How Long Does Vitamin D Supplement Stay In Your System
Vitamin D Stored in Fat Tissue During a 5-Year Intervention Affects Serum 25-Hydroxyvitamin D Levels the Following Year
Ieva Martinaityte, 1Tromsø Endocrine Research Group, Department of Clinical Medicine, UiT The Arctic University of Norway, 9019 Tromsø, Norway 2Division of Internal Medicine, University Hospital of North Norway, 9019 Tromsø, Norway Correspondence and Reprint Requests: Ieva Martinaityte, MD, Division of Internal Medicine, University Hospital of North Norway, Sykehusvegen 9038, 9019 Tromsø, Norway. E-mail ieva.martinaityte@unn.no. Search for other works by this author on: 1Tromsø Endocrine Research Group, Department of Clinical Medicine, UiT The Arctic University of Norway, 9019 Tromsø, Norway 2Division of Internal Medicine, University Hospital of North Norway, 9019 Tromsø, Norway Search for other works by this author on: 1Tromsø Endocrine Research Group, Department of Clinical Medicine, UiT The Arctic University of Norway, 9019 Tromsø, Norway Search for other works by this author on: 3Research Group for Bioactives–Analysis and Application, National Food Institute, Technical University of Denmark, 2800 Lyngby, Denmark Search for other works by this author on: 1Tromsø Endocrine Research Group, Department of Clinical Medicine, UiT The Arctic University of Norway, 9019 Tromsø, Norway 2Division of Internal Medicine, University Hospital of North Norway, 9019 Tromsø, Norway Search for other works by this author on:
Abstract
Context
Vitamin D and 25-hydroxyvitamin D [25(OH)D] are stored in adipose tissue, but the clinical relevance is uncertain.
Objective
To evaluate changes in serum 25(OH)D and adipose tissue vitamin D levels after stopping vitamin D supplementation.
Design
A prospective, double-blind cohort follow-up study.
Setting
Clinical Research Unit at University Hospital of North Norway.
Patients
Seventy-six subjects were included after participation in a 3- to 5-year prevention of type 2 diabetes study and were administered 20,000 IU of vitamin D or placebo per week.
Intervention
During the 12-month follow-up period, blood samples were drawn at the beginning and after 1, 3, 6, 9, and 12 months. Fat biopsies were taken at the start and end.
Main Outcome Measures
Changes in 25(OH)D level in serum and 25(OH)D and vitamin D levels in adipose tissue.
Results
Forty-one of 42 subjects who were given vitamin D and 33 of 34 subjects who were given placebo completed the study. At the inclusion, mean serum 25(OH)D levels were 122 and 71 nmol/L in the vitamin D and placebo groups, respectively. Serum 25(OH)D levels were significantly higher in the vitamin D group than in the placebo group throughout and were 84.5 and 73.1 nmol/L, respectively, after 12 months. In the vitamin D group, adipose tissue vitamin D levels decreased by 52% over 12 months.
Conclusion
Vitamin D and 25(OH)D stored in adipose tissue after 3 to 5 years of vitamin D supplementation may have a clinically relevant effect on serum 25(OH)D level the following year.
Vitamin D is obtained from the diet, with fatty fish being the main source, or from production in the skin after UVB exposure. Regardless of its origin, vitamin D that enters the circulation is either hydroxylated in the liver to 25-hydroxyvitamin D [25(OH)D] or stored mainly in adipose tissue (1–3). Similarly, 25(OH)D may be further hydroxylated in the kidneys to its active form, 1,25-dihydroxyvitamin D [1,25(OH)2D], stored and hydroxylated in small amounts in adipose and other tissues, or undergo 24-hydroxylation and be excreted mainly in the urine (3, 4).
Storage of vitamin D and 25(OH)D in adipose and other tissues has been well documented (2, 4, 5). Whether this storage is of clinical importance is uncertain (4), and some have argued that the amount stored has only negligible effects on serum 25(OH)D levels (1). However, serum 25(OH)D levels fall by only 20% to 40% during the winter in Nordic countries, in spite of a UVB winter that lasts >6 months (6). Furthermore, the traced half-life of serum 25(OH)D is about 15 to 25 days (7, 8), whereas the calculated half-life of serum 25(OH)D after intake of vitamin D is up to 82 days (9). Accordingly, it is reasonable to assume that vitamin D metabolites from adipose or other tissues are gradually released into the circulatory system (4, 10), which prevents serum 25(OH)D from falling to critically low levels during the winter (6).
However, to prove the importance of vitamin D storage, one must conduct a clinical study. We recently performed a 5-year intervention trial with vitamin D for the prevention of type 2 diabetes (T2D) (11). At the end of the trial, we performed adipose tissue biopsies (12) and followed up on subjects for the next year, when an additional biopsy was performed. This gave us the opportunity to compare the declining serum 25(OH)D levels in the vitamin D group with those in the placebo group and to relate these observations to the stored amount of vitamin D.
Materials and Methods
Study subjects
The subjects invited to the current study had participated in a double-blind, randomized, controlled trial for the prevention of T2D, a study that was previously described in detail (11, 13). Thus, 511 subjects with impaired glucose tolerance or impaired fasting glucose were included and randomly assigned to 20,000 IU of oral vitamin D3 (cholecalciferol) or an identical-looking placebo every week for 5 years. The exclusion criteria were primary hyperparathyroidism, granulomatous disorders, urolithiasis, cancer in the past 5 years, unstable angina pectoris, acute myocardial infarction or stroke in the past year, reduced kidney function (serum creatinine level >125 µmol/L in men and >105 µmol/L in women), pregnancy, lactation, and fertile age in women and no use of contraception. An oral glucose tolerance test was performed annually. By the end of the 5-year period or when excluded because of development of T2D, subjects were invited to participate in the present 12-month follow-up study.
Study design and measurements
This follow-up cohort study was prospective and double-blind, and the study outcomes were planned before data collection began. Nonfasting blood samples were drawn at inclusion (baseline), which was shortly after ending participation in the primary T2D prevention study (11), and then after 1, 3, 6, 9, and 12 months. At every visit, the subjects were interviewed for intake of vitamin D (as tablets or cod liver oil), calcium supplementation, use of a solarium, and sunny vacations.
During the 12-month study period, the subjects were not given any study medication, were asked not to take vitamin D supplementation (including cod liver oil) exceeding 400 IU per day, and were asked not to use a solarium more than twice a month or to stay in the tropics for more than 2 months of the next year. At inclusion and after 6 and 12 months, height and weight were measured with the subject wearing light clothes and no shoes. Body mass index (BMI) was calculated as weight divided by height squared. The subjects were also invited to have an optional fat biopsy taken from subcutaneous abdominal adipose tissue at inclusion and after 12 months. The biopsies were taken with the needle technique described by Mutch et al. (14).
Serum calcium and phosphate levels were analyzed by using the Hitachi 917 (Roche Diagnostics, Basel, Switzerland), with reagents from Boehringer-Mannheim (Mannheim, Germany). Serum parathyroid hormone (PTH) level was measured using an Immulite 2000 Intact PTH analyzer (Siemens Healthcare Diagnostics, Los Angeles, CA). Glycosylated hemoglobin, type A1c was analyzed by an immunoturbidimetric method using the Unimate 5 HbA1c (Hoffmann-La Roche, Basel, Switzerland). Serum 25(OH)D was analyzed in a batch at the end of the study using sensitive liquid chromatography with tandem mass spectrometry, as previously described (13). Serum 25(OH)D2 and 25(OH)D3 were measured simultaneously, with a limit of quantification of <4 nmol/L, a between-day coefficient of variation of <9%, and a within-day coefficient of variation of <2%. The accuracy of 25(OH)D3 was 104.6% over 3 days. In most of the samples, 25(OH)D2 was not measurable; therefore, we present values of the total 25(OH)D measured.
Vitamin D and 25(OH)D in adipose tissue were analyzed by the liquid chromatography with tandem mass spectrometry method, as previously described (15). In short, the saponified porcine adipose tissue and liver samples were liquid-liquid extracted and further cleaned up by a normal-phase, solid-phase extraction method. The mass spectrometer was operated in the positive multiple-reaction monitoring mode. The limit of quantification was <0.1 ng/g. The analyses of variation (using house-reference pork fat materials) was calculated to be 8.2% for vitamin D3 (5.8 ng/g) and 8.5% for 25(OH)D3 (2.4 ng/g). The accuracy was 82% to 97% for vitamin D3 and 113% to 124% for 25(OH)D3, whereas the precision was 1.4% to 16% assessed on spiking (12).
