The effect of daily intake of vitamin D-fortified yogurt drink, with and without added calcium, on serum adiponectin and sirtuins 1 and 6 in adult subjects with type 2 diabetes

Diabetes is the most prevalent metabolic disease worldwide and is projected to affect 700.2 million individuals by 2045, imposing substantial health, social, and economic burdens. Over 90% of cases are type 2 diabetes (T2D), largely driven by lifestyle factors and partly preventable through weight control, healthy diet, and physical activity. Vitamin D deficiency is common globally and has been linked to multiple diseases, including T2D. Prior studies suggest that improving vitamin D status may benefit glycemic control, insulin signaling, and inflammatory/oxidative biomarkers. Beyond direct effects on pancreatic β-cell function and insulin signaling, vitamin D may exert indirect effects via mediators such as adiponectin and sirtuins. Adiponectin, an adipokine from adipose tissue and muscle, improves insulin resistance via anti-inflammatory and antioxidative mechanisms, reduced hepatic glucose production, and enhanced skeletal muscle glucose uptake. Sirtuins (NAD+-dependent enzymes) are implicated in insulin resistance, inflammation, oxidative stress, and glucose homeostasis, with SIRT1 and SIRT6 particularly relevant to T2D. SIRT6 deficiency can impair glucose tolerance and β-cell function, while SIRT1 may enhance insulin sensitivity via inhibition of protein tyrosine phosphatase 1B. Research question: whether regular daily intake of vitamin D-fortified yogurt drink affects serum adiponectin, SIRT1, and SIRT6, and whether added calcium to the vitamin D-fortified yogurt modifies these effects.

Multiple reports link hypovitaminosis D to metabolic disorders including T2D, with evidence that raising vitamin D status can improve glycemic control and inflammatory/oxidative biomarkers. Experimental studies indicate vitamin D can modulate SIRT1 directly via the vitamin D receptor and related pathways (e.g., SIRT1/AMPK/GLUT4), and SIRT6 contributes to glucose homeostasis by enhancing insulin secretion and inhibiting gluconeogenesis and lipogenesis. Animal models show vitamin D insufficiency reduces SIRT1 activity, while supplementation upregulates SIRT1. The role of SIRT6 is complex, with both inhibition and enhancement showing potential benefits depending on context; however, strong evidence supports SIRT6’s importance in β-cell function. Adiponectin secretion is regulated by SIRT1 and Ero1-Lα, and SIRT1 can potentiate active vitamin D signaling via VDR deacetylation. The effect of calcium on diabetes-related outcomes (weight, insulin resistance) is debated, though mitochondrial matrix calcium may regulate sirtuin expression. Prior clinical trials by the same group showed vitamin D- or vitamin D+calcium-fortified yogurt drinks improved glycemic control and reduced inflammatory markers in T2D.

Design and participants: Secondary analysis using serum samples from a randomized, double-blind, placebo-controlled clinical trial (clinicaltrials.gov NCT01229891). Sample size was calculated with G*Power (power 80%, effect size 0.4), requiring at least 66 participants; 75 adults (30 men, 45 women), aged 30–60 years with confirmed T2D, were randomly selected. Exclusions: pregnancy/lactation, regular supplement use in prior 3 months, or diseases affecting vitamin D metabolism. Recruitment was from the Iranian Diabetes Society registry. Randomization and blinding: Participants were randomized to three groups for 12 weeks during late fall/winter (minimal dermal vitamin D synthesis). Interventions: (i) DY: 500 mL/day vitamin D-fortified yogurt drink providing 1000 IU vitamin D and 300 mg naturally occurring calcium; (ii) CDY: 500 mL/day vitamin D+calcium-fortified yogurt drink providing 1000 IU vitamin D and 500 mg calcium; (iii) PY: 500 mL/day plain yogurt drink with no detectable vitamin D and 300 mg calcium. Drinks were provided as two 250 mL bottles/day, preferably with lunch and dinner. Only the main investigator knew allocations; participants and research staff were blinded. Ethics: Approved by the Research Ethics Committee of NNFTRI; written informed consent obtained. Assessments: Baseline and post-intervention dietary intake (two 24-h recalls), anthropometry (weight, height; BMI calculated), and laboratory measures. Serum 25-hydroxyvitamin D [25(OH)D] measured by HPLC at a DEQAS-participating lab. HbA1c, adiponectin, and total body fat mass (FM) assessed per prior publications. Serum SIRT1 and SIRT6 were measured by enzyme immunoassay (ZellBio) using a microplate reader. Statistical analysis: Data presented as mean ± SD or 95% CI. Normality assessed by Shapiro–Wilk. Baseline comparisons by ANOVA (continuous) or chi-square (categorical). Associations by Pearson correlation. Multiple linear regression adjusted for baseline values assessed intervention effects; pairwise comparisons used Tukey’s adjustment. Analyses performed in STATA 16. Significance threshold p < 0.05.

