Abstract

Introduction

Low calcium intake and insufficient or deficient 25-hydroxyvitamin D (25[OH]D) levels are often observed in obese patients [1]. Consequently, interest in potential mechanisms by which calcium and 25[OH] D regulate body weight has increased. Evidence suggests that calcium and 25[OH]D increase whole body fat oxidation after single and multiple meals, and that calcium promotes a modest energy loss through increased fecal fat excretion [2]. The evidence is equivocal for a greater diet-induced thermogenesis, increased lipolysis, suppression of key lipogenic enzymes, decreased hunger or reduced energy/macronutrient intake [3]. The active form of vitamin D, 1,25-dihydroxy-vitamin D3, appears to decrease adipogenesis [4]. However, experiments in vitamin D receptor (VDR) null mice suggest a role for 25[OH]D to impair energy expenditure [5]. Overall, the role of 25[OH]D on energy expenditure and fat oxidation in humans remains controversial [6]. Emerging evidence suggests a potential improvement in insulin sensitivity following 25[OH]D supplementation that could increase substrate oxidation and promote weight loss. However, whether low levels of 25[OH]D are a cause or effect of insulin resistance and obesity remains unclear [7]. In addition, suppression of parathyroid hormone (PTH) by 25[OH]D supplementation could have a beneficial effect in obesity [8].

Several randomized controlled trials (RCTs) have evaluated the effect of 25[OH]D on weight loss. In a recent systematic review, the authors considered the data to be inadequate to draw conclusions and a meta-analysis was not attempted [9]. However, this review did not include data from studies published after 2010. Therefore, we performed a systematic review and meta-analysis of all RCTs published from January 1, 1998 to January 1, 2013 which compared 25[OH]D supplementation (cholecalciferol, or ergocalciferol) to placebo and had BMI change as either a primary or secondary outcome.

Methods

Eligible studies

We included only RCTs that studied orally administered 25[OH] D supplementation (cholecalciferol, or ergocalciferol) versus a placebo. Use of calcium supplements as the primary intervention strategy with 25[OH]D as a secondary supplement or vice versa was not restricted in our primary analysis. To be included in the primary analysis, we required reported administration of 25[OH]D supplementation and dosage, and baseline and follow-up BMI measurements. There was no restriction on age, gender, geographic location, or time of follow for study participants. We excluded uncontrolled trials, observational studies, and animal studies. Any studies that did not provide adequate or precise 25[OH]D administration (such as through 25[OH]D fortified foods or dietary sources of 25[OH]D) were not included. Those that did not contain a measureable or obtainable baseline or follow up BMI along with respective standard errors were excluded. Figure 1 shows the study selection process based on these criteria.

Figure 1

Flow chart of studies selected.

Quality assessment

The quality of each study was assessed using the scale provided in Martyn and Carroll [10] and a modified version of the scale provided in Downs and Black [11]. The Jadad scale consisted of three questions with a total score ranging from 0–5. The modified Downs scale consisted of 27 questions with total scores ranging from 0–28. For the Downs scale, question 27 was modified to a 0 or 1 scoring system based on whether or not a power calculation was reported. Three reviewers (K.D., D.L., and K.R.) independently rated the quality of all studies using both scales. In the case of discrepancies in item scores, the majority rated score was used. Quality scores used for our analysis reflect the total of majority score for each item on a given scale (Table 1).

Table 1

Quality assessment scores for selected articles

The Downs and Jadad scales assess the quality of reports of randomized controlled trials. The modified Downs scale consisted of 27 questions with total scores ranging from 0–28. The Jadad scale consisted of three questions with a total score ranging from 0–5. Higher scores indicate higher report quality for both scales.

The full quality scores for each study are located in Appendix B and Appendix C of the supplemental information.