All data were sent directly to the hospital's research department and returned as a final data file when the study and all analyses were completed. Until then, the investigators participating in the study were unaware of the randomization status of the subjects.
Statistical analyses
Residuals of serum 25(OH)D measurements and other dependent variables when applicable were assessed for normality by using the Shapiro-Wilk test and by visual inspection of the histogram and plots. Homogeneity was evaluated by Levene's test, and sphericity by Mauchly's test. Potential confounding variables were then added to the model. Missing values [serum 25(OH)D and other measurements] for one subject at the last visit in the placebo group, as well as a few height and weight values, were replaced by the series mean method. For intake of supplements and other questionnaire answers, the last observation carried forward was used.
Comparisons between vitamin D and placebo groups were conducted using the independent samples t test for continuous variables and the Fisher's exact test for frequencies. For not normally distributed data, the Mann-Whitney U test was used. The variation of continuous variables over time was evaluated by using the one-way analysis of variance test or the related-samples Wilcoxon signed rank test. The medians of concentrations in fat biopsies between the intervention groups were compared with the independent samples median test. Correlations between continuous variables were investigated using the Spearman test to avoid the influence of a few extreme observations.
The terminal half-life of 25(OH)D is the time required for the serum concentration of 25(OH)D to fall by 50% during the terminal phase (16). In our study, the terminal phase was determined visually, when the plot of the logarithmically transformed concentrations on time looked linear. The preterminal as well as the terminal half-lives of serum 25(OH)D in the vitamin D group were then calculated using the following formula on the web calculator http://www.calculator.net/half-life-calculator.html:
where ln2 is the natural logarithm of 2 and is a constant of 0.693.
Data are shown as mean ± standard deviation or median with interquartile range (IQR) when not normally distributed. The tests performed were two-sided, and a P value <0.05 was considered statistically significant. The data were analyzed with IBM SPSS Statistics 22 (SPSS Inc., Chicago, IL).
Power calculation
On the basis of previous studies, we expected mean serum 25(OH)D levels at the baseline of the follow-up study to be ∼130 nmol/L in the vitamin D group and ∼50 nmol/L in the placebo group (17). In accordance with our theory on storage of vitamin D in adipose and other tissues, we assumed that the half-life of serum 25(OH)D would be 50 days during the follow-up period. If so and assuming an input from diet/skin production sustaining a serum 25(OH)D level of 50 nmol/L, the serum 25(OH)D level in the vitamin D group would fall to 70 nmol/L after 3 months (∼2 half-lives). To show a difference between 70 and 50 nmol/L with a power of 0.8 and P < 0.05 and assuming a standard deviation for 25(OH)D of 17 nmol/L at this level (18), we had to include 38 subjects. Given the uncertainty of this calculation, we decided to include twice that number.
Ethics
This study was approved by the Regional Committee for Medical and Health Research Ethics (REK Nord). Only participants with valid written informed consent were included. The trial is registered at ClinicalTrials.gov (NCT01729013).
Results
Out of 511 subjects in the intervention study, 92 were invited to participate in the follow-up study, and 76 accepted the invitation and were included. Among these, 42 subjects had received vitamin D and 34 had received placebo in the intervention study. One subject from the vitamin D group withdrew after 1 month and was excluded from all but baseline analyses. One subject in the placebo group did not participate in the last 12-month visit but was included in the analyses.
Twenty-nine subjects (18 in the vitamin D group and 11 in the placebo group) agreed to have an abdominal fat biopsy at baseline, and 12 (eight in the vitamin D group and four in the placebo group) also had the biopsy performed after 12 months. There were no serious complications after the biopsy procedures except local bruising. The median weights of the fat biopsies were 454 mg (IQR, 216 mg) at inclusion and 558 mg (IQR, 170 mg) after 12 months.
Mean serum 25(OH)D levels at the baseline examination before the 5-year intervention study were 59.9 nmol/L in the vitamin D group and 61.1 nmol/L in the placebo group (13). The characteristics of the subjects at inclusion in this follow-up study are presented in Table 1. As expected, serum 25(OH)D level was higher and serum PTH level was lower in the vitamin D group than in the placebo group. Otherwise, the two groups were similar.
Table 1.
Characteristics of Subjects in the Vitamin D and Placebo Groups at Inclusion in the Follow-Up Study
| All Subjects | Subjects With Fat Biopsy at Baseline | Subjects With Fat Biopsies at Baseline and After 12 Months | ||||
|---|---|---|---|---|---|---|
| Vitamin D Group | Placebo Group | Vitamin D Group | Placebo Group | Vitamin D Group | Placebo Group | |
| N | 42 | 34 | 18 | 11 | 8 | 4 |
| Females, % | 40.5 | 47.1 | 16.7 | 18.2 | 12.5 | 0 |
| Age, y | 67.5 (10) | 67.0 (15) | 67.5 (6) | 65.0 (17) | 67.5 (4) | 77.0 (15) |
| Weight, kg | 88.1 (22.9) | 86.5 (21.8) | 96.9 (20.9) | 88.4 (36.9) | 96.3 (21.0) | 86.2 (19.2) |
| BMI, kg/m2 | 29.5 (6.0) | 28.3 (7.3) | 30.9 (6.2) | 28.7 (9.2) | 31.0 (6.0) | 27.8 (4.4) |
| Glycosylated hemoglobin, type A1c, % | 6.1 (0.4) | 6.0 (0.4) | 6.1 (0.4) | 6.1 (0.2) | 6.0 (0.5) | 6.1 (0.2) |
| Serum 25(OH)D, nmol/L (mean ± SD) | 121.7 ± 28.9 a | 70.9 ± 16.7 | 107.6 ± 25.3 a | 68.8 ± 20.8 | 110.6 ± 30.1 | 80.3 ± 21.8 |
| Serum calcium, mmol/L | 2.35 (0.15) | 2.35 (0.11) | 2.35 (0.14) | 2.38 (0.11) | 2.34 (0.18) | 2.33 (0.04) |
| Serum PTH, pmol/L | 4.5 (1.7) | 5.4 (2.1) | 4.7 (2.1) | 5.7 (3.9) | 4.1 (1.7) b | 6.3 (3.1) |
| Serum phosphate, mmol/L | 1.05 (0.16) | 1 (0.19) | 1.01 (0.17) | 0.97 (0.26) | 1.08 (0.21) | 0.93 (0.18) |
| Time in the primary study, y | 5.0 (1) | 5.0 (1) | 5.0 (1) | 4.0 (2) | 5.0 (0) | 5.0 (0) |
| Sunny vacation last 3 months, % | 16.7 | 11.8 | 11.1 | 0 | 12.5 | 0 |
| Used solarium last 3 months, % | 0 | 0 | 0 | 0 | 0 | 0 |
| Vitamin D supplementation, % | 59.5 | 47.1 | 61.1 | 54.5 | 75.0 | 75.0 |
| Calcium supplementation, % | 9.5 | 8.8 | 5.6 | 18.2 | 0 | 25.0 |
| All Subjects | Subjects With Fat Biopsy at Baseline | Subjects With Fat Biopsies at Baseline and After 12 Months | ||||
|---|---|---|---|---|---|---|
| Vitamin D Group | Placebo Group | Vitamin D Group | Placebo Group | Vitamin D Group | Placebo Group | |
| N | 42 | 34 | 18 | 11 | 8 | 4 |
| Females, % | 40.5 | 47.1 | 16.7 | 18.2 | 12.5 | 0 |
| Age, y | 67.5 (10) | 67.0 (15) | 67.5 (6) | 65.0 (17) | 67.5 (4) | 77.0 (15) |
| Weight, kg | 88.1 (22.9) | 86.5 (21.8) | 96.9 (20.9) | 88.4 (36.9) | 96.3 (21.0) | 86.2 (19.2) |
| BMI, kg/m2 | 29.5 (6.0) | 28.3 (7.3) | 30.9 (6.2) | 28.7 (9.2) | 31.0 (6.0) | 27.8 (4.4) |
| Glycosylated hemoglobin, type A1c, % | 6.1 (0.4) | 6.0 (0.4) | 6.1 (0.4) | 6.1 (0.2) | 6.0 (0.5) | 6.1 (0.2) |
| Serum 25(OH)D, nmol/L (mean ± SD) | 121.7 ± 28.9 a | 70.9 ± 16.7 | 107.6 ± 25.3 a | 68.8 ± 20.8 | 110.6 ± 30.1 | 80.3 ± 21.8 |
| Serum calcium, mmol/L | 2.35 (0.15) | 2.35 (0.11) | 2.35 (0.14) | 2.38 (0.11) | 2.34 (0.18) | 2.33 (0.04) |
| Serum PTH, pmol/L | 4.5 (1.7) | 5.4 (2.1) | 4.7 (2.1) | 5.7 (3.9) | 4.1 (1.7) b | 6.3 (3.1) |
| Serum phosphate, mmol/L | 1.05 (0.16) | 1 (0.19) | 1.01 (0.17) | 0.97 (0.26) | 1.08 (0.21) | 0.93 (0.18) |
| Time in the primary study, y | 5.0 (1) | 5.0 (1) | 5.0 (1) | 4.0 (2) | 5.0 (0) | 5.0 (0) |
| Sunny vacation last 3 months, % | 16.7 | 11.8 | 11.1 | 0 | 12.5 | 0 |
| Used solarium last 3 months, % | 0 | 0 | 0 | 0 | 0 | 0 |
| Vitamin D supplementation, % | 59.5 | 47.1 | 61.1 | 54.5 | 75.0 | 75.0 |
| Calcium supplementation, % | 9.5 | 8.8 | 5.6 | 18.2 | 0 | 25.0 |
The numbers represent median (IQR) for continuous variables if not specified otherwise. Abbreviations: SD, standard deviation.
