DBI_C was used for the first time to assess the correlations between diet quality and ECC among 2- to 5-year-old Chinese children. The indicators of DBI_C all involve food groups, but no nutrients, and thus avoid the tedious calculation needed by the nutrient-based diet quality assessment methods. Thus, DBI_C can be used to easily and rapidly analyze the diet quality of individuals and populations. Moreover, these indicators are expressed as two-way scores and can better intuitively reflect the problem and degree of dietary imbalance. Besides, taking the characteristics of Chinese preschool children into consideration, DBI_C is a better choice for assessing their diet quality.
In the present study, the possibility of the 2- to 5-year-old children having ECC or SECC is positively correlated with HBS and DQD, but not correlated with LBS. The analysis of the relationships between the DBI_C indicators (grains, vegetables, fruits, dairy, beans, animal foods, cooking oil, salt, drinking water, and food diversity) and ECC showed that the severity of caries among the 2- to 5-year-old children is positively correlated with the score of Grains and negatively with the score of Food diversity. Besides, ECC is not correlated with the scores of Vegetables, Fruits, Dairy, Beans, Animal foods, Cooking oil, Salt, or Drinking water.
The severity of caries among the studied children is aggravated with age, which is consistent with the previous study . This may be resulted from the bad dietary/snack habits and the chronic contact between teeth and caries-inducing foods during the growing period of children .
All the studied subjects suffer from low-grade diet imbalance, and suffer both insufficient diet intake and excessive intake. Nevertheless, the average scores of HBS, LBS and DQD in the caries-free group are all lower compared with the ECC group and the SECC group. These results indicate the caries-free children have higher diet quality and are more likely to follow recommendations on healthy diet. These results are consistent with other studies [18, 20] that the adherence to general health eating guidelines is significantly correlated to the lower possibility of SECC in children based on HEI-2005 scores. Owing to the rapid sociometric development, the nutrition statuses of Chinese children are improved [28, 29], but dietary imbalance is still common among Chinese children . Our study shows the average intake of grains and salt by 2- to 5-year-old Chinese children are higher than the recommended intake according to A Balanced Diet Pagoda for Chinese Preschool Children, but the average intake of vegetables, fruits, dairy, beans, drinking water, and food diversity are all lower than the recommended intake. Moreover, the probability of developing ECC is positively correlated with the degree of dietary imbalance. Dietary imbalance will lead to malnutrition with varying degrees, which in turn will result in the hypofunction of salivary glands, and the variation in saliva composition and the decline in buffering ability of saliva, increasing the risk of ECC [31, 32]. The diet quality of preschool children will affect their diet quality both at the school age and adulthood , and is associated with the incidence of chronic diseases and even death at the adulthood and old age [34, 35]. Thus, the dietary imbalance problems of preschool children deserves great attention globally. Necessary intervention measures can be adopted, such as to strengthen the guardians' health consciousness  and to conduct popular science and propaganda about nutritional health of young children.
The average score of Grains in the ECC group and the SECC group are both significantly higher than that of the caries-free group, and the severity of caries is positively correlated with the score of Grains. This result indicates that children with more grains intake will suffer more-severe caries. However, Nunn et al. found the scores of whole grains by caries-free children were significantly higher compared with ECC patients, and the increase in the intake of whole grains was associated with a decrease in the risk of ECC . In this study, DBI_C does not differentiate refined grains and whole grains. According to Scientific Research Report on Dietary Guidelines for Chinese Residents (2021) (abridged version) , the grains consumed by Chinese residents are dominated by refined rice and flour, but the intake of whole grains is deficient, which indirectly indicate that the 2- to 5-year-old Chinese children consume more refined grains than whole grains. Owing to the loss of the external bran layers and the mashing of endosperm during processing, the contents of dietary fibers, vitamins, minerals, essential fatty acids and plant chemical substances in refined grains are largely decreased , with the main component of refined grains as starch. Sarah Hancock et al. found that after exposure to starch- and sugar-containing processed foods, the pH of dental plaques was maintained below the critical level of 5.5 for longer time compared with the foods only containing high sucrose , indicating that the intake of refined grains, especially the sugary refined grains, is associated with the occurrence of caries.