• Participants: n = 75; mean age 50.7 ± 6.1 years; 60% women; groups balanced for age (p = 0.496) and gender (p = 0.513).
• Diet: No significant within- or between-group differences in energy, macronutrients, calcium, or vitamin D intakes (excluding fortified drink contributions).
• 25(OH)D: Significant increases in both fortified groups: DY 44.3 ± 18.5 to 75.7 ± 21.5 nmol/L (p < 0.001); CDY 38.1 ± 23.8 to 68.9 ± 23.9 nmol/L (p < 0.001); PY showed no increase (35.1 ± 22.7 to 32.3 ± 25.1 nmol/L, p = 0.343); between-group p < 0.001.
• Adiponectin: Significant within-group increases in DY and CDY: +60.4 ± 8.6 µg/L and +57.5 ± 6.4 µg/L, respectively (p < 0.001 both). Between-group differences not significant.
• Glycemic control (HbA1c): Decreased in DY (7.5 ± 1.8 to 6.7 ± 2.0%, p = 0.002) and CDY (8.0 ± 1.8 to 7.1 ± 1.4%, p = 0.027); increased in PY (7.6 ± 1.5 to 8.6 ± 1.4%, p = 0.001). Between-group after-intervention p = 0.027.
• Anthropometry: BMI decreased in DY (28.5 ± 3.9 to 27.5 ± 3.9 kg/m², p < 0.001) and CDY (28.1 ± 4.8 to 27.7 ± 4.9 kg/m², p = 0.006); FM% decreased in DY (32.7 ± 10.1 to 31.0 ± 9.8, p = 0.007) and CDY (35.3 ± 10.6 to 34.0 ± 10.0, p = 0.0225).
• Sirtuins: Significant increases only in CDY: SIRT1 3.59 ± 0.9 to 4.31 ± 0.7 ng/mL (p = 0.003), SIRT6 1.32 ± 0.4 to 1.81 ± 0.5 ng/mL (p = 0.001). DY and PY showed no significant sirtuin changes.
• Correlations (changes): SIRT1 correlated positively with SIRT6 (r = 0.375, p < 0.001), 25(OH)D (r = 0.336, p = 0.003), and adiponectin (r = 0.300, p = 0.008); inversely with HbA1c (r = -0.391, p = 0.001). SIRT6 correlated positively with 25(OH)D (r = 0.328, p = 0.004) and inversely with HbA1c (r = -0.252, p = 0.029). 25(OH)D inversely correlated with HbA1c.
• Regression (adjusted for baseline): vs PY, DY and CDY increased SIRT1 (B = 0.52, p = 0.002; B = 0.93, p < 0.001), 25(OH)D (DY B = 36.5, p < 0.001; CDY B = 34.3, p < 0.001), adiponectin (DY B = 40.6, p < 0.001; CDY B = 37.6, p = 0.001), and reduced BMI (DY B = -1.5, p < 0.001; CDY B = -0.8, p = 0.02) and FM% (DY B = -3.2, p < 0.001; CDY B = -2.4, p = 0.002). Only CDY increased SIRT6 (B = 0.51, p < 0.001). Both DY and CDY reduced HbA1c (B = -1.8 and -1.7, both p < 0.001).
• Mediation/associations (Table 5): Changes in 25(OH)D predicted increases in adiponectin (B = 0.38, p = 0.017) and decreases in HbA1c (B = -0.02, p < 0.001) and FM% (B = -0.03, p = 0.018). The 25(OH)D–adiponectin association became non-significant after adjusting for SIRT1, suggesting SIRT1 mediation. The 25(OH)D–HbA1c association remained significant after adjustment (model including HbA1c- and FM-changes: p = 0.011). Changes in SIRT6 were not associated with changes in HbA1c or adiponectin. 25(OH)D remained a significant predictor of FM% after adjusting for SIRT1 or SIRT6 (both p = 0.008).

This randomized, double-blind trial demonstrates that improving vitamin D status via daily fortified yogurt intake favorably modulates metabolic mediators in T2D. Both vitamin D-fortified drinks increased adiponectin, reduced HbA1c, BMI, and fat mass, while only the vitamin D+calcium formulation significantly increased circulating SIRT1 and SIRT6, indicating a potential enhancing role of added calcium on SIRT6. The observed relationships align with experimental data showing vitamin D’s direct interactions with SIRT1 through VDR and downstream AMPK/GLUT4 signaling, and the role of SIRT6 in glucose homeostasis and inflammation. The attenuation of the 25(OH)D–adiponectin association after controlling for SIRT1 suggests that vitamin D may increase adiponectin through SIRT1 upregulation. Conversely, the persistent association between 25(OH)D and HbA1c after SIRT1 adjustment implies SIRT1-independent pathways for vitamin D’s glycemic benefits. The greater effect of CDY on SIRT6 could reflect calcium’s regulatory influence on mitochondrial function and sirtuin expression, particularly in a population with suboptimal calcium intake. Collectively, these findings support vitamin D fortification as a feasible strategy to improve key metabolic markers in T2D and provide clinical evidence linking vitamin D status with sirtuin biology in humans.

Daily consumption of vitamin D–fortified yogurt drink for 12 weeks increased circulating SIRT1 and SIRT6 in adults with T2D, with the vitamin D+calcium–fortified product exerting a stronger effect on SIRT6. Vitamin D improved adiponectin likely via a SIRT1-dependent mechanism, while its effect on HbA1c appeared SIRT1-independent. These results elucidate potential mechanisms underlying vitamin D’s benefits on weight and glycemic control in T2D and support food fortification as a practical intervention. Future research should assess long-term effects, explore dose–response relationships, and evaluate additional sirtuins (e.g., SIRT3) to further clarify mechanistic pathways.

• Short intervention duration; findings reflect short-term effects and may not generalize to long-term outcomes. - Limited scope of sirtuin assessment; other relevant sirtuins (notably SIRT3) were not measured. - Generalizability may be limited to adults with T2D and similar baseline vitamin D and calcium intakes.