Seasonal variation and correlation analysis of vitamin D and parathyroid hormone in Hangzhou, Southeast China

Abstract This study aimed to describe the 25‐hydroxyvitamin D (25(OH)D) and parathyroid hormone (PTH) status of Southeast Chinese individuals influenced by season. The secondary aim was to determine the cutoff for sufficient 25(OH)D in a four‐season region. From January 2011 to June 2014, a total of 17 646 individuals were evaluated in our study. The serum levels of PTH were detected simultaneously in 5579 cases. A total of 25(OH)D and intact PTH were measured by the electrochemiluminescent immunoassay. The distribution of the concentration, prevalence and seasonal variability of 25(OH)D and PTH were studied. The mean 25(OH)D concentration in our study was 43.00(30.40) nmol/L. The prevalence of insufficiency (25(OH)D < 50 nmol/L) was 62.87% and that of deficiency (<30 nmol/L) was 28.54%. Mean serum 25(OH)D levels revealed a limited sinusoidal profile throughout the year and were significantly higher in Autumn. On the other hand, PTH levels showed an opposite response to seasonal effects relative to 25(OH)D. Age, BMI and daylight were not significantly correlated with 25(OH)D and serum PTH reached a plateau at higher values of serum 25(OH)D of 42.86 nmol/L. This study demonstrated that Vitamin D insufficiency is highly prevalent in Southeast China. The concentration of 25(OH)D in the male group was generally higher than that in the female group. Seasonal variation was an important aspect of 25(OH)D and PTH concentration. This study revealed that the optimal serum threshold of 25(OH)D for bone health should be between 40 and 50 nmol/L for Southeast Chinese individuals.

to 25-hydroxyvitamin D (25(OH)D), through an enzymatic process involving 25-hydroxylase. Within the renal tubular cell, 25(OH) D undergoes 1-alpha-hydroxylase and 24-alpha hydroxylase to form 1,25 (OH) 2, the active form of the vitamin. 2 The National Osteoporosis Society (NOS) recommends that the serum concentration of 25(OH)D is the best indicator of Vitamin D status in the human body. 3 And NOS consider a serum 25(OH)D threshold of 30 nmol/L for deficiency and potential for deficiency between 30 and 50 nmol/L. 4 However, there is no universal consensus on the level of serum 25(O)HD which constitutes sufficiency and the appropriate thresholds for vitamin D deficiency are debated.
A low vitamin D status is emerging as a very common condition worldwide. A recent study has suggested that 34% of Africans are vitamin D insufficiency (25(OH)D < 50 nmol/L), and 18.46% are deficient (25(OH)D < 30 nmol/L). 5 Meanwhile, prevalence rates of vitamin D insufficiency, defined as 25(OH)D < 50 nmol/L, of 24% (US), 6 37% (Canada), 7 40% (Europe) 8 and 58.7% (Korea) 9  From a clinical point of view, several factors may affect the production of vitamin D in the skin, including seasonal variation, gender, age, BMI and genetic factor. [13][14][15] On the other hand, low Vitamin D level often leads to an increase of parathyroid hormone (PTH) and calcium release from bone. The secondary hyperparathyroidism will cause osteoporosis and pathological fractures.
Recently, there has been considerable evidence that secondary hyperparathyroidism has developed for a long time before Vitamin D level reduces to deficiency. Therefore, it is suggested that the deficiency of Vitamin D should be defined as the lowest Vitamin D level that causes secondary hyperparathyroidism. As previously reported, this threshold, however, is not permanent and varies from 50 to 104 nmol/mL with different geographic location, country and season. [15][16][17][18]

| Clinical and biochemical measurements
All the subjects completed a standard questionnaire on demographic characteristics (eg age and sex) and medical history. Then, completed anthropometrical measurements with the assistance of trained staff using standard protocols. BMI was calculated as weight in kilograms divided by height in meters squared (kg/m 2 ). The exclusion criteria were as follows: (a) chronic liver and kidney disease, Based on several research articles and society recommendations, 3,19 we used the following groups: >75 nmol/L sufficient, 50-75 nmol/L moderate insufficient, 30-50 nmol/L insufficient, 20-30 nmol/L deficiency and <20 nmol/L severe deficiency.

| Statistical analysis
The baseline characteristics were presented using means with the standard deviations or medians with the interquartile ranges for continuous variables, and frequencies with percentages for categorical variables. Differences between the seasons were examined using the one-way ANOVA test and Kruskal-Wallis test for continuous and ranked variables, respectively. Differences between the 25(OH)D levels were examined using Kruskal-Wallis and chi-square test for ranked and nominal variables, respectively. The generalized linear model was fitted to explore the correlation between 25(OH)D and PTH adjusted for age, gender and seasons. The boxplot and bar chart were coded using R version 3.5.1 (R Core team). All other statistical analyses were performed using IBM SPSS Statistics (version 21.0). Throughout, we refer to statistical significance as a two-sided P < 0.05.

| Subjects characteristics
The participants' demographic and biochemical information was The serum PTH concentration median was 49.80 (40.05) pg/mL. Furthermore, as shown in Table 3, we correlated 25(OH)D with other relevant parameters and found that PTH was the strongest negative correlation parameter for 25(OH)D (P < 0.001), while season was the strongest positive correlation parameter (P < 0.001), and there was a significant difference between autumn and spring (P < 0.001). In the remaining parameters, gender was significantly correlated with 25(OH)D (P < 0.001). Age, BMI and daylight were

| Prevalence of serum 25(OH)D in the evaluated population of Southeast China
This study included 17 646 25(OH)D results from 2011 to 2015 were shown in Figure 1. The overall trend within one year was significantly higher in summer and autumn than in spring and winter.
As mentioned above, vitamin D insufficient was considered to be year compared with the previous (Figure 1).  Table 2, P < 0.001). In all groups, regardless of any season, the highest proportion was always in the insufficiency group (average 35.6%, ranging from 30.6% to 39.3%). The sufficiency group was always the lowest, average 11.0%, range from 7.4% to 16.6% (Figure 4, P < 0.001). Consistent with this, participants with a high PTH level (PTH > 65 pg/mL) accounted for 32.5% of the total population ( Table 2, P < 0.001).

| Determinants of 25(OH)D level and association with PTH the correlation between concentrations of serum 25(OH)D and PTH
The subgroup analysis of 25(OH)D found that the concentration of 25(OH)D in the male group was generally higher than that in  Although we did not find a significant difference between BMI and 25(OH) D, the mean values showed that 25(OH)D was slightly lower in obese participants (Table 2). Vitamin D is fat-soluble and distributed into fat, muscle, liver and serum. All of these compartments are increased in volume in obesity, so the whole body stores of vitamin D may be adequate. 20,21 Another unexpected finding from this study revealed that the increased starting in April, reaching a peak level in October, and then decreased to baseline levels by January (Figures 1 and 2).
We have shown that both 25(OH)D and PTH demonstrate a con- The present study has some limitations and one of them is the lack of information on vitamin supplementation usage of the participates whose test results were used. The second one is that we measured serum 25(OH)D and PTH in a single clinical centre and bone turnover markers were not measured. A further study based on subjects sampled on multicenter of the different regions would be carried out and more biochemical parameters, such as bone density and bone turnover markers should be investigated. Nevertheless, the present study is valuable because it was based on retrospective data employed from the largest study group ever in the Southeast China population which experiences four seasons.

| CON CLUS ION
In conclusion, our study demonstrates that vitamin D insufficiency

CO N FLI C T O F I NTE R E S T
The authors have no conflicts of interest to declare.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available because of privacy or ethical restrictions.