This observational study was the first to assess circulating 25(OH)D in over 50,000 children by HPLC–MS/MS, the current gold standard in vitamin D level assessment. Our data showed that (1) hypovitaminosis D was prevalent (65.60%) among children in Heilongjiang; (2) voluntary and iatrogenic vitamin D supplementation were inversely associated with the rates of pediatric hypovitaminosis D; (3) increased outdoor activity time was also linked with a lower risk of hypovitaminosis D, and (4) supplementation combined with outdoor time over 60 min/d may be a better way in preventing hypovitaminosis D among children in North China.
Latitude has a clear impact on the vitamin D status in children. However, we still lack of data for the vitamin D nutriture of children form different latitudes. Previous hospital-based cross-sectional studies indicated that 23.28% children in Huzhou (southeastern China, 30°2′–31°1′ N) and 33.60% in Hangzhou (southeastern China, 29°1′–30°3′ N) were found with hypovitaminosis D, whereas the prevalence increased to 65.91% in this study, which is consistent with early research focusing on children in high latitudes. Data from Hutterite communities (Canada, 49°2′–54°8′N) suggested that 76.00% of children suffered from hypovitaminosis D, indicating that hypovitaminosis D might be a common and serious problem in children in high latitudes. Skin is a key organ in vitamin D synthesis because 80–90% vitamin D that the human needs per day is produced in the skin from ultraviolet-B-activated 7-dehydrocholesterol. Evidence from Spain (36°0′–43°2′ N) suggested that production of over 1000 IU/d vitamin D in spring and summer depends on regular sunlight exposure, making outdoor activity an optimal way of getting vitamin D. However, our results showed that still 17.75% and 21.63% of the enrolled children whose outdoor time was over 60 min/d had a deficient and insufficient vitamin D status, respectively. Moreover, excessive ultraviolet radiation exposure throughout childhood appears to be particularly harmful. Children with sun protection by their parents are much less exposed to ultraviolet radiation. Therefore, vitamin D supplementation might be taken into account to prevent hypovitaminosis D among children living in Heilongjiang.
Numerous programs or health policies for vitamin D supplementation are available in almost all countries to prevent vitamin D deficiency or hypovitaminosis D in children[29–32]. In the guidelines aforementioned, some risk factors for vitamin D deficiency are recognized, including vegan diet, malabsorption syndromes, and reduced sunlight exposure; meanwhile, regular vitamin D supplementation and routine 25(OH)D testing are recommended for children. However, living at high latitude as an identified risk factor for vitamin D deficiency has not been explicitly mentioned for routine 25(OH)D testing in these guidelines. Despite of high incidence of hypovitaminosis D among children living in high latitudes was found in previous studies, few of them focused on related improvements. Approximately 30% of the total world population lives from 40° N to 60°N. Given the vitamin D status of children living at high latitudes, it is necessary to determine whether they need to be routinely screened for serum 25 (OH) D levels, followed by appropriate vitamin D supplementation as required. In this study, we found that vitamin D supplementation and outdoor time were all negatively associated with children’s vitamin D status, and routine 25(OH)D testing combined with vitamin D supplementation might be an effective approach for the prevention of hypovitaminosis D among children living at high latitudes, which were of great significance of addressing this global public health issue.
The pediatric health care system in China has been improved and consists of a proper examination interval and routine monitor network of serum 25(OH)D levels in children[22, 33]. Prescriptions were given by health professionals according to vitamin D status and the supplementary recommendation in the consensus of the Chinese Society of Osteoporosis and Bone Mineral Research. To the best of our knowledge, this was the first study to describe and evaluate the effect of iatrogenic supplementation on prevention of hypovitaminosis D among children, and iatrogenic supplementation had a 2.21-, and 5.24-fold decreased risk of hypovitaminosis D compared with voluntary supplementation and no supplementation, respectively. Besides, health professionals urge the parents to keep regular vitamin D supplementation. Minkowitz et al. reported that better adherence of guardians with low serum vitamin D levels to physicians’ orders to remind their children of daily intake of vitamin supplements. Although there were some unfavorable factors causing poor response to oral supplementation, pediatricians could improve the oral doses according to the follow-up serum 25(OH)D test results until the doses reached optimum. Noticeably, however, compliance of children with follow-up blood collection varied. No vitamin D supplementation due to guardians’ lack of awareness that might be the main cause, was like to expose their children to the increased risk of hypovitaminosis D and acute and chronic diseases including infectious diseases, autoimmune disorders and childhood dental caries. Thus, although iatrogenic supplementation has shown a better effect in preventing hypovitaminosis D than voluntary supplementation, which prevention strategy to follow depends on certain factors, in particular the local health policy and compliance of guardians with blood collection.
This study had several limitations. First, the vitamin D doses in the iatrogenic supplementation group remained inconsistent. Individualized vitamin D supplementations were prescribed by doctors according to the medical conditions of healthy children and those with vitamin D deficiency. Second, outdoor time might be a confounder. However, duration and sun-exposed skin areas of each child was not investigated in this study. Third, < 10–20% vitamin D that human needed per day was from diet, which played an important role in the vitamin D status of children. However, such information was unavailable in our study. We assumed that children in the same region had similar dietary patterns. A prospective study is warranted to explore the role of diet in addition to other confounders. Fourth, as the subjects in this study were all recruited from the outpatient department instead of the general pediatric population, their parents might be more concerned about their children’s vitamin D status or health condition, and their health status may be better. Therefore, demographic characteristics of the participants in this study could not represent that of the whole population of children living in North China.