Supplementary Material for: Low AMY1 gene copy numbers in saliva are associated with reduced dental caries experience in obese adults

Abstract:
Introduction
Evidence suggests that a low copy number (CN) (2–3) of the α-amylase 1 gene (AMY1) may reduce the risk of dental caries, although findings remain inconsistent. Variations observed between studies could potentially be explained by the modulation of third factors, such as obesity, which may amplify the cariogenic potential of high AMY1 CN. This cross-sectional study explored the relationship between AMY1 gene CN variation (CNV) and dental caries experience, assessed with the Decayed and Filled Surfaces (DFS) index in young adults, and whether this association varied by obesity status.
Methods
A total of 597 participants (52.4% female, aged 26 – 29 years) from the Norwegian Fit Futures 3 study (2021–2022) were included. Salivary AMY1 CN was measured using droplet digital polymerase chain reaction (ddPCR). Obesity was defined as BMI ≥30 kg/m². We analysed AMY1 CN as both continuous and categorical variables (2–3 (17%), 4–6 (53%), and 7–14 (30%) CN) using logistic regression.
Results
Although AMY1 CN alone was not associated with DFS, its interaction with obesity was significant (p = 0.036). Among obese individuals, a one-unit increase in AMY1 CN raised the probability of DFS ≥9 by 4.7 percentage points, with no association observed in non-obese individuals. In obese participants (n = 103), sex-adjusted odds ratios (ORs) for DFS ≥9 were as follows: OR 1.25 (95% CI: 1.01–1.53) per unit increase in AMY1 CN; OR 3.47 (95% CI: 1.23–9.79) for 4–6 vs. 2–3 CN, and OR 4.48 (95% CI: 1.35–14.89) for 7–14 vs. 2–3 CN.
Conclusion
This study found that AMY1 gene CN alone was not directly associated with dental caries in young adults. However, obesity was identified as an effect modifier in the relationship between salivary AMY1 CNV and dental caries experience. Specifically, dental caries experience was higher in obese individuals with above-average AMY1 CN. These findings highlight the complex interplay between genetic, metabolic, and behavioural factors in dental caries development. Further research is needed to confirm these results and to explore the underlying mechanisms of this interaction.