The epidemic investigation of serum 25-hydroxy vitamin D levels in the adults in Qujing area of Yunnan province in China

Abstract

Objective: To investigate the levels of serum 25-hydroxy vitamin D(25-OHD) and its related influence factors in the adults in Qujing area of Yunnan Province in China. Method: 25-OHD levels in the sera were detected with a 25-hydroxy vitamin D kit and read with LIAISON XL chemiluminescence analyzer. Data was statistically analyzed with software SPSS17.0. Results: The serum 25-OHD levels of the subjects were 12.08±5.92 ng/ml. Although the sera levels of 25-OHD were significantly higher in urban residents, men and young subjects than in rural residents, women, and old subjects, the 25-OHD levels were significantly lower compared to international standard values in almost all populations in Qujing area. Conclusion: Qujing is plateau area with affluent sunlight, however, serum 25-OHD levels of majority populations are low. The serum 25-OHD levels are affected by many factors. In order to avoid excessive diagnosis and treatment, the adjusted local standard values of serum 25-OHD are needed.

Vitamin D is an key hormone in the regulation of calcium and phosphorus metabolism and maintenance of normal bone mineral salt in human body [1-2]. Vitamin D deficiency increases the risk of osteomalacia and fracture [3]. Recent studies have demonstrated that the deficiency of vitamin D has also a correlation to immune dysfunction, tumor, cardiovascular diseases (CVD) [4], blood glucose metabolic disorder [5], polycystic ovary, and infertility, etc. It leads to a concern that vitamin D deficiency might be an increasingly serious globally health problem.

Many researches have revealed that the populations with different genetic backgrounds have displayed a state of Vitamin D insufficiency or deficiency [6]. With reference to the current diagnostic criteria: serum 25-hydroxy vitamin D (25-OHD)<20.0 ng/ml is diagnosed as deficiency, 20-30 ng/ml as insufficient, >30 ng/ml as appropriate. This study has used random sampling survey and standard immunology assay, to examine the levels of serum 25-OHD in 1078 subjects with different genders, different locations between the ages of 20-90, our results have demonstrated that majority of populations in a plateau area in China have a lower serum level of 25-OHD, suggesting that the new standard for values of serum 25-OHD should be made in China.

Subjects and methods

Subjects

1078 adults between 20-90 years old in Qujing area of Yunnan Province in China have been recruited during Oct. to Dec. 2012 without race and gender limit. Exclusion criteria are included: (1) Ages are less than 20 years old; (2) One with serious kidney disease and evident skin diseases who are sensitive to sunlight; (3) Patients with the malnutrition; (4) Patients who are suffering from parathyroid disease, paget’s disease and osteomalacia; (5) One who has used hormones, steroids, parathyroid hormone, calcitonin, double phosphonic acid salts, Vitamin D or other similar drugs or supplements; (6) Patients with malignant tumors; (7) One with Long-term diarrhea, digestion and absorption dysfunction. Study has been approved by an IRB from Qujing City First Hospital of Yunan Province in China.

Methods

A form with the questionnaire was sent to each attendee. The study was followed-up and attendee wad eliminated if dynamic information was not gained during the study period.

4-5 ml venous blood was drawn from each attendee and serum was prepared with the regular method. Serum was harvested following a 10-min centrifugal with 3500 rpm speed. Each serum was saved on cryogenic tubes and stored on -80°C refrigerator. When all samples were ready, 25 OHD levels in the sera were detected with a 25-hydroxy vitamin D kit (DiaSorin, USA) and analyzed with LIAISON XL chemiluminescence analyzer using a method manufacturer introduced. The final concentration was indicated by ng/ml with a minimum value of 4.0 ng/mL and the maximum value of 150.0 ng/mL. The intra-assay and inter-assay coefficients of variation between this range were <10%.

Bone density in each subject was examined with dual-energy X-ray absorptiometry in same day. DPX-L dual-energy absorptiometry (Lunar Co. USA) has been widely utilized in measuring bone density and its accuracy is 1% with CV (Bone mineral density repeat precision) <1%. Results have been reported as normal values, osteoporosis, and osteopenia. The diagnosis of bone loss has adopted the WHO diagnostic criteria as following: osteoporosis, the BMD (bone mass density) value is lower than the peak bone mass >2.5S (what is mean of S?); Osteopenia, BMD value is between 2.5S to 1.0S and >1.0S was viewed as normal bone density.

