Longitudinal Stability of Vitamin D Status and Its Association With Bone Mineral Density in Middle-Aged Australians

Kun Zhu; Michael Hunter; Jennie Hui; Kevin Murray; Alan James; Ee Mun Lim; Brian R. Cooke; John P. Walsh

J Endo Soc. 2023;7(2)

Abstract and Introduction

Abstract

Context: The skeletal effects of vitamin D remain controversial and it is uncertain whether variation in serum 25-hydroxyvitamin D (25OHD) levels over time influences bone mineral density (BMD).

Objective: We evaluated longitudinal stability of serum 25OHD and associations with changes in BMD in participants aged 46–70 years at baseline.

Methods: We studied 3698 Busselton Healthy Ageing Study participants (2040 female) with serum 25OHD and dual-energy x-ray absorptiometry (DXA) BMD assessments at baseline and at ~6 years follow-up. Restricted cubic splines were used to evaluate associations between changes in 25OHD and BMD.

Results: Mean season-corrected serum 25OHD was 81.3 ± 22.7 and 78.8 ± 23.1 nmol/L at baseline and 6 years, respectively, and showed moderate correlation (intraclass correlation coefficient: 0.724). Significant predictors of change in 25OHD concentration (Δ25OHD) included baseline 25OHD, change in body mass index and vitamin D supplementation at follow-up. Greater decline in serum 25OHD over time was associated with significantly greater reduction in BMD at total hip and femoral neck, but the magnitude of the differences was small (estimated differences 0.004 g/cm² and 0.005–0.007 g/cm², respectively, for lowest quartile of Δ25OHD compared with higher quartiles, adjusted for sex, baseline BMD, 25OHD, and demographics). No significant associations between Δ25OHD and lumbar spine BMD were observed. Increase in 25OHD levels was not associated with change in BMD.

Conclusions: In this predominantly vitamin D–replete middle-aged cohort, serum 25OHD showed moderate longitudinal stability. Declining serum 25OHD over time was associated with greater reduction in BMD at the total hip and femoral neck.

Introduction

Despite extensive research, the skeletal effects of insufficient vitamin D status in adults are still controversial. Observational studies, based on a single 25-hydroxyvitamin D (25OHD) measurement at baseline, have shown that circulating 25OHD levels are positively associated with bone mineral density (BMD) in middle to older aged adults, up to a threshold level (which in different studies varies widely from 50 nmol/L^[1–3] to 90–100 nmol/L),^[4,5] above which the relationship is attenuated or plateaus. By contrast, in Mendelian randomization studies (designed to minimize bias from confounding), genetically higher 25OHD status has not been positively associated with BMD.^[6,7] However, the Mendelian randomization studies have important limitations: only linear relationships between predictor and outcome variables were evaluated to date, and the single nucleotide polymorphisms (SNPs) included explain only a small percentage of the variance in 25OHD levels (2%-10%).^[8] Randomized controlled trials of vitamin D supplementation, recruiting mostly vitamin D–replete participants, have shown minimal effect on bone mass.^[9] The recent VITAL study of 2000 IU/day supplementation showed no significant effect overall on changes in dual-energy x-ray absorptiometry (DXA)-assessed BMD after 2 years in a subcohort of 771 participants (men ≥ 50 and women ≥ 55 years of age), although improved spine BMD (0.75%) and reduced bone loss (0.56%) at total hip were observed in a subgroup with low free 25OHD concentration at baseline (<14.2 pmol/L).^[10] Furthermore, no significant reduction in fracture risk was observed over 5.3 years follow-up in the whole cohort of 25 871 participants.^[11] The ViDA trial of 100 000 IU/month supplementation in 452 older adults showed ~0.5% reduction in bone loss at total hip and femoral neck after 2 years, and in a subgroup of participants with baseline serum 25OHD ≤ 30 nmol/L (n = 46), greater reduction in bone loss of 2% was observed at the spine and femoral neck.^[12]

