Dental caries is negatively correlated with body mass index among 7-9 years old children in Guangzhou, China

Bagramian et al. have documented that temporal trends showed a global decline in dental caries since the 1970s, yet recent data indicate an increase of the disease in both the primary and permanent dentition [38]. Based on the data from the United States National Health and Nutrition Examination Survey (NHANES), the prevalence of primary dental caries has remained unchanged (49.90 % in 1988–1994 vs. 51.17 % in 1999–2004, P > 0.05) among youths aged 6–11 years [39]. According to the report on The Physical Fitness and Health Research Of Chinese Students in 2005, the prevalence of primary dental caries and mean (SD) of dmft were 60.9 % and 3.56 (4.65) in children aged 5 years in urban areas in Guangdong, China, respectively. Both indices declined compared with the report in 1995 (76.55 % and 4.48, respectively) [3]. Similar to the secular trend, a further reduction for caries prevalence was indicated in our study in comparison to the results in 2005, which presented a 0.5-fold reduction (30.7 % vs. 60.9 %). The prevalence of dental caries in our data was relatively low, and the following reasons might be involved: firstly, we only examined the children in urban areas in Guangzhou. Higher living standard and better educational attainment improved children’s access to dental care and the availability of fluoride-containing products; secondly, China's one child policy has turned many children into the “Little Emperor” of their families. The parents paid more attention to their child’s health status including oral health.

Caries is usually regarded as a consequence of frequent ingestion of fermentable sugars, which can also lead to obesity. As a result, the link between caries and obesity has long been suspected. However, research results into the obesity-caries relationship in children have been mixed and inconclusive. In the data of NHANES III, Kopycka-Kedzierawski et al. suggested that overweight may be associated with decreased rates of caries in children aged 2-18 years [16]. Likewise, in a sample of 1951 in Philippine children aged 11-13 years, Benzian et al. also reported that there was a significant inverse association between caries and BMI and particularly between odontogenic infections and below-normal BMI [14]. A cross-sectional study conducted by Alkarimi et al. in Saudi Arabian children age 6-8 years old suggested an inverse linear association between dental caries and anthropometric outcomes [20]. A recent Chinese study among 744 8-year-old children also reported a weak negative association between caries severity and weight status [40]. In accordance with those findings, we also found that caries-free children were more overweight and obesity. However, in the study performed by Willerhausen et al. the authors found a significant positive association between weight and primary caries after adjusting for age in 1290 pupils aged 6-11 years [8]. Similarly, Marshall et al. reported that the greatest proportion of dental caries was found in the group of obese subjects in a sample of 413 children [6]. Other studies with limited sample size (less than 1,000) have demonstrated no correlation between BMI categories and the prevalence of dental caries [21, 23‐26]. The heterogeneities between studies might be attributed to: 1) disparity of sample size. Most previous study sample sizes were less than 1,000. Nevertheless, our findings agreed with the results from national surveys in US [16]; 2) non-uniformity of the criteria for BMI classification across studies. Some studies used the CDC centiles [6, 9, 13, 23, 26, 40], other studies employed the international age and gender appropriate data sets recommended by the IOTF [5, 10, 12] or WHO [14, 19, 20, 22]. And in the current paper, we applied the thresholds developed by Group of China Obesity Task Force [29] and also the above mentioned three criteria for evaluation, and we found that the results were similar; 3) different indices and definitions of caries. For example, some studies reported dmft, whereas others reported dmfs (decayed, missing, filled, surfaces); 4) variation in the confounders used for adjustment such as sex, race/ethnicity, socioeconomic and nutritional status. Underweight children are usually under-nutrition. Enamel hypoplasia, salivary glandular hypofunction and saliva compositional changes may be mechanisms through which malnutrition is associated with caries as reviewed by Psoter et al. [41]. And in turn, untreated caries might have caused severe pain and discomfort in children and, thus, reduced food intake. In addition, other symptoms induced by caries including infection, irritability, and disturbed sleeping habits can affect children's quality of life and thereby growth [42]. A longitudinal design will be helpful in understanding the course and consequences relationship.

The strengths of this study include a relatively large sample size, wide age range, and inclusion of both sexes and the use of a standardized diagnostic interview administered by trained interviewers. Also, we used restricted cubic spline regression to evaluate the shape of the relationship between BAZ and primary dental caries. Some potential limitations also warrant discussion. First, no causal association can be made as with any cross-sectional study. Second, dental caries detection was carried out visually and no X-rays were taken, thus the prevalence of dental caries and dmft value might be underestimated. Thirdly, other factors including energy-dense foods and drinks, family/parental socioeconomic status, vitamin D have also been observed to exert significant influences on the presence of dental caries and obesity as well [20, 43]. However, we have not evaluated those confounders in our data. Therefore, residual confounding could bias the observed associations. Finally, since the sample referred in urban Guangzhou citizens, more work is needed to evaluate whether these results extrapolate to rural samples and other racial or ethnic groups.