This study clarified the prevalence of MIH among children aged 7–9 years and showed regional differences in MIH prevalence throughout Japan.
The overall prevalence of MIH in this study was 19.8%, which is similar to the rates reported in Sweden (18.4%) [
12], Iraq (18.6%) [
13], Finland (19.3%) [
14], and Brazil (19.9%) [
15]. This similarity in MIH prevalence suggests that MIH is a common clinical finding among children worldwide. The loss of enamel from permanent teeth in children is a frequent occurrence. MIH is associated with many dental problems, such as hypersensitivity, esthetics, and rapid dental caries progression [
16]. Therefore, clinical management of MIH is essential in the field of pediatric dentistry.
MIH was more prevalent among participants in southwestern areas (Kinki, Chugoku, Shikoku, and Kyushu) than those in northeastern areas (Hokkaido and Tohoku) of Japan. These regional differences could generate hypotheses for the analysis of the etiology of MIH in Japan. Many potential causes of MIH have been proposed, such as prematurity, viral or bacterial infections, respiratory diseases, asthma, and frequent episodes of fever in early childhood or mothers who experienced problems during pregnancy [
7,
17]. However, these possible causes do not explain the regional differences in Japan. One potential explanation may be serum 25-hydroxyvitamin D concentrations in children or their mothers. Osteoporosis also occurs more frequently in southwestern areas than northeastern areas of Japan [
18]. Patients with osteoporosis have low serum 25-hydroxyvitamin D concentrations [
19]. Increases in serum 25-hydroxyvitamin D concentrations are significantly associated with a lower odds ratio of having MIH. Furthermore, higher serum 25-hydroxyvitamin D levels are related to a lower number of caries-affected permanent teeth [
20]. Endogenous vitamin D3 is synthesized in the skin through exposure to ultraviolet B radiation from sunlight, and exogenous vitamin D3 is obtained from dietary sources including fatty fish, fish liver oil, egg yolks, and mushrooms [
21,
22]. Vitamin D obtained from diet or ultraviolet B radiation undergoes hydroxylation in the liver, producing 25-hydroxyvitamin D, which is the active form of vitamin D [
23]. Thus, serum 25-hydroxyvitamin D concentrations may vary according to the differences in eating habits, daylight hours, and outside time by area [
24]. Japan is a very long country from the northeast to southwest direction. Since the daylight hours are longer and ultraviolet B radiation is greater in the southwestern part than the northeastern part of Japan, the prevalence of MIH may depend on the factors other than endogenous vitamin D3. Yaegashi et al. suggest that vitamin D intake, mainly from fish and fish products, is much higher in eastern areas of Hokkaido and Tohoku than in western areas of Japan. Thus, vitamin D intake itself, more than day length, might explain the lower incidence of hip fracture by osteoporosis [
25]. Nutrient intake from fish might explain the regional differences we observed in the prevalence of MIH in Japan.
In this study, the prevalence of MIH decreased with increasing age. Hypomineralized teeth are prone to dental caries over time [
26]. Although enamel hypomineralization would be masked by dental caries, the possibility of annual increases in MIH prevalence rates cannot be denied.
Our study has limitations. First, this study is not true random sampling. MIH has a difficult differential diagnosis from early caries, enamel hypoplasia after trauma, or amelogenesis imperfecta. Therefore, a dedicated specialist would be required to examine over 370 local clinics throughout Japan. Second, we did not evaluate socioeconomic conditions. However, almost all prefectures in Japan guarantee medical expenses for children. Since poverty rates, working poor rates, and child poverty rates in Kansai (and further west) and Tohoku (and further north) tend to be high [
27], random sampling from each area (community-based design) would be ideal for the study. However, such a study is not feasible when considering the necessity of cooperation from local communities/schools throughout Japan. Therefore, we selected a clinic-/hospital-based study design. True MIH may be slightly lower than this study, but MIH is found by the dentist of the clinic. It is thought that this study reflects the actual situation.