Vitamin D status in individuals is not necessarily stable over time. Only a few studies have evaluated repeated measures of 25OHD in individuals at intervals ranging from 1 to 14 years. The results show moderate stability, with higher correlation coefficients for short intervals of 1 to 3 years (0.66–0.75)^[13,14] than for longer intervals such as 5 years (0.61) or 14 years (0.39–0.52).^[15,16] There have been no previous studies examining changes in vitamin D status and changes in BMD in large cohorts, and the influence of variation in serum 25OHD levels over time on age-related bone loss is uncertain. Therefore, studies with more than one measure of 25OHD will advance understanding of the relationship between vitamin D and skeletal health, and also aid interpretation of previous studies in which 25OHD was measured only at baseline.

In the Busselton Healthy Ageing Study, a cohort study of a representative middle-aged population in Western Australia, serum 25OHD and DXA BMD were assessed in participants at baseline and after ~6 years. The aims of this study were to evaluate: (1) stability of vitamin D status over a 6-year period; (2) predictors of change in serum 25OHD; and (3) associations between changes in 25OHD and changes in BMD.

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Abstract and Introduction
Methods
Results
Discussion
References

Table 1. Characteristics of participants
	All (n = 3698)	Male (n = 1658)	Female (n = 2040)	P value^a
White, %	99.1	99.3	98.9	0.261
Measurement interval, years	6.2 ± 0.9	6.2 ± 0.9	6.2 ± 0.9	0.977
Baseline characteristics
Age, years	57.9 ± 5.7	58.1 ± 5.8	57.8 ± 5.7	0.122
Measured serum 25OHD, nmol/L	81.6 ± 24.4	85.1 ± 24.4	78.7 ± 23.9	<0.001
Season-corrected serum 25OHD, nmol/L	81.4 ± 23.1	85.1 ± 22.8	78.4 ± 23.0	<0.001
Body mass index, kg/m ²	28.0 ± 4.8	28.4 ± 4.1	27.7 ± 5.4	<0.001
Physical activity, % Low
Low	22.7	19.7	25.0	<0.001
Medium	31.9	26.4	36.5
High	45.4	53.9	38.5
Smoking current, %	7.8	8.4	7.3	0.180
Alcohol intake, %
0–7 glasses/week	50.1	33.5	63.5	<0.001
7–14 glasses/week	22.3	20.4	23.9
>14 glasses/week	27.6	46.1	12.6
Osteoporosis medication, %	0.5	0.1	0.9	<0.001
Avoidance of dairy products, %	1.6	1.1	2.1	0.030
Vitamin D supplement, %
Baseline	11.7	5.6	16.6	<0.001
6 years	12.8	5.0	19.2	<0.001
Change over 6 years
ΔMeasur ed serum 25OHD, nmol/L	−3.0 ± 24.2	−6.3 ± 22.5	−0.4 ± 25.2	<0.001
ΔSeason-corrected serum 25OHD, nmol/L	−2.9 ± 21.7	−6.5 ± 18.8	−0.1 ± 23.3	<0.001
ΔBody mass index, kg/m²	0.3 ± 2.0	0.2 ± 1.7	0.4 ± 2.2	0.034

Values are mean ± SD unless otherwise stated. Abbreviation: 25OHD, 25-hydroxyvitamin D.
^a P values obtained using Student t test or chi-square test for comparisons between males and females.

Table 2. Pearson's correlation coefficients and intraclass correlation coefficients for raw and season-corrected serum 25-hydroxyvitamin D values at baseline and 6 years
	Pearson's correlation coefficient		Intraclass correlation coefficient (95% CI)
	Raw values	Season-corrected values	Raw values	Season-corrected values
All (n = 3698)	0.505	0.567	0.671 (0.649, 0.692)	0.724 (0.705, 0.741)
Sex
Male (n = 1658)	0.552	0.648	0.710 (0.681, 0.737)	0.786 (0.765, 0.806)
Female (n = 2040)	0.477	0.520	0.645 (0.613, 0.675)	0.683 (0.655, 0.710)
BMI category
<30 kg/m² (n = 2633)	0.496	0.563	0.663 (0.637, 0.688)	0.720 (0.698, 0.741)
≥30 kg/m² (n = 1065)	0.480	0.526	0.648 (0.603, 0.688)	0.688 (0.648, 0.723)
Age group
<55 years (n = 1261)	0.521	0.576	0.685 (0.648, 0.718)	0.731 (0.700, 0.759)
≥55 years (n = 2437)	0.496	0.563	0.663 (0.636, 0.689)	0.720 (0.697, 0.741)
Vitamin D supplementation at either time point
No (n = 2935)	0.566	0.644	0.723 (0.702, 0.742)	0.784 (0.767, 0.799)
Yes (n = 763)	0.350	0.385	0.517 (0.444, 0.581)	0.555 (0.487, 0.614)

