While the proverb ‘Don’t look a gift horse in the mouth’ is advice about gratitude, it’s also a comment on how teeth have long been an indicator of age and health. For most people, poor oral health means cavities, root canals and crowns. But scientists have shown that its consequences extend far beyond the mouth and contribute to a number of systemic diseases and conditions.

Twenty years ago, U.S. Surgeon General David Satcher declared poor oral health “a silent epidemic”, largely because it was so pervasive and the impacts were so broad. Since then, more than 700 studies have explored the complex ecosystem of bacteria that lives in the mouth, and how unchecked it causes gum, or periodontal disease. That research has also linked chronic inflammation from periodontitis, a severe form of periodontal disease, to serious, life-threatening conditions–including heart disease, diabetes and potential associations with Alzheimer’s disease and cancer.

Scientists have now begun to reveal the molecular mechanisms and immunological feedback loops that underpin these connections, drawing an even clearer line between oral and general health. The implications are broad. Nearly half of adults over 30 have some form of periodontal disease, according to the CDC. To understand the significant risks that accompany this condition, it’s best to start with something rather small: the mouth microbiome.

The jungle in your mouth

Our mouth is the first port of entry as we eat, drink, talk and breathe, making it home to a diverse community of bacteria–the second-largest in the body after the gut. “This community is like a small city,” says Jeanie Suvan, an associate editor at the International Journal of Dental Hygiene, populated by 700-plus species living in concert with each other. Together, these bacteria form tough, sticky biofilms, known as plaque, that adhere to teeth and soft tissues, colonizing an area about the size of your hand.

“These bacteria are familiar to our immune system,” says Purnima Kumar, a periodontist and microbial ecologist at Ohio State University. “Some are acquired as infants, before we even have teeth.” Most are harmless or even helpful, but a dozen or so are pathogenic. Under normal circumstances, those bad bacteria are kept in check by the presence of good ones. However, like any ecosystem, the mouth microbiome requires balance.

Trouble arises when that balance is lost. Suvan says that changes in the oral ecosystem can trigger a biofilm buildup. That might be caused by imperfect care (even in just a few hard-to-reach places), missed dental cleaning sessions, smoking, or even changes in hormone levels or medications.

As biofilms build up, they can grow beneath the gum line. In that anaerobic environment, the bacteria Porphymonas gingivalis—known as the driver of periodontal (or gum) diseases—and other virulent bacteria proliferate, much like an invasive species. They secrete toxins that break down tissue. Soon, with normal tooth brushing, inflamed gums may start to bleed.

“That provokes immune defenses to attack, and that’s where the problems really start, with inflammation,” says Jan Potempa, a biochemist at the University of Louisville School of Dentistry. He likens it to a war, with waves of immune cells, such as neutrophils, macrophages and lymphocytes, attacking bacterial invaders. But the pathogens are difficult to eliminate, so the immune cells secrete signaling molecules called cytokines that recruit ever more soldiers to the fight.

Meanwhile, chemicals produced by the body as part of the immune response cause collateral damage, not unlike the destruction caused by bombing a city. These chemicals ulcerate deep pockets into the gums, creating new real estate for bacteria to colonize.

It becomes what Kumar calls “a freewheeling, self-perpetuating cycle”. Without treatment, this chronic infection progresses into full-blown periodontitis that damages gum tissue and the ligaments that anchor the teeth in their sockets. Bone, too, is destroyed. Teeth loosen, shift or fall out. More adults now lose teeth to periodontitis than to tooth decay, and it is considered among the world’s most prevalent chronic inflammatory diseases, affecting more than 700 million people. It can also be a harbinger of even more serious conditions.

From the mouth to the bod

Once periodontitis takes hold, scientists have learned that pathogenic bacteria can migrate into the bloodstream. In animal studies, DNA from both oral pathogens and live oral bacteria have been discovered in the heart, brain, joints, liver and lungs, in fetal tissue, and even inside cancerous tumors. 

That invasion sparks new, systemic battles. “The body doesn’t want them there and sends an influx of lymphocytes and macrophages,” says Thomas Van Dyke, a professor of oral medicine at Harvard University and vice president of clinical and translational research at the Forsyth Institute. That can create chronic inflammation.

Oral pathogens are drawn to such areas of chronic inflammation, Kumar says, “where the production of cytokines pulls them almost like a beacon of light.” The heart is one of those beacons, specifically arteries that are clogged by plaque: cholesterol and fat. The arrival of new pathogens, Kumar says, “jumpstarts another chain of events...a hyper-amped response and further buildup of plaque.” That can lead to increased risk of heart failure—or not. A decade-long study found that meticulous oral care indeed lowered that same risk.

Other diseases linked to inflammation have shown a connection to oral health, or lack thereof. Forty years of research on type 2 diabetes and gum disease has elucidated a unique two-way relationship. Diabetics are at high risk of periodontitis, at least double that of the general population. Onset is often early and severe, and blood sugar levels become hard to control because the proinflammatory cytokines fighting gum infection can cause insulin resistance.

In pregnant women, studies indicate that gum disease appears to raise the risk of premature birth. Oral bacteria are also a known cause of endocarditis, a potentially fatal heart valve infection and it may worsen rheumatoid arthritis.

Scientists have recently been investigating a connection to Alzheimer’s disease. P. gingivalis and other oral pathogens have been found lodged in the brain, while autopsies of Alzheimer’s patients found them embedded in the amyloid plaques that characterize the disease. The mechanism by which oral pathogens impact brain health, if any, is still unclear. But recent animal studies found that the presence of P. gingivalis appeared to impair neuron function, learning and memory. Much work remains.

Stopping the battle before it begins

Gum disease can burn largely under the radar for years, progressing slowly, but surely. Unlike other infections, it may not be painful–perhaps just a little chewing discomfort. The only real sign may be a little blood in the sink after brushing, which is often ignored. “If you were bleeding from anywhere else, would you consider it normal?” asks Suvan.

While brushing or flossing too hard or biting into a hard pizza crust may cause an occasional injury, bleeding, swollen, red gums are not normal. It is, however, fairly common. Between 60 and 80 percent of adults in the U.S. experience this at some time. “If you’re bleeding—you need to see your dentist before future damage can occur,” says Potempa. Detected early, gingivitis is easy to treat with deep cleaning, and can be prevented with good home care.

Full-blown periodontitis is far more serious. A root planing procedure that goes below the gumline can remove hidden plaque and reduce the depth of the ulcerated pockets so they can be kept clean at home. A periodontist may also be able to regenerate attachment of loose teeth.

Preventing this cascade of infection, Suvan says, is really just a question of consistent, thorough habits: brushing, flossing, rinsing and regular trips to a dental professional for cleanings. Amidst the current Covid-19 pandemic, many people have missed dental appointments despite strict surgical protocols that make visits safe. This places even greater emphasis on vigilant home care.

Maintaining a healthy mouth is important for so many reasons: allowing us to communicate, eat what we want–and smile. But we now know that it’s even more critical. “A healthy mouth is essential to having a healthy body,” Kumar says. 

To learn more about the connections between oral and general health, visit the dedicated page from Crest + Oral-B