Statistical analysis

All results have been used with SPSS17.0 software for statistical analysis. The average population of all ages was described using the mean and standard deviation. T test, analysis of variance and rank-sum test have been used to compare the differences of 25 OHD levels between urban and rural areas, different ages and different genders. Using multiple linear regression analysis the influence factors of vitamin D. The factors affecting Vitamin D levels have been analyzed with multiple linear regression analysis.

Results

The subjects consisted of 1078 adults, with an average age of 54.44±14.66 years, maximum 90 years old, minimum 20 years old. The average levels of 25-OHD were 12.08±5.92 ng/ml (maximum 62.4 ng/ml and minimal <4 ng/ml). Vitamin D levels were significantly higher in men than in women; in urban residents than in rural ones; while the ages of subjects increase, Vitamin D levels gradually decreased. The rates of Vitamin D deficiency were markedly increased in ≥60 group compared to <60 group and this trend can be observed in group with 40≤ and <60 than in group with 20≤ and <40. Nonetheless, referring to the current diagnostic criteria for vitamin D, vitamin D deficiency accounted for 98.7% in urban residents and 98.8% in rural residents. Similarly, 98.1% of men and 99.4% of woman also accounted for Vitamin D deficiency (Table 1).

Table 1.

Levels of serum Vitamin D in different groups

	N (%)	Mean ± SD (ng/ml)	Vitamin D appropriate (%)	Vitamin D insufficient (%)	Vitamin D deficiency (%)
20≤ and <40	164 (15.2)	15.70±5.95	1 (0.6)	140 (85.4)	23 (14.0)
40≤ and <60	499 (46.3)	12.89±5.89	4 (0.8)	336 (67.3)	159 (31.9)
≥60	415 (38.5)	9.67±4.86	3 (0.7)	159 (38.3)	253 (61.0)
Male	546 (50.6)	13.55±5.96	5 (0.9)	389 (71.2)	152 (27.8)
Female	532 (49.4)	10.56±5.48	3 (0.6)	246 (46.2)	283 (53.2)
Rural	555 (51.5)	10.92±5.20	7 (1.3)	353 (67.5)	163 (31.2)
Urban	523 (48.5)	13.31±6.37	1 (0.2)	282 (50.8)	272 (49.0)

We then compared the differences of serum 25-OHD levels between groups with ages and groups with genders and locations. As shown in Table 2, the Vitamin D levels were significantly higher in men than in women, in urban residents than in rural residents in all ages of groups (P<0.001).

Table 2.

Levels and correction of serum 25-OHD between ages, genders and locations

	Male	Female	Rural residents	Urban residents
20≤ and <40	17.23±5.12	14.10±6.36	15.96±6.38	15.21±5.06
40≤ and <60	14.15±5.93	11.64±5.59	13.83±6.09	11.89±5.51
≥60	11.42±5.44	7.81±3.25	10.66±5.86	9.06±4.00

We further adopted multiple linear stepwise regression analysis with α = 0.05 and with excluding standard β = 0.10 to determine the influence factors affecting the serum 25-OHD levels. To the end, when BMD results, genders and ages were entered to regression equation, the T test was used and p values were less than 0.05 and the difference was statistically significant (Table 3).

Table 3.

The influence factors affecting serum Vitamin D levels

Influence factors	B	Std. Error	Beta	t	P
Constant term	18.437	2.198		8.387	<0.001
Bone mineral density	1.077	0.478	0.126	2.254	0.025
Gender	-2.474	0.544	-0.191	-4.546	<0.001
Age	-0.065	0.022	-0.162	-2.909	0.004

Discussion

Vitamin D is one of fat-soluble Vitamins, belonging to the sterol compounds and consisting of D2, D3, D4 and D5. Only a small part of Vitamin D comes from the food since 90% of Vitamin D human body needed originate from ultraviolet radiation in sunlight skin (please add a reference).

