The present study is the first one to investigate vitamin D status among the pediatric population in Hangzhou, Zhejiang province, the southeast China. Our data demonstrate that the serum 25-hydroxyvitamin D levels among pediatric population decreased when the children became older, and the prevalence of vitamin D deficiency increased at the same time. The mean level of 25(OH) D among infants was much higher than that of any other stages. This is mainly because of the recommendation by the Pediatric Branch of Chinese Medical Association that all children receive 400 IU/day of vitamin D from two weeks after born until they are 2 years old[22
]. According to this recommendation, many of the children's health care providers will give parents advices about taking vitamin D supplementation in children's first two years of life. And there are many kinds of oral supplementations containing vitamins D and A for infants, made in aqua or oil drops and can be purchased in pharmacies in Hangzhou. So the parents are easy to get these kinds of supplementations for their infants. Furthermore the parents may will provide vitamin D fortified formula to their young children especially younger than 2 years according to the feeding recommendations for infants and toddlers [23
But as to the children older than 2 years, we don't have any formal recommendations on vitamin D supplementations. Fewer and fewer children take vitamin D supplements when they grow older than 2 years. This may be one of the causes of the finding in the present study that the mean serum 25(OH)D levels of the children aged 2-16 years were much lower than that of infants, and the prevalence of vitamin D deficiency among children older than 2 years was much higher than that of infants.
When compared with American children, our children had much higher prevalence of vitamin D level at < 50 nmol/L in most of stages, 2-5 years (22% vs 14%), 6-11 years (40% vs 20%) [17
] and adolescents (46% vs 28.8%)[24
], although the prevalence of 25(OH)D < 25 nmol/L among our children was similar to that of American children. If the threshold is changed to < 75 nmol/L, almost all children aged 6-16 years had low levels (88.3% among children aged 6-11 years and 89.6% among adolescents) and most children aged 2-5 years did as well (68.6%).
Hangzhou is located in the southeast of China at northern latitude 30°. The solar exposure in Hangzhou is more ample than in northern areas. It is reported that the prevalence of vitamin D deficiency among adolescent girls in Beijing was 89% [25
], much higher than that of our adolescent girls (52%). In Shanxi province, another northern area of China, the prevalence of 25(OH)D level < 50 nmol/L among children aged 12-24 months was 84.3% in spring and 8.1% in autumn[26
], much higher than that of our infants (7.7% in spring and 3.4% in autumn).
Given the concerns for vitamin D deficiency in older children and adolescents, in November 2008 the American Academy of Pediatrics released a new recommendation that all children receive 400 IU/day of vitamin D from their first days of life through adolescence [28
]. So we suggest that the recommendation on vitamin D supplementation for Chinese pediatric population should be revised to include older children and adolescents now that the low 25(OH)D status of Chinese children are more severe than that of American children.
In the present study, we also analyzed the vitamin D status in different seasons. We found that the mean levels of serum 25(OH)D among children aged 6-16 years changed according to seasons. The levels were very low in winter, increased gradually in spring and summer, and reached the peaks in autumn. This tendency was similar to that of American adolescents [24
]. But there were strange findings in our study, that the mean serum 25(OH)D levels among children aged 0-5 years in winter were as high as that in summer or autumn and significantly higher than that in spring. The possible reason is that the vitamin D supplementation for young children may be emphasized in winter when the sunlight is weak and the children will be dressed in many layers when they go outdoors. We did not find any significant differences between boys and girls at any stage. But the mean serum 25(OH)D level among adolescent girls was lower than that of boys, P
= 0.05. This significant difference might be hidden by some reasons such as the uneven distribution of subjects between genders, now that it was reported that the serum 25(OH)D level was lower among American adolescent girls than that of boys[29
]. The reason need to be explored in the future.
Although the subjects were not sampled from the whole pediatric population, they participated in the present study just for health examination with no medical status that might affect study results. Many of them took the examination before entered in kindergarten or school. Furthermore, the number of the sample was so large and the range of ages was also very big. So to some extent, the sample could represent the pediatric population of Hangzhou.
But there are still some limitations in the present study. The subjects were not sampled from the whole pediatric population in Hangzhou, and other possible relating factors of vitamin D status including intake of supplements, children's BMI and the time of physical activities were not collected. A further study based on subjects sampled on a population basis would be carried out and the possible relating factors of vitamin D status should be investigated. Furthermore, the optimal vitamin D level among children should to be certificated by more studies, now that adequate amount of vitamin D in all human populations for sustaining both innate and acquired immunity against infection is very important [8
]. While at the same time, some authors warned that the prevalence of vitamin D deficiency was over estimated and the evidence of the role of vitamin D for extraskeletal outcomes was inconsistent and inconclusive and need to be assessed by more randomized clinical trials [30