Results

Description of Studies Selected

Of the 30 studies reviewed [3,13–39], 9 were included in the meta-analysis (Table 2), including one article which was broken down by participant age group. In total, 1651 participants were analyzed, most of whom were women (n= 1401, [82.6%]) with a mean age of 47.9 years (SD ± 11.7). Mean follow-up varied between 6 to 192 weeks and mean daily dose of 25[OH]D supplementation varied from 200 IU to 1,110 IU per day, with most falling between 200 IU and 400 IU. Five of the 9 studies had calcium supplements in both the control and intervention groups [13–16] (between 800 mg to 1200 mg), while the other 4 studies did not use calcium supplements [17–19]. The mean BMI for the intervention and control groups were 30.7 kg/m² (SD ± 3.2) and 30.4 kg/m² (SD ± 3.3), respectively.

Table 2

Summary of articles selected for analysis.

Results of continuous data

Meta-analysis showed no statistically significant differences between the treatment group (25[OH]D supplementation) and the control group (placebo) for BMI change. Figure 2 shows the forest plot of the random effects model, displaying BMI change for each of the 9 controlled trials weighted by 1/within- study variance. According to the protocol we proceeded to compare the ending values of the two groups (mean differences in BMI). The point estimates and the 95% confidence intervals were similar in the majority of the studies with one value skewed to the left widely. Heterogeneity was significant p value (p<0.0001). Of the 9 studies, five suggested a beneficial effect of 25[OH]D supplementation for the end point of BMI change (weight loss), whereas 3 showed a negative effect (weight gain) and one with non-perceptible change.

Figure 2

Forest plot of BMI change for each of the 9 trials weighted by 1/within-study variance using random effects model.

Meta-analysis showed no statistically significant differences in BMI change by use of 25[OH]D supplementation (p value=0.8). The I² showed that 0% of the effect of variance in dispersion in the true effects, and the T² reflected no heterogeneity between studies on the absolute scale. Figure 3 displays the funnel plots which suggested no evidence of publication bias. The test of Begg and Mazumdar, and of Egger suggested no publication bias, both with a p value of 0.18 (Figure 4).

Figure 3

Funnel plot with pseudo 95% confidence limits for each of the 9 trials to assess publication bias in the association between change in BMI and vitamin D supplementation.

Figure 4

Egger plot to assess publication bias in the association between change in BMI and vitamin D supplementation.

Two univariate linear meta-regressions were performed, using the 9 trials. As expected, no significant association was noted between 25[OH]D supplementation and BMI change after controlling for BMI or percentage of female participants in the study. Therefore, additional analyses sub-divided the studies according to baseline BMI values (i.e. high and low groups). Although not statistically significant, there may have been a trend towards a beneficial effect of 25[OH]D supplementation in subjects with higher baseline BMI values. Similar results were observed using a meta-regression analysis. No association was noted between 25[OH]D supplementation and BMI change after adjusting for baseline BMI groups (Adjusted R²=0%, I²=0%). Adjusting for baseline BMI as a continuous variable also demonstrated no associations between 25[OH]D supplementation and BMI change (R²=0%, I²=0%) (Table 3).

Table 3

Results of univariate meta-regression on summary estimates of BMI change by use of vitamin D supplementation

Each analysis adjusted for one covariate only due to limited number of studies (+BMI as a continuous variable, *prevalence of BMI ≥ 30 kg/m² and percentage of female study participants). Beta-coefficient represents the adjusted summary estimate of BMI change by use of vitamin D supplementation vs. no vitamin d supplementation. A negative beta-coefficient indicates a decrease in BMI with use of vitamin D supplementation after adjusting for the covariate. All 95% confidence intervals (CI) include zero indicating no significant difference in BMI change by use of vitamin D supplementation after adjusting for the covariate.

A sensitivity analysis was performed including only high quality studies based on quality assessment scores (Downs scores ≥ 23 and Jadad scores ≥ 4 were included). Again, the results showed no statistically significant differences between the intervention and control groups for BMI change.

