In this study, a multi—stage cluster sampling method was employed. Fifteen kindergartens in Shijiazhuang, Hebei Province were selected. These kindergartens were numbered and chosen using a simple randomization method based on a computer—generated random sequence. Ultimately, 380 preschool children from the 15 selected kindergartens were included in this study.

Inclusion criteria:

Aged between 3 and 6 years old;

Based on the preliminary results of previous studies, the prevalence of ECC among Chinese preschool children is approximately 74% (20). According to the sample—size calculation formula, to estimate a 10% width of the 95% confidence interval, approximately 295 children are required [n = 4 × 1.96² × P × (1 − P)/L², where n is the number of participants, P is the estimated disease prevalence, and L is the width of the confidence interval, which is 0.10 in this study]. Assuming an 80% response rate, at least 369 children should be initially recruited. In this study, 380 preschool children were finally recruited as the study subjects.

Parents or primary caregivers completed a structured questionnaire covering:

Child basic information: age (months), sex, only child (yes/no), recent morbidity (past 2 weeks).

Feeding history: exclusive breastfeeding (first 6 months; yes/no), duration of breastfeeding (months), bottle feeding (yes/no), night-time feeding during infancy (frequency per night), night-time feeding after tooth eruption (never/occasionally/≥3 nights per week).

Current diet (past 3 months) using a brief food frequency module: frequency of sweet foods (candies, cookies, cakes, chocolates) and sugar-sweetened beverages (sodas/juices/sweetened milk/tea) categorized as never/≤1 time per week/2–4 times per week/≥5 times per week; frequency of fruit and vegetable intake (days per week), dairy intake (servings per day), and protein-rich foods (meat/eggs/beans; days per week).

Oral hygiene: toothbrushing frequency (per day), bedtime toothbrushing (yes/no), use of fluoride toothpaste (yes/no), initiation age of toothbrushing (years), dental visits in the past 12 months (yes/no).

Micronutrient supplementation: vitamin D and calcium (ever use: yes/no), current use (yes/no), cumulative duration (<6 months/6–<12 months/12–<24 months/≥24 months), typical daily dose if known, and source (drops/tablets/formula).

Parental information: parental age, education level, and self-rated oral health knowledge; parental oral-health knowledge score derived from 10 items (correct = 1, incorrect/unknown = 0), summed to a total score (0–10); a score ≥7 was considered “qualified” knowledge.

The full questionnaire (English translation) has been provided as Supplementary Material (Supplementary File S1), including item wording, response options, and coding rules.

Two trained dentists performed visual–tactile examinations in the kindergarten setting following the WHO Oral Health Surveys: Basic Methods (5th edition). Examinations were conducted under adequate lighting using disposable plane mouth mirrors and WHO CPI probes; cotton rolls/gauze were used to remove debris and gently dry tooth surfaces when needed. Caries experience was recorded at the tooth level using the dmft index, where dmft = d + m + f: d = number of carious teeth with cavitated lesions into dentine (including teeth with restorations accompanied by secondary caries), m = number of teeth missing due to caries (excluding teeth lost due to trauma or orthodontic reasons), and f = number of filled teeth without secondary caries. We did not classify lesions as “active” or “arrested” beyond the WHO diagnostic threshold. White spot lesions (initial, non-cavitated) were not included in dmft and were not used to define ECC status (21).

Age (months), sex, standing height (cm), and body weight (kg) were collected during the same visit. Height was measured to the nearest 0.1 cm using a wall-mounted stadiometer with children barefoot and standing upright; weight was measured to the nearest 0.1 kg using a calibrated digital scale with light clothing. Two measurements were taken for each parameter; if they differed by >0.5 cm (height) or >0.2 kg (weight), a third measurement was obtained, and the two closest values were averaged. Anthropometric Z-scores were computed using WHO growth standards: Among them, HAZ < −2 indicates short stature, −2 ≤ HAZ ≤ 2 indicates normal stature, and HAZ > 2 indicates tall stature; WAZ < −2 indicates low weight, −2 ≤ WAZ ≤ 2 indicates normal weight, and WAZ > 2 indicates overweight; WHZ < −2 indicates underweight, −2 ≤ WHZ ≤ 2 indicates normal weight, and WHZ−>−2 indicates obesity (22).

ECC was defined as dmft ≥ 1. ECC prevalence was calculated as the number of children with ECC divided by the total number examined.

In this study, SPSS 25.0 software was used for statistical analysis. Measurement data were expressed as mean ± standard deviation. The independent—samples t-test was used for comparisons between two groups, and one—way analysis of variance was used for comparisons among multiple groups. Count data were expressed as the number of cases and percentages, and the X²-test was used for comparisons between groups. Multivariate logistic regression analysis was used for the analysis of influencing factors, and Spearman analysis was used for correlation analysis. A P-value < 0.05 was considered statistically significant. Details of variable coding (e.g., sweet-food frequency categories) are provided in Supplementary File S1.

In this research, a total of 380 preschool children were enlisted. The legal guardians of all the recruited children gave their written informed consents, and all the children went through oral inspections. Among them, 157 children who did not have early childhood caries (ECC) were placed in the healthy group, whereas 223 children with ECC were incorporated into the ECC group. The prevalence rate of ECC was 58.68%. Among the children being examined, the aggregate number of decayed teeth was 889, and the average number of decayed teeth was (2.34 ± 2.35). No statistically notable disparities were found in the prevalence of ECC and the average number of decayed teeth between different genders (P > 0.05). Moreover, no statistically significant differences were detected in the prevalence of ECC and the average number of decayed teeth across various age groups (P > 0.05) (Table 1, Figure 1).

In univariate comparisons, children with ECC differed significantly from caries-free peers across feeding, hygiene, and parental factors. The ECC group reported a longer duration of night-time feeding and a higher mean number of night feeds (both P < 0.001), as well as a higher daily frequency of sweet-food consumption over the past year (P = 0.0011). By contrast, the proportions with continuous vitamin D supplementation for ≥2 years (P = 0.0037) and continuous calcium supplementation for ≥2 years (P = 0.0106) were lower in the ECC group. Bedtime toothbrushing, use of fluoride toothpaste, and receipt of an annual oral examination were also less common among children with ECC (all P < 0.001). In addition, parents of caries-free children were more likely to achieve a qualified level of oral-health knowledge (P < 0.001) (Table 2, Figure 2).

A multivariate Logistic regression analysis was conducted with the presence or absence of ECC as the dependent variable and the variables with significant differences in Table 2 as independent variables. The results showed that the average daily frequency of consuming sweet foods was an independent risk factor for the occurrence of dental caries. In contrast, continuous vitamin D supplementation for ≥2 years, brushing teeth before bedtime, using fluoride toothpaste, having annual oral examinations, and having a qualified cognitive level of oral health knowledge were protective factors for preventing dental caries (Table 3).

Among the 380 examined preschool children, 214 (56.32%) had normal growth and development, 48 (12.63%) had low body weight, 65 (17.10%) had short stature, and 53 (13.95%) were underweight (Table 4, Figure 3).

Among the 380 examined children, 49 children with dental caries had short stature, accounting for 75.38% of all short—statured children; 39 children with dental caries had low body weight, accounting for 81.25% of all low—weight children; and 41 children with dental caries were underweight, accounting for 77.36% of all underweight children. The proportions of short—statured, low—weight, and underweight children in the dental caries group were higher than those in the healthy group (Table 5, Figure 4).

The results of Spearman's correlation analysis showed that there was a negative correlation between the number of dental caries and the Weight—for—Height Z-score (WHZ) in preschool children (r < 0, P < 0.05) (Table 6).