a P < 0.01 vs the placebo group [independent samples t test for serum 25(OH)D; Mann-Whitney U test for not normally distributed data; Fisher's exact test for frequencies].
b P < 0.05 vs the placebo group [independent samples t test for serum 25(OH)D; Mann-Whitney U test for not normally distributed data; Fisher's exact test for frequencies].
Table 1.
Characteristics of Subjects in the Vitamin D and Placebo Groups at Inclusion in the Follow-Up Study
| All Subjects | Subjects With Fat Biopsy at Baseline | Subjects With Fat Biopsies at Baseline and After 12 Months | ||||
|---|---|---|---|---|---|---|
| Vitamin D Group | Placebo Group | Vitamin D Group | Placebo Group | Vitamin D Group | Placebo Group | |
| N | 42 | 34 | 18 | 11 | 8 | 4 |
| Females, % | 40.5 | 47.1 | 16.7 | 18.2 | 12.5 | 0 |
| Age, y | 67.5 (10) | 67.0 (15) | 67.5 (6) | 65.0 (17) | 67.5 (4) | 77.0 (15) |
| Weight, kg | 88.1 (22.9) | 86.5 (21.8) | 96.9 (20.9) | 88.4 (36.9) | 96.3 (21.0) | 86.2 (19.2) |
| BMI, kg/m2 | 29.5 (6.0) | 28.3 (7.3) | 30.9 (6.2) | 28.7 (9.2) | 31.0 (6.0) | 27.8 (4.4) |
| Glycosylated hemoglobin, type A1c, % | 6.1 (0.4) | 6.0 (0.4) | 6.1 (0.4) | 6.1 (0.2) | 6.0 (0.5) | 6.1 (0.2) |
| Serum 25(OH)D, nmol/L (mean ± SD) | 121.7 ± 28.9 a | 70.9 ± 16.7 | 107.6 ± 25.3 a | 68.8 ± 20.8 | 110.6 ± 30.1 | 80.3 ± 21.8 |
| Serum calcium, mmol/L | 2.35 (0.15) | 2.35 (0.11) | 2.35 (0.14) | 2.38 (0.11) | 2.34 (0.18) | 2.33 (0.04) |
| Serum PTH, pmol/L | 4.5 (1.7) | 5.4 (2.1) | 4.7 (2.1) | 5.7 (3.9) | 4.1 (1.7) b | 6.3 (3.1) |
| Serum phosphate, mmol/L | 1.05 (0.16) | 1 (0.19) | 1.01 (0.17) | 0.97 (0.26) | 1.08 (0.21) | 0.93 (0.18) |
| Time in the primary study, y | 5.0 (1) | 5.0 (1) | 5.0 (1) | 4.0 (2) | 5.0 (0) | 5.0 (0) |
| Sunny vacation last 3 months, % | 16.7 | 11.8 | 11.1 | 0 | 12.5 | 0 |
| Used solarium last 3 months, % | 0 | 0 | 0 | 0 | 0 | 0 |
| Vitamin D supplementation, % | 59.5 | 47.1 | 61.1 | 54.5 | 75.0 | 75.0 |
| Calcium supplementation, % | 9.5 | 8.8 | 5.6 | 18.2 | 0 | 25.0 |
| All Subjects | Subjects With Fat Biopsy at Baseline | Subjects With Fat Biopsies at Baseline and After 12 Months | ||||
|---|---|---|---|---|---|---|
| Vitamin D Group | Placebo Group | Vitamin D Group | Placebo Group | Vitamin D Group | Placebo Group | |
| N | 42 | 34 | 18 | 11 | 8 | 4 |
| Females, % | 40.5 | 47.1 | 16.7 | 18.2 | 12.5 | 0 |
| Age, y | 67.5 (10) | 67.0 (15) | 67.5 (6) | 65.0 (17) | 67.5 (4) | 77.0 (15) |
| Weight, kg | 88.1 (22.9) | 86.5 (21.8) | 96.9 (20.9) | 88.4 (36.9) | 96.3 (21.0) | 86.2 (19.2) |
| BMI, kg/m2 | 29.5 (6.0) | 28.3 (7.3) | 30.9 (6.2) | 28.7 (9.2) | 31.0 (6.0) | 27.8 (4.4) |
| Glycosylated hemoglobin, type A1c, % | 6.1 (0.4) | 6.0 (0.4) | 6.1 (0.4) | 6.1 (0.2) | 6.0 (0.5) | 6.1 (0.2) |
| Serum 25(OH)D, nmol/L (mean ± SD) | 121.7 ± 28.9 a | 70.9 ± 16.7 | 107.6 ± 25.3 a | 68.8 ± 20.8 | 110.6 ± 30.1 | 80.3 ± 21.8 |
| Serum calcium, mmol/L | 2.35 (0.15) | 2.35 (0.11) | 2.35 (0.14) | 2.38 (0.11) | 2.34 (0.18) | 2.33 (0.04) |
| Serum PTH, pmol/L | 4.5 (1.7) | 5.4 (2.1) | 4.7 (2.1) | 5.7 (3.9) | 4.1 (1.7) b | 6.3 (3.1) |
| Serum phosphate, mmol/L | 1.05 (0.16) | 1 (0.19) | 1.01 (0.17) | 0.97 (0.26) | 1.08 (0.21) | 0.93 (0.18) |
| Time in the primary study, y | 5.0 (1) | 5.0 (1) | 5.0 (1) | 4.0 (2) | 5.0 (0) | 5.0 (0) |
| Sunny vacation last 3 months, % | 16.7 | 11.8 | 11.1 | 0 | 12.5 | 0 |
| Used solarium last 3 months, % | 0 | 0 | 0 | 0 | 0 | 0 |
| Vitamin D supplementation, % | 59.5 | 47.1 | 61.1 | 54.5 | 75.0 | 75.0 |
| Calcium supplementation, % | 9.5 | 8.8 | 5.6 | 18.2 | 0 | 25.0 |
The numbers represent median (IQR) for continuous variables if not specified otherwise. Abbreviations: SD, standard deviation.
a P < 0.01 vs the placebo group [independent samples t test for serum 25(OH)D; Mann-Whitney U test for not normally distributed data; Fisher's exact test for frequencies].
b P < 0.05 vs the placebo group [independent samples t test for serum 25(OH)D; Mann-Whitney U test for not normally distributed data; Fisher's exact test for frequencies].