Discussion

Based on the published literature to date, our pooled results indicate that oral 25[OH]D supplementation does not impact BMI change. Neither fixed-effect nor random effects meta-analyses showed any significant associations between 25[OH]D supplementation and BMI change. Specifically, in the fixed-effect analysis, we observed no variance in the differences between true values (I²=0, p=0.7 for heterogeneity chisquared test), suggesting that the true effect was the same in all of the studies. Findings were similar with the use of a random-effects analysis. Further analyses which adjusted for the percentage of females in the study, the dose of 25[OH]D supplementation, the co-administration of calcium, or the length of follow-up revealed no contribution of these factors to the potential association between 25[OH]D and BMI change. Most studies included populations with at least 50% females, and only one study focused on men. Gender does impact one’s ability to lose weight [40–43] and perhaps heterogeneity of effects exists between different genders. In contrast, the 25[OH]D dose was fairly consistent across studies, except for one outlier [17] The appropriate dose of 25[OH]D supplementation necessary to significantly impact BMI change remains unknown, and it is possible that higher doses of 25[OH]D could be more effective. Calcium has been suggested as another potential weight-loss aid [3,7,8] and the addition of this supplement in some of the studies at various doses further confounds the potential effects of 25[OH]D supplementation. Although calcium and 25[OH]D have been shown to increase whole body fat oxidation after meals [3] the exact effect of calcium on body weight, however, remains controversial [3,22,24–26].

One factor not measured in most studies was the baseline 25[OH]D level of study participants. Evidence suggests that a substantial number of adult Americans have low levels of 25[OH]D [44], and low serum levels of 25[OH]D have been shown to be inversely correlated with higher BMI values [21,45,46] and abnormal cardiovascular risk markers [47,48]. A higher baseline 25[OH]D level has also been correlated with an enhanced response to hypocaloric diets and loss of body fat in women [21]. Evidence from the 4th Tromso study in Norway, which included nearly 20 000 participants, suggests that dietary 25[OH]D intake may inversely correlate with BMI [46]. Shahar et al. [22] demonstrated that increases in serum 25[OH]D levels were associated with an increase in the likelihood of diet-induced weight loss [22]. Furthermore, an adequate response of 25[OH]D levels to oral supplementation may be inversely associated with baseline BMI, suggesting that body size should be considered when determining the appropriate dose [23]. While 25[OH]D supplementation over a relatively short-period can normalize serum levels, the effects are not likely maintained upon cessation of supplementation [49,50]. Therefore, the benefits from 25[OH]D supplementation may require prolonged dosing in order to maintain the desired effect.

Other potential confounders which varied across studies included diet, physical activity, and instruction/counseling. Due to the limited number of studies identified, further sub-analyses were not feasible, although the contribution of these variables remains possible. Additional factors not specifically discussed in this subset of studies that may also play a role in BMI change include race, socioeconomic status, type of community (urban vs. rural), and education.

Our quality assessments of each study revealed significant variability in the specified criteria for each assessment. Nevertheless, adjusting for these factors using meta-regression analyses did not change our findings of no significant differences in BMI change by 25[OH]D supplementation. The small number of trials included also limited the number of variables that could be examined simultaneously in meta-regression. While our initial search yielded a larger number of studies, the trials that met our inclusion criteria markedly decreased with further scrutiny. Several studies appeared to evaluate 25[OH] D supplementation and BMI change, but the data were presented as change in weight, adiposity, waist circumference or specific follow-up data needed for analysis were not provided [23,27–29]. Most studies reviewed focused on BMI and not on weight or other adiposity measures. BMI reflects both fat mass and muscle mass and may not be the best measure of adiposity. One randomized, double-blinded, placebo-controlled trial of >36,000 women demonstrated a small effect of calcium plus cholecalciferol on weight gain over several years [28]. The exclusion of quality studies such as this may have influenced our findings. Additionally, our findings may not be generalizable to non-obese populations since most studies selected for analyses included only obese populations.

Conclusions

This study represents the first meta-analysis to evaluate the association between 25[OH]D supplementation and BMI change. Our extensive evaluation of each study, including two independent quality assessments, yielded a thorough and important review of the current literature. Although no significant association between oral 25[OH]D supplementation and BMI change was noted, the possibility of a small (undetected) correlation remains possible. Larger studies would be required to fully explore a potential smaller association, which could potentially add to our limited understanding of the complexities of weight management.

Acknowledgment

References