Our findings showed that improvement of vitamin D status via daily intake of D-fortified yogurt drink resulted in a significant increase in serum concentrations of sirtuins 1 and 6. These findings are in accord with several experimental studies. For example, in rats fed on low vitamin D diet, secretion of SIRT1 was decreased [37] whereas in mice fed on high fat diet, vitamin D3 supplementation resulted in up-regulation of SIRT1 [38]. Current evidence shows that association of vitamin D and sirtuins may be through both direct and indirect pathways. Direct association of vitamin D with SIRT1, through vitamin D receptor (VDR), has been shown in experimental models [39–41]. On the other hand, vitamin D-induced up-regulation of SIRT1 together with pAMPK and GLUT-4 in adipose tissue suggests a role for these insulin-independent signaling molecules in glycemic control through vitamin D [38].
Sirtuin 6 contributes to glucose homeostasis by enhancing insulin secretion and inhibiting gluconeogenesis as well as lipogenesis [33]. In macrophages, SIRT6 suppresses obesity-induced inflammation and insulin resistance [28]. In fat-specific SIRT6 knockout mice fed on a high-fat diet, there was an augmented tendency to obesity, inflammation and insulin resistance. An under-expression of SIRT6 and related reduced adipose triacylglycerol lipase activity was observed in obese subjects [42]. Nevertheless, various studies have documented that both inhibition and enhancing SIRT6 may improve glucose tolerance in T2D. In murine model of T2D, inhibition of SIRT6 for ten days resulted in over-expression of muscular GLUT-1 and GLUT-4, enhanced glycolysis, decreased serum insulin as well as blood lipid concentrations and improved oral glucose tolerance [43]. On the other hand, experimental studies provided strong evidence for SIRT6 in pancreatic β-cells function [44, 45]. Notwithstanding, our findings provide clinical evidence for vitamin D-induced increased SIRT6, which was accompanied by a formerly reported significant improvement of glycemic status in T2D subjects [14].
We found consuming yogurt drink fortified with both vitamin D and calcium is more favorable to increase SIRT6. It has been shown that mitochondrial matrix calcium has a regulatory effect on sirtuin expression [46]. The effect of calcium intake on different aspects of diabetes including body weight and insulin resistance has been vastly studied [47–49] but still is controversial [50]. It is noteworthy that the mean calcium intake in our participants was about 700 mg/d, considerably less than the recommended intake for this age group. It is therefore likely that supplementing calcium intake in CDY group might, at least in part, contribute to its SIRT6 enhancing effect.
Disappearance of the association of changes of serum 25(OH)D and adiponectin concentrations following adjustment for changes of serum SIRT1 indicates a SIRT1-mediated effect of vitamin D on adiponectin secretion. Thus, vitamin D up-regulates SIRT1, as demonstrated in both animal model [51] and randomized clinical trial [21] and then SIRT1 in turn regulates adiponectin secretion. This finding is in accord with the report of regulation of adiponectin secretion by SIRT1 and endoplasmic reticulum oxidoreductase Ero1-Lα [52]. It is also documented that SIRT1 can potentiate 1,25-dihydroxycalciferol, the active form of vitamin D, via enzymatic deacetylation of VDR [53].
In the current study, there was a significant decrease in BMI in both vitamin D-supplemented groups despite no significant change in energy intake during 12 weeks intervention period. Though the enhancing effect of dairy calcium intake on weight loss in subjects with diabetes has already been reported [54], we found no significant change in PY group. Along the same line, a prospective study in Australia demonstrated an association of higher circulating 25(OH)D concentrations, but not dietary calcium intake, with lower risk of diabetes in adults [55].
In this study, reduction of BMI in both vitamin D supplemented groups was independent of changes of serum SIRT1 and SIRT6 concentrations. In oppose to this finding, it has been shown that vitamin D may have a fat-storing inhibitory effect on adipocytes which is mediated by NAD and SIRT1[56]. It is, therefore, likely that the effect of vitamin D on body weight may be mediated through both SIRT1-dependent and SIRT1-independent pathways. In a study, adipocyte and muscle cell culture media were treated by adding sera obtained from the overweight/obese subjects fed a low or high dairy diet for four weeks. The results demonstrated activation of SIRT1 and SIRT1-independent pathways in media treated with high-dairy dieters' sera resulting in enhanced mitochondrial biogenesis [57]. The regulatory action of SIRT1 on energy metabolism has been reported earlier [58].
The limitations of present study must be acknowledged. Firstly, the short term effects observed in this study does not necessarily reflect any possible long-term effects. Secondly, the other sirtuins with possible effect on pancreatic β-cell function, notably SIRT3 [59, 60], were not examined, either.