During the 12-month follow-up period, body weight, serum calcium and PTH, exposure to UVB, and intake of vitamin D and calcium supplements showed only small variations within the vitamin D and placebo groups, with sporadic differences between groups. In the vitamin D group, serum PTH level was lowest in subjects at the 1-month visit and there were fewer subjects visiting the tropics at the 9-month visit and a higher intake of vitamin D at the 12-month visit than in subjects in the placebo group (Table 2).
Table 2.
Characteristics of Subjects in the Vitamin D Group (N = 42 at Baseline, N = 41 From Month 1) and the Placebo Group (N = 34) During the 12-Month Follow-up
| Intervention Group | Baseline | 1 Month | 3 Months | 6 Months | 9 Months | 12 Months | |
|---|---|---|---|---|---|---|---|
| BMI, kg/m2 | Vitamin D group | 29.5 (6.0) | — | — | 28.9 (5.4) | — | 29.2 (5.1) |
| Placebo group | 28.3 (7.3) | — | — | 28.3 (6.5) | — | 28.7 (7.1) | |
| Serum 25(OH)D, nmol/L (mean ± SD) | Vitamin D group a | 121.7 ± 28.9 b | 106.4 ± 24.5 b | 93.0 ± 22.4 b | 89.3 ± 20.6 b | 88.8 ± 22.5 b | 84.5 ± 21.1 c |
| Placebo group | 70.9 ± 16.7 | 69.4 ± 17.3 | 68.9 ± 19.7 | 70.1 ± 16.7 | 72.3 ± 19.1 | 73.1 ± 19.8 | |
| Fat 25(OH)D, ng/g d | Vitamin D group | 3.8 (0.9) b | — | — | — | — | 2.4 (1.0) |
| Placebo group | 2.5 (1.0) | — | — | — | — | 3.8 (2.3) | |
| Fat vitamin D (cholecalciferol), ng/g d | Vitamin D group | 209 (130) c , e | — | — | — | — | 100 (111) |
| Placebo group | 32 (65) | — | — | — | — | 56 (125) | |
| Serum calcium, mmol/L | Vitamin D group | 2.35 (0.15) | 2.36 (0.09) | 2.34 (0.08) | 2.36 (0.12) | 2.34 (0.10) | 2.33 (0.13) |
| Placebo group | 2.35 (0.11) | 2.34 (0.11) | 2.34 (0.09) | 2.34 (0.10) | 2.35 (0.09) | 2.35 (0.09) | |
| Serum PTH, pmol/L | Vitamin D group | 4.5 (1.7) | 4.6 (1.6) c | 4.5 (1.9) | 4.8 (2.3) | 4.9 (2.7) | 4.8 (2.4) |
| Placebo group | 5.4 (2.1) | 5.5 (2.4) | 5.3 (3.5) | 5.4 (1.7) | 5.2 (2.2) | 5.1 (2.3) | |
| Serum phosphate, mmol/L | Vitamin D group | 1.05 (0.16) | 1.05 (0.29) | 1.05 (0.27) | 1.06 (0.19) | 1.10 (0.28) | 1.04 (0.23) |
| Placebo group | 1.00 (0.19) | 1.03 (0.18) | 1.10 (0.21) | 1.06 (0.16) | 1.07 (0.19) | 1.04 (0.20) | |
| Sunny vacation, wk (median; min–max) | Vitamin D group | 0 (0); 0–3 | 0; 0–1.5 | 0; 0–2 | 0; 0–3 | 0; 0–1 c | 0; 0–4 |
| Placebo group | 0; 0–3 | 0; 0–1.5 | 0; 0–2 | 0; 0–4 | 0; 0–2 | 0; 0–4 | |
| Use of solarium, times (median; min–max) | Vitamin D group | 0 | 0 | 0 | 0 | 0 | 0 |
| Placebo group | 0 | 0; 0–1 | 0; 0–3 | 0; 0–8 | 0; 0–2 | 0 | |
| Vitamin D supplementation, IU/d | Vitamin D group | 200 (400) | 200 (400) | 200 (400) | 200 (400) | 200 (400) | 200 (400) c |
| Placebo group | 0 (363) | 0 (400) | 140 (400) | 140 (400) | 190 (400) | 0 (325) | |
| Calcium supplementation, mg/d (median; min–max) | Vitamin D group | 0; 0–1000 | 0; 0–1000 | 0; 0–1,000 | 0; 0–1000 | 0; 0–1000 | 0; 0–1000 |
| Placebo group | 0; 0–500 | 0; 0–500 | 0; 0–500 | 0; 0–500 | 0; 0–500 | 0; 0–500 |
| Intervention Group | Baseline | 1 Month | 3 Months | 6 Months | 9 Months | 12 Months | |
|---|---|---|---|---|---|---|---|
| BMI, kg/m2 | Vitamin D group | 29.5 (6.0) | — | — | 28.9 (5.4) | — | 29.2 (5.1) |
| Placebo group | 28.3 (7.3) | — | — | 28.3 (6.5) | — | 28.7 (7.1) | |
| Serum 25(OH)D, nmol/L (mean ± SD) | Vitamin D group a | 121.7 ± 28.9 b | 106.4 ± 24.5 b | 93.0 ± 22.4 b | 89.3 ± 20.6 b | 88.8 ± 22.5 b | 84.5 ± 21.1 c |
| Placebo group | 70.9 ± 16.7 | 69.4 ± 17.3 | 68.9 ± 19.7 | 70.1 ± 16.7 | 72.3 ± 19.1 | 73.1 ± 19.8 | |
| Fat 25(OH)D, ng/g d | Vitamin D group | 3.8 (0.9) b | — | — | — | — | 2.4 (1.0) |
| Placebo group | 2.5 (1.0) | — | — | — | — | 3.8 (2.3) | |
| Fat vitamin D (cholecalciferol), ng/g d | Vitamin D group | 209 (130) c , e | — | — | — | — | 100 (111) |
| Placebo group | 32 (65) | — | — | — | — | 56 (125) | |
| Serum calcium, mmol/L | Vitamin D group | 2.35 (0.15) | 2.36 (0.09) | 2.34 (0.08) | 2.36 (0.12) | 2.34 (0.10) | 2.33 (0.13) |
| Placebo group | 2.35 (0.11) | 2.34 (0.11) | 2.34 (0.09) | 2.34 (0.10) | 2.35 (0.09) | 2.35 (0.09) | |
| Serum PTH, pmol/L | Vitamin D group | 4.5 (1.7) | 4.6 (1.6) c | 4.5 (1.9) | 4.8 (2.3) | 4.9 (2.7) | 4.8 (2.4) |
| Placebo group | 5.4 (2.1) | 5.5 (2.4) | 5.3 (3.5) | 5.4 (1.7) | 5.2 (2.2) | 5.1 (2.3) | |
| Serum phosphate, mmol/L | Vitamin D group | 1.05 (0.16) | 1.05 (0.29) | 1.05 (0.27) | 1.06 (0.19) | 1.10 (0.28) | 1.04 (0.23) |
| Placebo group | 1.00 (0.19) | 1.03 (0.18) | 1.10 (0.21) | 1.06 (0.16) | 1.07 (0.19) | 1.04 (0.20) | |
| Sunny vacation, wk (median; min–max) | Vitamin D group | 0 (0); 0–3 | 0; 0–1.5 | 0; 0–2 | 0; 0–3 | 0; 0–1 c | 0; 0–4 |
| Placebo group | 0; 0–3 | 0; 0–1.5 | 0; 0–2 | 0; 0–4 | 0; 0–2 | 0; 0–4 | |
| Use of solarium, times (median; min–max) | Vitamin D group | 0 | 0 | 0 | 0 | 0 | 0 |
| Placebo group | 0 | 0; 0–1 | 0; 0–3 | 0; 0–8 | 0; 0–2 | 0 | |
| Vitamin D supplementation, IU/d | Vitamin D group | 200 (400) | 200 (400) | 200 (400) | 200 (400) | 200 (400) | 200 (400) c |
| Placebo group | 0 (363) | 0 (400) | 140 (400) | 140 (400) | 190 (400) | 0 (325) | |
| Calcium supplementation, mg/d (median; min–max) | Vitamin D group | 0; 0–1000 | 0; 0–1000 | 0; 0–1,000 | 0; 0–1000 | 0; 0–1000 | 0; 0–1000 |
| Placebo group | 0; 0–500 | 0; 0–500 | 0; 0–500 | 0; 0–500 | 0; 0–500 | 0; 0–500 |
The numbers represent median (IQR) for continuous variables if not specified otherwise.
a P < 0.01 linear trend within the group.
b P < 0.01 vitamin D vs placebo group, one-way analysis of variance [independent samples median test for fat 25(OH)D and fat vitamin D].
c P < 0.05 vitamin D vs placebo group, independent samples Mann-Whitney U test [one-way analysis of variance for serum 25(OH)D].
d At baseline, N = 18 and N = 11 in vitamin D and placebo groups, respectively. After 12 months, N = 8 and N = 4 in the vitamin D and placebo groups, respectively.
e P < 0.05 for baseline vs the 12-month value within the vitamin D group; related-samples Wilcoxon signed rank test.
