Vitamin D deficiency and insufficiency are global problems [15-17]. Our study found that a high prevalence of vitamin D deficiency and insufficiency among children aged 0-6 years old in a Chinese population, especially in the preschool-aged children.
As for serum 25(OH)D concentration, only 34.7% of the children were sufficient, 43.2% were insufficient, and 22.1% were deficient in the total population. The prevalence of vitamin D deficiency (<50 nmol/L) increased by age, with 17.9%, 21.2% and 48.1% in the infants (0-1 years of age), toddlers (1-3 years of age) and preschoolers (3-6 years of age) groups, respectively. Consistent with our findings, the prevalence of vitamin D deficiency increased by age in American children, with a deficiency prevalence of 14%, 20%, and 28.8% in children aged 2-5 years, 6-11 years and adolescents, respectively [12, 18]. In addition, a dramatic seasonal variation of vitamin D deficiency and insufficiency was observed in our present study; the levels were very low in autumn, increased gradually in spring and winter, and reached a peak in summer. This is consistent with the results reported in other studies [13, 19-22]. There are many reasons for vitamin deficiency and insufficiency. The content of vitamin D in the skin of human body exceeds 90%, and the vitamin D obtained from dietary intake for only about 5% to 10% . The origin of Vitamin D in infants is mainly from milk and exogenous supplementation; the Pediatrics Branch of Chinese Medical Association recommends that all children receive no less than 400 IU/day of vitamin D from two weeks to two years after birth . However, when the children reach ages older than 2 years, milk is no longer the main food, and the origin of Vitamin D is mainly from outdoor activities. Studies have shown that children have lower levels of vitamin D due to less outdoor activities [24-26].
It is known that humans obtain vitamin D from exposure to UV-B rays in natural sunlight and dietary intake (including supplements) . For most people, the majority (80-90%) of the 25 (OH) D in the circulation is produced by the skin's 7-dehydrocholesterol by ultraviolet B radiation . Few foods contain vitamin D precursors, sunlight exposure is the primary factor affecting the vitamin D status in children. Also, the circulating 25(OH)D is regularly influenced by season [28, 29]. The seasonal variation in vitamin D status correlated well with the seasonal variation in the intensity of solar UVB light [30, 31]. Additionally, the condition of UVB on skin radiation determines the effect of temperature on vitamin D status [32, 33]. A few studies have investigated the association between 25(OH)D levels and air temperature and revealed that the 25(OH)D levels were fairly consistent with the changes in air temperature . Interestingly, our research found a nonlinear curve for the association between 25(OH)D levels and air temperature in the pediatric population. Our research showed that the 25(OH)D levels decreased with increasing air temperatures in the temperature range of 0-10℃. Some children may take vitamin D supplements due to less outdoor activities during the transition from winter to spring. However, vitamin D supplementation may decline as air temperatures rise, which is a probable reason for the decreasing vitamin D levels. The 25(OH)D levels increased to a high level when the air temperature increased from 10℃ to 24℃. It is probable that the children had more outdoor activities in this comparatively more comfortable temperature range. However, when the air temperature exceeded 24℃, the 25(OH)D levels again declined slightly with rising air temperatures. Children spent less time outdoors due to high air temperatures, which is a possible reason for the decreasing vitamin D levels.
There were some limitations of our study worth mention. The socioeconomic status of the participants has not been provided in this study. Additionally, our study did not collect information regarding the children’s dietary intake, vitamin D supplements, body mass index, time of physical activities or amount of sunlight exposure. These factors might affect the vitamin D status of young children.