To our knowledge, this study provides the first evidence on the relationship between country-level prevalence of ECC, malnutrition, and anemia. However, not all types of malnutrition were associated with ECC. We demonstrated a positive and significant relationship between country-level prevalence of overweight and ECC in children 0-2-years of age, with higher prevalence of overweight associated with higher ECC prevalence. There was also an inverse relationship between country-level prevalence of anemia and ECC in 3-5-years olds with lower prevalence of anemia in countries with higher prevalence of ECC.
Overweight was associated with ECC only in children ≤ 2-years of age. The relationship between ECC, and childhood growth and development is not entirely clear [5, 31]. The existing studies on the relationship between ECC and nutritional status provide conflicting results – some found no association [32-34], others demonstrated a positive relationship, while some provided inconclusive results [31, 35]. Recent Canadian studies reported that preschool children with severe ECC undergoing dental rehabilitation were more likely to have higher BMI z-scores than caries-free controls [11, 12, 21, 23]. The inconsistent findings of the previous studies may be due to differences in the methods used for nutritional assessments, age range cut-offs, and confounders of dental caries, including differences in definition and severity of ECC .
The higher prevalence of overweight in countries with higher ECC estimates in 0-2-year-olds may reflect the findings by El Tantawi et al.  who reported a higher ECC prevalence in countries with greater economic growth. ECC and overweight/obesity share common risk factors – high frequency and quantity of free sugar consumption , food insecurity [37, 38], low socioeconomic status [39-41], residence in urban slums [42, 43] and rural areas [44, 45]. Growing economies are most likely to be undergoing nutrition transitions from traditional diets to low-quality, processed, high-sugar, high-fat, carbohydrate-dense food and beverages poor in micronutrients [46, 47] that predisposes to overweight and high ECC prevalence. Our results might suggest that having ECC and being overweight have shared risk factors that are related to the macro-economic status of the country. A common risk factor approach [20, 48] may be used to address both ECC and overweight problems; with global action to control these health problems giving priority to countries with greater economic growth.
The few studies that assessed the relationship between ECC and nutritional status frequently included age ranges larger than that for ECC. Four studies conducted amongst preschool children, showed no association between ECC and overweight in children 3 years of age , 2-5-year-olds , and 2-6-year-olds [51, 52]. These findings highlight the need for appropriate age groupings when studying ECC, as the relationship between ECC and nutritional status seems to be modulated by age. However, Davidson et al.  found that severe ECC was associated with obesity in 2-5-year-olds thereby highlighting two additional dimensions to determining the association between ECC and overweight – the severity of ECC and the severity of overweight. Interestingly, they found an association between the two extremes of the phenomena studied. This also implies that enrolling those with milder forms of caries and nutritional status may downplay potential relationships . Therefore, we suggest that future studies on ECC and nutritional status not only ensure that ECC is analyzed by age groups 0-2-year-olds and 3-5-year-olds, but also ECC and malnutrition data should include the extremes of the variables, with emphasis placed on studying severe levels of ECC like using the WHO Significant Caries Index. We exercise caution with the correlation we observed may be an artifact as correlational analysis at the macro-level is usually larger than it will otherwise be for individuals .
Anemia, a complication of malnutrition and other factors that are not malnutrition related [24, 36], was inversely associated with ECC in older preschool children. Anemia may not necessarily be a direct result of ECC, but may be related to increased milk consumption in early childhood . There is evidence suggesting that in developed countries where the intake of milk is high, the risk of anemia is also high [54, 55]. Anemia from high milk intakes results from early weaning of the child, and introduction of other foods with low iron bioavailability. Milk also has a negative effect on non-heme and heme iron absorption . Developed countries do not have malnutrition as a major health crisis , and have lower prevalence of ECC . Recent Canadian studies reported that preschool children with severe ECC undergoing dental rehabilitation were more likely to have iron deficiency anemia than caries-free controls [11, 12, 21, 23]. Future studies are required to explore this reported finding.
The present findings have implications for parents of young children. Feeding practices leading to malnutrition and/or anemia may be associated with greater risk for ECC. Health education programs to improve parental dietary choices and sponsored healthy meals in kindergartens and similar gatherings may reduce the risk of these diseases. In addition, health providers managing malnutrition, anemia and/or ECC are in a position to provide better and more comprehensive care to children by screening for either of the diseases, and referring to for specialized care. Structuring primary health care services to provide integrated dental and pediatric care may be of benefit in addressing the dual burden of ECC and malnutrition. Dentists caring for patients with severe ECC should be aware that the children may have undiagnosed nutritional disease that might warrant further investigation.
One of the strengths of the study was that collated data on malnutrition included those from the Demographic Health Survey with high quality data . However, there is the risk of over-representing children of living mothers since the anthropometric variables used for nutritional status assessment are only available for those who are alive. The sample may therefore have under-represented the presence of malnutrition in infants and preschool children [60, 61]. We controlled for the gross national income per capita, but were also unable to control for all possible confounders as these remain largely unknown due to lack of data. One of these confounders is sex, which although clearly plays a role in nutrition , the clarity of its role as a risk factor for ECC is poor. Controlling for these factors may further attenuate the relationships we established in this study. Our use of the z-scores adjusted for both age and sex to determine nutritional status allowed for more meaningful reporting of means . We did not use the body mass index to assess nutritional status because it is meant to be used in children ≥ 2 years of age whereas we focused on 2-year-olds and younger, and 3 to 5-year-olds.
A study limitation was use of the World Health Organization’s criteria for assessing caries by many of the epidemiological surveys . This assessment tool does not include non-cavitated lesions: only 15% of ECC surveys reported non-cavitated and/or cavitated as the caries detection level . The ECC prevalence for many countries may therefore be under-reported. Our study analysis was also limited by the fact that only a minor portion of the global ECC prevalence estimates was based on national surveys making the generalizability of our study finding challenging. ECC is under-studied in many parts of the world and true population estimates are often unknown. Also, although we split the data on ECC into two group - two age groups 0-2 and 3-5-year-olds – it was not possible to split these estimates for wasting, stunting and overweight as the data was not available. Further, our study is cross-sectional and therefore, the direction of the observed relationships cannot be ascertained: whether ECC increases the risk of overweight in 0-2-year-old children, or being overweight increases the risk for caries. However, a carefully designed longitudinal study might be able to answer the questions this study and past studies have raised