Introduction

In 1948 Albright et al. investigated bone development in diabetic children linking for the first time diabetes with bone mass loss [1]. Since then several studies have examined the relationship between type 1 diabetes (T1D) and skeletal disorders showing an impaired peak of bone mass and, consequently, an increased risk of osteoporosis and fractures [2], [3]. A large variety of mechanisms have been proposed to explain this finding, (such as hyperglycemia, inflammation and generation of advanced-glycation end products into the bone matrix), but most studies have focused on the lack of anabolic effect of insulin on osteoblasts, indicating T1D as a condition of low bone turnover, meaning both osteoblast and osteoclast functions are suppressed [4], [5], [6].

Vitamin D deficiency has been associated with T1D [7] and several studies suggest that vitamin D may exert immunomodulatory effects. [7], [8]. At high doses calcitriol prevents insulitis and the development of experimental diabetes, by acting on the defective suppressor cellular function, or cytokine expression modulation [7], [9]. Moreover, supplementation with vitamin D during early childhood may decrease the risk of developing T1D [10], [11]. Contrary to what was expected, we have found that calcitriol supplementation is not effective in improving β-cell function or reducing insulin requirement in patients with newly diagnosed T1D [12].

However, a large amount of data suggests the importance of vitamin D to preserve bone accrual in young subjects [13], [14]. Vitamin D supplementation in deficient children and adolescents restores mineralization and may improve bone health in deficient children and adolescents [15]. The beneficial effect of vitamin D is even more pronounced when supplementation starts in infancy or at the beginning of the adolescence [16], [17], and seems to be primarily dependent on a reduction of bone resorption [18]. The achievement of peak of bone mass plays a major role in determining bone health throughout life. A 10% rise in bone mass during adolescence or early adulthood can halve the risk of an osteoporotic fracture in older age [19].

Nevertheless, according to our previous study, vitamin D deficiency is common in young T1D patients [20], a condition that may contribute to lower BMD and increases fracture risk [21], [22]. While several evidences support the efficacy of vitamin D supplementation at any age, no data are available in T1D patients in terms of bone turnover and fracture prevention.

In addition, new research lines link bone metabolism to pancreatic function. Thus, it has been shown that osteocalcin (OC), an osteoblast-derived protein, has endocrine effects acting on islet cells by stimulating β-cell proliferation and insulin secretion [23], [24]. As an insulin secretagogue, OC stimulates the expression of insulin genes [24], [25]. These findings have been partially confirmed in humans, showing an inverse correlation between OC and glucose homeostasis [26]. However, no data are available in young T1D patients on OC and its effect on parameters of β-cell function.

Therefore, we conducted a post-hoc analysis using sera collected in a previous clinical trial, aiming to investigate the effect of 1 year calcitriol administration on β-cell function [12], on another parameter, the bone turnover. Moreover, we also tested in the same patients the relationship between OC and β-cell function in patients with recent onset of T1D.

Materials and Methods

Results

We analyzed bone turnover in 27 T1D patients (<12 weeks duration). Clinical features of patients are reported in Table 1.

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Table 1. Clinical and biochemical features of the study population at baseline.

https://doi.org/10.1371/journal.pone.0056488.t001

At baseline, no significant differences were found in terms of age and BMI between males and females. OC levels were significantly lower in female than in male patients (17.8±3.1 ng/ml vs. 43.9±6.9 ng/ml; P<0.01. Figure 1) while no other metabolic parameters as HbA1c, C-peptide differed between genders. Analyzing the whole group of patients, baseline OC levels were negatively correlated to age (r = −0.59, P = 0.02. Figure 2) while no significant correlations were found in relation to HbA1c, C-peptide or insulin requirement. BMI was positively correlated to age (r = 0.44, P<0.05) and to C-peptide (r = 0.58, P<0.001).

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Figure 1. Osteocalcin levels at baseline according to gender.

Osteocalcin levels were significantly higher in males than females (P<0.01).

https://doi.org/10.1371/journal.pone.0056488.g001

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Figure 2. Correlation between osteocalcin levels and age in type 1 diabetes subjects at diagnosis.

Baseline OC levels were negatively correlated to age (r = −0.59, P = 0.02). Dashed curves denote upper and lower 95% confidence intervals.

https://doi.org/10.1371/journal.pone.0056488.g002

At 1 year follow-up, OC and β-CrossLaps levels did not change significantly compared to diagnosis nor in the placebo not in the calcitriol-treated group. No significant differences were observed between calcitriol and placebo groups for OC (37.2±11.8 ng/ml vs 25.8±7.4 ng/ml; P = 0.39) and β-CrossLaps (0.33±0.07 ng/ml vs 0.50±0.12 ng/ml; P = 0.29) (table 2).

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Table 2. Biochemical markers of bone turnover during the trial.

https://doi.org/10.1371/journal.pone.0056488.t002

No significant correlations were found at 12 months in the placebo group between OC and parameters of metabolic control, such as HbA1c, C-peptide or insulin requirement. Changes in any metabolic parameters across time were unrelated to OC levels.

