Reductions in body weight and percent fat mass increase the vitamin D status of obese subjects: a systematic review and metaregression analysis

Associations of vitamin D with insulin resistance, obesity, type 2 diabetes, and metabolic syndrome

The aim of this study is to determine the relationships of vitamin D with diabetes, insulin resistance obesity, and metabolic syndrome. Intra cellular vitamin D receptors and the 1-α hydroxylase enzyme are distributed ubiquitously in all tissues suggesting a multitude of functions of vitamin D. It plays an indirect but an important role in carbohydrate and lipid metabolism as reflected by its association with type 2 diabetes (T2D), metabolic syndrome, insulin secretion, insulin resistance, polycystic ovarian syndrome, and obesity. Peer-reviewed papers, related to the topic were extracted using key words, from PubMed, Medline, and other research databases. Correlations of vitamin D with diabetes, insulin resistance and metabolic syndrome were examined for this evidence-based review. In addition to the well-studied musculoskeletal effects, vitamin D decreases the insulin resistance, severity of T2D, prediabetes, metabolic syndrome, inflammation, and autoimmunity. Vitamin D exerts autocrine and paracrine effects such as direct intra-cellular effects via its receptors and the local production of 1,25(OH)₂D₃, especially in muscle and pancreatic β-cells. It also regulates calcium homeostasis and calcium flux through cell membranes, and activation of a cascade of key enzymes and cofactors associated with metabolic pathways. Cross-sectional, observational, and ecological studies reported inverse correlations between vitamin D status with hyperglycemia and glycemic control in patients with T2D, decrease the rate of conversion of prediabetes to diabetes, and obesity. However, no firm conclusions can be drawn from current studies, because (A) studies were underpowered; (B) few were designed for glycemic outcomes, (C) the minimum (or median) serum 25(OH) D levels achieved are not measured or reported; (D) most did not report the use of diabetes medications; (E) some trials used too little (F) others used too large, unphysiological and infrequent doses of vitamin D; and (G) relative paucity of rigorous clinical data on the effects of vitamin D sufficiency on non-calcium endpoints. Although a large number of observational studies support improving T2D, insulin resistance, obesity, and metabolic syndrome with vitamin D adequacy, there is a lack of conclusive evidence from randomized control clinical trials that, these disorders are prevented following optimization of serum levels of 25(OH)D. However, none of the currently conducted clinical studies would resolve these issues. Thus, specifically designed, new clinical studies are needed to be conducted in well-defined populations, following normalizing the serum vitamin D levels in vitamin D deficient prediabetes subjects, to test the hypothesis that hypovitaminosis D worsens these disorders and correction would alleviate it.

Effect of vitamin D deficiency in developed countries

INTRODUCTION

Vitamin D deficiency is common worldwide with adverse effects on skeletal health. In recent years, there has been great interest in non-classical actions of vitamin D. Basic research has uncovered actions in a range of non-skeletal tissues, and observational studies have identified inverse relationships with risk of a number of disease states including sarcopenia, obesity, the metabolic syndrome, cardiovascular disease, cancer and autoimmune diseases.

SOURCES OF DATA

PubMed, Medline and Cochrane Systematic Reviews.

AREAS OF AGREEMENT

Current evidence supports the use of vitamin D supplementation in deficiency to improve skeletal outcomes such as falls/fracture risk and bone mineral density.

AREAS OF CONTROVERSY

There is debate reflected in guidelines on optimal thresholds for circulating levels of vitamin D. Further studies are required to refine dosing regimens and treatment target levels of vitamin D.

GROWING POINTS

A number of studies have investigated the extra-skeletal effects of vitamin D deficiency but causality in humans has yet to be confirmed.

AREAS TIMELY FOR DEVELOPING RESEARCH

Large-scale randomized controlled trials incorporating data on vitamin D status at baseline and follow up, adverse events, and comparison of dosing regimens are required. It is imperative that studies are carried out with a diverse range of participants (age, gender and ethnicity), and settings to allow for a more individualized approach. In addition, we would advocate incorporating cutting-edge research tools into human studies to advance our understanding of the mechanisms of vitamin D action in extra-skeletal disease. This may involve multi-metabolite analysis of vitamin D metabolites, or unbiased approaches to assess regulation of gene/protein expression in tissues of interest.

