Risk Factors for Bone Fragility and the Effect of a High-Fiber Diet on Bone Health in Type 2 Diabetes: Clinical and Preclinical Studies

Patients with type 2 diabetes mellitus (T2DM) have an increased fracture risk despite higher to normal bone mineral density (BMD). This suggests that bone quality aspects are impaired in patients with T2DM, yet the underlying mechanisms are not fully understood. Given the recent worldwide rise in the prevalence of diabetes, diabetic bone disease is becoming an emergent health hazard. Thus, the management and treatment of diabetes are crucial to prevent further loss of quality of life and diabetic bone complications. Hence, the overall objective of this thesis is to study the underlying mechanisms of T2DM-induced bone fragility and the effect of nutritional therapy on diabetic bone disease. The general objective was addressed by two studies. Study 1: Deficits in bone microarchitecture, identifiable with high-resolution peripheral quantitative computed tomography (HR-pQCT), may contribute to increased bone fragility in T2DM. Previous studies of HR-pQCT parameters and T2DM have not reported separate results for men. Thus, we sought to investigate the association between T2DM and HR-pQCT parameters of the distal radius, distal tibia, and diaphyseal tibia in older men using data from the Osteoporotic Fractures in Men (MrOS) study. Additionally, in T2DM participants, we examined the association between diabetes-related factors and HR-pQCT parameters and assessed whether HR-pQCT predicts incident non-vertebral fracture. Analyses included 1789 men (mean age 84.44 ± 4.21 years, T2DM n=302) who underwent HR-pQCT scanning at visit 4. The results show that after adjustment for age, race, clinic site, and BMI, total cross-sectional area (Tt.AR) was significantly smaller in T2DM than non-T2DM at all three scan sites. At the diaphyseal tibia, cortical BMD was higher in T2DM compared to non-T2DM (p=0.0054). There were no differences in trabecular indices or cortical porosity between T2DM and non-T2DM. Estimated failure load was lower in T2DM compared to non-T2DM at the diaphyseal tibia (p=0.0142). Interestingly, in T2DM, diabetes duration ≥ 10 years was significantly associated with lower Tt.AR and trabecular area, and higher trabecular BMD, trabecular thickness, and cortical porosity at the distal radius, as well as increased trabecular thickness at the distal tibia and cortical BMD at the diaphyseal tibia than those with a diabetes duration < 10 years. Moreover, insulin use was significantly associated with lower cortical area fraction, cortical thickness, total BMD, and failure load at the distal radius, lower trabecular BMD and failure load at the distal tibia, and lower cortical thickness and failure load at the diaphyseal tibia compared to the non-insulin users. Finally, we found that lower cortical BMD, cortical area fraction, and cortical thickness of the distal sites were independently associated with an increased risk of incident non-vertebral fracture in T2DM. In conclusion, reductions in bone size may contribute to diabetic skeletal fragility in older men. Longer diabetes duration and insulin use were associated with smaller bone size and cortical bone deterioration in T2DM men. Study 2: The development of an effective intervention to prevent diabetic bone fragility is urgently needed. As lifestyle intervention represents an effective option for diabetes management and treatment, there is potential for an effect on bone health. Consuming dietary fiber is highly recommended for patients with T2DM. However, the effect of increasing dietary fiber intake on metabolism and bone health is still unclear. Thus, we investigated the effect of a high-fiber diet (HFD) on metabolic parameters and bone health in T2DM patients. For that, T2DM patients were randomized to follow a HFD (38g/day) or to make no diet changes for 12 weeks. We found that three months of HFD intervention is effective in improving metabolic outcomes (body weight and glucose control) but is not enough to make changes in bone in T2DM patients. Next, to further understand the possible mechanisms that link HFD and diabetic bone disease, we moved to a T2D mice model (Tallyho) fed a HFD (20% fermentable fiber pectin) or normal chow diet (3.1% fiber) for 8 weeks. The results show that HFD significantly improved glucose metabolism in T2D mice. Interestingly, gene expression analysis of brown adipose tissue showed that the HFD group exhibited increased mRNA levels of brown fat-related genes (PRDM16, p=0.0002; UCP1, p=0.0411). Furthermore, gene expression of insulin-like growth factor 1 (IGF-1) and adiponectin (Adipo Q) was increased after HFD in gonadal fat (IGF-1, p=0.0082; Adipo Q, p=0.0005). Regarding the bone phenotype, no differences were observed in trabecular and cortical parameters at the femur, whereas HFD significantly reduced vertebral BMD, bone volume per total volume (BV/TV), trabecular number (Tb. N), and increased trabecular separation (Tb. Sp). Mechanical testing showed no significant changes in both groups. In line with the uCT, gene expression analysis of the tibia demonstrated that HFD resulted in increased sclerostin (SOST) expression (p=0.0401) and decreased expression of LDL receptor-related protein 5 (Lrp5) (p=0.0446),bone formation (RUNX2, p=0.0131; ALP, p=0.0568, trend), and mineralization (Phex, p=0.0005; MEPE, p=0.0170) markers. However, histomorphometry and serum bone turnover markers showed that HFD reduced osteoclasts number and increased serum concentration of the bone formation marker P1NP. Interestingly, increased SOST expression correlated with worse bone microarchitecture, including lower BV/TV (r=-0.5189, p=0.0573, trend), Tb.N (r=-0.5490, p=0.0421), BMD (r=-0.5873, p=0.0272), and increased Tb.sp (r=0.5927, p=0.0255). Altogether, these findings show that HFD improves glucose metabolism and metabolic activity of brown adipose tissue but appears to induce bone loss in T2D mice. Moreover, our data suggests that sclerostin could be associated with HFD-induced bone loss in T2DM.