1,25(OH)2-D is an active ingredient of Vitamin D, however, its half-life is fairly short. 25-OHD is the synthesis precursor of 1,25(OH)2-D, its concentration in blood is usually highest and most stable with the longest half-life. 25-OHD has been viewed as a good indicator to reflect the levels of Vitamin D in the body [7,8].

Vitamin D deficiency has becoming a worldwide health problem. More than 40% American suffer from Vitamin D deficiency or insufficient and the rates of Vitamin D deficiency or insufficient are even higher in China [7]. Researches have shown that the levels of serum 25-OHD in both men (9.534±4.4 ng/mL) and women (10.26±5.07 ng/ml) in the summer of Shenyang in China were evidently lower than the international standard. The average levels of serum 25-OHD in Beijing adults in the summer is 18.9±6.5 ng/ml whereas this values were slightly lower in the older population of Chongqing (14.62±7.97 ng/mL) [11]. Although Guiyan city of Guizhou Province in China is considered as one of locations lacking sunlight, the levels of serum 25-OHD in the adults were not lower as expected (21±10 mg/L) but the prevalence of Vitamin D insufficient and deficiency is still as high as 82% [12]. Interestingly, the levels of 25-OHD of men in Lhasa in Tibet reach to 30.5±14.5 ng/ml and women to 27.64±14.1 ng/ml in the summer, implicating the sufficient sunlight is important for maintenance of 25-OHD levels [10].

The reasons of different Vitamin D levels in various areas can be explained for the different altitudes, different ultraviolet irradiation, weather, seasons, and diet, etc. This research has showed that 25-OHD levels in Qujing area were similarly lower to other plain areas in China. Qujing area belongs to the plateau region, but we did not observe the obvious regional and geographical advantages for serum 25-OHD levels in the adults. It is possible that the samples have been collected in autumn and winter when sunshine time is short.

Our survey has also revealed that 25-OHD levels were related to ages. The obvious lower levels of serum Vitamin D in elderly populations in Qujing area may be related to the following factors; (1) Outdoor activity of the old has decreased and therefore their sunshine time is shortened. Eventually, skin synthesis of Vitamin D is reduced; (2) The intake of meat foods is limited; (3) Body mass index (BMI) is increased; (4) 7-dehydrogenation cholesterol levels in the skin are reduced. Previous studies have shown that the levels of 7-dehydrogenation cholesterol in the shin have negatively correlated to ages. The levels of 7-dehydrogenation cholesterol in the shin of 70 year-old people are only 25% of young people. Lapatsanis [13] has reported that 25-OHD levels decreased with increasing ages, the results are consistent with our finding. Kuchuk [14] observed that 25-OHD levels of men was significantly higher than women between 65-68 years old population in Netherlands, that is also similar to our observations. It is likely that the frequency of outdoor activities and meat food intake are higher in older male than in old female.

Our results also revealed that average 25-OHD levels were lower in rural residents than in urban residents. It may be related to unreasonable diet structure and lack of the extra vitamin D supplements in the rural area. It has been known that the intake amounts of animal food are usually lower in rural residents than in urban residents.

Our study also investigates the correction between 25-OHD levels and races, genders, ages, locations, smoking, drinking, outdoor activities, bone mineral density, intake amounts of milk and education degrees by multiple linear stepwise regression analysis, to the end, the 25-OHD levels are associated with bone mineral density, sex, and ages. Namely, 25-OHD positively correlated to bone mineral density whereas negatively correlated to female and ages. In our study, we excluded the influence factors including the weather, milk intake and education degrees. More than 80 studies from multi countries have showed that ordinary food does not provide adequate vitamin D without special Vitamin D food [15]. There is no correlation between 25-OHD with smoking and drinking. Because there is no specific survey questionnaire regarding to the timing and manner of outdoor activities in this study, it keeps an open question whether the outdoors is involved in affecting 25-OHD levels in Qujing area.

Taken together, our study indicates that the prevalence rate of Vitamin D insufficient and deficiency is high referring to the international standard. The results are similar to other domestic areas and plateau does not display the obvious regional or geographical advantage in the winter. We next will study the 25-OHD levels in the different seasons and enlarge the sample sizes. We also suggest that a new standard for serum 25-OHD levels should be made to truly reflect values of this assay in this area.