Table 2.
Characteristics of Subjects in the Vitamin D Group (N = 42 at Baseline, N = 41 From Month 1) and the Placebo Group (N = 34) During the 12-Month Follow-up
| Intervention Group | Baseline | 1 Month | 3 Months | 6 Months | 9 Months | 12 Months | |
|---|---|---|---|---|---|---|---|
| BMI, kg/m2 | Vitamin D group | 29.5 (6.0) | — | — | 28.9 (5.4) | — | 29.2 (5.1) |
| Placebo group | 28.3 (7.3) | — | — | 28.3 (6.5) | — | 28.7 (7.1) | |
| Serum 25(OH)D, nmol/L (mean ± SD) | Vitamin D group a | 121.7 ± 28.9 b | 106.4 ± 24.5 b | 93.0 ± 22.4 b | 89.3 ± 20.6 b | 88.8 ± 22.5 b | 84.5 ± 21.1 c |
| Placebo group | 70.9 ± 16.7 | 69.4 ± 17.3 | 68.9 ± 19.7 | 70.1 ± 16.7 | 72.3 ± 19.1 | 73.1 ± 19.8 | |
| Fat 25(OH)D, ng/g d | Vitamin D group | 3.8 (0.9) b | — | — | — | — | 2.4 (1.0) |
| Placebo group | 2.5 (1.0) | — | — | — | — | 3.8 (2.3) | |
| Fat vitamin D (cholecalciferol), ng/g d | Vitamin D group | 209 (130) c , e | — | — | — | — | 100 (111) |
| Placebo group | 32 (65) | — | — | — | — | 56 (125) | |
| Serum calcium, mmol/L | Vitamin D group | 2.35 (0.15) | 2.36 (0.09) | 2.34 (0.08) | 2.36 (0.12) | 2.34 (0.10) | 2.33 (0.13) |
| Placebo group | 2.35 (0.11) | 2.34 (0.11) | 2.34 (0.09) | 2.34 (0.10) | 2.35 (0.09) | 2.35 (0.09) | |
| Serum PTH, pmol/L | Vitamin D group | 4.5 (1.7) | 4.6 (1.6) c | 4.5 (1.9) | 4.8 (2.3) | 4.9 (2.7) | 4.8 (2.4) |
| Placebo group | 5.4 (2.1) | 5.5 (2.4) | 5.3 (3.5) | 5.4 (1.7) | 5.2 (2.2) | 5.1 (2.3) | |
| Serum phosphate, mmol/L | Vitamin D group | 1.05 (0.16) | 1.05 (0.29) | 1.05 (0.27) | 1.06 (0.19) | 1.10 (0.28) | 1.04 (0.23) |
| Placebo group | 1.00 (0.19) | 1.03 (0.18) | 1.10 (0.21) | 1.06 (0.16) | 1.07 (0.19) | 1.04 (0.20) | |
| Sunny vacation, wk (median; min–max) | Vitamin D group | 0 (0); 0–3 | 0; 0–1.5 | 0; 0–2 | 0; 0–3 | 0; 0–1 c | 0; 0–4 |
| Placebo group | 0; 0–3 | 0; 0–1.5 | 0; 0–2 | 0; 0–4 | 0; 0–2 | 0; 0–4 | |
| Use of solarium, times (median; min–max) | Vitamin D group | 0 | 0 | 0 | 0 | 0 | 0 |
| Placebo group | 0 | 0; 0–1 | 0; 0–3 | 0; 0–8 | 0; 0–2 | 0 | |
| Vitamin D supplementation, IU/d | Vitamin D group | 200 (400) | 200 (400) | 200 (400) | 200 (400) | 200 (400) | 200 (400) c |
| Placebo group | 0 (363) | 0 (400) | 140 (400) | 140 (400) | 190 (400) | 0 (325) | |
| Calcium supplementation, mg/d (median; min–max) | Vitamin D group | 0; 0–1000 | 0; 0–1000 | 0; 0–1,000 | 0; 0–1000 | 0; 0–1000 | 0; 0–1000 |
| Placebo group | 0; 0–500 | 0; 0–500 | 0; 0–500 | 0; 0–500 | 0; 0–500 | 0; 0–500 |
| Intervention Group | Baseline | 1 Month | 3 Months | 6 Months | 9 Months | 12 Months | |
|---|---|---|---|---|---|---|---|
| BMI, kg/m2 | Vitamin D group | 29.5 (6.0) | — | — | 28.9 (5.4) | — | 29.2 (5.1) |
| Placebo group | 28.3 (7.3) | — | — | 28.3 (6.5) | — | 28.7 (7.1) | |
| Serum 25(OH)D, nmol/L (mean ± SD) | Vitamin D group a | 121.7 ± 28.9 b | 106.4 ± 24.5 b | 93.0 ± 22.4 b | 89.3 ± 20.6 b | 88.8 ± 22.5 b | 84.5 ± 21.1 c |
| Placebo group | 70.9 ± 16.7 | 69.4 ± 17.3 | 68.9 ± 19.7 | 70.1 ± 16.7 | 72.3 ± 19.1 | 73.1 ± 19.8 | |
| Fat 25(OH)D, ng/g d | Vitamin D group | 3.8 (0.9) b | — | — | — | — | 2.4 (1.0) |
| Placebo group | 2.5 (1.0) | — | — | — | — | 3.8 (2.3) | |
| Fat vitamin D (cholecalciferol), ng/g d | Vitamin D group | 209 (130) c , e | — | — | — | — | 100 (111) |
| Placebo group | 32 (65) | — | — | — | — | 56 (125) | |
| Serum calcium, mmol/L | Vitamin D group | 2.35 (0.15) | 2.36 (0.09) | 2.34 (0.08) | 2.36 (0.12) | 2.34 (0.10) | 2.33 (0.13) |
| Placebo group | 2.35 (0.11) | 2.34 (0.11) | 2.34 (0.09) | 2.34 (0.10) | 2.35 (0.09) | 2.35 (0.09) | |
| Serum PTH, pmol/L | Vitamin D group | 4.5 (1.7) | 4.6 (1.6) c | 4.5 (1.9) | 4.8 (2.3) | 4.9 (2.7) | 4.8 (2.4) |
| Placebo group | 5.4 (2.1) | 5.5 (2.4) | 5.3 (3.5) | 5.4 (1.7) | 5.2 (2.2) | 5.1 (2.3) | |
| Serum phosphate, mmol/L | Vitamin D group | 1.05 (0.16) | 1.05 (0.29) | 1.05 (0.27) | 1.06 (0.19) | 1.10 (0.28) | 1.04 (0.23) |
| Placebo group | 1.00 (0.19) | 1.03 (0.18) | 1.10 (0.21) | 1.06 (0.16) | 1.07 (0.19) | 1.04 (0.20) | |
| Sunny vacation, wk (median; min–max) | Vitamin D group | 0 (0); 0–3 | 0; 0–1.5 | 0; 0–2 | 0; 0–3 | 0; 0–1 c | 0; 0–4 |
| Placebo group | 0; 0–3 | 0; 0–1.5 | 0; 0–2 | 0; 0–4 | 0; 0–2 | 0; 0–4 | |
| Use of solarium, times (median; min–max) | Vitamin D group | 0 | 0 | 0 | 0 | 0 | 0 |
| Placebo group | 0 | 0; 0–1 | 0; 0–3 | 0; 0–8 | 0; 0–2 | 0 | |
| Vitamin D supplementation, IU/d | Vitamin D group | 200 (400) | 200 (400) | 200 (400) | 200 (400) | 200 (400) | 200 (400) c |
| Placebo group | 0 (363) | 0 (400) | 140 (400) | 140 (400) | 190 (400) | 0 (325) | |
| Calcium supplementation, mg/d (median; min–max) | Vitamin D group | 0; 0–1000 | 0; 0–1000 | 0; 0–1,000 | 0; 0–1000 | 0; 0–1000 | 0; 0–1000 |
| Placebo group | 0; 0–500 | 0; 0–500 | 0; 0–500 | 0; 0–500 | 0; 0–500 | 0; 0–500 |
The numbers represent median (IQR) for continuous variables if not specified otherwise.
