Type 2 diabetes may increase salivary sugars, shifting oral biofilms toward caries

In the study, the researchers examined whether plasma-to-saliva migration of sugars contributes to caries pathogenesis by changing oral bacterial community composition and function. Using metabolomic profiling and microbial sequencing, the team identified increased salivary glucose and fructose associated with hyperglycaemia and linked these changes to a microbial shift consistent with heightened caries risk.

“We developed a novel method for untargeted metabolomic profiling of gland-derived saliva that preserves intact metabolite profiles before modification by the oral microbiome,” senior author Dr Masae Kuboniwa, associate professor in the Department of Preventive Dentistry at the University of Osaka in Japan, said in a press release. “This allowed us to understand the changes in these metabolites between the blood and saliva, and their subsequent changes after exposure to the oral microbiome,” she added.

To distinguish host-derived metabolic signals from microbiome-modified profiles, the team compared gland-derived saliva—reflecting host metabolism with minimal bacterial influence—with whole saliva and plasma samples from participants with and without Type 2 diabetes. The analyses indicated that hyperglycaemia was associated with increased transfer of glucose and fructose from plasma to saliva. Microbial sequencing was then used to evaluate how elevated salivary sugars relate to the composition of supragingival biofilms.

“The increase of these metabolites in saliva fuelled changes in the oral microbiome, enriching cariogenic bacteria such as Streptococcus mutans (S. mutans) and reducing the abundance of health-associated species like Streptococcus sanguinis, shifting oral biofilm metabolism toward glycolysis and carbohydrate degradation,” explained lead author Dr Akito Sakanaka, associate professor in the same department. “This shift in the microbial population increases acid production, which erodes tooth enamel and strongly links diabetes to dental caries,” he noted.

Importantly, the researchers also observed that improved glycaemic control was associated with reduced plasma-to-saliva transfer of sugars—particularly fructose—along with partial reversal of the microbiome imbalance linked with cariogenicity. In a co-culture biofilm model using S. mutans and S. sanguinis, the proportion of S. mutans increased in a nutrient-rich medium containing fructose, suggesting that combined exposure to glucose and fructose may favour S. mutans under competitive conditions.

Overall, the findings support a mechanism in which hyperglycaemia increases salivary sugar availability via blood-to-saliva migration, potentially promoting supragingival biofilm changes associated with caries development and plaque accumulation. The authors suggest that optimising glycaemic control may therefore contribute to reducing caries risk in people with Type 2 diabetes, alongside its established relevance for periodontal health.

The study, titled “Diabetes alters the supragingival microbiome through plasma-to-saliva migration of glucose and fructose”, was published online on 4 December 2025 in Microbiome.