Study On The Factors Influencing The Bioavailability Of Vitamin D₂

Objective:Previous animal studies have shown that vitamin D is mainly stored in adipocytes.Many clinical studies have found that vitamin D deficiency is more common in individuals with high body mass index（BMI）.However,the impact of human body fat content on the required dose of vitamin D supplementation is not fully understood.Intramuscular injection of vitamin D2 is a common method to correct vitamin D deficiency among Chinese people.However,in the existing studies,the effect of Intramuscular injection of vitamin D₂on the degree and duration of the elevation of serum total 25（OH）D level,as well as on blood calcium,urine calcium,and bone turnover markers is still unclear.Previous studies have used oral administration of vitamin D₃to observe changes in vitamin D bioavailability,but inconsistent results have emerged due to the inability to rule out the effects of sunlight and compliance.Therefore,the present study utilized single or multiple intramuscular injection of vitamin D₂for research,which not only excluded the influence of sunlight but also ensured the compliance of the subjects.The purpose of this study was to investigate the impact of fat content and distribution differences on the dose required for vitamin D supplementation,as well as the long-term bioavailability and safety of single and intermittent multiple intramuscular injection of vitamin D₂in Chinese people.Methods:The whole study consisted of two randomized controlled trials,in which 80 subjects were respectively included in the study on the influence of different doses of vitamin D₂and body composition on the bioavailability of vitamin D₂by single intramuscular injection.In the study of single intramuscular injection of vitamin D₂,the subjects were divided into four groups:the no-treatment group and the intramuscular injection of 200,000,400,000 or 600,000 IU vitamin D₂groups.We collected the blood and 24-hour urine of subjects before and after administration.To evaluate the of vitamin D level in subjects,liquid chromatography tandem mass spectrometry（LC-MS/MS）was used to measure serum 25-hydroxyvitamin D[25（OH）D]levels and analyze the effect of BMI on bioavailability in different groups.Enzyme-linked immunosorbent assay（ELISA）was used to measure vitamin D binding protein（DBP）levels.In studies of the effects of body composition on vitamin D₂bioavailability,the subjects were divided into the control group with a single intramuscular injection of 600,000 IU vitamin D₂and the experimental groups with multiple intermittent intramuscular injection of vitamin D₂until 25（OH）D reaches the adequate level.The impact of differences in body fat content and distribution on the required dose of vitamin D supplementation was clarified by examining subject body composition by dual-energy X-ray absorptiometry（DXA）.In addition,serum calcium and 24-hour urine calcium were detected at various follow-up time point to monitor the occurrence of hypercalcemia and hypercalciuria.Results:After a single intramuscular vitamin D₂,there was no significant correlation between the BMI and serum 25（OH）D₂.After intramuscular administration of 600,000 IU vitamin D₂,serum 25（OH）D₂elevations were linearly negatively correlated with total body fat content,limb fat content,and trunk fat content,and linearly positively correlated with total body lean mass content but were not correlated with total body bone mineral content.Multiple intermittent intramuscular injection of vitamin D₂raised serum 25（OH）D levels to over 30 ng/m L.The subjects were divided into 1,200,000 IU,2,400,000,3,600,000 IU vitamin D₂groups according to the amount of intramuscular injection vitamin D₂when 25（OH）D levels reached to the sufficient level.To reach the sufficient 25（OH）D level,the group who received 3,600,000 IU vitamin D₂need not only higher doses but also more time than the group that just received 1,200,000 IU vitamin D₂.Moreover,the subjects received3,600,000 IU vitamin D₂reached a lower peak and plateau on the level of25（OH）D.In addition,subjects with higher body fat percentage,especially higher limb fat content,needed higher doses of vitamin D₂to achieve vitamin D sufficiency.In addition,bone mineral content did not affect vitamin D absorption.Although the 25（OH）D₃levels were reduced by intramuscular injection of vitamin D₂,the total 25（OH）D levels increased 5.0,7.0,or 10.3 ng/m L in average after injection of 200,000 IU,400,000 IU,or 600,000 IU D₂at least 12 weeks.In addition,there were no significant changes in serum calcium,24-hour urine calcium and markers of bone turnover after single intramuscular injection of vitamin D₂,but the level of i PTH significantly decreased.There was no significant difference in the effect on serum DBP levels between the no treatment group and 600,000 IU D₂group across the time points.After multiple intermittent intramuscular injections of vitamin D₂,serum calcium and 24-hour urine calcium of the subjects were not significantly different from the baseline.Conclusion:The difference in body fat content is the major factor influencing vitamin D₂bioavailability.People with high body fat content need to supplement higher doses of vitamin D.However,the bone and muscle content or BMI does not affect the bioavailability of vitamin D₂.A single intramuscular injection of 200,000,400,000,or 600,000 units increases the average serum total 25（OH）D levels of 4.9,7.8,or 11.6ng/m L of vitamin D₂within 12 weeks.These three supplementary doses reduce i PTH,but do not affect the levels of blood calcium,urinary calcium,bone turnover markers,and DBP.