Introduction

Oral health has received increased focus with more attention paid to the oral cavity, as medicine advances continuously. It goes beyond dental aesthetics and disease prevention. Several research confirm the strong and complex links between oral health and systemic diseases, such as the increased risk of endocarditis after dental procedures. Specifically, periodontal disease is linked to diabetes, metabolic syndrome, obesity, eating disorders, liver disease, cardiovascular disease, Alzheimer's disease, rheumatoid arthritis, adverse pregnancy outcomes, and cancer.

As the body's first gateway, oral health often mirrors its overall state and can significantly influence the development and progression of systemic diseases. Thus, understanding this association is crucial for better disease prevention and management and overall health maintenance.

Fig. 1 Periodontal disease associations with systemic diseases and conditions. (Kapila YL. 2021).

Connection Between Oral Health and Common Systemic Diseases

Oral Health and Cardiovascular Diseases (CVDs)

CVDs, the most common non-communicable disease, are the leading cause of death and disability worldwide, with a huge socio-economic impact. Numerous studies have shown that oral diseases, especially periodontitis, are strongly associated with the risk of developing several CVDs, such as ischaemic heart disease, cerebrovascular disease, heart failure, atrial fibrillation, and peripheral arterial disease.

Periodontitis, a chronic inflammatory disease mainly caused by bacteria and the second most common oral disease worldwide, presents with main symptoms of red, swollen, and bleeding gums, formation of periodontal pockets, and loosening or loss of teeth. Its pathogenic bacteria are not only locally present in the oral cavity but can also enter the circulatory system, triggering a systemic inflammatory response. Inflammatory mediators like CRP, IL-1β, IL-6, and TNF-α can be detected in the bloodstream at disease onset, and increase with periodontitis developing, affecting cardiovascular and overall health. Their circulation in the bloodstream promotes the development of atherosclerosis and triggers the blockage of blood vessels, which leads to the development of cardiovascular diseases such as coronary heart disease. Additionally, periodontitis development activates systemic immune pathways, leading to and maintaining a possible non-specific inflammatory state.

Fig. 2 Natural progression of periodontitis in predisposed individuals. (Serena A., et al. 2024).

Oral Health and Diabetes Mellitus (DM)

DM is a common systemic metabolic disease with a two-way relationship with oral health.

On the one hand, hyperglycemia triggers immune and salivary dysfunction that affects periodontal tissues, making diabetic patients more susceptible to oral diseases such as periodontitis due to poor glycaemic control and reduced immunity. Studies have shown that children with type 1 diabetes are more vulnerable to gingivitis and calculus than children without diabetes, even with similar oral hygiene practices and plaque counts. High blood glucose levels provide a favorable environment for oral bacteria to grow and multiply, and they affect wound healing and tissue repair.

On the other hand, oral infections and inflammation can adversely affect diabetes control. Increased inflammatory factors caused by periodontitis can interfere with insulin action, making it difficult to control blood glucose. Some studies have reported that people with periodontitis have a higher prevalence of diabetes than their peers without periodontitis. Conversely, treating inflammation-related oral diseases can lead to significant reductions in blood glucose levels. In addition, oral pain and discomfort can affect the diet and nutrient intake, exacerbating diabetes in patients.

Fig. 3 Potential oral complications of diabetes mellitus. (Borgnakke W.S. 2019).

Oral Health and Respiratory Diseases

The oral cavity is an ecologically balanced system composed of a wide range of microorganisms, and diverse evidence shows that oral microorganisms are strongly associated with respiratory diseases.

On the one hand, there is substance exchange between the oral cavity and the respiratory tract, oral bacteria can enter the lungs via the respiratory tract, and behaviors such as coughing can also cause respiratory mucus to enter the oral cavity. A large number of bacteria in the oral cavity will enter the respiratory tract with breathing and swallowing, triggering serious respiratory diseases such as pneumonia, when suffering from oral diseases such as periodontitis. In addition, a variety of respiratory pathogens can be detected in dental plaque, periodontal pockets, and saliva, suggesting that the diverse environment of the oral microecosystem can provide potential host sites for common respiratory pathogens. When host resistance is low, they may invade the respiratory tract and cause disease, such as SARS-CoV-2, which can be transmitted and incubated in saliva.

On the other hand, some respiratory diseases also affect oral health. For example, patients with chronic obstructive pulmonary disease may suffer from dry mouth due to prolonged dyspnoea and coughing. Dry mouth is one of the risk factors for respiratory diseases and will reduce saliva secretion, making it easier for oral bacteria to grow and increasing the chances of dental caries, periodontitis, and other oral diseases.

Fig. 4 Co-infection with oral microbes strengthens the pathogenicity of respiratory pathogens. (Dong J.J., et al. 2022).

Oral Health and Pregnancy Complications

For pregnant women, oral health is of great importance. Pregnancy is a special physiological state that brings about temporary changes in a woman's body structure, hormone levels, metabolism, and immune system, which are significant in maintaining a stable state for mother and child.

Firstly, hormonal changes during pregnancy will affect oral health, easily triggering oral diseases such as gingivitis. Women are especially prone to gum problems during pregnancy, and according to statistics, the prevalence of maternal periodontitis is high at 61%.

Secondly, poor oral health may increase the risk of pregnancy complications. If a pregnant woman suffers from oral diseases such as periodontitis, bacteria and their toxins in the mouth can enter the bloodstream. These bacteria and toxins may stimulate an inflammatory response in the body, affecting the normal functioning of the placenta and leading to a poor outcome. Studies have shown that pregnant women with severe periodontitis have a significantly higher risk of preterm labor and low birth weight babies than those with healthy oral.

Mechanism of Oral Health Affecting Systemic Diseases

The main mechanisms by which oral health affects systemic diseases are bacterial transmission, immune response, and nutrient metabolism mechanisms.

Firstly, oral bacteria can enter the bloodstream through inflamed tissues and reach all parts of the body. For example, periodontitis bacteria entering the bloodstream can affect the heart, kidneys, etc., increasing the risk of systemic diseases. Secondly, oral inflammation triggers a systemic immune response, releasing inflammatory factors, putting the body in a chronic inflammatory state, and increasing the incidence of cardiovascular disease, diabetes, and other morbidities. Thirdly, oral health affects nutrient intake and metabolism, damaged or missing teeth affect chewing, which in turn affects the body's normal metabolism and physiological functions, increasing the risk of systemic diseases.

Conclusion

In summary, poor oral health increases the risk of various systemic diseases such as cardiovascular diseases, diabetes, respiratory diseases, and pregnancy complications. This association is primarily mediated through mechanisms such as bacterial transmission, immune response, and nutrient metabolism. Bacteria in the oral cavity can enter the bloodstream, and affect the functions of various organs in the body. Meanwhile, systemic diseases also hurt oral health, thus forming a vicious cycle.

Therefore, recognizing the association between oral health and systemic diseases is extremely important for us to take active preventive and therapeutic measures. Future studies and efforts are needed to provide evidence and promote the inclusion of oral health care in overall health.