Abbreviation: BMI, body mass index.

Table 3. Predictors of change in season-corrected serum 25-hydroxyvitamin D levels over 6 years
	Regression coefficients (95% CI)	P value	R^2a
Season-correct serum 25OHD at baseline, nmol/L	−0.423 (−0.449, −0.396)	<0.001	0.180
Sex—male	−2.021 (−3.352, −0.691)	0.003	0.002
Caucasian	−0.194 (−6.341, 5.953)	0.951	—
Baseline age, years	0.039 (−0.064, 0.143)	0.457	—
Baseline BMI, kg/m²	−0.295 (−0.423, −0.167)	<0.001	0.004
Change in BMI over 6 years, kg/m²	−1.256 (−1.558, −0.954)	<0.001	0.013
Current smoking	−1.110 (−3.315, 1.096)	0.324	—
Alcohol intake 7–14 glasses/week	0.424 (−1.081, 1.928)	0.581	—
Alcohol intake ≥14 glasses/week	1.892 (0.382, 3.402)	0.014	0.001
Moderate level of physical activity	−0.590 (−2.207, 1.027)	0.474	—
High level of physical activity	−0.058 (−1.603, 1.487)	0.941	—
Vitamin D supplement use at baseline	−0.643 (−2.541, 1.255)	0.507	—
Vitamin D supplement use at 6 years	17.276 (15.440, 19.112)	<0.001	0.065
Total variance explained			0.300

Abbreviation: 25OHD, 25-hydroxyvitamin D; BMI, body mass index.
^a R ² for individual predictor variable refers to semi-partial R ², which is the proportion of the variance associated uniquely with the predictor.

Table 4. Estimated mean baseline bone mineral density at mid-quartile levels of baseline serum 25-hydroxyvitamin D
	Q1	Q2	Q3	Q4
All (n = 3698), mid-quartile 25OHD	56.6 nmol/L	72.1 nmol/L	86.0 nmol/L	107.6 nmol/L
Total hip BMD, g/cm ²	1.083 (1.038, 1.127)	1.094 (1.050, 1.138)^a	1.102 (1.057, 1.146)^a,b	1.111 (1.066, 1.155)^a,b
Femoral neck BMD, g/cm ²	0.982 (0.939, 1.025)	0.997 (0.954, 1.039)^a	1.007 (0.964, 1.050)^a,b	1.010 (0.966, 1.053)^a,b
Male (n = 1658), mid-quartile 25OHD	60.8 nmol/L	76.0 nmol/L	89.4 nmol/L	110.8 nmol/L
Lumbar spine BMD, g/cm²	1.161 (1.051, 1.271)	1.175 (1.066, 1.285)	1.175 (1.065, 1.284)	1.190 (1.079, 1.301)^a
Female (n = 2040), mid-quartile 25OHD	53.9 nmol/L	69.2 nmol/L	83.1 nmol/L	103.7 nmol/L
Lumbar spine BMD, g/cm²	1.187 (1.118, 1.257)	1.196 (1.126, 1.265)	1.209 (1.138, 1.280)	1.199 (1.128, 1.271)

Values are restricted cubic spline estimated least square mean (95% CI) for the mid-quartile levels of baseline serum 25-hydroxyvitamin D (25OHD); analysis for lumbar spine BMD was made for males and females separately due to significant interaction with sex.
Abbreviations: 25OHD, 25-hydroxyvitamin D; BMD, bone mineral density; BMI, body mass index.
^a P < 0.05 vs Q1.
^b P < 0.05 vs Q2, adjusted for sex, race, and baseline age, BMI, physical activity, smoking, alcohol intake, osteoporosis medication, vitamin D supplement, and dairy avoidance.