a P < 0.01 linear trend within the group.
b P < 0.01 vitamin D vs placebo group, one-way analysis of variance [independent samples median test for fat 25(OH)D and fat vitamin D].
c P < 0.05 vitamin D vs placebo group, independent samples Mann-Whitney U test [one-way analysis of variance for serum 25(OH)D].
d At baseline, N = 18 and N = 11 in vitamin D and placebo groups, respectively. After 12 months, N = 8 and N = 4 in the vitamin D and placebo groups, respectively.
e P < 0.05 for baseline vs the 12-month value within the vitamin D group; related-samples Wilcoxon signed rank test.
Serum 25(OH)D levels were significantly higher in the vitamin D group than in the placebo group during the entire 12-month follow-up period (Fig. 1; Table 2); this result was not confounded by the length of vitamin D supplementation, exposure to UVB, intake of vitamin D or calcium supplements, serum PTH, phosphates, calcium, age, sex, month of sampling, or BMI. There was a significant positive correlation between serum 25(OH)D levels at baseline and after 12 months in both groups (ρ = 0.82, P < 0.01 in the vitamin D group and ρ = 0.88, P < 0.01 in the placebo group) (Table 3).
Figure 1.
Serum 25(OH)D levels (mean ± standard deviation) over 12 months after 3 to 5 years of intervention with vitamin D or placebo.
Figure 1.
Serum 25(OH)D levels (mean ± standard deviation) over 12 months after 3 to 5 years of intervention with vitamin D or placebo.
Table 3.
Correlations (Spearman ρ) Between 25(OH)D and Vitamin D (Cholecalciferol) in Serum and Fat at Baseline and After 12 Months in the Vitamin D and Placebo Groups
| Baseline | 12 Months | |||||||
|---|---|---|---|---|---|---|---|---|
| Intervention Group | Serum 25(OH)D (nmol/L) | Fat 25(OH)D (ng/g) | Fat Vitamin D (ng/g) | Serum 25(OH)D (nmol/L) | Fat 25(OH)D (ng/g) | Fat Vitamin D (ng/g) | ||
| Baseline | Serum 25(OH)D, nmol/L | Vitamin D group | 1 (N = 42) | 0.81 a (N = 18) | 0.62 a (N = 18) | 0.82 a (N = 41) | 0.74 b (N = 8) | 0.69 (N = 8) |
| Placebo group | 1 (N = 34) | 0.86 a (N = 11) | 0.45 (N = 11) | 0.88 a (N = 34) | −0.60 (N = 4) | 0.20 (N = 4) | ||
| Fat 25(OH)D, ng/g | Vitamin D group | 0.81 a (N = 18) | 1 (N = 18) | 0.68 a (N = 18) | 0.76 a (N = 17) | 0.40 (N = 7) | 0.57 (N = 7) | |
| Placebo group | 0.86 a (N = 11) | 1 (N = 11) | 0.45 (N = 11) | 0.92 a (N = 11) | −0.60 (N = 4) | 0.20 (N = 4) | ||
| Fat vitamin D, ng/g | Vitamin D group | 0.62 a (N = 18) | 0.68 a (N = 18) | 1 (N = 18) | 0.80 a (N = 17) | 0.20 (N = 7) | 1.0 a (N = 7) | |
| Placebo group | 0.45 (N = 11) | 0.45 (N = 11) | 1 (N = 11) | 0.45 (N = 11) | 0.20 (N = 4) | 1.0 a (N = 4) | ||
| 12 Months | Serum 25(OH)D, nmol/L | Vitamin D group | 0.82 a (N = 41) | 0.76 a (N = 17) | 0.80 a (N = 17) | 1 (N = 41) | 0.78 b (N = 17) | 0.81 b (N = 17) |
| Placebo group | 0.88 a (N = 34) | 0.92 a (N = 11) | 0.45 (N = 11) | 1 (N = 34) | −0.60 (N = 4) | 0.20 (N = 4) | ||
| Fat 25(OH)D, ng/g | Vitamin D group | 0.74 b (N = 8) | 0.40 (N = 7) | 0.20 (N = 7) | 0.78 b (N = 8) | 1 (N = 8) | 0.89 b (N = 8) | |
| Placebo group | −0.60 (N = 4) | −0.60 (N = 4) | 0.20 (N = 4) | −0.60 (N = 4) | 1 (N = 4) | 0.20 (N = 4) | ||
| Fat vitamin D, ng/g | Vitamin D group | 0.69 (N = 8) | 0.57 (N = 7) | 1.0 a (N = 7) | 0.81 b (N = 8) | 0.89 b (N = 8) | 1 (N = 8) | |
| Placebo group | 0.20 (N = 4) | 0.20 (N = 4) | 1.0 a (N = 4) | 0.20 (N = 4) | 0.20 (N = 4) | 1 (N = 4) | ||
| Baseline | 12 Months | |||||||
|---|---|---|---|---|---|---|---|---|
| Intervention Group | Serum 25(OH)D (nmol/L) | Fat 25(OH)D (ng/g) | Fat Vitamin D (ng/g) | Serum 25(OH)D (nmol/L) | Fat 25(OH)D (ng/g) | Fat Vitamin D (ng/g) | ||
| Baseline | Serum 25(OH)D, nmol/L | Vitamin D group | 1 (N = 42) | 0.81 a (N = 18) | 0.62 a (N = 18) | 0.82 a (N = 41) | 0.74 b (N = 8) | 0.69 (N = 8) |
| Placebo group | 1 (N = 34) | 0.86 a (N = 11) | 0.45 (N = 11) | 0.88 a (N = 34) | −0.60 (N = 4) | 0.20 (N = 4) | ||
| Fat 25(OH)D, ng/g | Vitamin D group | 0.81 a (N = 18) | 1 (N = 18) | 0.68 a (N = 18) | 0.76 a (N = 17) | 0.40 (N = 7) | 0.57 (N = 7) | |
| Placebo group | 0.86 a (N = 11) | 1 (N = 11) | 0.45 (N = 11) | 0.92 a (N = 11) | −0.60 (N = 4) | 0.20 (N = 4) | ||
| Fat vitamin D, ng/g | Vitamin D group | 0.62 a (N = 18) | 0.68 a (N = 18) | 1 (N = 18) | 0.80 a (N = 17) | 0.20 (N = 7) | 1.0 a (N = 7) | |
| Placebo group | 0.45 (N = 11) | 0.45 (N = 11) | 1 (N = 11) | 0.45 (N = 11) | 0.20 (N = 4) | 1.0 a (N = 4) | ||
| 12 Months | Serum 25(OH)D, nmol/L | Vitamin D group | 0.82 a (N = 41) | 0.76 a (N = 17) | 0.80 a (N = 17) | 1 (N = 41) | 0.78 b (N = 17) | 0.81 b (N = 17) |
| Placebo group | 0.88 a (N = 34) | 0.92 a (N = 11) | 0.45 (N = 11) | 1 (N = 34) | −0.60 (N = 4) | 0.20 (N = 4) | ||
| Fat 25(OH)D, ng/g | Vitamin D group | 0.74 b (N = 8) | 0.40 (N = 7) | 0.20 (N = 7) | 0.78 b (N = 8) | 1 (N = 8) | 0.89 b (N = 8) | |
| Placebo group | −0.60 (N = 4) | −0.60 (N = 4) | 0.20 (N = 4) | −0.60 (N = 4) | 1 (N = 4) | 0.20 (N = 4) | ||
| Fat vitamin D, ng/g | Vitamin D group | 0.69 (N = 8) | 0.57 (N = 7) | 1.0 a (N = 7) | 0.81 b (N = 8) | 0.89 b (N = 8) | 1 (N = 8) | |
| Placebo group | 0.20 (N = 4) | 0.20 (N = 4) | 1.0 a (N = 4) | 0.20 (N = 4) | 0.20 (N = 4) | 1 (N = 4) | ||
a P < 0.01 with the Spearman ρ test.
b P < 0.05 with the Spearman ρ test.
