobed with four to ten small lateral lobes and a large terminal lobe. But beauty is in the eye of the beholder so others might find it attractive. And there might be more important weeds than dandelion, but why destroy a good story.

And why bad? It is actually a plant that accumulates a lot of nitrate. In a vegetable survey published in 2008 as part of an EFSA opinion, average nitrate levels varied from a low of 20-30 mg/kg in Brussels sprouts and peas to a high of 4,667 mg/kg in rucola. Other leafy green vegetables also had relatively high nitrate levels with lettuce at 2,026 mg/kg and spinach at 1,066 mg/kg, but nowhere near the levels found in rucola.

Nitrate is a naturally occurring compound playing an important role in the nutrition and function of plants as part of the nitrogen cycle. Higher levels of nitrate tend to be found in leaves whereas lower levels occur in seeds or tubers. Thus leafy green vegetables generally have higher nitrate concentrations than other vegetable groups. Human exposure to nitrate comes mainly from vegetables. Now, nitrate as such is relatively non-toxic, so what’s the problem?

The metabolites of nitrate and their reaction products, nitrite, nitric oxide and N-nitroso compounds, have raised concern because of adverse health effects like methaemoglobinaemia in young children (blue baby syndrome) and potential cancer in adults. When we consume nitrate it is easily absorbed and circulated in the blood stream. In humans, about 25% of ingested nitrate is secreted in saliva of which at least 20% is converted to nitrite by the microbial flora we all carry in the mouth. We swallow most of the saliva and that provides about 80% of our nitrite intake. Sounds complicated , but that is the way it is.

Vegetables provide more nitrite than processed meat products (Photo: YoGeek Mami)

Processed meat products are often blamed for high nitrite intake but this is plain wrong. Vegetables are the real culprits in this respect.

When nitrite reaches the acidic conditions of the stomach it changes into nitrous acid that after further changes reacts with amines in other food sources like fish, meat and cheese and form nitrosamines.  These N-nitroso compounds are known carcinogens. A rucola and parmesan cheese salad could be the ultimate source for nitrosamine formation.

And now the good! But to complicate things it is not all bad. Recent research indicates that nitrite have antimicrobial activity and participates in defending against infections. Other nitrate metabolites, e.g. nitric oxide, have important physiological roles in lowering blood pressure. Thus, despite being a major source of nitrate, increased consumption of vegetables is widely recommended because of their generally agreed beneficial effects for health.

But the beneficial side doesn’t end there. Vegetables provide other biologically active substances as well as nutrients like pro-vitamin A, vitamin C, calcium, iron, folate, potassium, magnesium, digestible carbohydrates and non-digestible carbohydrates (fibre), protein. In addition, vegetables lack saturated fat and trans fatty acids and are low in sodium which are all good for health. What more can you ask for?

And looking at the vitamin C content in vegetables it can actually prevent the formation of carcinogenic nitrosamines. Maybe this is the reason why epidemiological studies have not found an association between dietary nitrate intake and cancer. On the contrary we are encouraged to eat more vegetables with the World Health Organization (WHO) recommending that we all eat at least 400 g of fruit and vegetables a day.

An occasional rucola pizza is no problem

So what to do? There is actually an Acceptable Daily Intake (ADI) for nitrate of 3.7 mg/kg bodyweight per day, or 222 mg nitrate per day for a 60 kg adult established by the former European Scientific Committee on Food (SCF) and reconfirmed by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) in 2002. This means that if you stay below this limit on average over your lifetime you should be safe. You can safely eat close to 50 g of rucola in a day before exceeding this limit as long as you don’t consume other food or water with appreciable levels of nitrate during the same day. And an occasional rucola pizza should also be fine. But don’t make it a common habit.

It is also possible to reduce the bacterial flora in the mouth responsible for nitrite formation by regularly using antibacterial mouth wash. But you will miss out on the lowering effect on blood pressure so this might not be a good idea. It is also worth noting that people on antacids to reduce the acidity in the stomach face less risk of nitrosamine formation. But again the acidic condition is quite helpful for normal digestion so don’t try this if you don’t have to because of another health issue.