Discussion

The natural history of diabetes is related to long term complications that impact life quality and expectancy. The skeleton is an important target of diabetes complications and T1D patients exhibit bone loss and increased risk of fracture compared with healthy age-matched subjects [2]. Because T1D typically occurs in young age, the disruption in bone homeostasis affects bone accrual and peak of bone mass. A metanalysis confirmed the deleterious impact of T1D on bone health, showing a lower Z-score at lumbar spine and at proximal femur in patients with T1D compared to healthy subjects and consequent increased risk of hip fracture (RR = 6.94) [3].

The potential effect of calcitriol on bone turnover in T1D has not been fully evaluated. Administration of pharmacological doses of calcitriol in NOD mice showed a near two-fold increase of osteocalcin levels [9]. According to our knowledge, this is the first study assessing the effect of calcitriol supplementation on bone turnover in T1D patients. In this study 12 month intervention with the active form of vitamin D was not effective in modifying bone turnover markers.

To date, there are no available trials investigating the effect of calcitriol on bone metabolism in new diagnosed T1D young patients. Intervention studies conducted in adults and post-menopausal women have yielded variable results, ranging from no effective to a reduction of bone turnover [27]. Contrasting findings have been reported also by studies with vitamin D₃, especially in young people and adolescents. Schou et al. have shown that vitamin D supplementation in healthy children did not affect bone turnover significantly [28]. Conversely, another study on young subjects treated with D₃ showed that, similarly to an antiresorptive agent, vitamin D lowers primarily bone resorption [18].

This is also the first study to evaluate the association between OC levels and pancreatic function in T1D patients. Although limited by the small sample size, our results might suggest that metabolic action of OC is not relevant for controlling β-cell function in T1D. To date, few human studies explored the relationship between OC and glucose homeostasis and most of these were conducted in healthy subjects or in patients with type 2 diabetes (T2D) and limited by the cross-sectional design. OC levels have been reported to be lower in T2D compared to healthy subjects [29], inversely related to body mass index, fat mass, and plasma glucose [30], [31], [32], [33]. Our data follow a different trend, showing a lack of association between OC and β-cell function. Although human studies in patients with T2D and in animals support a positive feedback between osteoblast and β-cells, we may speculate that in a condition of continuous autoimmune damage against β-cells such as in T1D, the OC may be uneffective on controlling β-cell function. The relationship between OC and metabolic control in T1D subjects has been also investigated in a recent cross-sectional study by Thrailkill et al. [34]. In contrast to our findings, Thraikill et al. reported on a positive effect of OC on endogenous insulin production (assessed by authors as C-peptide/glucose ratio) [34], but several differences in term of sample size, population (new-onset vs. long-standing diabetes), design of the study (longitudinal vs. cross-sectional) and measure of endogenous insulin production (C-peptide vs. C-peptide/glucose ratio) limit the comparison between our study and the one conducted by Thraikill. In our study, OC levels were lower in females than males. This difference has been shown in healthy subjects, as a consequence of the effect of sex hormones on skeletal tissues during late-pubertal and post-pubertal stages [35], [36]. Other studies reproduced this difference also among young patients with T1D [37], [38]. Although OC levels are physiologically lower in females than males, we cannot exclude that diabetes-induced osteoblast disruption might amplify the difference we reported [39].

The significance of our findings may be limited by the lack of BMD measurements and the under-carboxylated osteocalcin (ucOC) dosage. However, a large body of clinical evidence suggests that total OC correlates to glucose homeostasis [26] similarly to ucOC [31], [40]. Moreover, although we did not measure serum levels of calcitirol, the same dosage was used by Walter et al. who shown significant raise of calcitiol serum concentrations in T1D after 9 months of treatment [41]. Finally, given the limited number of studied subjects and the post-hoc nature of the study, we cannot definitely rule out from this study the interplay between OC and pancreatic function in T1D patients and further studies, in larger cohorts of patients, are required.

In conclusion, the results of this study indicate that supplementation with 0.25ug calcitriol per day to patients with new-onset T1D does not affect circulating markers of bone turnover. However, larger ad-hoc trials with vitamin D investigating bone turnover and fracture outcomes in T1D are needed. OC levels were unrelated to β-cell function and other metabolic parameters suggesting that OC is ineffective to control pancreatic function in presence of aggressive autoimmune destruction. To disentangle the role of OC in T1D more studies are needed, with larger cohorts and longer follow-up.

Acknowledgments

Calcitriol and placebo tablets were prepared by Pharmacist Dr Raffaele Marzano who is greatly acknowledged for his contribution.

Investigators of the IMDIAB group are: Anguissola GB, Astorri E, Barchetta I, Benevento D, Beretta Anguissola G, Bizzarri C, Buzzetti R, Capizzi M, Cappa M, Cassone Faldetta MR, Cavallo MG, Cipolloni L, Cipponeri E, Crinò A, Di Stasio E, Fioriti E, Ghirlanda G, Guglielmi C, Lauria A, Maggi D, Manfrini S, Matteoli MC, Maurizi AR, Moretti C, Napoli N, Palermo A, Pitocco D, Portuesi R, Pozzilli P, Spera S, Strollo R, Tuccinardi D, Valente L, Visalli N.