Table 3.
Correlations (Spearman ρ) Between 25(OH)D and Vitamin D (Cholecalciferol) in Serum and Fat at Baseline and After 12 Months in the Vitamin D and Placebo Groups
| Baseline | 12 Months | |||||||
|---|---|---|---|---|---|---|---|---|
| Intervention Group | Serum 25(OH)D (nmol/L) | Fat 25(OH)D (ng/g) | Fat Vitamin D (ng/g) | Serum 25(OH)D (nmol/L) | Fat 25(OH)D (ng/g) | Fat Vitamin D (ng/g) | ||
| Baseline | Serum 25(OH)D, nmol/L | Vitamin D group | 1 (N = 42) | 0.81 a (N = 18) | 0.62 a (N = 18) | 0.82 a (N = 41) | 0.74 b (N = 8) | 0.69 (N = 8) |
| Placebo group | 1 (N = 34) | 0.86 a (N = 11) | 0.45 (N = 11) | 0.88 a (N = 34) | −0.60 (N = 4) | 0.20 (N = 4) | ||
| Fat 25(OH)D, ng/g | Vitamin D group | 0.81 a (N = 18) | 1 (N = 18) | 0.68 a (N = 18) | 0.76 a (N = 17) | 0.40 (N = 7) | 0.57 (N = 7) | |
| Placebo group | 0.86 a (N = 11) | 1 (N = 11) | 0.45 (N = 11) | 0.92 a (N = 11) | −0.60 (N = 4) | 0.20 (N = 4) | ||
| Fat vitamin D, ng/g | Vitamin D group | 0.62 a (N = 18) | 0.68 a (N = 18) | 1 (N = 18) | 0.80 a (N = 17) | 0.20 (N = 7) | 1.0 a (N = 7) | |
| Placebo group | 0.45 (N = 11) | 0.45 (N = 11) | 1 (N = 11) | 0.45 (N = 11) | 0.20 (N = 4) | 1.0 a (N = 4) | ||
| 12 Months | Serum 25(OH)D, nmol/L | Vitamin D group | 0.82 a (N = 41) | 0.76 a (N = 17) | 0.80 a (N = 17) | 1 (N = 41) | 0.78 b (N = 17) | 0.81 b (N = 17) |
| Placebo group | 0.88 a (N = 34) | 0.92 a (N = 11) | 0.45 (N = 11) | 1 (N = 34) | −0.60 (N = 4) | 0.20 (N = 4) | ||
| Fat 25(OH)D, ng/g | Vitamin D group | 0.74 b (N = 8) | 0.40 (N = 7) | 0.20 (N = 7) | 0.78 b (N = 8) | 1 (N = 8) | 0.89 b (N = 8) | |
| Placebo group | −0.60 (N = 4) | −0.60 (N = 4) | 0.20 (N = 4) | −0.60 (N = 4) | 1 (N = 4) | 0.20 (N = 4) | ||
| Fat vitamin D, ng/g | Vitamin D group | 0.69 (N = 8) | 0.57 (N = 7) | 1.0 a (N = 7) | 0.81 b (N = 8) | 0.89 b (N = 8) | 1 (N = 8) | |
| Placebo group | 0.20 (N = 4) | 0.20 (N = 4) | 1.0 a (N = 4) | 0.20 (N = 4) | 0.20 (N = 4) | 1 (N = 4) | ||
| Baseline | 12 Months | |||||||
|---|---|---|---|---|---|---|---|---|
| Intervention Group | Serum 25(OH)D (nmol/L) | Fat 25(OH)D (ng/g) | Fat Vitamin D (ng/g) | Serum 25(OH)D (nmol/L) | Fat 25(OH)D (ng/g) | Fat Vitamin D (ng/g) | ||
| Baseline | Serum 25(OH)D, nmol/L | Vitamin D group | 1 (N = 42) | 0.81 a (N = 18) | 0.62 a (N = 18) | 0.82 a (N = 41) | 0.74 b (N = 8) | 0.69 (N = 8) |
| Placebo group | 1 (N = 34) | 0.86 a (N = 11) | 0.45 (N = 11) | 0.88 a (N = 34) | −0.60 (N = 4) | 0.20 (N = 4) | ||
| Fat 25(OH)D, ng/g | Vitamin D group | 0.81 a (N = 18) | 1 (N = 18) | 0.68 a (N = 18) | 0.76 a (N = 17) | 0.40 (N = 7) | 0.57 (N = 7) | |
| Placebo group | 0.86 a (N = 11) | 1 (N = 11) | 0.45 (N = 11) | 0.92 a (N = 11) | −0.60 (N = 4) | 0.20 (N = 4) | ||
| Fat vitamin D, ng/g | Vitamin D group | 0.62 a (N = 18) | 0.68 a (N = 18) | 1 (N = 18) | 0.80 a (N = 17) | 0.20 (N = 7) | 1.0 a (N = 7) | |
| Placebo group | 0.45 (N = 11) | 0.45 (N = 11) | 1 (N = 11) | 0.45 (N = 11) | 0.20 (N = 4) | 1.0 a (N = 4) | ||
| 12 Months | Serum 25(OH)D, nmol/L | Vitamin D group | 0.82 a (N = 41) | 0.76 a (N = 17) | 0.80 a (N = 17) | 1 (N = 41) | 0.78 b (N = 17) | 0.81 b (N = 17) |
| Placebo group | 0.88 a (N = 34) | 0.92 a (N = 11) | 0.45 (N = 11) | 1 (N = 34) | −0.60 (N = 4) | 0.20 (N = 4) | ||
| Fat 25(OH)D, ng/g | Vitamin D group | 0.74 b (N = 8) | 0.40 (N = 7) | 0.20 (N = 7) | 0.78 b (N = 8) | 1 (N = 8) | 0.89 b (N = 8) | |
| Placebo group | −0.60 (N = 4) | −0.60 (N = 4) | 0.20 (N = 4) | −0.60 (N = 4) | 1 (N = 4) | 0.20 (N = 4) | ||
| Fat vitamin D, ng/g | Vitamin D group | 0.69 (N = 8) | 0.57 (N = 7) | 1.0 a (N = 7) | 0.81 b (N = 8) | 0.89 b (N = 8) | 1 (N = 8) | |
| Placebo group | 0.20 (N = 4) | 0.20 (N = 4) | 1.0 a (N = 4) | 0.20 (N = 4) | 0.20 (N = 4) | 1 (N = 4) | ||
a P < 0.01 with the Spearman ρ test.
b P < 0.05 with the Spearman ρ test.
In the vitamin D group but not the placebo group, serum 25(OH)D level had a linear negative trend over time. The decline in serum 25(OH)D level was fastest during the first 3 months (Fig. 1). Thereafter, there was a steady and approximately linear fall in logarithmic 25(OH)D concentrations, and this phase was considered the terminal phase of 25(OH)D. In this phase, the half-life of serum 25(OH)D in the vitamin D group was 255 days, whereas the half-life in the preterminal phase (0 to 3 months) was 83.4 days.
At baseline, those given vitamin D had significantly more vitamin D and 25(OH)D stored in adipose tissue than subjects in the placebo group (a median of 209 ng/g vs 32 ng/g for vitamin D and a median of 3.8 ng/g vs 2.5 ng/g for 25(OH)D, respectively) (Table 2). Levels of vitamin D in the fat of those given vitamin D decreased significantly over 12 months, by 52% (calculated as medians), and were similar to those in the placebo group at the end of the study (Table 2).