The impact of cholecalciferol supplementation on the systemic inflammatory profile: a systematic review and meta-analysis of high-quality randomized controlled trials

Causal links between vitamin D status [25(OH)D] and systemic inflammation were examined through a systematic review of randomized controlled trials (RCTs). Selected RCTs were ⩾12 weeks, conducted in adults free of acute inflammatory disease, and of high-quality (Jadad score ⩾3). Of 14 studies that met our criteria, 9 studies (15 study arms) permitted extraction of data. There was no effect on the weighted mean difference (WMD) of IL-6 (WMD (95% confidence interval)=0.1, (-0.166, 0.366) pg/ml, P=0.462) or C-reactive protein (CRP) (WMD=-0.324, (-1.007, 0.359) mg/l, P=0.352). Subgroup analyses of trials achieving ⩾80 nmol/l indicated a trend for lower CRP (WMD=-0.834, (-1.726, 0.058) mg/l, P=0.067), however heterogeneity was significant (I²=66.7%, P=0.017). Studies employing a low dose (<1000 IU/d) showed increased CRP (WMD=0.615, (0.132, 1.098), P=0.013). In contrast, ⩾1000 IU/d had a favourable effect on CRP (WMD=-0.939, (-1.805, -0.073), P=0.034) but heterogeneity was significant (I²=61.3%, P=0.017). Meta-regression indicated that older age predicted a significant decrease in IL-6 (β=-0.02, (-0.034, -0.006) pg/ml, P=0.013) and CRP (β=-0.06, (-0.103, -0.017), P=0.01), whereas a greater percentage of females (β=0.027, (0.011, 0.044), P=0.004) and longer study duration independently predicted a higher WMD for CRP (β=0.049, (0.018, 0.079), P=0.005). Available high-quality RCTs did not support a beneficial effect of cholecalciferol on systemic IL-6 and CRP. Future studies should consider the confounding effects of age, gender and study duration, while possibly targeting an achieved 25(OH)D ⩾80 nmol/l.

A systematic review of the interrelation between diet- and surgery-induced weight loss and vitamin D status

Obesity is a major global health problem and has been associated with vitamin D deficiency. Intentional weight loss may alter vitamin D status and, conversely, vitamin D supplementation has been hypothesized to aid in weight loss. A systematic literature search in PubMed/Medline identified 3173 articles of which 37 studies (randomized controlled trials (RCT) [n=17], non-RCTs [n=20]) are summarized as effect of: (I) diet-induced weight loss on vitamin D status (n=7), (II) vitamin D supplementation on diet-induced weight loss (n=11), (III) surgery-induced weight loss on vitamin D status (n=15), and (IV) vitamin D supplementation after surgery-induced weight loss on vitamin D status (n=5). While all studies on the effect of diet-induced weight loss on vitamin D status have consistently reported increased vitamin D levels, the targeted percentage of weight loss that is necessary for an increase has varied between 5% and >10%. N=11 RCTs testing the effect of vitamin D supplementation observe that vitamin D supplementation does not result in increased weight loss, but may affect body fat loss. Vitamin D deficiency and subsequent hyperparathyroidism have been detected in post-surgery patients, and there is evidence that vitamin D supplementation improves these post-surgery complications. We review the current evidence addressing the role of vitamin D status and supplementation in diet- and surgery-induced weight loss. Subsequently, we highlight gaps in current research and suggest directions for future research including differences in vitamin D supplementation dosages, indoor vs. outdoor exercise, and the assessment of vitamin D status in different body pools.

Changes in circulating vitamin D levels with loss of adipose tissue

PURPOSE OF REVIEW

Low vitamin D levels have been extensively reported in obesity. Thus, the pandemic of obesity has been paralleled by a high prevalence of low vitamin D status. Given the well documented associations linking poor vitamin D status to adverse health outcomes (diabetes, cardiovascular disease, cancers, all-cause mortality), a proper understanding of the mechanisms linking excess adiposity to low vitamin D status is key to identify and implement effective interventions to replenish vitamin D levels in obese individuals. In this review, we will discuss recent literature investigating the effects of adipose tissue volume loss through energy restriction and/or physical activity on circulating 25-hydroxyvitamin D [25(OH)D] levels.