Table 5. Estimated mean changes in bone mineral density over 6 years at mid-quartile levels of changes in serum 25-hydroxyvitamin D
	Q1	Q2	Q3	Q4
All (n = 3698), mid-quartile Δ25OHD	−23.7 nmol/L	−9.2 nmol/L	1.4 nmol/L	18.6 nmol/L
ΔTotal hip BMD, g/cm²	−0.025 (−0.041, −0.010)	−0.021 (−0.037, −0.006)^a	−0.021 (−0.036, −0.005)^a	−0.023 (−0.039, −0.007)
ΔFemoral neck BMD, g/cm²	−0.028 (−0.045, −0.011)	−0.023 (−0.040, −0.007)^a	−0.021 (−0.038, −0.005)^a	−0.023 (−0.040, −0.006)^a
Male (n = 1658), mid-quartile Δ25OHD	−25.4 nmol/L	−11.0 nmol/L	−1.8 nmol/L	11.6 nmol/L
ΔLumbar spine BMD, g/cm²	0.005 (−0.032, 0.042)	0.008 (−0.029, 0.046)	0.010 (−0.027, 0.047)	0.007 (−0.030, 0.045)
Female (n = 2040), mid-quartile Δ25OHD	−22.3 nmol/L	−7.0 nmol/L	4.7 nmol/L	23.7 nmol/L
ΔLumbar spine BMD, g/cm²	−0.044 (−0.071, −0.016)	−0.041 (−0.068, −0.014)	−0.042 (−0.069, −0.015)	−0.043 (−0.070, −0.015)

Values are restricted cubic spline estimated least square mean (95% CI) for the mid-quartile levels of change in serum 25-hydroxyvitamin D over 6 years (Δ25OHD); analysis for lumbar spine BMD was made for males and females separately due to significant interaction with sex.
Abbreviations: 25OHD, 25-hydroxyvitamin D; BMD, bone mineral density; BMI, body mass index.
^a P < 0.05 vs Q1, adjusted for sex, race, measurement interval, and baseline BMD, 25OHD, age, BMI, physical activity, smoking, alcohol intake, osteoporosis medication, vitamin D supplement, and dairy avoidance.

Table 6. Estimated mean changes in bone mineral density over 6 years by vitamin D status tracking groups
	Low n = 99	Decreasing n = 796	Increasing n = 538	Medium n = 750	High n = 1515	P value
ΔTotal hip BMD, g/cm²	−0.027 ± 0.004	−0.032 ± 0.002	−0.028 ± 0.002	−0.028 ± 0.002	−0.027 ± 0.001	0.076
ΔFemoral neck BMD, g/cm²	−0.037 ± 0.005	−0.036 ± 0.002^a,b,c	−0.031 ± 0.002	−0.030 ± 0.002	−0.032 ± 0.001	0.052
ΔLumbar spine BMD, g/cm²	0.001 ± 0.006	−0.005 ± 0.002	−0.006 ± 0.003	−0.004 ± 0.002	−0.004 ± 0.002	0.883

Values are estimated mean ± SEM. Vitamin D status tracking groups defined as: low (both serum 25-hydroxyvitamin D measures <50 nmol/L), decreasing (moved to lower category), increasing (moved to higher category), medium (both measures 50–74.9 nmol/L) or high (both measures ≥75 nmol/L).
Abbreviation: BMD, bone mineral density.
^a P < 0.05 vs Increasing.
^b P < 0.05 vs Medium.
^c P < 0.05 vs High, general linear model adjusted for sex, race, measurement interval, and baseline BMD, age, BMI, physical activity, smoking, alcohol intake, osteoporosis medication, vitamin D supplement, and dairy avoidance.
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