There were significant positive intercorrelations between baseline serum 25(OH)D, fat 25(OH)D, and fat vitamin D levels (ρ, 0.62 to 0.81, P < 0.01) in the vitamin D group. Furthermore, there was a significant correlation between fat vitamin D at baseline and after 12 months (ρ = 0.89, P < 0.01) and a nonsignificant correlation between fat 25(OH)D at baseline and after 12 months (ρ = 0.40). Similar associations were seen in the placebo group (Table 3).
Discussion
When the weekly intake of 20,000 IU of vitamin D or placebo for 3 to 5 years ceased, we observed significantly higher serum 25(OH)D levels in the vitamin D group than in the placebo group over the entire follow-up period of 12 months. The difference between the two groups decreased from 51 nmol/L at the start of the follow-up to 11 nmol/L at the end of the study. This proves that long-term intake of vitamin D within the physiological range leads to storage in adipose tissue and probably also in other tissues of possible clinical importance. To our knowledge, these results have not been demonstrated before.
Vitamin D and its metabolites appear to have several functions in adipose tissue. Vitamin D is significantly stored and may then be slowly released from the adipose tissue when serum vitamin D levels decrease (10), or it may be hydroxylated within adipose tissue to 25(OH)D and 1,25(OH)2D (4). Because the serum vitamin D half-life is known to be 15 to 24 hours (19), long-term effects after high doses of oral vitamin D or UVB light would be impossible without storage either as vitamin D or as 25(OH)D. However, the amount of 25(OH)D found in adipose tissue was fairly small. It is therefore most likely that 25(OH)D in adipose tissue is more important as a substrate for 1α-hydroxylase to produce the active form 1,25(OH)2D and thereby regulate local metabolism than for storage and later release as 25(OH)D into the circulation.
In our study, the calculated half-life of serum 25(OH)D in the preterminal phase (0 to 3 months) was 83.4 days. This is almost identical to the half-life of 82 days found by Oliveri et al. (9). In their study, subjects were followed up for 2 months after a bolus of 100,000 IU of vitamin D with supplementation of 4800 IU of vitamin D per day from day 7 to day 20, and the half-life was calculated on the basis of the area under the curve (9). Similarly, in a study by Cipriani et al. (20), the serum 25(OH)D half-life was calculated to be 74 days (day 30 to day 90) after a single oral dose of 600,000 IU of vitamin D. Meanwhile, the terminal half-life of serum 25(OH)D (months 3 to 12) in our vitamin D group was 255 days, indicating a much slower decrease of serum 25(OH)D after the first 3 months.
Our approach to calculate half-life using 25(OH)D levels after subtraction of the levels in the placebo group minimized confounding due to ongoing low-dose vitamin D supplementation. This also enabled us to evaluate the half-life of circulating 25(OH)D accumulated as a result of high-dose vitamin D supplementation. If serum half-life were calculated using per se levels of 25(OH)D, the preterminal and terminal half-lives would have been even longer. Because the traced half-life of serum 25(OH)D is only about 15 to 25 days (7, 8), the gradual release of vitamin D stored in adipose and other tissues can at least partly explain the slowly declining serum 25(OH)D levels (4).
However, it should be stressed that in comparison with other studies on storage of vitamin D in adipose tissue (Supplemental Table 1), our randomized, controlled trial was very long, which might have caused saturation of vitamin D deposition at some time. Furthermore, only Heaney et al. (1) took biopsies both before and after the intervention with vitamin D. Thus, Heaney et al. (1) reported a mean baseline fat vitamin D level of 76 ± 16 ng/g, which was higher than the level in our placebo group (32 to 56 ng/g). After 3 months of supplementation with 50,000 IU of vitamin D weekly, the mean fat vitamin D level in the study by Heaney et al. (1) had increased to 180 ng/g, which was only slightly lower than the 209 ng/g in our vitamin D group after 20,000 IU per week administered for 3 to 5 years. Meanwhile, the mean serum 25(OH)D level at the end of the intervention in the study by Heaney et al. (1) was ∼55 nmol/L higher than the level in our vitamin D group at baseline. Thus, the fairly similar accumulation of vitamin D in the short-term study by Heaney et al. (1) and in our long-term study could indicate either that storage capacity for vitamin D is limited in adipose tissue or that a similar steady state in relation to input of vitamin D and level of serum 25(OH)D had been reached.
It should be noted that the circulating levels of 25(OH)D in the vitamin D group declined fairly rapidly. For adequate saturation of the 25-hydroxylase to zero-order kinetics, approximately 15 to 20 nmol/L of circulating vitamin D is required, which corresponds to a daily intake of approximately 2000 IU (21). Accordingly, the release of vitamin D from adipose and other tissues in our study was limited even after long-term high-dose vitamin D supplementation.
In another study, Heaney et al. (22) calculated that to maintain a serum 25(OH)D level of 80 nmol/L, a daily input or supply of 4000 IU (80 µg) from diet, production in the skin, or release from the body's vitamin D stores was needed. Given no vitamin D from the diet and no UVB exposure, the adipose tissue vitamin D stores after 3 months of supplementation with 50,000 IU per week would be rapidly depleted (within approximately 50 days), and the authors concluded that vitamin D stored in adipose tissue was of relatively minor importance (1). In contrast, our study, in which serum 25(OH)D levels were actually followed up over time after supplementation was stopped, showed prolonged benefit of stored vitamin D, which may indicate that significant storage also occurred in other tissues than fat.
However, it is worth mentioning that the calculations of adipose tissue are very rough; as in other studies in which vitamin D in adipose tissues has been reported, the levels appeared to vary not only with intake of vitamin D and type of subjects included but also with which fat compartment the biopsy was taken from (Supplemental Table 1) (23–27). Thus, in an autopsy study by Lawson et al. (24), the mean vitamin D level in perirenal fat was 45 ng/g, whereas in axillary fat it was 116 ng/g; similarly in a study by Pramyothin et al. (25), there was 297 ng/g of vitamin D in subcutaneous fat, 120 ng/g in mesenteric fat, and only 60 ng/g in omental fat. Nevertheless, as in our study, the number of subjects included in all these studies was small, and larger sample sizes are needed to draw firm conclusions.
Our study also had several other limitations. Although the intake of vitamin D supplements and sun exposure were registered and used as covariates, the diet was not standardized, nor was the intake of vitamin D from the diet calculated. We did not measure serum vitamin D levels, which could have given additional valuable information. We took biopsies only from subcutaneous adipose tissue, and we had no information on vitamin D storage in other tissues. The prevalence of vitamin D supplementation in the study group was high and could have interfered with the serum 25(OH)D levels. However, our method of calculating half-life using Δ values minimized bias caused by supplementation. Our study does not provide information on whether the storage and release of vitamin D in adipose tissue is an active or passive process; because we have biopsies from only one vitamin D group, we cannot reach conclusions about dose-response relationships.
Our study also has some strengths. It was double-blind and prospective; external factors such as sun exposure and BMI were comparable in the two groups. In addition, for serum 25(OH)D levels during the follow-up period, we had sufficient power to detect a significant difference between the two groups even after 12 months.
In conclusion, our study strongly indicates that vitamin D stored in adipose and possibly other tissues is physiologically important and may partly explain why serum 25(OH)D levels do not fall to critically low levels during the winter. However, this has to be tested experimentally with a design that is more similar to, or better mimics, ordinary summer and winter periods than our study design.
Abbreviations
-
25(OH)D
-
1,25(OH)2D
-
BMI
-
IQR
-
PTH
-
T2D
Acknowledgments
We acknowledge the assistance of the staff at the Clinical Research Unit (in particular, Aslaug Jakobsen and Birthe Angermo) and the Department of Medical Biochemistry at the University Hospital of North Norway.
Financial Support: This work was supported by grants from the Novo Nordisk Foundation (R195-A16126), the North Norway Regional Health Authorities (6856/SFP1029-12), UiT The Arctic University of Norway, the Norwegian Diabetes Association, the Research Council of Norway (184766), and University Hospital of North Norway (SFP1215-14). The funding institutions had no role in the study design, data analysis and interpretation, decision to publish, or preparation of the manuscript.
Clinical Trial Information : ClinicalTrials.gov no. NCT01729013 (registered 13 November 2012).
Disclosure Summary: The authors have nothing to disclose.
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Supplementary data
How Long Does Vitamin D Supplement Stay In Your System
Source: https://academic.oup.com/jcem/article/102/10/3731/4036364
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