RECENT FINDINGS

Improvements of circulating 25(OH)D levels with adiposity loss through lifestyle interventions without supplementation is being reported by a growing number of studies, including recent randomized controlled trials.

SUMMARY

Low 25(OH)D is one of the metabolic disturbances associated with excess adiposity, particularly visceral adiposity. Recommendations for the treatment of obesity-related vitamin D deficiency should emphasize the role of visceral adiposity loss through healthy lifestyle habits, in conjunction with weight-adjusted vitamin D supplementation, not only to replenish 25(OH)D levels but also to address other visceral adiposity-related disturbances, such as insulin resistance, inflammation, hypertension, and dyslipidemia.

Vitamin D status and weight loss: a systematic review and meta-analysis of randomized and nonrandomized controlled weight-loss trials

BACKGROUND

Obesity is associated with lower concentrations of serum 25-hydroxyvitamin D; however, uncertainty exists as to the direction of causation. To date, meta-analyses of randomized controlled vitamin D-supplementation trials have shown no effect of raising circulating vitamin D on body weight, although several weight-loss-intervention trials have reported an increase in circulating vitamin D after weight reduction.

OBJECTIVE

We undertook a systematic review and meta-analysis of randomized and nonrandomized controlled trials to determine whether weight loss compared with weight maintenance leads to an increase in serum 25-hydroxyvitamin D.

DESIGN

A systematic search for controlled weight-loss-intervention studies published up to 31 March 2016 was performed. Studies that included participants of any age with changes in adiposity and serum 25-hydroxyvitamin D as primary or secondary outcomes were considered eligible.

RESULTS

We identified 4 randomized controlled trials (n = 2554) and 11 nonrandomized controlled trials (n = 917) for inclusion in the meta-analysis. Random assignment to weight loss compared with weight maintenance resulted in a greater increase in serum 25-hydroxyvitamin D with a mean difference of 3.11 nmol/L (95% CI: 1.38, 4.84 nmol/L) between groups, whereas a mean difference of 4.85 nmol/L (95% CI: 2.59, 7.12 nmol/L) was observed in nonrandomized trials. No evidence for a dose-response effect of weight loss on the change in serum 25-hydroxyvitamin D was shown overall.

CONCLUSIONS

Our results indicate that vitamin D status may be marginally improved with weight loss in comparison with weight maintenance under similar conditions of supplemental vitamin D intake. Although additional studies in unsupplemented individuals are needed to confirm these findings, our results support the view that the association between obesity and lower serum 25-hydroxyvitamin D may be due to reversed causation with increased adiposity leading to suboptimal concentrations of circulating vitamin D. This trial was registered at www.crd.york.ac.uk/PROSPERO/ as CRD42015023836.

Vitamin D status in young Swedish women with anorexia nervosa during intensive weight gain therapy

PURPOSE

Anorexia nervosa (AN) is associated with reduced bone mass and an increased fracture risk. The aim was to evaluate the vitamin D status and the association with body mass index (BMI), fat mass and bone mineral density (BMD) in patients with severe AN during a prospective intervention study of intensive nutrition therapy.

METHODS

This study comprised 25 Swedish female AN patients (20.1 ± 2.3 years), who were treated as inpatients for 12 weeks with a high-energy diet. Serum 25-hydroxyvitamin D (25(OH)D), calcium, phosphate and parathyroid hormone (PTH) were measured. BMD and body composition were assessed by dual-energy X-ray absorptiometry at study start and after 12 weeks.

RESULTS

Twenty-two patients completed the study. The mean weight gain was 9.9 kg and BMI (mean ± SD) increased from 15.5 ± 0.9 to 19.0 ± 0.9 kg/m², P < 0.0001. Fat mass increased from median 12 to 27 %. The median serum 25(OH)D level was 84 nmol/L at baseline, which decreased to 76 nmol/L, P < 0.05. PTH increased from median 21.9 to 30.0 ng/L, P < 0.0001. BMC increased during the study period, P < 0.001.

CONCLUSIONS

Serum 25(OH)D levels were adequate both at study start and completion, however, nominally decreased after the 12-week nutritional intervention. PTH increased subsequently, which coincide with the decreased 25(OH)D levels. The reduction in 25(OH)D could be due to an increased storage of vitamin D related to the increase in fat mass since vitamin D is sequestered in adipose tissue.