In the present study, the score of Vegetables are significantly different among the three groups, but logistic regression analysis shows the score of Vegetables is not an independent influence factor of ECC or SECC, suggesting there may be other confounders. This result is opposite to two other studies [20, 40] that the intake of vegetables is significantly related to a lower ECC risk. Mastication of vegetables, which are featured with fiber characteristics and self-cleaning ability, can stimulate saliva flow and enhance the acid neutralizing ability of saliva, which help with the cleaning of oral fermentable carbohydrates [41, 42]. However, the structures, physicochemical properties and nutrition effect of dietary fibers are easily affected by the processing methods . Some parents think children's chewing ability is weak because of young ages, and thus cut vegetables into fine pieces and boil them very soft, thereby altering the functions and nutrition of fibers and decreasing the anti-caries capacity of fibers. This may explain that the vegetable consumption in the present study is not significantly related to the severity of caries.
Moreover, the average score of Fruits among children with ECC or SECC is higher than that of caries-free children, but with no significant differences among the three groups, which is inconsistent with the result of a previous study that the intake of fruits is negatively correlated with the severity of caries . However, some fruits such as oranges are rich in organic acids (e.g. citric acid, malic acid, oxalic acid, tartaric acid) that can decrease the oral cavity pH, and excessive intake of such fruits may induce dental erosion and demineralization . The seasonal differences in the intake of fruits also affect the results.
Compared with the other two groups, the SECC group shows the highest intake of dairy, but without significant difference. Various components of dairy products are considered to be able to prevent caries, including the minerals (e.g. Ca, P), proteins (e.g. casein, lactoal-bumin) and lipids (e.g. essential fatty acids, nonessential fatty acids) in milk [45, 46], and casein phosphopeptides in yogurt and cheese [47, 48]. In vivo and in vitro demineralization / remineralization experiments also prove the low cariogenicity ability and potential caries-protective role of dairy . However, to improve the taste and flavor of products, most commercial milk and milk producers add sucrose and seasoners, which complicate the research on the caries-inducing ability of dairy products. Moreover, yogurt and cheese both are sticky foods; when children eat too much such foods and do not clean teeth in time, these foods become the substrates of bacteria and may induce ECC.
Children in the caries-free group has the highest score of Food diversity compared to children in the ECC and SECC groups. The severity of caries is negatively correlated with food diversity. It is indicated that the degree of caries is lower among children consuming more diverse foods. The majority of micronutrients originate from daily diets, and food diversity is proved to be positively correlated with the intake of dietary micronutrients . Thus, food diversity is a valuable indicator to predict whether the macronutrients or micronutrients taken by children are sufficient or not [50, 51]. However, For children with monophagism, deficiency in vitamin D, iron, calcium and albumin, and protein-energy malnutrition may induce enamel defects, enamel surface roughness and dental plaque accumulation, thereby leading to caries [32, 52–54]. Thus, balanced diet with diverse foods is rather necessary.
Image-based methods provide more feasible and extensive choices for diet assessment and are closer to the daily life of participants. Compared with the solo use of traditional assessment methods, the participants can supplement image details to decrease the food under-reporting, which improve the accuracy of diet assessment [24, 55]. Though many meaningful valuable results are obtained, this study still has some limitations. Firstly, in this cross-sectional study, the results only reflect associations, rather than causality. Secondly, DBI_C is a scale based on food group indicators, and cannot assess the concrete nutrient intake of children and thereby cannot analyze the relationship between nutrients and ECC. Thirdly, the image-based methods is dependent on image data, but China is huge in area and has diverse and complex dietary cultures, which make Chinese diets one of the most complex dietary systems. In the case of some unrecorded food types, it cannot be identified or analyzed.